--[[
	* Copyright (c) Roblox Corporation. All rights reserved.
	* Licensed under the MIT License (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	*     https://opensource.org/licenses/MIT
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
]]
local __DEV__ = _G.__DEV__
local Array = script.Parent
local Packages = Array.Parent.Parent
local types = require(Packages.ES7Types)
type Array<T> = types.Array<T>
type reduceFn<T, U> = (previousValue: U, currentValue: T, currentIndex: number, array: Array<T>) -> U

-- Implements Javascript's `Array.prototype.reduce` as defined below
-- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce
-- TODO Luau: when Luau supports overloads, use them here so that reduceFn can correctly use T when initialValue (U) isn't supplied
return function<T, U>(array: Array<T>, callback: reduceFn<T, U>, initialValue: U?): U
	if __DEV__ then
		if typeof(array) ~= "table" then
			error(string.format("Array.reduce called on %s", typeof(array)))
		end
		if typeof(callback) ~= "function" then
			error("callback is not a function")
		end
	end

	local length = #array

	local value: T | U
	local initial = 1

	if initialValue ~= nil then
		value = initialValue
	else
		initial = 2
		if length == 0 then
			error("reduce of empty array with no initial value")
		end
		value = array[1]
	end

	for i = initial, length do
		value = callback(value :: U, array[i], i, array)
	end

	return value :: U
end