ractive/src/viewmodel/prototype/merge/mapOldToNewIndex.js

Next-generation DOM manipulation

export default function ( oldArray, newArray ) {
	var usedIndices, firstUnusedIndex, newIndices, changed;

	usedIndices = {};
	firstUnusedIndex = 0;

	newIndices = oldArray.map( function ( item, i ) {
		var index, start, len;

		start = firstUnusedIndex;
		len = newArray.length;

		do {
			index = newArray.indexOf( item, start );

			if ( index === -1 ) {
				changed = true;
				return -1;
			}

			start = index + 1;
		} while ( usedIndices[ index ] && start < len );

		// keep track of the first unused index, so we don't search
		// the whole of newArray for each item in oldArray unnecessarily
		if ( index === firstUnusedIndex ) {
			firstUnusedIndex += 1;
		}

		if ( index !== i ) {
			changed = true;
		}

		usedIndices[ index ] = true;
		return index;
	});

	newIndices.unchanged = !changed;
	return newIndices;
}