--[[
	Iterates serially over the given an array of values, calling the predicate callback on each before continuing.
	If the predicate returns a Promise, we wait for that Promise to resolve before continuing to the next item
	in the array. If the Promise the predicate returns rejects, the Promise from Promise.each is also rejected with
	the same value.

	Returns a Promise containing an array of the return values from the predicate for each item in the original list.
]]

function Promise.each(list, predicate)
	assert(type(list) == "table", string.format(ERROR_NON_LIST, "Promise.each"))
	assert(type(predicate) == "function", string.format(ERROR_NON_FUNCTION, "Promise.each"))

	return Promise._new(debug.traceback(nil, 2), function(resolve, reject, onCancel)
		local results = {}
		local promisesToCancel = {}

		local cancelled = false

		local function cancel()
			for _, promiseToCancel in ipairs(promisesToCancel) do
				promiseToCancel:cancel()
			end
		end

		onCancel(function()
			cancelled = true

			cancel()
		end)

		-- We need to preprocess the list of values and look for Promises.
		-- If we find some, we must register our andThen calls now, so that those Promises have a consumer
		-- from us registered. If we don't do this, those Promises might get cancelled by something else
		-- before we get to them in the series because it's not possible to tell that we plan to use it
		-- unless we indicate it here.

		local preprocessedList = {}

		for index, value in ipairs(list) do
			if Promise.is(value) then
				if value:getStatus() == Promise.Status.Cancelled then
					cancel()
					return reject(Error.new({
						error = "Promise is cancelled",
						kind = Error.Kind.AlreadyCancelled,
						context = string.format(
							"The Promise that was part of the array at index %d passed into Promise.each was already cancelled when Promise.each began.\n\nThat Promise was created at:\n\n%s",
							index,
							value._source
						),
					}))
				elseif value:getStatus() == Promise.Status.Rejected then
					cancel()
					return reject(select(2, value:await()))
				end

				-- Chain a new Promise from this one so we only cancel ours
				local ourPromise = value:andThen(function(...)
					return ...
				end)

				table.insert(promisesToCancel, ourPromise)
				preprocessedList[index] = ourPromise
			else
				preprocessedList[index] = value
			end
		end

		for index, value in ipairs(preprocessedList) do
			if Promise.is(value) then
				local success
				success, value = value:await()

				if not success then
					cancel()
					return reject(value)
				end
			end

			if cancelled then
				return
			end

			local predicatePromise = Promise.resolve(predicate(value, index))

			table.insert(promisesToCancel, predicatePromise)

			local success, result = predicatePromise:await()

			if not success then
				cancel()
				return reject(result)
			end

			results[index] = result
		end

		resolve(results)
	end)
end
