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

---
 util/promise.lua | 133 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 133 insertions(+)
 create mode 100644 util/promise.lua

(limited to 'util/promise.lua')

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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