--!strict
local Sift = script.Parent.Parent

local Util = require(Sift.Util)
local Copy = require(script.Parent.copy)

type Callback<T> = (index: number) -> T
type Updater<T> = (currentValue: T, index: number) -> T

local function call<T>(callback: Callback<T>, index: number)
	if type(callback) == "function" then
		return callback(index)
	end
end

--[=[
	@function update
	@within Array

	@param array {T} -- The array to update.
	@param index number -- The index to update.
	@param updater? (value: T, index: number) -> T -- The updater function.
	@param callback? (index: number) -> T -- The callback function.
	@return {T} -- The updated array.

	Updates an array at the given index. If the value at the given index does
	not exist, `callback` will be called, and its return value will be used
	as the value at the given index.

	```lua
	local array = { 1, 2, 3 }

	local new = Update(array, 2, function(value)
		return value + 1
	end) -- { 2, 3, 3 }

	local new = Update(array, 4, function(value)
		return value + 1
	end, function(value)
		return 10
	end) -- { 1, 2, 3, 10 }
	```
]=]
local function update<T>(
	array: { T },
	index: number,
	updater: Updater<T>?,
	callback: Callback<T>?
): { T }
	local length = #array
	local result = Copy(array)

	if index < 1 then
		index += length
	end

	updater = if type(updater) == "function" then updater else Util.func.returned

	if result[index] ~= nil then
		result[index] = updater(result[index], index)
	else
		result[index] = call(callback, index)
	end

	return result
end

return update