<?php
/**
 * Class GF256
 *
 * @created      16.01.2021
 * @author       ZXing Authors
 * @author       Smiley <smiley@chillerlan.net>
 * @copyright    2021 Smiley
 * @license      Apache-2.0
 */
declare(strict_types=1);

namespace chillerlan\QRCode\Common;

use chillerlan\QRCode\QRCodeException;

use function array_fill;

/**
 * This class contains utility methods for performing mathematical operations over
 * the Galois Fields. Operations use a given primitive polynomial in calculations.
 *
 * Throughout this package, elements of the GF are represented as an int
 * for convenience and speed (but at the cost of memory).
 *
 *
 * @author Sean Owen
 * @author David Olivier
 */
final class GF256{

	/**
	 * irreducible polynomial whose coefficients are represented by the bits of an int,
	 * where the least-significant bit represents the constant coefficient
	 */
#	private int $primitive = 0x011D;

	private const logTable = [
		  0, // the first value is never returned, index starts at 1
		       0,   1,  25,   2,  50,  26, 198,   3, 223,  51, 238,  27, 104, 199,  75,
		  4, 100, 224,  14,  52, 141, 239, 129,  28, 193, 105, 248, 200,   8,  76, 113,
		  5, 138, 101,  47, 225,  36,  15,  33,  53, 147, 142, 218, 240,  18, 130,  69,
		 29, 181, 194, 125, 106,  39, 249, 185, 201, 154,   9, 120,  77, 228, 114, 166,
		  6, 191, 139,  98, 102, 221,  48, 253, 226, 152,  37, 179,  16, 145,  34, 136,
		 54, 208, 148, 206, 143, 150, 219, 189, 241, 210,  19,  92, 131,  56,  70,  64,
		 30,  66, 182, 163, 195,  72, 126, 110, 107,  58,  40,  84, 250, 133, 186,  61,
		202,  94, 155, 159,  10,  21, 121,  43,  78, 212, 229, 172, 115, 243, 167,  87,
		  7, 112, 192, 247, 140, 128,  99,  13, 103,  74, 222, 237,  49, 197, 254,  24,
		227, 165, 153, 119,  38, 184, 180, 124,  17,  68, 146, 217,  35,  32, 137,  46,
		 55,  63, 209,  91, 149, 188, 207, 205, 144, 135, 151, 178, 220, 252, 190,  97,
		242,  86, 211, 171,  20,  42,  93, 158, 132,  60,  57,  83,  71, 109,  65, 162,
		 31,  45,  67, 216, 183, 123, 164, 118, 196,  23,  73, 236, 127,  12, 111, 246,
		108, 161,  59,  82,  41, 157,  85, 170, 251,  96, 134, 177, 187, 204,  62,  90,
		203,  89,  95, 176, 156, 169, 160,  81,  11, 245,  22, 235, 122, 117,  44, 215,
		 79, 174, 213, 233, 230, 231, 173, 232, 116, 214, 244, 234, 168,  80,  88, 175,
	];

	private const expTable = [
		  1,   2,   4,   8,  16,  32,  64, 128,  29,  58, 116, 232, 205, 135,  19,  38,
		 76, 152,  45,  90, 180, 117, 234, 201, 143,   3,   6,  12,  24,  48,  96, 192,
		157,  39,  78, 156,  37,  74, 148,  53, 106, 212, 181, 119, 238, 193, 159,  35,
		 70, 140,   5,  10,  20,  40,  80, 160,  93, 186, 105, 210, 185, 111, 222, 161,
		 95, 190,  97, 194, 153,  47,  94, 188, 101, 202, 137,  15,  30,  60, 120, 240,
		253, 231, 211, 187, 107, 214, 177, 127, 254, 225, 223, 163,  91, 182, 113, 226,
		217, 175,  67, 134,  17,  34,  68, 136,  13,  26,  52, 104, 208, 189, 103, 206,
		129,  31,  62, 124, 248, 237, 199, 147,  59, 118, 236, 197, 151,  51, 102, 204,
		133,  23,  46,  92, 184, 109, 218, 169,  79, 158,  33,  66, 132,  21,  42,  84,
		168,  77, 154,  41,  82, 164,  85, 170,  73, 146,  57, 114, 228, 213, 183, 115,
		230, 209, 191,  99, 198, 145,  63, 126, 252, 229, 215, 179, 123, 246, 241, 255,
		227, 219, 171,  75, 150,  49,  98, 196, 149,  55, 110, 220, 165,  87, 174,  65,
		130,  25,  50, 100, 200, 141,   7,  14,  28,  56, 112, 224, 221, 167,  83, 166,
		 81, 162,  89, 178, 121, 242, 249, 239, 195, 155,  43,  86, 172,  69, 138,   9,
		 18,  36,  72, 144,  61, 122, 244, 245, 247, 243, 251, 235, 203, 139,  11,  22,
		 44,  88, 176, 125, 250, 233, 207, 131,  27,  54, 108, 216, 173,  71, 142,   1,
	];

	/**
	 * Implements both addition and subtraction -- they are the same in GF(size).
	 *
	 * @return int sum/difference of a and b
	 */
	public static function addOrSubtract(int $a, int $b):int{
		return ($a ^ $b);
	}

	/**
	 * @return GenericGFPoly the monomial representing coefficient * x^degree
	 * @throws \chillerlan\QRCode\QRCodeException
	 */
	public static function buildMonomial(int $degree, int $coefficient):GenericGFPoly{

		if($degree < 0){
			throw new QRCodeException('degree < 0');
		}

		$coefficients    = array_fill(0, ($degree + 1), 0);
		$coefficients[0] = $coefficient;

		return new GenericGFPoly($coefficients);
	}

	/**
	 * @return int 2 to the power of $a in GF(size)
	 */
	public static function exp(int $a):int{

		if($a < 0){
			$a += 255;
		}
		elseif($a >= 256){
			$a -= 255;
		}

		return self::expTable[$a];
	}

	/**
	 * @return int base 2 log of $a in GF(size)
	 * @throws \chillerlan\QRCode\QRCodeException
	 */
	public static function log(int $a):int{

		if($a < 1){
			throw new QRCodeException('$a < 1');
		}

		return self::logTable[$a];
	}

	/**
	 * @return int multiplicative inverse of a
	 * @throws \chillerlan\QRCode\QRCodeException
	 */
	public static function inverse(int $a):int{

		if($a === 0){
			throw new QRCodeException('$a === 0');
		}

		return self::expTable[(256 - self::logTable[$a] - 1)];
	}

	/**
	 * @return int product of a and b in GF(size)
	 */
	public static function multiply(int $a, int $b):int{

		if($a === 0 || $b === 0){
			return 0;
		}

		return self::expTable[((self::logTable[$a] + self::logTable[$b]) % 255)];
	}

}
