"use strict";

module.exports = function generateLookupTable(items, getKey) {
	let lookupTable = new Map();

	function setItem(key, value) {
		// TODO: Add a fast path for unique keys (no array wrapping), so that the index can contain both single items and arrays?
		if (lookupTable.has(key)) {
			lookupTable.get(key).push(value);
		} else {
			lookupTable.set(key, [ value ]);
		}
	}

	items.forEach((item) => {
		let key = getKey(item);

		if (key != null) {
			if (Array.isArray(key)) {
				// This item should be available under multiple keys
				for (let keyItem of key) {
					setItem(keyItem, item);
				}
			} else {
				setItem(key, item);
			}
		}
	});

	return lookupTable;
};