From e557cfb0123ef1e7fe366f25e38bbbccaa36ff4c Mon Sep 17 00:00:00 2001
From: Matthew Wild <mwild1@gmail.com>
Date: Wed, 10 Oct 2018 17:45:19 +0100
Subject: util.promise: ES6-like API for promises

---
 spec/util_promise_spec.lua | 262 +++++++++++++++++++++++++++++++++++++++++++++
 util/promise.lua           | 133 +++++++++++++++++++++++
 2 files changed, 395 insertions(+)
 create mode 100644 spec/util_promise_spec.lua
 create mode 100644 util/promise.lua

diff --git a/spec/util_promise_spec.lua b/spec/util_promise_spec.lua
new file mode 100644
index 00000000..f0aec64c
--- /dev/null
+++ b/spec/util_promise_spec.lua
@@ -0,0 +1,262 @@
+local promise = require "util.promise";
+
+describe("util.promise", function ()
+	describe("new()", function ()
+		it("returns a promise object", function ()
+			assert(promise.new());
+		end);
+	end);
+	it("notifies immediately for fulfilled promises", function ()
+		local p = promise.new(function (resolve)
+			resolve("foo");
+		end);
+		local cb = spy.new(function (v)
+			assert.equal("foo", v);
+		end);
+		p:next(cb);
+		assert.spy(cb).was_called(1);
+	end);
+	it("notifies on fulfilment of pending promises", function ()
+		local r;
+		local p = promise.new(function (resolve)
+			r = resolve;
+		end);
+		local cb = spy.new(function (v)
+			assert.equal("foo", v);
+		end);
+		p:next(cb);
+		assert.spy(cb).was_called(0);
+		r("foo");
+		assert.spy(cb).was_called(1);
+	end);
+	it("allows chaining :next() calls", function ()
+		local r;
+		local result;
+		local p = promise.new(function (resolve)
+			r = resolve;
+		end);
+		local cb1 = spy.new(function (v)
+			assert.equal("foo", v);
+			return "bar";
+		end);
+		local cb2 = spy.new(function (v)
+			assert.equal("bar", v);
+			result = v;
+		end);
+		p:next(cb1):next(cb2);
+		assert.spy(cb1).was_called(0);
+		assert.spy(cb2).was_called(0);
+		r("foo");
+		assert.spy(cb1).was_called(1);
+		assert.spy(cb2).was_called(1);
+		assert.equal("bar", result);
+	end);
+	it("supports multiple :next() calls on the same promise", function ()
+		local r;
+		local result;
+		local p = promise.new(function (resolve)
+			r = resolve;
+		end);
+		local cb1 = spy.new(function (v)
+			assert.equal("foo", v);
+			result = v;
+		end);
+		local cb2 = spy.new(function (v)
+			assert.equal("foo", v);
+			result = v;
+		end);
+		p:next(cb1);
+		p:next(cb2);
+		assert.spy(cb1).was_called(0);
+		assert.spy(cb2).was_called(0);
+		r("foo");
+		assert.spy(cb1).was_called(1);
+		assert.spy(cb2).was_called(1);
+		assert.equal("foo", result);		
+	end);
+	it("automatically rejects on error", function ()
+		local r;
+		local p = promise.new(function (resolve)
+			r = resolve;
+			error("oh no");
+		end);
+		local cb = spy.new(function () end);
+		local err_cb = spy.new(function (v)
+			assert.equal("oh no", v);
+		end);
+		p:next(cb, err_cb);
+		assert.spy(cb).was_called(0);
+		assert.spy(err_cb).was_called(1);
+		r("foo");
+		assert.spy(cb).was_called(0);
+		assert.spy(err_cb).was_called(1);
+	end);
+	it("supports reject()", function ()
+		local r, result;
+		local p = promise.new(function (resolve, reject)
+			r = reject;
+		end);
+		local cb = spy.new(function () end);
+		local err_cb = spy.new(function (v)
+			result = v;
+			assert.equal("oh doh", v);
+		end);
+		p:next(cb, err_cb);
+		assert.spy(cb).was_called(0);
+		assert.spy(err_cb).was_called(0);
+		r("oh doh");
+		assert.spy(cb).was_called(0);
+		assert.spy(err_cb).was_called(1);
+		assert.equal("oh doh", result);		
+	end);
+	it("supports chaining of rejected promises", function ()
+		local r, result;
+		local p = promise.new(function (resolve, reject)
+			r = reject;
+		end);
+		local cb = spy.new(function () end);
+		local err_cb = spy.new(function (v)
+			result = v;
+			assert.equal("oh doh", v);
+			return "ok"
+		end);
+		local cb2 = spy.new(function (v)
+			result = v;
+		end);
+		local err_cb2 = spy.new(function (v) end);
+		p:next(cb, err_cb):next(cb2, err_cb2)
+		assert.spy(cb).was_called(0);
+		assert.spy(err_cb).was_called(0);
+		assert.spy(cb2).was_called(0);
+		assert.spy(err_cb2).was_called(0);
+		r("oh doh");
+		assert.spy(cb).was_called(0);
+		assert.spy(err_cb).was_called(1);
+		assert.spy(cb2).was_called(1);
+		assert.spy(err_cb2).was_called(0);
+		assert.equal("ok", result);		
+	end);
+
+	describe("race()", function ()
+		it("works with fulfilled promises", function ()
+			local p1, p2 = promise.resolve("yep"), promise.resolve("nope");
+			local p = promise.race({ p1, p2 });
+			local result;
+			p:next(function (v)
+				result = v;
+			end);
+			assert.equal("yep", result);
+		end);
+		it("works with pending promises", function ()
+			local r1, r2;
+			local p1, p2 = promise.new(function (resolve) r1 = resolve end), promise.new(function (resolve) r2 = resolve end);
+			local p = promise.race({ p1, p2 });
+
+			local result;
+			local cb = spy.new(function (v)
+				result = v;
+			end);
+			p:next(cb);
+			assert.spy(cb).was_called(0);
+			r2("yep");
+			r1("nope");
+			assert.spy(cb).was_called(1);
+			assert.equal("yep", result);
+		end);
+	end);
+	describe("all()", function ()
+		it("works with fulfilled promises", function ()
+			local p1, p2 = promise.resolve("yep"), promise.resolve("nope");
+			local p = promise.all({ p1, p2 });
+			local result;
+			p:next(function (v)
+				result = v;
+			end);
+			assert.same({ "yep", "nope" }, result);
+		end);
+		it("works with pending promises", function ()
+			local r1, r2;
+			local p1, p2 = promise.new(function (resolve) r1 = resolve end), promise.new(function (resolve) r2 = resolve end);
+			local p = promise.all({ p1, p2 });
+
+			local result;
+			local cb = spy.new(function (v)
+				result = v;
+			end);
+			p:next(cb);
+			assert.spy(cb).was_called(0);
+			r2("yep");
+			assert.spy(cb).was_called(0);
+			r1("nope");
+			assert.spy(cb).was_called(1);
+			assert.same({ "nope", "yep" }, result);
+		end);
+		it("rejects if any promise rejects", function ()
+			local r1, r2;
+			local p1 = promise.new(function (resolve, reject) r1 = reject end);
+			local p2 = promise.new(function (resolve, reject) r2 = reject end);
+			local p = promise.all({ p1, p2 });
+
+			local result;
+			local cb = spy.new(function (v)
+				result = v;
+			end);
+			local cb_err = spy.new(function (v)
+				result = v;
+			end);
+			p:next(cb, cb_err);
+			assert.spy(cb).was_called(0);
+			assert.spy(cb_err).was_called(0);
+			r2("fail");
+			assert.spy(cb).was_called(0);
+			assert.spy(cb_err).was_called(1);
+			r1("nope");
+			assert.spy(cb).was_called(0);
+			assert.spy(cb_err).was_called(1);
+			assert.equal("fail", result);
+		end);
+	end);
+	describe("catch()", function ()
+		it("works", function ()
+			local result;
+			local p = promise.new(function (resolve)
+				error({ foo = true });
+			end);
+			local cb1 = spy.new(function (v)
+				result = v;
+			end);
+			assert.spy(cb1).was_called(0);
+			p:catch(cb1);
+			assert.spy(cb1).was_called(1);
+			assert.same({ foo = true }, result);
+		end);
+	end);
+	it("promises may be resolved by other promises", function ()
+		local r1, r2;
+		local p1, p2 = promise.new(function (resolve) r1 = resolve end), promise.new(function (resolve) r2 = resolve end);
+
+		local result;
+		local cb = spy.new(function (v)
+			result = v;
+		end);
+		p1:next(cb);
+		assert.spy(cb).was_called(0);
+
+		r1(p2);
+		assert.spy(cb).was_called(0);
+		r2("yep");
+		assert.spy(cb).was_called(1);
+		assert.equal("yep", result);
+	end);
+	describe("reject()", function ()
+		it("returns a rejected promise", function ()
+			local p = promise.reject("foo");
+			local cb = spy.new(function (v)
+				result = v;
+			end);			
+			p:next(cb);
+			assert.spy(cb).was_called(1);
+			assert.spy(cb).was_called_with("foo");
+		end);
+	end);
+end);
diff --git a/util/promise.lua b/util/promise.lua
new file mode 100644
index 00000000..7184f5fb
--- /dev/null
+++ b/util/promise.lua
@@ -0,0 +1,133 @@
+local promise_methods = {};
+local promise_mt = { __name = "promise", __index = promise_methods };
+
+local function is_promise(o)
+	local mt = getmetatable(o);
+	return mt == promise_mt;
+end
+
+local function next_pending(self, on_fulfilled, on_rejected)
+	table.insert(self._pending_on_fulfilled, on_fulfilled);
+	table.insert(self._pending_on_rejected, on_rejected);
+end
+
+local function next_fulfilled(promise, on_fulfilled, on_rejected) -- luacheck: ignore 212/on_rejected
+	on_fulfilled(promise.value);
+end
+
+local function next_rejected(promise, on_fulfilled, on_rejected) -- luacheck: ignore 212/on_fulfilled
+	on_rejected(promise.reason);
+end
+
+local function promise_settle(promise, new_state, new_next, cbs, value)
+	if promise._state ~= "pending" then
+		return;
+	end
+	promise._state = new_state;
+	promise._next = new_next;
+	for _, cb in ipairs(cbs) do
+		cb(value);
+	end
+	return true;
+end
+
+local function new_resolve_functions(p)
+	local resolved = false;
+	local function _resolve(v)
+		if resolved then return; end
+		resolved = true;
+		if is_promise(v) then
+			v:next(new_resolve_functions(p));
+		elseif promise_settle(p, "fulfilled", next_fulfilled, p._pending_on_fulfilled, v) then
+			p.value = v;
+		end
+
+	end
+	local function _reject(e)
+		if resolved then return; end
+		resolved = true;
+		if promise_settle(p, "rejected", next_rejected, p._pending_on_rejected, e) then
+			p.reason = e;
+		end
+	end
+	return _resolve, _reject;
+end
+
+local function new(f)
+	local p = setmetatable({ _state = "pending", _next = next_pending, _pending_on_fulfilled = {}, _pending_on_rejected = {} }, promise_mt);
+	if f then
+		local resolve, reject = new_resolve_functions(p);
+		local ok, ret = pcall(f, resolve, reject);
+		if not ok and p._state == "pending" then
+			reject(ret);
+		end
+	end
+	return p;
+end
+
+local function wrap_handler(f, resolve, reject)
+	return function (param)
+		local ok, ret = pcall(f, param);
+		if ok then
+			resolve(ret);
+		else
+			reject(ret);
+		end
+	end;
+end
+
+function promise_methods:next(on_fulfilled, on_rejected)
+	return new(function (resolve, reject)
+		self:_next(
+			on_fulfilled and wrap_handler(on_fulfilled, resolve, reject) or nil,
+			on_rejected and wrap_handler(on_rejected, resolve, reject) or nil
+		);
+	end);
+end
+
+function promise_methods:catch(on_rejected)
+	return self:next(nil, on_rejected);
+end
+
+local function all(promises)
+	return new(function (resolve, reject)
+		local count, total, results = 0, #promises, {};
+		for i = 1, total do
+			promises[i]:next(function (v)
+				results[i] = v;
+				count = count + 1;
+				if count == total then
+					resolve(results);
+				end
+			end, reject);
+		end
+	end);
+end
+
+local function race(promises)
+	return new(function (resolve, reject)
+		for i = 1, #promises do
+			promises[i]:next(resolve, reject);
+		end
+	end);
+end
+
+local function resolve(v)
+	return new(function (_resolve)
+		_resolve(v);
+	end);
+end
+
+local function reject(v)
+	return new(function (_reject)
+		_reject(v);
+	end);
+end
+
+return {
+	new = new;
+	resolve = resolve;
+	reject = reject;
+	all = all;
+	race = race;
+}
-- 
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