index.js

module.exports = function memize( fn, options ) {
	var size = 0,
		maxSize, head, tail;

	if ( options && options.maxSize ) {
		maxSize = options.maxSize;
	}

	function memoized( /* ...args */ ) {
		var node = head,
			len = arguments.length,
			args, i;

		searchCache: while ( node ) {
			// Check whether node arguments match arguments
			for ( i = 0; i < len; i++ ) {
				if ( node.args[ i ] !== arguments[ i ] ) {
					node = node.next;
					continue searchCache;
				}
			}

			// At this point we can assume we've found a match

			// Surface matched node to head if not already
			if ( node !== head ) {
				const beforeNode = node.prev;
				const afterNode = node.next;
				beforeNode.next = afterNode;

				if ( ! afterNode ) {
					tail = beforeNode;
				} else {
					afterNode.prev = beforeNode;
				}

				node.next = head;
				node.prev = null;

				head.prev = node;
				head = node;
			}

			// Return immediately
			return node.val;
		}

		// No cached value found. Continue to insertion phase:

		// Create a copy of arguments (avoid leaking deoptimization)
		args = new Array( len );
		for ( i = 0; i < len; i++ ) {
			args[ i ] = arguments[ i ];
		}

		node = {
			args: args,

			// Generate the result from original function
			val: fn.apply( null, args )
		};

		// Don't need to check whether node is already head, since it would
		// have been returned above already if it was

		// Shift existing head down list
		if ( head ) {
			head.prev = node;
			node.next = head;
		} else {
			// If no head, follows that there's no tail (at initial or reset)
			tail = node;
		}

		// Trim tail if we're reached max size and are pending cache insertion
		if ( size === maxSize ) {
			tail = tail.prev;
			tail.next = null;
		} else {
			size++;
		}

		head = node;

		return node.val;
	}

	memoized.clear = function() {
		head = null;
		tail = null;
		size = 0;
	};

	if ( process.env.NODE_ENV === 'test' ) {
		// Cache is not exposed in the public API, but used in tests to ensure
		// expected list progression
		memoized.getCache = function() {
			return [ head, tail, size ];
		};
	}

	return memoized;
};