<?php

/**

 * A gettext Plural-Forms parser.

 *

 * @since 4.9.0

 */

class Plural_Forms {

	/**

	 * Operator characters.

	 *

	 * @since 4.9.0

	 * @var string OP_CHARS Operator characters.

	 */

	const OP_CHARS = '|&><!=%?:';

	/**

	 * Valid number characters.

	 *

	 * @since 4.9.0

	 * @var string NUM_CHARS Valid number characters.

	 */

	const NUM_CHARS = '0123456789';

	/**

	 * Operator precedence.

	 *

	 * Operator precedence from highest to lowest. Higher numbers indicate

	 * higher precedence, and are executed first.

	 *

	 * @see https://en.wikipedia.org/wiki/Operators_in_C_and_C%2B%2B#Operator_precedence

	 *

	 * @since 4.9.0

	 * @var array $op_precedence Operator precedence from highest to lowest.

	 */

	protected static $op_precedence = array(

		'%' => 6,

		'<' => 5,

		'<=' => 5,

		'>' => 5,

		'>=' => 5,

		'==' => 4,

		'!=' => 4,

		'&&' => 3,

		'||' => 2,

		'?:' => 1,

		'?' => 1,

		'(' => 0,

		')' => 0,

	);

	/**

	 * Tokens generated from the string.

	 *

	 * @since 4.9.0

	 * @var array $tokens List of tokens.

	 */

	protected $tokens = array();

	/**

	 * Cache for repeated calls to the function.

	 *

	 * @since 4.9.0

	 * @var array $cache Map of $n => $result

	 */

	protected $cache = array();

	/**

	 * Constructor.

	 *

	 * @since 4.9.0

	 *

	 * @param string $str Plural function (just the bit after `plural=` from Plural-Forms)

	 */

	public function __construct( $str ) {

		$this->parse( $str );

	}

	/**

	 * Parse a Plural-Forms string into tokens.

	 *

	 * Uses the shunting-yard algorithm to convert the string to Reverse Polish

	 * Notation tokens.

	 *

	 * @since 4.9.0

	 *

	 * @param string $str String to parse.

	 */

	protected function parse( $str ) {

		$pos = 0;

		$len = strlen( $str );

		// Convert infix operators to postfix using the shunting-yard algorithm.

		$output = array();

		$stack = array();

		while ( $pos < $len ) {

			$next = substr( $str, $pos, 1 );

			switch ( $next ) {

				// Ignore whitespace

				case ' ':

				case "\t":

					$pos++;

					break;

				// Variable (n)

				case 'n':

					$output[] = array( 'var' );

					$pos++;

					break;

				// Parentheses

				case '(':

					$stack[] = $next;

					$pos++;

					break;

				case ')':

					$found = false;

					while ( ! empty( $stack ) ) {

						$o2 = $stack[ count( $stack ) - 1 ];

						if ( $o2 !== '(' ) {

							$output[] = array( 'op', array_pop( $stack ) );

							continue;

						}

						// Discard open paren.

						array_pop( $stack );

						$found = true;

						break;

					}

					if ( ! $found ) {

						throw new Exception( 'Mismatched parentheses' );

					}

					$pos++;

					break;

				// Operators

				case '|':

				case '&':

				case '>':

				case '<':

				case '!':

				case '=':

				case '%':

				case '?':

					$end_operator = strspn( $str, self::OP_CHARS, $pos );

					$operator = substr( $str, $pos, $end_operator );

					if ( ! array_key_exists( $operator, self::$op_precedence ) ) {

						throw new Exception( sprintf( 'Unknown operator "%s"', $operator ) );

					}

					while ( ! empty( $stack ) ) {

						$o2 = $stack[ count( $stack ) - 1 ];

						// Ternary is right-associative in C

						if ( $operator === '?:' || $operator === '?' ) {

							if ( self::$op_precedence[ $operator ] >= self::$op_precedence[ $o2 ] ) {

								break;

							}

						} elseif ( self::$op_precedence[ $operator ] > self::$op_precedence[ $o2 ] ) {

							break;

						}

						$output[] = array( 'op', array_pop( $stack ) );

					}

					$stack[] = $operator;

					$pos += $end_operator;

					break;

				// Ternary "else"

				case ':':

					$found = false;

					$s_pos = count( $stack ) - 1;

					while ( $s_pos >= 0 ) {

						$o2 = $stack[ $s_pos ];

						if ( $o2 !== '?' ) {

							$output[] = array( 'op', array_pop( $stack ) );

							$s_pos--;

							continue;

						}

						// Replace.

						$stack[ $s_pos ] = '?:';

						$found = true;

						break;

					}

					if ( ! $found ) {

						throw new Exception( 'Missing starting "?" ternary operator' );

					}

					$pos++;

					break;

				// Default - number or invalid

				default:

					if ( $next >= '0' && $next <= '9' ) {

						$span = strspn( $str, self::NUM_CHARS, $pos );

						$output[] = array( 'value', intval( substr( $str, $pos, $span ) ) );

						$pos += $span;

						continue;

					}

					throw new Exception( sprintf( 'Unknown symbol "%s"', $next ) );

			}

		}

		while ( ! empty( $stack ) ) {

			$o2 = array_pop( $stack );

			if ( $o2 === '(' || $o2 === ')' ) {

				throw new Exception( 'Mismatched parentheses' );

			}

			$output[] = array( 'op', $o2 );

		}

		$this->tokens = $output;

	}

	/**

	 * Get the plural form for a number.

	 *

	 * Caches the value for repeated calls.

	 *

	 * @since 4.9.0

	 *

	 * @param int $num Number to get plural form for.

	 * @return int Plural form value.

	 */

	public function get( $num ) {

		if ( isset( $this->cache[ $num ] ) ) {

			return $this->cache[ $num ];

		}

		return $this->cache[ $num ] = $this->execute( $num );

	}

	/**

	 * Execute the plural form function.

	 *

	 * @since 4.9.0

	 *

	 * @param int $n Variable "n" to substitute.

	 * @return int Plural form value.

	 */

	public function execute( $n ) {

		$stack = array();

		$i = 0;

		$total = count( $this->tokens );

		while ( $i < $total ) {

			$next = $this->tokens[$i];

			$i++;

			if ( $next[0] === 'var' ) {

				$stack[] = $n;

				continue;

			} elseif ( $next[0] === 'value' ) {

				$stack[] = $next[1];

				continue;

			}

			// Only operators left.

			switch ( $next[1] ) {

				case '%':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 % $v2;

					break;

				case '||':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 || $v2;

					break;

				case '&&':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 && $v2;

					break;

				case '<':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 < $v2;

					break;

				case '<=':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 <= $v2;

					break;

				case '>':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 > $v2;

					break;

				case '>=':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 >= $v2;

					break;

				case '!=':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 != $v2;

					break;

				case '==':

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 == $v2;

					break;

				case '?:':

					$v3 = array_pop( $stack );

					$v2 = array_pop( $stack );

					$v1 = array_pop( $stack );

					$stack[] = $v1 ? $v2 : $v3;

					break;

				default:

					throw new Exception( sprintf( 'Unknown operator "%s"', $next[1] ) );

			}

		}

		if ( count( $stack ) !== 1 ) {

			throw new Exception( 'Too many values remaining on the stack' );

		}

		return (int) $stack[0];

	}

}