var PRECEDENCE, OPENERS, TERMINATORS, PATTERN;

/**
 * Operator precedence mapping.
 *
 * @type {Object}
 */
PRECEDENCE = {
	'(': 9,
	'!': 8,
	'*': 7,
	'/': 7,
	'%': 7,
	'+': 6,
	'-': 6,
	'<': 5,
	'<=': 5,
	'>': 5,
	'>=': 5,
	'==': 4,
	'!=': 4,
	'&&': 3,
	'||': 2,
	'?': 1,
	'?:': 1,
};

/**
 * Characters which signal pair opening, to be terminated by terminators.
 *
 * @type {string[]}
 */
OPENERS = [ '(', '?' ];

/**
 * Characters which signal pair termination, the value an array with the
 * opener as its first member. The second member is an optional operator
 * replacement to push to the stack.
 *
 * @type {string[]}
 */
TERMINATORS = {
	')': [ '(' ],
	':': [ '?', '?:' ],
};

/**
 * Pattern matching operators and openers.
 *
 * @type {RegExp}
 */
PATTERN = /<=|>=|==|!=|&&|\|\||\?:|\(|!|\*|\/|%|\+|-|<|>|\?|\)|:/;

/**
 * Given a C expression, returns the equivalent postfix (Reverse Polish)
 * notation terms as an array.
 *
 * If a postfix string is desired, simply `.join( ' ' )` the result.
 *
 * @example
 *
 * ```js
 * import postfix from '@tannin/postfix';
 *
 * postfix( 'n > 1' );
 * // ⇒ [ 'n', '1', '>' ]
 * ```
 *
 * @param {string} expression C expression.
 *
 * @return {string[]} Postfix terms.
 */
export default function postfix( expression ) {
	var terms = [],
		stack = [],
		match, operator, term, element;

	while ( ( match = expression.match( PATTERN ) ) ) {
		operator = match[ 0 ];

		// Term is the string preceding the operator match. It may contain
		// whitespace, and may be empty (if operator is at beginning).
		term = expression.substr( 0, match.index ).trim();
		if ( term ) {
			terms.push( term );
		}

		while ( ( element = stack.pop() ) ) {
			if ( TERMINATORS[ operator ] ) {
				if ( TERMINATORS[ operator ][ 0 ] === element ) {
					// Substitution works here under assumption that because
					// the assigned operator will no longer be a terminator, it
					// will be pushed to the stack during the condition below.
					operator = TERMINATORS[ operator ][ 1 ] || operator;
					break;
				}
			} else if ( OPENERS.indexOf( element ) >= 0 || PRECEDENCE[ element ] < PRECEDENCE[ operator ] ) {
				// Push to stack if either an opener or when pop reveals an
				// element of lower precedence.
				stack.push( element );
				break;
			}

			// For each popped from stack, push to terms.
			terms.push( element );
		}

		if ( ! TERMINATORS[ operator ] ) {
			stack.push( operator );
		}

		// Slice matched fragment from expression to continue match.
		expression = expression.substr( match.index + operator.length );
	}

	// Push remainder of operand, if exists, to terms.
	expression = expression.trim();
	if ( expression ) {
		terms.push( expression );
	}

	// Pop remaining items from stack into terms.
	return terms.concat( stack.reverse() );
}