<?php
/**
 * Binary-to-text PHP Utilities
 *
 * @package     binary-to-text-php
 * @link        https://github.com/ademarre/binary-to-text-php
 * @author      Andre DeMarre
 * @copyright   2009-2013 Andre DeMarre
 * @license     http://opensource.org/licenses/MIT  MIT
 */

/**
 * Class for binary-to-text encoding with a base of 2^n
 *
 * The Base2n class is for binary-to-text conversion. It employs a 
 * generalization of the algorithms used by many encoding schemes that 
 * use a fixed number of bits to encode each character. In other words, 
 * the base is a power of 2.
 *
 * Earlier versions of this class were named 
 * FixedBitNotation and FixedBitEncoding.
 *
 * @package binary-to-text-php
 */
class Base2n
{
    protected $_chars;
    protected $_bitsPerCharacter;
    protected $_radix;
    protected $_rightPadFinalBits;
    protected $_padFinalGroup;
    protected $_padCharacter;
    protected $_caseSensitive;
    protected $_charmap;

    /**
     * Constructor
     *
     * @param   integer $bitsPerCharacter   Bits to use for each encoded character
     * @param   string  $chars              Base character alphabet
     * @param   boolean $caseSensitive      To decode in a case-sensitive manner
     * @param   boolean $rightPadFinalBits  How to encode last character
     * @param   boolean $padFinalGroup      Add padding to end of encoded output
     * @param   string  $padCharacter       Character to use for padding
     *
     * @throws  InvalidArgumentException    for incompatible parameters
     */
    public function __construct(
        $bitsPerCharacter, 
        $chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ-_', 
        $caseSensitive = TRUE, $rightPadFinalBits = FALSE, 
        $padFinalGroup = FALSE, $padCharacter = '=')
    {
        // Ensure validity of $chars
        if (!is_string($chars) || ($charLength = strlen($chars)) < 2) {
            throw new InvalidArgumentException('$chars must be a string of at least two characters');
        }

        // Ensure validity of $padCharacter
        if ($padFinalGroup) {
            if (!is_string($padCharacter) || !isset($padCharacter[0])) {
                throw new InvalidArgumentException('$padCharacter must be a string of one character');
            }

            if ($caseSensitive) {
                $padCharFound = strpos($chars, $padCharacter[0]);
            } else {
                $padCharFound = stripos($chars, $padCharacter[0]);
            }

            if ($padCharFound !== FALSE) {
                throw new InvalidArgumentException('$padCharacter can not be a member of $chars');
            }
        }

        // Ensure validity of $bitsPerCharacter
        if (!is_int($bitsPerCharacter)) {
            throw new InvalidArgumentException('$bitsPerCharacter must be an integer');
        }

        if ($bitsPerCharacter < 1) {
            // $bitsPerCharacter must be at least 1
            throw new InvalidArgumentException('$bitsPerCharacter can not be less than 1');

        } elseif ($charLength < 1 << $bitsPerCharacter) {
            // Character length of $chars is too small for $bitsPerCharacter
            // Find greatest acceptable value of $bitsPerCharacter
            $bitsPerCharacter = 1;
            $radix = 2;

            while ($charLength >= ($radix <<= 1) && $bitsPerCharacter < 8) {
                $bitsPerCharacter++;
            }

            $radix >>= 1;
            throw new InvalidArgumentException(
                    '$bitsPerCharacter can not be more than ' . $bitsPerCharacter 
                  . ' given $chars length of ' . $charLength 
                  . ' (max radix ' . $radix . ')');

        } elseif ($bitsPerCharacter > 8) {
            // $bitsPerCharacter must not be greater than 8
            throw new InvalidArgumentException('$bitsPerCharacter can not be greater than 8');

        } else {
            $radix = 1 << $bitsPerCharacter;
        }

        $this->_chars             = $chars;
        $this->_bitsPerCharacter  = $bitsPerCharacter;
        $this->_radix             = $radix;
        $this->_rightPadFinalBits = $rightPadFinalBits;
        $this->_padFinalGroup     = $padFinalGroup;
        $this->_padCharacter      = $padCharacter[0];
        $this->_caseSensitive     = $caseSensitive;
    }

    /**
     * Encode a string
     *
     * @param   string  $rawString  Binary data to encode
     * @return  string
     */
    public function encode($rawString)
    {
        // Unpack string into an array of bytes
        $bytes = unpack('C*', $rawString);
        $byteCount = count($bytes);

        $encodedString = '';
        $byte = array_shift($bytes);
        $bitsRead = 0;

        $chars             = $this->_chars;
        $bitsPerCharacter  = $this->_bitsPerCharacter;
        $rightPadFinalBits = $this->_rightPadFinalBits;
        $padFinalGroup     = $this->_padFinalGroup;
        $padCharacter      = $this->_padCharacter;

        $charsPerByte = 8 / $bitsPerCharacter;
        $encodedLength = $byteCount * $charsPerByte;

        // Generate encoded output; each loop produces one encoded character
        for ($c = 0; $c < $encodedLength; $c++) {

            // Get the bits needed for this encoded character
            if ($bitsRead + $bitsPerCharacter > 8) {
                // Not enough bits remain in this byte for the current character
                // Save the remaining bits before getting the next byte
                $oldBitCount = 8 - $bitsRead;
                $oldBits = $byte ^ ($byte >> $oldBitCount << $oldBitCount);
                $newBitCount = $bitsPerCharacter - $oldBitCount;

                if (!$bytes) {
                    // Last bits; match final character and exit loop
                    if ($rightPadFinalBits) $oldBits <<= $newBitCount;
                    $encodedString .= $chars[$oldBits];

                    if ($padFinalGroup) {
                        // Array of the lowest common multiples of $bitsPerCharacter and 8, divided by 8
                        $lcmMap = array(1 => 1, 2 => 1, 3 => 3, 4 => 1, 5 => 5, 6 => 3, 7 => 7, 8 => 1);
                        $bytesPerGroup = $lcmMap[$bitsPerCharacter];
                        $pads = $bytesPerGroup * $charsPerByte - ceil((strlen($rawString) % $bytesPerGroup) * $charsPerByte);
                        $encodedString .= str_repeat($padCharacter, $pads);
                    }

                    break;
                }

                // Get next byte
                $byte = array_shift($bytes);
                $bitsRead = 0;

            } else {
                $oldBitCount = 0;
                $newBitCount = $bitsPerCharacter;
            }

            // Read only the needed bits from this byte
            $bits = $byte >> 8 - ($bitsRead + ($newBitCount));
            $bits ^= $bits >> $newBitCount << $newBitCount;
            $bitsRead += $newBitCount;

            if ($oldBitCount) {
                // Bits come from seperate bytes, add $oldBits to $bits
                $bits = ($oldBits << $newBitCount) | $bits;
            }

            $encodedString .= $chars[$bits];
        }

        return $encodedString;
    }

    /**
     * Decode a string
     *
     * @param   string  $encodedString  Data to decode
     * @param   boolean $strict         Returns NULL if $encodedString contains an undecodable character
     * @return  string
     */
    public function decode($encodedString, $strict = FALSE)
    {
        if (!$encodedString || !is_string($encodedString)) {
            // Empty string, nothing to decode
            return '';
        }

        $chars             = $this->_chars;
        $bitsPerCharacter  = $this->_bitsPerCharacter;
        $radix             = $this->_radix;
        $rightPadFinalBits = $this->_rightPadFinalBits;
        $padFinalGroup     = $this->_padFinalGroup;
        $padCharacter      = $this->_padCharacter;
        $caseSensitive     = $this->_caseSensitive;

        // Get index of encoded characters
        if ($this->_charmap) {
            $charmap = $this->_charmap;

        } else {
            $charmap = array();

            for ($i = 0; $i < $radix; $i++) {
                $charmap[$chars[$i]] = $i;
            }

            $this->_charmap = $charmap;
        }

        // The last encoded character is $encodedString[$lastNotatedIndex]
        $lastNotatedIndex = strlen($encodedString) - 1;

        // Remove trailing padding characters
        if ($padFinalGroup) {
            while ($encodedString[$lastNotatedIndex] == $padCharacter) {
                $encodedString = substr($encodedString, 0, $lastNotatedIndex);
                $lastNotatedIndex--;
            }
        }

        $rawString = '';
        $byte = 0;
        $bitsWritten = 0;

        // Convert each encoded character to a series of unencoded bits
        for ($c = 0; $c <= $lastNotatedIndex; $c++) {

            if (!isset($charmap[$encodedString[$c]]) && !$caseSensitive) {
                // Encoded character was not found; try other case
                if (isset($charmap[$cUpper = strtoupper($encodedString[$c])])) {
                    $charmap[$encodedString[$c]] = $charmap[$cUpper];

                } elseif (isset($charmap[$cLower = strtolower($encodedString[$c])])) {
                    $charmap[$encodedString[$c]] = $charmap[$cLower];
                }
            }

            if (isset($charmap[$encodedString[$c]])) {
                $bitsNeeded = 8 - $bitsWritten;
                $unusedBitCount = $bitsPerCharacter - $bitsNeeded;

                // Get the new bits ready
                if ($bitsNeeded > $bitsPerCharacter) {
                    // New bits aren't enough to complete a byte; shift them left into position
                    $newBits = $charmap[$encodedString[$c]] << $bitsNeeded - $bitsPerCharacter;
                    $bitsWritten += $bitsPerCharacter;

                } elseif ($c != $lastNotatedIndex || $rightPadFinalBits) {
                    // Zero or more too many bits to complete a byte; shift right
                    $newBits = $charmap[$encodedString[$c]] >> $unusedBitCount;
                    $bitsWritten = 8; //$bitsWritten += $bitsNeeded;

                } else {
                    // Final bits don't need to be shifted
                    $newBits = $charmap[$encodedString[$c]];
                    $bitsWritten = 8;
                }

                $byte |= $newBits;

                if ($bitsWritten == 8 || $c == $lastNotatedIndex) {
                    // Byte is ready to be written
                    $rawString .= pack('C', $byte);

                    if ($c != $lastNotatedIndex) {
                        // Start the next byte
                        $bitsWritten = $unusedBitCount;
                        $byte = ($charmap[$encodedString[$c]] ^ ($newBits << $unusedBitCount)) << 8 - $bitsWritten;
                    }
                }

            } elseif ($strict) {
                // Unable to decode character; abort
                return NULL;
            }
        }

        return $rawString;
    }
}
