<?php
/**
 * Encode / Decode Base64 VLQ.
 *
 * @private
 */
class Less_SourceMap_Base64VLQ {

	/**
	 * Shift
	 *
	 * @var int
	 */
	private $shift = 5;

	/**
	 * Mask
	 *
	 * @var int
	 */
	private $mask = 0x1F; // == (1 << shift) == 0b00011111

	/**
	 * Continuation bit
	 *
	 * @var int
	 */
	private $continuationBit = 0x20; // == (mask - 1 ) == 0b00100000

	/**
	 * Char to integer map
	 *
	 * @var array
	 */
	private $charToIntMap = [
		'A' => 0, 'B' => 1, 'C' => 2, 'D' => 3, 'E' => 4, 'F' => 5, 'G' => 6,
		'H' => 7, 'I' => 8, 'J' => 9, 'K' => 10, 'L' => 11, 'M' => 12, 'N' => 13,
		'O' => 14, 'P' => 15, 'Q' => 16, 'R' => 17, 'S' => 18, 'T' => 19, 'U' => 20,
		'V' => 21, 'W' => 22, 'X' => 23, 'Y' => 24, 'Z' => 25, 'a' => 26, 'b' => 27,
		'c' => 28, 'd' => 29, 'e' => 30, 'f' => 31, 'g' => 32, 'h' => 33, 'i' => 34,
		'j' => 35, 'k' => 36, 'l' => 37, 'm' => 38, 'n' => 39, 'o' => 40, 'p' => 41,
		'q' => 42, 'r' => 43, 's' => 44, 't' => 45, 'u' => 46, 'v' => 47, 'w' => 48,
		'x' => 49, 'y' => 50, 'z' => 51, 0 => 52, 1 => 53, 2 => 54, 3 => 55, 4 => 56,
		5 => 57, 6 => 58, 7 => 59, 8 => 60, 9 => 61, '+' => 62, '/' => 63,
	];

	/**
	 * Integer to char map
	 *
	 * @var array
	 */
	private $intToCharMap = [
		0 => 'A', 1 => 'B', 2 => 'C', 3 => 'D', 4 => 'E', 5 => 'F', 6 => 'G',
		7 => 'H', 8 => 'I', 9 => 'J', 10 => 'K', 11 => 'L', 12 => 'M', 13 => 'N',
		14 => 'O', 15 => 'P', 16 => 'Q', 17 => 'R', 18 => 'S', 19 => 'T', 20 => 'U',
		21 => 'V', 22 => 'W', 23 => 'X', 24 => 'Y', 25 => 'Z', 26 => 'a', 27 => 'b',
		28 => 'c', 29 => 'd', 30 => 'e', 31 => 'f', 32 => 'g', 33 => 'h', 34 => 'i',
		35 => 'j', 36 => 'k', 37 => 'l', 38 => 'm', 39 => 'n', 40 => 'o', 41 => 'p',
		42 => 'q', 43 => 'r', 44 => 's', 45 => 't', 46 => 'u', 47 => 'v', 48 => 'w',
		49 => 'x', 50 => 'y', 51 => 'z', 52 => '0', 53 => '1', 54 => '2', 55 => '3',
		56 => '4', 57 => '5', 58 => '6', 59 => '7', 60 => '8', 61 => '9', 62 => '+',
		63 => '/',
	];

	/**
	 * Constructor
	 */
	public function __construct() {
		// I leave it here for future reference
		// foreach(str_split('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/') as $i => $char)
		// {
		//	 $this->charToIntMap[$char] = $i;
		//	 $this->intToCharMap[$i] = $char;
		// }
	}

	/**
	 * Convert from a two-complement value to a value where the sign bit is
	 * is placed in the least significant bit.	For example, as decimals:
	 *	 1 becomes 2 (10 binary), -1 becomes 3 (11 binary)
	 *	 2 becomes 4 (100 binary), -2 becomes 5 (101 binary)
	 * We generate the value for 32 bit machines, hence -2147483648 becomes 1, not 4294967297,
	 * even on a 64 bit machine.
	 * @param int $aValue
	 */
	public function toVLQSigned( $aValue ) {
		return 0xffffffff & ( $aValue < 0 ? ( ( -$aValue ) << 1 ) + 1 : ( $aValue << 1 ) + 0 );
	}

	/**
	 * Convert to a two-complement value from a value where the sign bit is
	 * is placed in the least significant bit. For example, as decimals:
	 *	 2 (10 binary) becomes 1, 3 (11 binary) becomes -1
	 *	 4 (100 binary) becomes 2, 5 (101 binary) becomes -2
	 * We assume that the value was generated with a 32 bit machine in mind.
	 * Hence
	 *	 1 becomes -2147483648
	 * even on a 64 bit machine.
	 * @param int $aValue
	 */
	public function fromVLQSigned( $aValue ) {
		return $aValue & 1 ? $this->zeroFill( ~$aValue + 2, 1 ) | ( -1 - 0x7fffffff ) : $this->zeroFill( $aValue, 1 );
	}

	/**
	 * Return the base 64 VLQ encoded value.
	 *
	 * @param int $aValue The value to encode
	 * @return string The encoded value
	 */
	public function encode( $aValue ) {
		$encoded = '';
		$vlq = $this->toVLQSigned( $aValue );
		do {
			$digit = $vlq & $this->mask;
			$vlq = $this->zeroFill( $vlq, $this->shift );
			if ( $vlq > 0 ) {
				$digit |= $this->continuationBit;
			}
			$encoded .= $this->base64Encode( $digit );
		} while ( $vlq > 0 );

		return $encoded;
	}

	/**
	 * Return the value decoded from base 64 VLQ.
	 *
	 * @param string $encoded The encoded value to decode
	 * @return int The decoded value
	 */
	public function decode( $encoded ) {
		$vlq = 0;
		$i = 0;
		do {
			$digit = $this->base64Decode( $encoded[$i] );
			$vlq |= ( $digit & $this->mask ) << ( $i * $this->shift );
			$i++;
		} while ( $digit & $this->continuationBit );

		return $this->fromVLQSigned( $vlq );
	}

	/**
	 * Right shift with zero fill.
	 *
	 * @param int $a number to shift
	 * @param int $b number of bits to shift
	 * @return int
	 */
	public function zeroFill( $a, $b ) {
		return ( $a >= 0 ) ? ( $a >> $b ) : ( $a >> $b ) & ( PHP_INT_MAX >> ( $b - 1 ) );
	}

	/**
	 * Encode single 6-bit digit as base64.
	 *
	 * @param int $number
	 * @return string
	 * @throws Exception If the number is invalid
	 */
	public function base64Encode( $number ) {
		if ( $number < 0 || $number > 63 ) {
			throw new Exception( "Invalid number \"$number\" given. Must be between 0 and 63." );
		}
		return $this->intToCharMap[$number];
	}

	/**
	 * Decode single 6-bit digit from base64
	 *
	 * @param string $char
	 * @return int
	 * @throws Exception If the number is invalid
	 */
	public function base64Decode( $char ) {
		if ( !array_key_exists( $char, $this->charToIntMap ) ) {
			throw new Exception( sprintf( 'Invalid base 64 digit "%s" given.', $char ) );
		}
		return $this->charToIntMap[$char];
	}

}
