--[[
	Returns a new promise that:
		* is resolved when all input promises resolve
		* is rejected if ANY input promises reject
]]
function Promise._all(traceback, promises, amount)
	if type(promises) ~= "table" then
		error(string.format(ERROR_NON_LIST, "Promise.all"), 3)
	end

	-- We need to check that each value is a promise here so that we can produce
	-- a proper error rather than a rejected promise with our error.
	for i, promise in pairs(promises) do
		if not Promise.is(promise) then
			error(string.format(ERROR_NON_PROMISE_IN_LIST, "Promise.all", tostring(i)), 3)
		end
	end

	-- If there are no values then return an already resolved promise.
	if #promises == 0 or amount == 0 then
		return Promise.resolve({})
	end

	return Promise._new(traceback, function(resolve, reject, onCancel)
		-- An array to contain our resolved values from the given promises.
		local resolvedValues = {}
		local newPromises = {}

		-- Keep a count of resolved promises because just checking the resolved
		-- values length wouldn't account for promises that resolve with nil.
		local resolvedCount = 0
		local rejectedCount = 0
		local done = false

		local function cancel()
			for _, promise in ipairs(newPromises) do
				promise:cancel()
			end
		end

		-- Called when a single value is resolved and resolves if all are done.
		local function resolveOne(i, ...)
			if done then
				return
			end

			resolvedCount = resolvedCount + 1

			if amount == nil then
				resolvedValues[i] = ...
			else
				resolvedValues[resolvedCount] = ...
			end

			if resolvedCount >= (amount or #promises) then
				done = true
				resolve(resolvedValues)
				cancel()
			end
		end

		onCancel(cancel)

		-- We can assume the values inside `promises` are all promises since we
		-- checked above.
		for i, promise in ipairs(promises) do
			newPromises[i] = promise:andThen(
				function(...)
					resolveOne(i, ...)
				end,
				function(...)
					rejectedCount = rejectedCount + 1

					if amount == nil or #promises - rejectedCount < amount then
						cancel()
						done = true

						reject(...)
					end
				end
			)
		end

		if done then
			cancel()
		end
	end)
end

function Promise.all(...)
	local promises = {...}

	-- check if we've been given a list of promises, not just a variable number of promises
	if type(promises[1]) == "table" and not Promise.is(promises[1]) then
		-- we've been given a table of promises already
		promises = promises[1]
	end

	return Promise._all(debug.traceback(nil, 2), promises)
end

function Promise.fold(list, callback, initialValue)
	assert(type(list) == "table", "Bad argument #1 to Promise.fold: must be a table")
	assert(type(callback) == "function", "Bad argument #2 to Promise.fold: must be a function")

	local accumulator = Promise.resolve(initialValue)
	return Promise.each(list, function(resolvedElement, i)
		accumulator = accumulator:andThen(function(previousValueResolved)
			return callback(previousValueResolved, resolvedElement, i)
		end)
	end):andThenReturn(accumulator)
end

function Promise.some(promises, amount)
	assert(type(amount) == "number", "Bad argument #2 to Promise.some: must be a number")

	return Promise._all(debug.traceback(nil, 2), promises, amount)
end

function Promise.any(promises)
	return Promise._all(debug.traceback(nil, 2), promises, 1):andThen(function(values)
		return values[1]
	end)
end

function Promise.allSettled(promises)
	if type(promises) ~= "table" then
		error(string.format(ERROR_NON_LIST, "Promise.allSettled"), 2)
	end

	-- We need to check that each value is a promise here so that we can produce
	-- a proper error rather than a rejected promise with our error.
	for i, promise in pairs(promises) do
		if not Promise.is(promise) then
			error(string.format(ERROR_NON_PROMISE_IN_LIST, "Promise.allSettled", tostring(i)), 2)
		end
	end

	-- If there are no values then return an already resolved promise.
	if #promises == 0 then
		return Promise.resolve({})
	end

	return Promise._new(debug.traceback(nil, 2), function(resolve, _, onCancel)
		-- An array to contain our resolved values from the given promises.
		local fates = {}
		local newPromises = {}

		-- Keep a count of resolved promises because just checking the resolved
		-- values length wouldn't account for promises that resolve with nil.
		local finishedCount = 0

		-- Called when a single value is resolved and resolves if all are done.
		local function resolveOne(i, ...)
			finishedCount = finishedCount + 1

			fates[i] = ...

			if finishedCount >= #promises then
				resolve(fates)
			end
		end

		onCancel(function()
			for _, promise in ipairs(newPromises) do
				promise:cancel()
			end
		end)

		-- We can assume the values inside `promises` are all promises since we
		-- checked above.
		for i, promise in ipairs(promises) do
			newPromises[i] = promise:finally(
				function(...)
					resolveOne(i, ...)
				end
			)
		end
	end)
end