Plural_Forms::parse( string $str )

	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;
	}