Plural_Forms

	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
		 *
		 * @throws Exception If there is a syntax or parsing error with the string.
		 *
		 * @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;
							break;
						}

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

		/**
		 * Execute the plural form function.
		 *
		 * @since 4.9.0
		 *
		 * @throws Exception If the plural form value cannot be calculated.
		 *
		 * @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 ( 'var' === $next[0] ) {
					$stack[] = $n;
					continue;
				} elseif ( 'value' === $next[0] ) {
					$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];
		}
	}