<?php

/**
 * @package KantPHP
 * @author  Zhenqiang Zhang <zhenqiang.zhang@hotmail.com>
 * @copyright (c) KantPHP Studio, All rights reserved.
 * @license http://www.opensource.org/licenses/BSD-3-Clause  The BSD 3-Clause License
 */

namespace Kant\Helper;

use Kant\Kant;
use Kant\Exception\InvalidParamException;
use Kant\Database\ActiveRecordInterface;
use Kant\Validators\StringValidator;
use Kant\Http\Request;
use Kant\Model\Model;

/**
 * BaseHtml provides concrete implementation for [[Html]].
 *
 * Do not use BaseHtml. Use [[Html]] instead.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @since 2.0
 */
class BaseHtml
{

	/**
	 *
	 * @var array list of void elements (element name => 1)
	 * @see http://www.w3.org/TR/html-markup/syntax.html#void-element
	 */
	public static $voidElements = [
		'area' => 1,
		'base' => 1,
		'br' => 1,
		'col' => 1,
		'command' => 1,
		'embed' => 1,
		'hr' => 1,
		'img' => 1,
		'input' => 1,
		'keygen' => 1,
		'link' => 1,
		'meta' => 1,
		'param' => 1,
		'source' => 1,
		'track' => 1,
		'wbr' => 1
	];

	/**
	 *
	 * @var array the preferred order of attributes in a tag. This mainly affects the order of the attributes
	 *      that are rendered by [[renderTagAttributes()]].
	 */
	public static $attributeOrder = [
		'type',
		'id',
		'class',
		'name',
		'value',
		'href',
		'src',
		'action',
		'method',
		'selected',
		'checked',
		'readonly',
		'disabled',
		'multiple',
		'size',
		'maxlength',
		'width',
		'height',
		'rows',
		'cols',
		'alt',
		'title',
		'rel',
		'media'
	];

	/**
	 *
	 * @var array list of tag attributes that should be specially handled when their values are of array type.
	 *      In particular, if the value of the `data` attribute is `['name' => 'xyz', 'age' => 13]`, two attributes
	 *      will be generated instead of one: `data-name="xyz" data-age="13"`.
	 * @since 2.0.3
	 */
	public static $dataAttributes = [
		'data',
		'data-ng',
		'ng'
	];

	/**
	 * Encodes special characters into HTML entities.
	 * The [[\Kant\Application::charset|application charset]] will be used for encoding.
	 * 
	 * @param string $content
	 *            the content to be encoded
	 * @param boolean $doubleEncode
	 *            whether to encode HTML entities in `$content`. If false,
	 *            HTML entities in `$content` will not be further encoded.
	 * @return string the encoded content
	 * @see decode()
	 * @see http://www.php.net/manual/en/function.htmlspecialchars.php
	 */
	public static function encode($content, $doubleEncode = true)
	{
		return htmlspecialchars($content, ENT_QUOTES | ENT_SUBSTITUTE, Kant::$app ? Kant::$app->charset : 'UTF-8', $doubleEncode);
	}

	/**
	 * Decodes special HTML entities back to the corresponding characters.
	 * This is the opposite of [[encode()]].
	 * 
	 * @param string $content
	 *            the content to be decoded
	 * @return string the decoded content
	 * @see encode()
	 * @see http://www.php.net/manual/en/function.htmlspecialchars-decode.php
	 */
	public static function decode($content)
	{
		return htmlspecialchars_decode($content, ENT_QUOTES);
	}

	/**
	 * Generates a complete HTML tag.
	 * 
	 * @param string|boolean|null $name
	 *            the tag name. If $name is `null` or `false`, the corresponding content will be rendered without any tag.
	 * @param string $content
	 *            the content to be enclosed between the start and end tags. It will not be HTML-encoded.
	 *            If this is coming from end users, you should consider [[encode()]] it to prevent XSS attacks.
	 * @param array $options
	 *            the HTML tag attributes (HTML options) in terms of name-value pairs.
	 *            These will be rendered as the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            
	 *            For example when using `['class' => 'my-class', 'target' => '_blank', 'value' => null]` it will result in the
	 *            html attributes rendered like this: `class="my-class" target="_blank"`.
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated HTML tag
	 * @see beginTag()
	 * @see endTag()
	 */
	public static function tag($name, $content = '', $options = [])
	{
		if ($name === null || $name === false) {
			return $content;
		}
		$html = "<$name" . static::renderTagAttributes($options) . '>';
		return isset(static::$voidElements[strtolower($name)]) ? $html : "$html$content</$name>";
	}

	/**
	 * Generates a start tag.
	 * 
	 * @param string|boolean|null $name
	 *            the tag name. If $name is `null` or `false`, the corresponding content will be rendered without any tag.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated start tag
	 * @see endTag()
	 * @see tag()
	 */
	public static function beginTag($name, $options = [])
	{
		if ($name === null || $name === false) {
			return '';
		}
		return "<$name" . static::renderTagAttributes($options) . '>';
	}

	/**
	 * Generates an end tag.
	 * 
	 * @param string|boolean|null $name
	 *            the tag name. If $name is `null` or `false`, the corresponding content will be rendered without any tag.
	 * @return string the generated end tag
	 * @see beginTag()
	 * @see tag()
	 */
	public static function endTag($name)
	{
		if ($name === null || $name === false) {
			return '';
		}
		return "</$name>";
	}

	/**
	 * Generates a style tag.
	 * 
	 * @param string $content
	 *            the style content
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated style tag
	 */
	public static function style($content, $options = [])
	{
		return static::tag('style', $content, $options);
	}

	/**
	 * Generates a script tag.
	 * 
	 * @param string $content
	 *            the script content
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated script tag
	 */
	public static function script($content, $options = [])
	{
		return static::tag('script', $content, $options);
	}

	/**
	 * Generates a link tag that refers to an external CSS file.
	 * 
	 * @param array|string $url
	 *            the URL of the external CSS file. This parameter will be processed by [[Url::to()]].
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The following option is specially handled:
	 *            
	 *            - condition: specifies the conditional comments for IE, e.g., `lt IE 9`. When this is specified,
	 *            the generated `link` tag will be enclosed within the conditional comments. This is mainly useful
	 *            for supporting old versions of IE browsers.
	 *            - noscript: if set to true, `link` tag will be wrapped into `<noscript>` tags.
	 *            
	 *            The rest of the options will be rendered as the attributes of the resulting link tag. The values will
	 *            be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated link tag
	 * @see Url::to()
	 */
	public static function cssFile($url, $options = [])
	{
		if (!isset($options['rel'])) {
			$options['rel'] = 'stylesheet';
		}
		$options['href'] = Url::to($url);

		if (isset($options['condition'])) {
			$condition = $options['condition'];
			unset($options['condition']);
			return self::wrapIntoCondition(static::tag('link', '', $options), $condition);
		} elseif (isset($options['noscript']) && $options['noscript'] === true) {
			unset($options['noscript']);
			return '<noscript>' . static::tag('link', '', $options) . '</noscript>';
		} else {
			return static::tag('link', '', $options);
		}
	}

	/**
	 * Generates a script tag that refers to an external JavaScript file.
	 * 
	 * @param string $url
	 *            the URL of the external JavaScript file. This parameter will be processed by [[Url::to()]].
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The following option is specially handled:
	 *            
	 *            - condition: specifies the conditional comments for IE, e.g., `lt IE 9`. When this is specified,
	 *            the generated `script` tag will be enclosed within the conditional comments. This is mainly useful
	 *            for supporting old versions of IE browsers.
	 *            
	 *            The rest of the options will be rendered as the attributes of the resulting script tag. The values will
	 *            be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated script tag
	 * @see Url::to()
	 */
	public static function jsFile($url, $options = [])
	{
		$options['src'] = Url::to($url);
		if (isset($options['condition'])) {
			$condition = $options['condition'];
			unset($options['condition']);
			return self::wrapIntoCondition(static::tag('script', '', $options), $condition);
		} else {
			return static::tag('script', '', $options);
		}
	}

	/**
	 * Wraps given content into conditional comments for IE, e.g., `lt IE 9`.
	 * 
	 * @param string $content
	 *            raw HTML content.
	 * @param string $condition
	 *            condition string.
	 * @return string generated HTML.
	 */
	private static function wrapIntoCondition($content, $condition)
	{
		if (strpos($condition, '!IE') !== false) {
			return "<!--[if $condition]><!-->\n" . $content . "\n<!--<![endif]-->";
		}
		return "<!--[if $condition]>\n" . $content . "\n<![endif]-->";
	}

	/**
	 * Generates the meta tags containing CSRF token information.
	 * 
	 * @return string the generated meta tags
	 * @see Request::enableCsrfValidation
	 */
	public static function csrfMetaTags()
	{
		$request = Kant::$app->getRequest();
		if ($request instanceof Request && $request->enableCsrfValidation) {
			return static::tag('meta', '', [
						'name' => 'csrf-param',
						'content' => $request->csrfParam
					]) . "\n    " . static::tag('meta', '', [
						'name' => 'csrf-token',
						'content' => $request->getCsrfToken()
					]) . "\n";
		} else {
			return '';
		}
	}

	/**
	 * Generates a form start tag.
	 * 
	 * @param array|string $action
	 *            the form action URL. This parameter will be processed by [[Url::to()]].
	 * @param string $method
	 *            the form submission method, such as "post", "get", "put", "delete" (case-insensitive).
	 *            Since most browsers only support "post" and "get", if other methods are given, they will
	 *            be simulated using "post", and a hidden input will be added which contains the actual method type.
	 *            See [[\Kant\Http\Request::methodParam]] for more details.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 *            Special options:
	 *            
	 *            - `csrf`: whether to generate the CSRF hidden input. Defaults to true.
	 *            
	 * @return string the generated form start tag.
	 * @see endForm()
	 */
	public static function beginForm($action = '', $method = 'post', $options = [])
	{
		$action = Url::to($action);

		$hiddenInputs = [];

		$request = Kant::$app->getRequest();
		if ($request instanceof Request) {
			if (strcasecmp($method, 'get') && strcasecmp($method, 'post')) {
				// simulate PUT, DELETE, etc. via POST
				$hiddenInputs[] = static::hiddenInput($request->methodParam, $method);
				$method = 'post';
			}
			$csrf = ArrayHelper::remove($options, 'csrf', true);

			if ($csrf && $request->enableCsrfValidation && strcasecmp($method, 'post') === 0) {
				$hiddenInputs[] = static::hiddenInput($request->csrfParam, $request->getCsrfToken());
			}
		}

		if (!strcasecmp($method, 'get') && ($pos = strpos($action, '?')) !== false) {
			// query parameters in the action are ignored for GET method
			// we use hidden fields to add them back
			foreach (explode('&', substr($action, $pos + 1)) as $pair) {
				if (($pos1 = strpos($pair, '=')) !== false) {
					$hiddenInputs[] = static::hiddenInput(urldecode(substr($pair, 0, $pos1)), urldecode(substr($pair, $pos1 + 1)));
				} else {
					$hiddenInputs[] = static::hiddenInput(urldecode($pair), '');
				}
			}
			$action = substr($action, 0, $pos);
		}

		$options['action'] = $action;
		$options['method'] = $method;
		$form = static::beginTag('form', $options);
		if (!empty($hiddenInputs)) {
			$form .= "\n" . implode("\n", $hiddenInputs);
		}

		return $form;
	}

	/**
	 * Generates a form end tag.
	 * 
	 * @return string the generated tag
	 * @see beginForm()
	 */
	public static function endForm()
	{
		return '</form>';
	}

	/**
	 * Generates a hyperlink tag.
	 * 
	 * @param string $text
	 *            link body. It will NOT be HTML-encoded. Therefore you can pass in HTML code
	 *            such as an image tag. If this is coming from end users, you should consider [[encode()]]
	 *            it to prevent XSS attacks.
	 * @param array|string|null $url
	 *            the URL for the hyperlink tag. This parameter will be processed by [[Url::to()]]
	 *            and will be used for the "href" attribute of the tag. If this parameter is null, the "href" attribute
	 *            will not be generated.
	 *            
	 *            If you want to use an absolute url you can call [[Url::to()]] yourself, before passing the URL to this method,
	 *            like this:
	 *            
	 *            ```php
	 *            Html::a('link text', Url::to($url, true))
	 *            ```
	 *            
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated hyperlink
	 * @see \Kant\Helper\Url::to()
	 */
	public static function a($text, $url = null, $options = [])
	{
		if ($url !== null) {
			Kant::info($url);
			if (is_array($url)) {
				$options['href'] = Url::to($url[0], array_slice($url, 1));
			} else {
				$options['href'] = Url::to($url);
			}
		}
		return static::tag('a', $text, $options);
	}

	/**
	 * Generates a mailto hyperlink.
	 * 
	 * @param string $text
	 *            link body. It will NOT be HTML-encoded. Therefore you can pass in HTML code
	 *            such as an image tag. If this is coming from end users, you should consider [[encode()]]
	 *            it to prevent XSS attacks.
	 * @param string $email
	 *            email address. If this is null, the first parameter (link body) will be treated
	 *            as the email address and used.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated mailto link
	 */
	public static function mailto($text, $email = null, $options = [])
	{
		$options['href'] = 'mailto:' . ($email === null ? $text : $email);
		return static::tag('a', $text, $options);
	}

	/**
	 * Generates an image tag.
	 * 
	 * @param array|string $src
	 *            the image URL. This parameter will be processed by [[Url::to()]].
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated image tag
	 */
	public static function img($src, $options = [])
	{
		$options['src'] = Url::to($src);
		if (!isset($options['alt'])) {
			$options['alt'] = '';
		}
		return static::tag('img', '', $options);
	}

	/**
	 * Generates a label tag.
	 * 
	 * @param string $content
	 *            label text. It will NOT be HTML-encoded. Therefore you can pass in HTML code
	 *            such as an image tag. If this is is coming from end users, you should [[encode()]]
	 *            it to prevent XSS attacks.
	 * @param string $for
	 *            the ID of the HTML element that this label is associated with.
	 *            If this is null, the "for" attribute will not be generated.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated label tag
	 */
	public static function label($content, $for = null, $options = [])
	{
		$options['for'] = $for;
		return static::tag('label', $content, $options);
	}

	/**
	 * Generates a button tag.
	 * 
	 * @param string $content
	 *            the content enclosed within the button tag. It will NOT be HTML-encoded.
	 *            Therefore you can pass in HTML code such as an image tag. If this is is coming from end users,
	 *            you should consider [[encode()]] it to prevent XSS attacks.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated button tag
	 */
	public static function button($content = 'Button', $options = [])
	{
		if (!isset($options['type'])) {
			$options['type'] = 'button';
		}
		return static::tag('button', $content, $options);
	}

	/**
	 * Generates a submit button tag.
	 *
	 * Be careful when naming form elements such as submit buttons. According to the [jQuery documentation](https://api.jquery.com/submit/) there
	 * are some reserved names that can cause conflicts, e.g. `submit`, `length`, or `method`.
	 *
	 * @param string $content
	 *            the content enclosed within the button tag. It will NOT be HTML-encoded.
	 *            Therefore you can pass in HTML code such as an image tag. If this is is coming from end users,
	 *            you should consider [[encode()]] it to prevent XSS attacks.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated submit button tag
	 */
	public static function submitButton($content = 'Submit', $options = [])
	{
		$options['type'] = 'submit';
		return static::button($content, $options);
	}

	/**
	 * Generates a reset button tag.
	 * 
	 * @param string $content
	 *            the content enclosed within the button tag. It will NOT be HTML-encoded.
	 *            Therefore you can pass in HTML code such as an image tag. If this is is coming from end users,
	 *            you should consider [[encode()]] it to prevent XSS attacks.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated reset button tag
	 */
	public static function resetButton($content = 'Reset', $options = [])
	{
		$options['type'] = 'reset';
		return static::button($content, $options);
	}

	/**
	 * Generates an input type of the given type.
	 * 
	 * @param string $type
	 *            the type attribute.
	 * @param string $name
	 *            the name attribute. If it is null, the name attribute will not be generated.
	 * @param string $value
	 *            the value attribute. If it is null, the value attribute will not be generated.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated input tag
	 */
	public static function input($type, $name = null, $value = null, $options = [])
	{
		if (!isset($options['type'])) {
			$options['type'] = $type;
		}
		$options['name'] = $name;
		$options['value'] = $value === null ? null : (string) $value;
		return static::tag('input', '', $options);
	}

	/**
	 * Generates an input button.
	 * 
	 * @param string $label
	 *            the value attribute. If it is null, the value attribute will not be generated.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated button tag
	 */
	public static function buttonInput($label = 'Button', $options = [])
	{
		$options['type'] = 'button';
		$options['value'] = $label;
		return static::tag('input', '', $options);
	}

	/**
	 * Generates a submit input button.
	 *
	 * Be careful when naming form elements such as submit buttons. According to the [jQuery documentation](https://api.jquery.com/submit/) there
	 * are some reserved names that can cause conflicts, e.g. `submit`, `length`, or `method`.
	 *
	 * @param string $label
	 *            the value attribute. If it is null, the value attribute will not be generated.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated button tag
	 */
	public static function submitInput($label = 'Submit', $options = [])
	{
		$options['type'] = 'submit';
		$options['value'] = $label;
		return static::tag('input', '', $options);
	}

	/**
	 * Generates a reset input button.
	 * 
	 * @param string $label
	 *            the value attribute. If it is null, the value attribute will not be generated.
	 * @param array $options
	 *            the attributes of the button tag. The values will be HTML-encoded using [[encode()]].
	 *            Attributes whose value is null will be ignored and not put in the tag returned.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated button tag
	 */
	public static function resetInput($label = 'Reset', $options = [])
	{
		$options['type'] = 'reset';
		$options['value'] = $label;
		return static::tag('input', '', $options);
	}

	/**
	 * Generates a text input field.
	 * 
	 * @param string $name
	 *            the name attribute.
	 * @param string $value
	 *            the value attribute. If it is null, the value attribute will not be generated.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated text input tag
	 */
	public static function textInput($name, $value = null, $options = [])
	{
		return static::input('text', $name, $value, $options);
	}

	/**
	 * Generates a hidden input field.
	 * 
	 * @param string $name
	 *            the name attribute.
	 * @param string $value
	 *            the value attribute. If it is null, the value attribute will not be generated.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated hidden input tag
	 */
	public static function hiddenInput($name, $value = null, $options = [])
	{
		return static::input('hidden', $name, $value, $options);
	}

	/**
	 * Generates a password input field.
	 * 
	 * @param string $name
	 *            the name attribute.
	 * @param string $value
	 *            the value attribute. If it is null, the value attribute will not be generated.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated password input tag
	 */
	public static function passwordInput($name, $value = null, $options = [])
	{
		return static::input('password', $name, $value, $options);
	}

	/**
	 * Generates a file input field.
	 * To use a file input field, you should set the enclosing form's "enctype" attribute to
	 * be "multipart/form-data". After the form is submitted, the uploaded file information
	 * can be obtained via $_FILES[$name] (see PHP documentation).
	 * 
	 * @param string $name
	 *            the name attribute.
	 * @param string $value
	 *            the value attribute. If it is null, the value attribute will not be generated.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated file input tag
	 */
	public static function fileInput($name, $value = null, $options = [])
	{
		return static::input('file', $name, $value, $options);
	}

	/**
	 * Generates a text area input.
	 * 
	 * @param string $name
	 *            the input name
	 * @param string $value
	 *            the input value. Note that it will be encoded using [[encode()]].
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated text area tag
	 */
	public static function textarea($name, $value = '', $options = [])
	{
		$options['name'] = $name;
		return static::tag('textarea', static::encode($value), $options);
	}

	/**
	 * Generates a radio button input.
	 * 
	 * @param string $name
	 *            the name attribute.
	 * @param boolean $checked
	 *            whether the radio button should be checked.
	 * @param array $options
	 *            the tag options in terms of name-value pairs.
	 *            See [[booleanInput()]] for details about accepted attributes.
	 *            
	 * @return string the generated radio button tag
	 */
	public static function radio($name, $checked = false, $options = [])
	{
		return static::booleanInput('radio', $name, $checked, $options);
	}

	/**
	 * Generates a checkbox input.
	 * 
	 * @param string $name
	 *            the name attribute.
	 * @param boolean $checked
	 *            whether the checkbox should be checked.
	 * @param array $options
	 *            the tag options in terms of name-value pairs.
	 *            See [[booleanInput()]] for details about accepted attributes.
	 *            
	 * @return string the generated checkbox tag
	 */
	public static function checkbox($name, $checked = false, $options = [])
	{
		return static::booleanInput('checkbox', $name, $checked, $options);
	}

	/**
	 * Generates a boolean input.
	 * 
	 * @param string $type
	 *            the input type. This can be either `radio` or `checkbox`.
	 * @param string $name
	 *            the name attribute.
	 * @param boolean $checked
	 *            whether the checkbox should be checked.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The following options are specially handled:
	 *            
	 *            - uncheck: string, the value associated with the uncheck state of the checkbox. When this attribute
	 *            is present, a hidden input will be generated so that if the checkbox is not checked and is submitted,
	 *            the value of this attribute will still be submitted to the server via the hidden input.
	 *            - label: string, a label displayed next to the checkbox. It will NOT be HTML-encoded. Therefore you can pass
	 *            in HTML code such as an image tag. If this is is coming from end users, you should [[encode()]] it to prevent XSS attacks.
	 *            When this option is specified, the checkbox will be enclosed by a label tag.
	 *            - labelOptions: array, the HTML attributes for the label tag. Do not set this option unless you set the "label" option.
	 *            
	 *            The rest of the options will be rendered as the attributes of the resulting checkbox tag. The values will
	 *            be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated checkbox tag
	 * @since 2.0.9
	 */
	protected static function booleanInput($type, $name, $checked = false, $options = [])
	{
		$options['checked'] = (bool) $checked;
		$value = array_key_exists('value', $options) ? $options['value'] : '1';
		if (isset($options['uncheck'])) {
			// add a hidden field so that if the checkbox is not selected, it still submits a value
			$hidden = static::hiddenInput($name, $options['uncheck']);
			unset($options['uncheck']);
		} else {
			$hidden = '';
		}
		if (isset($options['label'])) {
			$label = $options['label'];
			$labelOptions = isset($options['labelOptions']) ? $options['labelOptions'] : [];
			unset($options['label'], $options['labelOptions']);
			$content = static::label(static::input($type, $name, $value, $options) . ' ' . $label, null, $labelOptions);
			return $hidden . $content;
		} else {
			return $hidden . static::input($type, $name, $value, $options);
		}
	}

	/**
	 * Generates a drop-down list.
	 * 
	 * @param string $name
	 *            the input name
	 * @param string $selection
	 *            the selected value
	 * @param array $items
	 *            the option data items. The array keys are option values, and the array values
	 *            are the corresponding option labels. The array can also be nested (i.e. some array values are arrays too).
	 *            For each sub-array, an option group will be generated whose label is the key associated with the sub-array.
	 *            If you have a list of data models, you may convert them into the format described above using
	 *            [[\Kant\Helper\ArrayHelper::map()]].
	 *            
	 *            Note, the values and labels will be automatically HTML-encoded by this method, and the blank spaces in
	 *            the labels will also be HTML-encoded.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The following options are specially handled:
	 *            
	 *            - prompt: string, a prompt text to be displayed as the first option;
	 *            - options: array, the attributes for the select option tags. The array keys must be valid option values,
	 *            and the array values are the extra attributes for the corresponding option tags. For example,
	 *            
	 *            ```php
	 *            [
	 *            'value1' => ['disabled' => true],
	 *            'value2' => ['label' => 'value 2'],
	 *            ];
	 *            ```
	 *            
	 *            - groups: array, the attributes for the optgroup tags. The structure of this is similar to that of 'options',
	 *            except that the array keys represent the optgroup labels specified in $items.
	 *            - encodeSpaces: bool, whether to encode spaces in option prompt and option value with `&nbsp;` character.
	 *            Defaults to false.
	 *            - encode: bool, whether to encode option prompt and option value characters.
	 *            Defaults to `true`. This option is available since 2.0.3.
	 *            
	 *            The rest of the options will be rendered as the attributes of the resulting tag. The values will
	 *            be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated drop-down list tag
	 */
	public static function dropDownList($name, $selection = null, $items = [], $options = [])
	{
		if (!empty($options['multiple'])) {
			return static::listBox($name, $selection, $items, $options);
		}
		$options['name'] = $name;
		unset($options['unselect']);
		$selectOptions = static::renderSelectOptions($selection, $items, $options);
		return static::tag('select', "\n" . $selectOptions . "\n", $options);
	}

	/**
	 * Generates a list box.
	 * 
	 * @param string $name
	 *            the input name
	 * @param string|array $selection
	 *            the selected value(s)
	 * @param array $items
	 *            the option data items. The array keys are option values, and the array values
	 *            are the corresponding option labels. The array can also be nested (i.e. some array values are arrays too).
	 *            For each sub-array, an option group will be generated whose label is the key associated with the sub-array.
	 *            If you have a list of data models, you may convert them into the format described above using
	 *            [[\Kant\Helper\ArrayHelper::map()]].
	 *            
	 *            Note, the values and labels will be automatically HTML-encoded by this method, and the blank spaces in
	 *            the labels will also be HTML-encoded.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The following options are specially handled:
	 *            
	 *            - prompt: string, a prompt text to be displayed as the first option;
	 *            - options: array, the attributes for the select option tags. The array keys must be valid option values,
	 *            and the array values are the extra attributes for the corresponding option tags. For example,
	 *            
	 *            ```php
	 *            [
	 *            'value1' => ['disabled' => true],
	 *            'value2' => ['label' => 'value 2'],
	 *            ];
	 *            ```
	 *            
	 *            - groups: array, the attributes for the optgroup tags. The structure of this is similar to that of 'options',
	 *            except that the array keys represent the optgroup labels specified in $items.
	 *            - unselect: string, the value that will be submitted when no option is selected.
	 *            When this attribute is set, a hidden field will be generated so that if no option is selected in multiple
	 *            mode, we can still obtain the posted unselect value.
	 *            - encodeSpaces: bool, whether to encode spaces in option prompt and option value with `&nbsp;` character.
	 *            Defaults to false.
	 *            - encode: bool, whether to encode option prompt and option value characters.
	 *            Defaults to `true`. This option is available since 2.0.3.
	 *            
	 *            The rest of the options will be rendered as the attributes of the resulting tag. The values will
	 *            be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated list box tag
	 */
	public static function listBox($name, $selection = null, $items = [], $options = [])
	{
		if (!array_key_exists('size', $options)) {
			$options['size'] = 4;
		}
		if (!empty($options['multiple']) && !empty($name) && substr_compare($name, '[]', - 2, 2)) {
			$name .= '[]';
		}
		$options['name'] = $name;
		if (isset($options['unselect'])) {
			// add a hidden field so that if the list box has no option being selected, it still submits a value
			if (!empty($name) && substr_compare($name, '[]', - 2, 2) === 0) {
				$name = substr($name, 0, - 2);
			}
			$hidden = static::hiddenInput($name, $options['unselect']);
			unset($options['unselect']);
		} else {
			$hidden = '';
		}
		$selectOptions = static::renderSelectOptions($selection, $items, $options);
		return $hidden . static::tag('select', "\n" . $selectOptions . "\n", $options);
	}

	/**
	 * Generates a list of checkboxes.
	 * A checkbox list allows multiple selection, like [[listBox()]].
	 * As a result, the corresponding submitted value is an array.
	 * 
	 * @param string $name
	 *            the name attribute of each checkbox.
	 * @param string|array $selection
	 *            the selected value(s).
	 * @param array $items
	 *            the data item used to generate the checkboxes.
	 *            The array keys are the checkbox values, while the array values are the corresponding labels.
	 * @param array $options
	 *            options (name => config) for the checkbox list container tag.
	 *            The following options are specially handled:
	 *            
	 *            - tag: string|false, the tag name of the container element. False to render checkbox without container.
	 *            See also [[tag()]].
	 *            - unselect: string, the value that should be submitted when none of the checkboxes is selected.
	 *            By setting this option, a hidden input will be generated.
	 *            - encode: boolean, whether to HTML-encode the checkbox labels. Defaults to true.
	 *            This option is ignored if `item` option is set.
	 *            - separator: string, the HTML code that separates items.
	 *            - itemOptions: array, the options for generating the checkbox tag using [[checkbox()]].
	 *            - item: callable, a callback that can be used to customize the generation of the HTML code
	 *            corresponding to a single item in $items. The signature of this callback must be:
	 *            
	 *            ```php
	 *            function ($index, $label, $name, $checked, $value)
	 *            ```
	 *            
	 *            where $index is the zero-based index of the checkbox in the whole list; $label
	 *            is the label for the checkbox; and $name, $value and $checked represent the name,
	 *            value and the checked status of the checkbox input, respectively.
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated checkbox list
	 */
	public static function checkboxList($name, $selection = null, $items = [], $options = [])
	{
		if (substr($name, - 2) !== '[]') {
			$name .= '[]';
		}

		$formatter = ArrayHelper::remove($options, 'item');
		$itemOptions = ArrayHelper::remove($options, 'itemOptions', []);
		$encode = ArrayHelper::remove($options, 'encode', true);
		$separator = ArrayHelper::remove($options, 'separator', "\n");
		$tag = ArrayHelper::remove($options, 'tag', 'div');

		$lines = [];
		$index = 0;
		foreach ($items as $value => $label) {
			$checked = $selection !== null && (!ArrayHelper::isTraversable($selection) && !strcmp($value, $selection) || ArrayHelper::isTraversable($selection) && ArrayHelper::isIn($value, $selection));
			if ($formatter !== null) {
				$lines[] = call_user_func($formatter, $index, $label, $name, $checked, $value);
			} else {
				$lines[] = static::checkbox($name, $checked, array_merge($itemOptions, [
							'value' => $value,
							'label' => $encode ? static::encode($label) : $label
				]));
			}
			$index ++;
		}

		if (isset($options['unselect'])) {
			// add a hidden field so that if the list box has no option being selected, it still submits a value
			$name2 = substr($name, - 2) === '[]' ? substr($name, 0, - 2) : $name;
			$hidden = static::hiddenInput($name2, $options['unselect']);
			unset($options['unselect']);
		} else {
			$hidden = '';
		}

		$visibleContent = implode($separator, $lines);

		if ($tag === false) {
			return $hidden . $visibleContent;
		}

		return $hidden . static::tag($tag, $visibleContent, $options);
	}

	/**
	 * Generates a list of radio buttons.
	 * A radio button list is like a checkbox list, except that it only allows single selection.
	 * 
	 * @param string $name
	 *            the name attribute of each radio button.
	 * @param string|array $selection
	 *            the selected value(s).
	 * @param array $items
	 *            the data item used to generate the radio buttons.
	 *            The array keys are the radio button values, while the array values are the corresponding labels.
	 * @param array $options
	 *            options (name => config) for the radio button list container tag.
	 *            The following options are specially handled:
	 *            
	 *            - tag: string|false, the tag name of the container element. False to render radio buttons without container.
	 *            See also [[tag()]].
	 *            - unselect: string, the value that should be submitted when none of the radio buttons is selected.
	 *            By setting this option, a hidden input will be generated.
	 *            - encode: boolean, whether to HTML-encode the checkbox labels. Defaults to true.
	 *            This option is ignored if `item` option is set.
	 *            - separator: string, the HTML code that separates items.
	 *            - itemOptions: array, the options for generating the radio button tag using [[radio()]].
	 *            - item: callable, a callback that can be used to customize the generation of the HTML code
	 *            corresponding to a single item in $items. The signature of this callback must be:
	 *            
	 *            ```php
	 *            function ($index, $label, $name, $checked, $value)
	 *            ```
	 *            
	 *            where $index is the zero-based index of the radio button in the whole list; $label
	 *            is the label for the radio button; and $name, $value and $checked represent the name,
	 *            value and the checked status of the radio button input, respectively.
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated radio button list
	 */
	public static function radioList($name, $selection = null, $items = [], $options = [])
	{
		$formatter = ArrayHelper::remove($options, 'item');
		$itemOptions = ArrayHelper::remove($options, 'itemOptions', []);
		$encode = ArrayHelper::remove($options, 'encode', true);
		$separator = ArrayHelper::remove($options, 'separator', "\n");
		$tag = ArrayHelper::remove($options, 'tag', 'div');
		// add a hidden field so that if the list box has no option being selected, it still submits a value
		$hidden = isset($options['unselect']) ? static::hiddenInput($name, $options['unselect']) : '';
		unset($options['unselect']);

		$lines = [];
		$index = 0;
		foreach ($items as $value => $label) {
			$checked = $selection !== null && (!ArrayHelper::isTraversable($selection) && !strcmp($value, $selection) || ArrayHelper::isTraversable($selection) && ArrayHelper::isIn($value, $selection));
			if ($formatter !== null) {
				$lines[] = call_user_func($formatter, $index, $label, $name, $checked, $value);
			} else {
				$lines[] = static::radio($name, $checked, array_merge($itemOptions, [
							'value' => $value,
							'label' => $encode ? static::encode($label) : $label
				]));
			}
			$index ++;
		}
		$visibleContent = implode($separator, $lines);

		if ($tag === false) {
			return $hidden . $visibleContent;
		}

		return $hidden . static::tag($tag, $visibleContent, $options);
	}

	/**
	 * Generates an unordered list.
	 * 
	 * @param array|\Traversable $items
	 *            the items for generating the list. Each item generates a single list item.
	 *            Note that items will be automatically HTML encoded if `$options['encode']` is not set or true.
	 * @param array $options
	 *            options (name => config) for the radio button list. The following options are supported:
	 *            
	 *            - encode: boolean, whether to HTML-encode the items. Defaults to true.
	 *            This option is ignored if the `item` option is specified.
	 *            - separator: string, the HTML code that separates items. Defaults to a simple newline (`"\n"`).
	 *            This option is available since version 2.0.7.
	 *            - itemOptions: array, the HTML attributes for the `li` tags. This option is ignored if the `item` option is specified.
	 *            - item: callable, a callback that is used to generate each individual list item.
	 *            The signature of this callback must be:
	 *            
	 *            ```php
	 *            function ($item, $index)
	 *            ```
	 *            
	 *            where $index is the array key corresponding to `$item` in `$items`. The callback should return
	 *            the whole list item tag.
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated unordered list. An empty list tag will be returned if `$items` is empty.
	 */
	public static function ul($items, $options = [])
	{
		$tag = ArrayHelper::remove($options, 'tag', 'ul');
		$encode = ArrayHelper::remove($options, 'encode', true);
		$formatter = ArrayHelper::remove($options, 'item');
		$separator = ArrayHelper::remove($options, 'separator', "\n");
		$itemOptions = ArrayHelper::remove($options, 'itemOptions', []);

		if (empty($items)) {
			return static::tag($tag, '', $options);
		}

		$results = [];
		foreach ($items as $index => $item) {
			if ($formatter !== null) {
				$results[] = call_user_func($formatter, $item, $index);
			} else {
				$results[] = static::tag('li', $encode ? static::encode($item) : $item, $itemOptions);
			}
		}

		return static::tag($tag, $separator . implode($separator, $results) . $separator, $options);
	}

	/**
	 * Generates an ordered list.
	 * 
	 * @param array|\Traversable $items
	 *            the items for generating the list. Each item generates a single list item.
	 *            Note that items will be automatically HTML encoded if `$options['encode']` is not set or true.
	 * @param array $options
	 *            options (name => config) for the radio button list. The following options are supported:
	 *            
	 *            - encode: boolean, whether to HTML-encode the items. Defaults to true.
	 *            This option is ignored if the `item` option is specified.
	 *            - itemOptions: array, the HTML attributes for the `li` tags. This option is ignored if the `item` option is specified.
	 *            - item: callable, a callback that is used to generate each individual list item.
	 *            The signature of this callback must be:
	 *            
	 *            ```php
	 *            function ($item, $index)
	 *            ```
	 *            
	 *            where $index is the array key corresponding to `$item` in `$items`. The callback should return
	 *            the whole list item tag.
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated ordered list. An empty string is returned if `$items` is empty.
	 */
	public static function ol($items, $options = [])
	{
		$options['tag'] = 'ol';
		return static::ul($items, $options);
	}

	/**
	 * Generates a label tag for the given model attribute.
	 * The label text is the label associated with the attribute, obtained via [[Model::getAttributeLabel()]].
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            The following options are specially handled:
	 *            
	 *            - label: this specifies the label to be displayed. Note that this will NOT be [[encode()|encoded]].
	 *            If this is not set, [[Model::getAttributeLabel()]] will be called to get the label for display
	 *            (after encoding).
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated label tag
	 */
	public static function activeLabel($model, $attribute, $options = [])
	{
		$for = ArrayHelper::remove($options, 'for', static::getInputId($model, $attribute));
		$attribute = static::getAttributeName($attribute);
		$label = ArrayHelper::remove($options, 'label', static::encode($model->getAttributeLabel($attribute)));
		return static::label($label, $for, $options);
	}

	/**
	 * Generates a hint tag for the given model attribute.
	 * The hint text is the hint associated with the attribute, obtained via [[Model::getAttributeHint()]].
	 * If no hint content can be obtained, method will return an empty string.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            If a value is null, the corresponding attribute will not be rendered.
	 *            The following options are specially handled:
	 *            
	 *            - hint: this specifies the hint to be displayed. Note that this will NOT be [[encode()|encoded]].
	 *            If this is not set, [[Model::getAttributeHint()]] will be called to get the hint for display
	 *            (without encoding).
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated hint tag
	 * @since 2.0.4
	 */
	public static function activeHint($model, $attribute, $options = [])
	{
		$attribute = static::getAttributeName($attribute);
		$hint = isset($options['hint']) ? $options['hint'] : $model->getAttributeHint($attribute);
		if (empty($hint)) {
			return '';
		}
		$tag = ArrayHelper::remove($options, 'tag', 'div');
		unset($options['hint']);
		return static::tag($tag, $hint, $options);
	}

	/**
	 * Generates a summary of the validation errors.
	 * If there is no validation error, an empty error summary markup will still be generated, but it will be hidden.
	 * 
	 * @param Model|Model[] $models
	 *            the model(s) whose validation errors are to be displayed
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The following options are specially handled:
	 *            
	 *            - header: string, the header HTML for the error summary. If not set, a default prompt string will be used.
	 *            - footer: string, the footer HTML for the error summary.
	 *            - encode: boolean, if set to false then the error messages won't be encoded.
	 *            
	 *            The rest of the options will be rendered as the attributes of the container tag. The values will
	 *            be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 * @return string the generated error summary
	 */
	public static function errorSummary($models, $options = [])
	{
		$header = isset($options['header']) ? $options['header'] : '<p>' . Kant::t('kant', 'Please fix the following errors:') . '</p>';
		$footer = ArrayHelper::remove($options, 'footer', '');
		$encode = ArrayHelper::remove($options, 'encode', true);
		unset($options['header']);

		$lines = [];
		if (!is_array($models)) {
			$models = [
				$models
			];
		}
		foreach ($models as $model) {
			/* @var $model Model */
			foreach ($model->getFirstErrors() as $error) {
				$lines[] = $encode ? Html::encode($error) : $error;
			}
		}

		if (empty($lines)) {
			// still render the placeholder for client-side validation use
			$content = '<ul></ul>';
			$options['style'] = isset($options['style']) ? rtrim($options['style'], ';') . '; display:none' : 'display:none';
		} else {
			$content = '<ul><li>' . implode("</li>\n<li>", $lines) . '</li></ul>';
		}
		return Html::tag('div', $header . $content . $footer, $options);
	}

	/**
	 * Generates a tag that contains the first validation error of the specified model attribute.
	 * Note that even if there is no validation error, this method will still return an empty error tag.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The values will be HTML-encoded
	 *            using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 *            
	 *            The following options are specially handled:
	 *            
	 *            - tag: this specifies the tag name. If not set, "div" will be used.
	 *            See also [[tag()]].
	 *            - encode: boolean, if set to false then the error message won't be encoded.
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated label tag
	 */
	public static function error($model, $attribute, $options = [])
	{
		$attribute = static::getAttributeName($attribute);
		$error = $model->getFirstError($attribute);
		$tag = ArrayHelper::remove($options, 'tag', 'div');
		$encode = ArrayHelper::remove($options, 'encode', true);
		return Html::tag($tag, $encode ? Html::encode($error) : $error, $options);
	}

	/**
	 * Generates an input tag for the given model attribute.
	 * This method will generate the "name" and "value" tag attributes automatically for the model attribute
	 * unless they are explicitly specified in `$options`.
	 * 
	 * @param string $type
	 *            the input type (e.g. 'text', 'password')
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated input tag
	 */
	public static function activeInput($type, $model, $attribute, $options = [])
	{
		$name = isset($options['name']) ? $options['name'] : static::getInputName($model, $attribute);
		$value = isset($options['value']) ? $options['value'] : static::getAttributeValue($model, $attribute);
		if (!array_key_exists('id', $options)) {
			$options['id'] = static::getInputId($model, $attribute);
		}
		return static::input($type, $name, $value, $options);
	}

	/**
	 * If `maxlength` option is set true and the model attribute is validated by a string validator,
	 * the `maxlength` option will take the value of [[\Kant\Validators\StringValidator::max]].
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs.
	 */
	private static function normalizeMaxLength($model, $attribute, &$options)
	{
		if (isset($options['maxlength']) && $options['maxlength'] === true) {
			unset($options['maxlength']);
			$attrName = static::getAttributeName($attribute);
			foreach ($model->getActiveValidators($attrName) as $validator) {
				if ($validator instanceof StringValidator && $validator->max !== null) {
					$options['maxlength'] = $validator->max;
					break;
				}
			}
		}
	}

	/**
	 * Generates a text input tag for the given model attribute.
	 * This method will generate the "name" and "value" tag attributes automatically for the model attribute
	 * unless they are explicitly specified in `$options`.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            The following special options are recognized:
	 *            
	 *            - maxlength: integer|boolean, when `maxlength` is set true and the model attribute is validated
	 *            by a string validator, the `maxlength` option will take the value of [[\Kant\Validators\StringValidator::max]].
	 *            This is available since version 2.0.3.
	 *            
	 * @return string the generated input tag
	 */
	public static function activeTextInput($model, $attribute, $options = [])
	{
		self::normalizeMaxLength($model, $attribute, $options);
		return static::activeInput('text', $model, $attribute, $options);
	}

	/**
	 * Generates a hidden input tag for the given model attribute.
	 * This method will generate the "name" and "value" tag attributes automatically for the model attribute
	 * unless they are explicitly specified in `$options`.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated input tag
	 */
	public static function activeHiddenInput($model, $attribute, $options = [])
	{
		return static::activeInput('hidden', $model, $attribute, $options);
	}

	/**
	 * Generates a password input tag for the given model attribute.
	 * This method will generate the "name" and "value" tag attributes automatically for the model attribute
	 * unless they are explicitly specified in `$options`.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            The following special options are recognized:
	 *            
	 *            - maxlength: integer|boolean, when `maxlength` is set true and the model attribute is validated
	 *            by a string validator, the `maxlength` option will take the value of [[\Kant\Validators\StringValidator::max]].
	 *            This option is available since version 2.0.6.
	 *            
	 * @return string the generated input tag
	 */
	public static function activePasswordInput($model, $attribute, $options = [])
	{
		self::normalizeMaxLength($model, $attribute, $options);
		return static::activeInput('password', $model, $attribute, $options);
	}

	/**
	 * Generates a file input tag for the given model attribute.
	 * This method will generate the "name" and "value" tag attributes automatically for the model attribute
	 * unless they are explicitly specified in `$options`.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 * @return string the generated input tag
	 */
	public static function activeFileInput($model, $attribute, $options = [])
	{
		// add a hidden field so that if a model only has a file field, we can
		// still use isset($_POST[$modelClass]) to detect if the input is submitted
		$hiddenOptions = [
			'id' => null,
			'value' => ''
		];
		if (isset($options['name'])) {
			$hiddenOptions['name'] = $options['name'];
		}
		return static::activeHiddenInput($model, $attribute, $hiddenOptions) . static::activeInput('file', $model, $attribute, $options);
	}

	/**
	 * Generates a textarea tag for the given model attribute.
	 * The model attribute value will be used as the content in the textarea.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. These will be rendered as
	 *            the attributes of the resulting tag. The values will be HTML-encoded using [[encode()]].
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            The following special options are recognized:
	 *            
	 *            - maxlength: integer|boolean, when `maxlength` is set true and the model attribute is validated
	 *            by a string validator, the `maxlength` option will take the value of [[\Kant\Validators\StringValidator::max]].
	 *            This option is available since version 2.0.6.
	 *            
	 * @return string the generated textarea tag
	 */
	public static function activeTextarea($model, $attribute, $options = [])
	{
		$name = isset($options['name']) ? $options['name'] : static::getInputName($model, $attribute);
		if (isset($options['value'])) {
			$value = $options['value'];
			unset($options['value']);
		} else {
			$value = static::getAttributeValue($model, $attribute);
		}
		if (!array_key_exists('id', $options)) {
			$options['id'] = static::getInputId($model, $attribute);
		}
		self::normalizeMaxLength($model, $attribute, $options);
		return static::textarea($name, $value, $options);
	}

	/**
	 * Generates a radio button tag together with a label for the given model attribute.
	 * This method will generate the "checked" tag attribute according to the model attribute value.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs.
	 *            See [[booleanInput()]] for details about accepted attributes.
	 *            
	 * @return string the generated radio button tag
	 */
	public static function activeRadio($model, $attribute, $options = [])
	{
		return static::activeBooleanInput('radio', $model, $attribute, $options);
	}

	/**
	 * Generates a checkbox tag together with a label for the given model attribute.
	 * This method will generate the "checked" tag attribute according to the model attribute value.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs.
	 *            See [[booleanInput()]] for details about accepted attributes.
	 *            
	 * @return string the generated checkbox tag
	 */
	public static function activeCheckbox($model, $attribute, $options = [])
	{
		return static::activeBooleanInput('checkbox', $model, $attribute, $options);
	}

	/**
	 * Generates a boolean input
	 * This method is mainly called by [[activeCheckbox()]] and [[activeRadio()]].
	 * 
	 * @param string $type
	 *            the input type. This can be either `radio` or `checkbox`.
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $options
	 *            the tag options in terms of name-value pairs.
	 *            See [[booleanInput()]] for details about accepted attributes.
	 * @return string the generated input element
	 * @since 2.0.9
	 */
	protected static function activeBooleanInput($type, $model, $attribute, $options = [])
	{
		$name = isset($options['name']) ? $options['name'] : static::getInputName($model, $attribute);
		$value = static::getAttributeValue($model, $attribute);

		if (!array_key_exists('value', $options)) {
			$options['value'] = '1';
		}
		if (!array_key_exists('uncheck', $options)) {
			$options['uncheck'] = '0';
		}
		if (!array_key_exists('label', $options)) {
			$options['label'] = static::encode($model->getAttributeLabel(static::getAttributeName($attribute)));
		}

		$checked = "$value" === "{$options['value']}";

		if (!array_key_exists('id', $options)) {
			$options['id'] = static::getInputId($model, $attribute);
		}

		return static::$type($name, $checked, $options);
	}

	/**
	 * Generates a drop-down list for the given model attribute.
	 * The selection of the drop-down list is taken from the value of the model attribute.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $items
	 *            the option data items. The array keys are option values, and the array values
	 *            are the corresponding option labels. The array can also be nested (i.e. some array values are arrays too).
	 *            For each sub-array, an option group will be generated whose label is the key associated with the sub-array.
	 *            If you have a list of data models, you may convert them into the format described above using
	 *            [[\Kant\Helper\ArrayHelper::map()]].
	 *            
	 *            Note, the values and labels will be automatically HTML-encoded by this method, and the blank spaces in
	 *            the labels will also be HTML-encoded.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The following options are specially handled:
	 *            
	 *            - prompt: string, a prompt text to be displayed as the first option;
	 *            - options: array, the attributes for the select option tags. The array keys must be valid option values,
	 *            and the array values are the extra attributes for the corresponding option tags. For example,
	 *            
	 *            ```php
	 *            [
	 *            'value1' => ['disabled' => true],
	 *            'value2' => ['label' => 'value 2'],
	 *            ];
	 *            ```
	 *            
	 *            - groups: array, the attributes for the optgroup tags. The structure of this is similar to that of 'options',
	 *            except that the array keys represent the optgroup labels specified in $items.
	 *            - encodeSpaces: bool, whether to encode spaces in option prompt and option value with `&nbsp;` character.
	 *            Defaults to false.
	 *            - encode: bool, whether to encode option prompt and option value characters.
	 *            Defaults to `true`. This option is available since 2.0.3.
	 *            
	 *            The rest of the options will be rendered as the attributes of the resulting tag. The values will
	 *            be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated drop-down list tag
	 */
	public static function activeDropDownList($model, $attribute, $items, $options = [])
	{
		if (empty($options['multiple'])) {
			return static::activeListInput('dropDownList', $model, $attribute, $items, $options);
		} else {
			return static::activeListBox($model, $attribute, $items, $options);
		}
	}

	/**
	 * Generates a list box.
	 * The selection of the list box is taken from the value of the model attribute.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $items
	 *            the option data items. The array keys are option values, and the array values
	 *            are the corresponding option labels. The array can also be nested (i.e. some array values are arrays too).
	 *            For each sub-array, an option group will be generated whose label is the key associated with the sub-array.
	 *            If you have a list of data models, you may convert them into the format described above using
	 *            [[\Kant\Helper\ArrayHelper::map()]].
	 *            
	 *            Note, the values and labels will be automatically HTML-encoded by this method, and the blank spaces in
	 *            the labels will also be HTML-encoded.
	 * @param array $options
	 *            the tag options in terms of name-value pairs. The following options are specially handled:
	 *            
	 *            - prompt: string, a prompt text to be displayed as the first option;
	 *            - options: array, the attributes for the select option tags. The array keys must be valid option values,
	 *            and the array values are the extra attributes for the corresponding option tags. For example,
	 *            
	 *            ```php
	 *            [
	 *            'value1' => ['disabled' => true],
	 *            'value2' => ['label' => 'value 2'],
	 *            ];
	 *            ```
	 *            
	 *            - groups: array, the attributes for the optgroup tags. The structure of this is similar to that of 'options',
	 *            except that the array keys represent the optgroup labels specified in $items.
	 *            - unselect: string, the value that will be submitted when no option is selected.
	 *            When this attribute is set, a hidden field will be generated so that if no option is selected in multiple
	 *            mode, we can still obtain the posted unselect value.
	 *            - encodeSpaces: bool, whether to encode spaces in option prompt and option value with `&nbsp;` character.
	 *            Defaults to false.
	 *            - encode: bool, whether to encode option prompt and option value characters.
	 *            Defaults to `true`. This option is available since 2.0.3.
	 *            
	 *            The rest of the options will be rendered as the attributes of the resulting tag. The values will
	 *            be HTML-encoded using [[encode()]]. If a value is null, the corresponding attribute will not be rendered.
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated list box tag
	 */
	public static function activeListBox($model, $attribute, $items, $options = [])
	{
		return static::activeListInput('listBox', $model, $attribute, $items, $options);
	}

	/**
	 * Generates a list of checkboxes.
	 * A checkbox list allows multiple selection, like [[listBox()]].
	 * As a result, the corresponding submitted value is an array.
	 * The selection of the checkbox list is taken from the value of the model attribute.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $items
	 *            the data item used to generate the checkboxes.
	 *            The array keys are the checkbox values, and the array values are the corresponding labels.
	 *            Note that the labels will NOT be HTML-encoded, while the values will.
	 * @param array $options
	 *            options (name => config) for the checkbox list container tag.
	 *            The following options are specially handled:
	 *            
	 *            - tag: string|false, the tag name of the container element. False to render checkbox without container.
	 *            See also [[tag()]].
	 *            - unselect: string, the value that should be submitted when none of the checkboxes is selected.
	 *            You may set this option to be null to prevent default value submission.
	 *            If this option is not set, an empty string will be submitted.
	 *            - encode: boolean, whether to HTML-encode the checkbox labels. Defaults to true.
	 *            This option is ignored if `item` option is set.
	 *            - separator: string, the HTML code that separates items.
	 *            - itemOptions: array, the options for generating the checkbox tag using [[checkbox()]].
	 *            - item: callable, a callback that can be used to customize the generation of the HTML code
	 *            corresponding to a single item in $items. The signature of this callback must be:
	 *            
	 *            ```php
	 *            function ($index, $label, $name, $checked, $value)
	 *            ```
	 *            
	 *            where $index is the zero-based index of the checkbox in the whole list; $label
	 *            is the label for the checkbox; and $name, $value and $checked represent the name,
	 *            value and the checked status of the checkbox input.
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated checkbox list
	 */
	public static function activeCheckboxList($model, $attribute, $items, $options = [])
	{
		return static::activeListInput('checkboxList', $model, $attribute, $items, $options);
	}

	/**
	 * Generates a list of radio buttons.
	 * A radio button list is like a checkbox list, except that it only allows single selection.
	 * The selection of the radio buttons is taken from the value of the model attribute.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $items
	 *            the data item used to generate the radio buttons.
	 *            The array keys are the radio values, and the array values are the corresponding labels.
	 *            Note that the labels will NOT be HTML-encoded, while the values will.
	 * @param array $options
	 *            options (name => config) for the radio button list container tag.
	 *            The following options are specially handled:
	 *            
	 *            - tag: string|false, the tag name of the container element. False to render radio button without container.
	 *            See also [[tag()]].
	 *            - unselect: string, the value that should be submitted when none of the radio buttons is selected.
	 *            You may set this option to be null to prevent default value submission.
	 *            If this option is not set, an empty string will be submitted.
	 *            - encode: boolean, whether to HTML-encode the checkbox labels. Defaults to true.
	 *            This option is ignored if `item` option is set.
	 *            - separator: string, the HTML code that separates items.
	 *            - itemOptions: array, the options for generating the radio button tag using [[radio()]].
	 *            - item: callable, a callback that can be used to customize the generation of the HTML code
	 *            corresponding to a single item in $items. The signature of this callback must be:
	 *            
	 *            ```php
	 *            function ($index, $label, $name, $checked, $value)
	 *            ```
	 *            
	 *            where $index is the zero-based index of the radio button in the whole list; $label
	 *            is the label for the radio button; and $name, $value and $checked represent the name,
	 *            value and the checked status of the radio button input.
	 *            
	 *            See [[renderTagAttributes()]] for details on how attributes are being rendered.
	 *            
	 * @return string the generated radio button list
	 */
	public static function activeRadioList($model, $attribute, $items, $options = [])
	{
		return static::activeListInput('radioList', $model, $attribute, $items, $options);
	}

	/**
	 * Generates a list of input fields.
	 * This method is mainly called by [[activeListBox()]], [[activeRadioList()]] and [[activeCheckBoxList()]].
	 * 
	 * @param string $type
	 *            the input type. This can be 'listBox', 'radioList', or 'checkBoxList'.
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for the format
	 *            about attribute expression.
	 * @param array $items
	 *            the data item used to generate the input fields.
	 *            The array keys are the input values, and the array values are the corresponding labels.
	 *            Note that the labels will NOT be HTML-encoded, while the values will.
	 * @param array $options
	 *            options (name => config) for the input list. The supported special options
	 *            depend on the input type specified by `$type`.
	 * @return string the generated input list
	 */
	protected static function activeListInput($type, $model, $attribute, $items, $options = [])
	{
		$name = isset($options['name']) ? $options['name'] : static::getInputName($model, $attribute);
		$selection = static::getAttributeValue($model, $attribute);
		if (!array_key_exists('unselect', $options)) {
			$options['unselect'] = '';
		}
		if (!array_key_exists('id', $options)) {
			$options['id'] = static::getInputId($model, $attribute);
		}
		return static::$type($name, $selection, $items, $options);
	}

	/**
	 * Renders the option tags that can be used by [[dropDownList()]] and [[listBox()]].
	 * 
	 * @param string|array $selection
	 *            the selected value(s). This can be either a string for single selection
	 *            or an array for multiple selections.
	 * @param array $items
	 *            the option data items. The array keys are option values, and the array values
	 *            are the corresponding option labels. The array can also be nested (i.e. some array values are arrays too).
	 *            For each sub-array, an option group will be generated whose label is the key associated with the sub-array.
	 *            If you have a list of data models, you may convert them into the format described above using
	 *            [[\Kant\Helper\ArrayHelper::map()]].
	 *            
	 *            Note, the values and labels will be automatically HTML-encoded by this method, and the blank spaces in
	 *            the labels will also be HTML-encoded.
	 * @param array $tagOptions
	 *            the $options parameter that is passed to the [[dropDownList()]] or [[listBox()]] call.
	 *            This method will take out these elements, if any: "prompt", "options" and "groups". See more details
	 *            in [[dropDownList()]] for the explanation of these elements.
	 *            
	 * @return string the generated list options
	 */
	public static function renderSelectOptions($selection, $items, &$tagOptions = [])
	{
		$lines = [];
		$encodeSpaces = ArrayHelper::remove($tagOptions, 'encodeSpaces', false);
		$encode = ArrayHelper::remove($tagOptions, 'encode', true);
		if (isset($tagOptions['prompt'])) {
			$prompt = $encode ? static::encode($tagOptions['prompt']) : $tagOptions['prompt'];
			if ($encodeSpaces) {
				$prompt = str_replace(' ', '&nbsp;', $prompt);
			}
			$lines[] = static::tag('option', $prompt, [
						'value' => ''
			]);
		}

		$options = isset($tagOptions['options']) ? $tagOptions['options'] : [];
		$groups = isset($tagOptions['groups']) ? $tagOptions['groups'] : [];
		unset($tagOptions['prompt'], $tagOptions['options'], $tagOptions['groups']);
		$options['encodeSpaces'] = ArrayHelper::getValue($options, 'encodeSpaces', $encodeSpaces);
		$options['encode'] = ArrayHelper::getValue($options, 'encode', $encode);

		foreach ($items as $key => $value) {
			if (is_array($value)) {
				$groupAttrs = isset($groups[$key]) ? $groups[$key] : [];
				if (!isset($groupAttrs['label'])) {
					$groupAttrs['label'] = $key;
				}
				$attrs = [
					'options' => $options,
					'groups' => $groups,
					'encodeSpaces' => $encodeSpaces,
					'encode' => $encode
				];
				$content = static::renderSelectOptions($selection, $value, $attrs);
				$lines[] = static::tag('optgroup', "\n" . $content . "\n", $groupAttrs);
			} else {
				$attrs = isset($options[$key]) ? $options[$key] : [];
				$attrs['value'] = (string) $key;
				if (!array_key_exists('selected', $attrs)) {
					$attrs['selected'] = $selection !== null && (!ArrayHelper::isTraversable($selection) && !strcmp($key, $selection) || ArrayHelper::isTraversable($selection) && ArrayHelper::isIn($key, $selection));
				}
				$text = $encode ? static::encode($value) : $value;
				if ($encodeSpaces) {
					$text = str_replace(' ', '&nbsp;', $text);
				}
				$lines[] = static::tag('option', $text, $attrs);
			}
		}

		return implode("\n", $lines);
	}

	/**
	 * Renders the HTML tag attributes.
	 *
	 * Attributes whose values are of boolean type will be treated as
	 * [boolean attributes](http://www.w3.org/TR/html5/infrastructure.html#boolean-attributes).
	 *
	 * Attributes whose values are null will not be rendered.
	 *
	 * The values of attributes will be HTML-encoded using [[encode()]].
	 *
	 * The "data" attribute is specially handled when it is receiving an array value. In this case,
	 * the array will be "expanded" and a list data attributes will be rendered. For example,
	 * if `'data' => ['id' => 1, 'name' => 'kant']`, then this will be rendered:
	 * `data-id="1" data-name="kant"`.
	 * Additionally `'data' => ['params' => ['id' => 1, 'name' => 'kant'], 'status' => 'ok']` will be rendered as:
	 * `data-params='{"id":1,"name":"kant"}' data-status="ok"`.
	 *
	 * @param array $attributes
	 *            attributes to be rendered. The attribute values will be HTML-encoded using [[encode()]].
	 * @return string the rendering result. If the attributes are not empty, they will be rendered
	 *         into a string with a leading white space (so that it can be directly appended to the tag name
	 *         in a tag. If there is no attribute, an empty string will be returned.
	 */
	public static function renderTagAttributes($attributes)
	{
		if (count($attributes) > 1) {
			$sorted = [];
			foreach (static::$attributeOrder as $name) {
				if (isset($attributes[$name])) {
					$sorted[$name] = $attributes[$name];
				}
			}
			$attributes = array_merge($sorted, $attributes);
		}

		$html = '';
		foreach ($attributes as $name => $value) {
			if (is_bool($value)) {
				if ($value) {
					$html .= " $name";
				}
			} elseif (is_array($value)) {
				if (in_array($name, static::$dataAttributes)) {
					foreach ($value as $n => $v) {
						if (is_array($v)) {
							$html .= " $name-$n='" . Json::htmlEncode($v) . "'";
						} else {
							$html .= " $name-$n=\"" . static::encode($v) . '"';
						}
					}
				} elseif ($name === 'class') {
					if (empty($value)) {
						continue;
					}
					$html .= " $name=\"" . static::encode(implode(' ', $value)) . '"';
				} elseif ($name === 'style') {
					if (empty($value)) {
						continue;
					}
					$html .= " $name=\"" . static::encode(static::cssStyleFromArray($value)) . '"';
				} else {
					$html .= " $name='" . Json::htmlEncode($value) . "'";
				}
			} elseif ($value !== null) {
				$html .= " $name=\"" . static::encode($value) . '"';
			}
		}

		return $html;
	}

	/**
	 * Adds a CSS class (or several classes) to the specified options.
	 * If the CSS class is already in the options, it will not be added again.
	 * If class specification at given options is an array, and some class placed there with the named (string) key,
	 * overriding of such key will have no effect. For example:
	 *
	 * ```php
	 * $options = ['class' => ['persistent' => 'initial']];
	 * Html::addCssClass($options, ['persistent' => 'override']);
	 * var_dump($options['class']); // outputs: array('persistent' => 'initial');
	 * ```
	 *
	 * @param array $options
	 *            the options to be modified.
	 * @param string|array $class
	 *            the CSS class(es) to be added
	 */
	public static function addCssClass(&$options, $class)
	{
		if (isset($options['class'])) {
			if (is_array($options['class'])) {
				$options['class'] = self::mergeCssClasses($options['class'], (array) $class);
			} else {
				$classes = preg_split('/\s+/', $options['class'], - 1, PREG_SPLIT_NO_EMPTY);
				$options['class'] = implode(' ', self::mergeCssClasses($classes, (array) $class));
			}
		} else {
			$options['class'] = $class;
		}
	}

	/**
	 * Merges already existing CSS classes with new one.
	 * This method provides the priority for named existing classes over additional.
	 * 
	 * @param array $existingClasses
	 *            already existing CSS classes.
	 * @param array $additionalClasses
	 *            CSS classes to be added.
	 * @return array merge result.
	 */
	private static function mergeCssClasses(array $existingClasses, array $additionalClasses)
	{
		foreach ($additionalClasses as $key => $class) {
			if (is_int($key) && !in_array($class, $existingClasses)) {
				$existingClasses[] = $class;
			} elseif (!isset($existingClasses[$key])) {
				$existingClasses[$key] = $class;
			}
		}
		return array_unique($existingClasses);
	}

	/**
	 * Removes a CSS class from the specified options.
	 * 
	 * @param array $options
	 *            the options to be modified.
	 * @param string|array $class
	 *            the CSS class(es) to be removed
	 */
	public static function removeCssClass(&$options, $class)
	{
		if (isset($options['class'])) {
			if (is_array($options['class'])) {
				$classes = array_diff($options['class'], (array) $class);
				if (empty($classes)) {
					unset($options['class']);
				} else {
					$options['class'] = $classes;
				}
			} else {
				$classes = preg_split('/\s+/', $options['class'], - 1, PREG_SPLIT_NO_EMPTY);
				$classes = array_diff($classes, (array) $class);
				if (empty($classes)) {
					unset($options['class']);
				} else {
					$options['class'] = implode(' ', $classes);
				}
			}
		}
	}

	/**
	 * Adds the specified CSS style to the HTML options.
	 *
	 * If the options already contain a `style` element, the new style will be merged
	 * with the existing one. If a CSS property exists in both the new and the old styles,
	 * the old one may be overwritten if `$overwrite` is true.
	 *
	 * For example,
	 *
	 * ```php
	 * Html::addCssStyle($options, 'width: 100px; height: 200px');
	 * ```
	 *
	 * @param array $options
	 *            the HTML options to be modified.
	 * @param string|array $style
	 *            the new style string (e.g. `'width: 100px; height: 200px'`) or
	 *            array (e.g. `['width' => '100px', 'height' => '200px']`).
	 * @param boolean $overwrite
	 *            whether to overwrite existing CSS properties if the new style
	 *            contain them too.
	 * @see removeCssStyle()
	 * @see cssStyleFromArray()
	 * @see cssStyleToArray()
	 */
	public static function addCssStyle(&$options, $style, $overwrite = true)
	{
		if (!empty($options['style'])) {
			$oldStyle = is_array($options['style']) ? $options['style'] : static::cssStyleToArray($options['style']);
			$newStyle = is_array($style) ? $style : static::cssStyleToArray($style);
			if (!$overwrite) {
				foreach ($newStyle as $property => $value) {
					if (isset($oldStyle[$property])) {
						unset($newStyle[$property]);
					}
				}
			}
			$style = array_merge($oldStyle, $newStyle);
		}
		$options['style'] = is_array($style) ? static::cssStyleFromArray($style) : $style;
	}

	/**
	 * Removes the specified CSS style from the HTML options.
	 *
	 * For example,
	 *
	 * ```php
	 * Html::removeCssStyle($options, ['width', 'height']);
	 * ```
	 *
	 * @param array $options
	 *            the HTML options to be modified.
	 * @param string|array $properties
	 *            the CSS properties to be removed. You may use a string
	 *            if you are removing a single property.
	 * @see addCssStyle()
	 */
	public static function removeCssStyle(&$options, $properties)
	{
		if (!empty($options['style'])) {
			$style = is_array($options['style']) ? $options['style'] : static::cssStyleToArray($options['style']);
			foreach ((array) $properties as $property) {
				unset($style[$property]);
			}
			$options['style'] = static::cssStyleFromArray($style);
		}
	}

	/**
	 * Converts a CSS style array into a string representation.
	 *
	 * For example,
	 *
	 * ```php
	 * print_r(Html::cssStyleFromArray(['width' => '100px', 'height' => '200px']));
	 * // will display: 'width: 100px; height: 200px;'
	 * ```
	 *
	 * @param array $style
	 *            the CSS style array. The array keys are the CSS property names,
	 *            and the array values are the corresponding CSS property values.
	 * @return string the CSS style string. If the CSS style is empty, a null will be returned.
	 */
	public static function cssStyleFromArray(array $style)
	{
		$result = '';
		foreach ($style as $name => $value) {
			$result .= "$name: $value; ";
		}
		// return null if empty to avoid rendering the "style" attribute
		return $result === '' ? null : rtrim($result);
	}

	/**
	 * Converts a CSS style string into an array representation.
	 *
	 * The array keys are the CSS property names, and the array values
	 * are the corresponding CSS property values.
	 *
	 * For example,
	 *
	 * ```php
	 * print_r(Html::cssStyleToArray('width: 100px; height: 200px;'));
	 * // will display: ['width' => '100px', 'height' => '200px']
	 * ```
	 *
	 * @param string $style
	 *            the CSS style string
	 * @return array the array representation of the CSS style
	 */
	public static function cssStyleToArray($style)
	{
		$result = [];
		foreach (explode(';', $style) as $property) {
			$property = explode(':', $property);
			if (count($property) > 1) {
				$result[trim($property[0])] = trim($property[1]);
			}
		}
		return $result;
	}

	/**
	 * Returns the real attribute name from the given attribute expression.
	 *
	 * An attribute expression is an attribute name prefixed and/or suffixed with array indexes.
	 * It is mainly used in tabular data input and/or input of array type. Below are some examples:
	 *
	 * - `[0]content` is used in tabular data input to represent the "content" attribute
	 * for the first model in tabular input;
	 * - `dates[0]` represents the first array element of the "dates" attribute;
	 * - `[0]dates[0]` represents the first array element of the "dates" attribute
	 * for the first model in tabular input.
	 *
	 * If `$attribute` has neither prefix nor suffix, it will be returned back without change.
	 * 
	 * @param string $attribute
	 *            the attribute name or expression
	 * @return string the attribute name without prefix and suffix.
	 * @throws InvalidParamException if the attribute name contains non-word characters.
	 */
	public static function getAttributeName($attribute)
	{
		if (preg_match('/(^|.*\])([\w\.]+)(\[.*|$)/', $attribute, $matches)) {
			return $matches[2];
		} else {
			throw new InvalidParamException('Attribute name must contain word characters only.');
		}
	}

	/**
	 * Returns the value of the specified attribute name or expression.
	 *
	 * For an attribute expression like `[0]dates[0]`, this method will return the value of `$model->dates[0]`.
	 * See [[getAttributeName()]] for more details about attribute expression.
	 *
	 * If an attribute value is an instance of [[ActiveRecordInterface]] or an array of such instances,
	 * the primary value(s) of the AR instance(s) will be returned instead.
	 *
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression
	 * @return string|array the corresponding attribute value
	 * @throws InvalidParamException if the attribute name contains non-word characters.
	 */
	public static function getAttributeValue($model, $attribute)
	{
		if (!preg_match('/(^|.*\])([\w\.]+)(\[.*|$)/', $attribute, $matches)) {
			throw new InvalidParamException('Attribute name must contain word characters only.');
		}
		$attribute = $matches[2];
		$value = $model->$attribute;
		if ($matches[3] !== '') {
			foreach (explode('][', trim($matches[3], '[]')) as $id) {
				if ((is_array($value) || $value instanceof \ArrayAccess) && isset($value[$id])) {
					$value = $value[$id];
				} else {
					return null;
				}
			}
		}

		if (is_array($value)) {
			foreach ($value as $i => $v) {
				if ($v instanceof ActiveRecordInterface) {
					$v = $v->getPrimaryKey(false);
					$value[$i] = is_array($v) ? json_encode($v) : $v;
				}
			}
		} elseif ($value instanceof ActiveRecordInterface) {
			$value = $value->getPrimaryKey(false);

			return is_array($value) ? json_encode($value) : $value;
		}

		return $value;
	}

	/**
	 * Generates an appropriate input name for the specified attribute name or expression.
	 *
	 * This method generates a name that can be used as the input name to collect user input
	 * for the specified attribute. The name is generated according to the [[Model::formName|form name]]
	 * of the model and the given attribute name. For example, if the form name of the `Post` model
	 * is `Post`, then the input name generated for the `content` attribute would be `Post[content]`.
	 *
	 * See [[getAttributeName()]] for explanation of attribute expression.
	 *
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression
	 * @return string the generated input name
	 * @throws InvalidParamException if the attribute name contains non-word characters.
	 */
	public static function getInputName($model, $attribute)
	{
		$formName = $model->formName();
		if (!preg_match('/(^|.*\])([\w\.]+)(\[.*|$)/', $attribute, $matches)) {
			throw new InvalidParamException('Attribute name must contain word characters only.');
		}
		$prefix = $matches[1];
		$attribute = $matches[2];
		$suffix = $matches[3];
		if ($formName === '' && $prefix === '') {
			return $attribute . $suffix;
		} elseif ($formName !== '') {
			return $formName . $prefix . "[$attribute]" . $suffix;
		} else {
			throw new InvalidParamException(get_class($model) . '::formName() cannot be empty for tabular inputs.');
		}
	}

	/**
	 * Generates an appropriate input ID for the specified attribute name or expression.
	 *
	 * This method converts the result [[getInputName()]] into a valid input ID.
	 * For example, if [[getInputName()]] returns `Post[content]`, this method will return `post-content`.
	 * 
	 * @param Model $model
	 *            the model object
	 * @param string $attribute
	 *            the attribute name or expression. See [[getAttributeName()]] for explanation of attribute expression.
	 * @return string the generated input ID
	 * @throws InvalidParamException if the attribute name contains non-word characters.
	 */
	public static function getInputId($model, $attribute)
	{
		$name = strtolower(static::getInputName($model, $attribute));
		return str_replace([
			'[]',
			'][',
			'[',
			']',
			' ',
			'.'
				], [
			'',
			'-',
			'-',
			'',
			'-',
			'-'
				], $name);
	}

	/**
	 * Escapes regular expression to use in JavaScript
	 * 
	 * @param string $regexp
	 *            the regular expression to be escaped.
	 * @return string the escaped result.
	 * @since 2.0.6
	 */
	public static function escapeJsRegularExpression($regexp)
	{
		$pattern = preg_replace('/\\\\x\{?([0-9a-fA-F]+)\}?/', '\u$1', $regexp);
		$deliminator = substr($pattern, 0, 1);
		$pos = strrpos($pattern, $deliminator, 1);
		$flag = substr($pattern, $pos + 1);
		if ($deliminator !== '/') {
			$pattern = '/' . str_replace('/', '\\/', substr($pattern, 1, $pos - 1)) . '/';
		} else {
			$pattern = substr($pattern, 0, $pos + 1);
		}
		if (!empty($flag)) {
			$pattern .= preg_replace('/[^igm]/', '', $flag);
		}

		return $pattern;
	}

}
