--[[
The MIT License (MIT)
=====================

Copyright © `2015` `Colin Fein`

Permission is hereby granted, free of charge, to any person
obtaining a copy of this software and associated documentation
files (the “Software”), to deal in the Software without
restriction, including without limitation the rights to use,
copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the
Software is furnished to do so, subject to the following
conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
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NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
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OTHER DEALINGS IN THE SOFTWARE.
]]

-- Port of https://github.com/rhysbrettbowen/promise_impl/blob/master/promise.js
-- and https://github.com/rhysbrettbowen/Aplus
--
local queue = {}

local State = {
  PENDING   = 'pending',
  FULFILLED = 'fulfilled',
  REJECTED  = 'rejected',
}

local passthrough = function(x) return x end
local errorthrough = function(x) error(x) end

local function callable_table(callback)
  local mt = getmetatable(callback)
  return type(mt) == 'table' and type(mt.__call) == 'function'
end

local function is_callable(value)
  local t = type(value)
  return t == 'function' or (t == 'table' and callable_table(value))
end

local transition, resolve, run

local Promise = {
  is_promise = true,
  state = State.PENDING
}
Promise.mt = { __index = Promise }

local do_async = function(callback)
  if Promise.async then
    Promise.async(callback)
  else
    table.insert(queue, callback)
  end
end

local reject = function(promise, reason)
  transition(promise, State.REJECTED, reason)
end

local fulfill = function(promise, value)
  transition(promise, State.FULFILLED, value)
end

transition = function(promise, state, value)
  if promise.state == state
    or promise.state ~= State.PENDING
    or ( state ~= State.FULFILLED and state ~= State.REJECTED )
  then
    return
  end

  promise.state = state
  promise.value = value
  run(promise)
end

function Promise:next(on_fulfilled, on_rejected)
  local promise = Promise.new()

  table.insert(self.queue, {
    fulfill = is_callable(on_fulfilled) and on_fulfilled or nil,
    reject = is_callable(on_rejected) and on_rejected or nil,
    promise = promise
  })

  run(self)

  return promise
end

resolve = function(promise, x)
  if promise == x then
    reject(promise, 'TypeError: cannot resolve a promise with itself')
    return
  end
  
  local x_type = type(x)

  if x_type ~= 'table' then
    fulfill(promise, x)
    return
  end
  
  -- x is a promise in the current implementation
  if x.is_promise then 
    -- 2.3.2.1 if x is pending, resolve or reject this promise after completion
    if x.state == State.PENDING then
      x:next(
        function(value)
          resolve(promise, value)
        end,
        function(reason)
          reject(promise, reason)
        end
      )
      return
    end
    -- if x is not pending, transition promise to x's state and value
    transition(promise, x.state, x.value)
    return
  end

  local called = false
  -- 2.3.3.1. Catches errors thrown by __index metatable
  local success, reason = pcall(function()
    local next = x.next
    if is_callable(next) then
      next(
        x,
        function(y) 
          if not called then
            resolve(promise, y)
            called = true
          end
        end,
        function(r)
          if not called then
            reject(promise, r)
            called = true
          end
        end
      )
    else
      fulfill(promise, x)
    end
  end)

  if not success then
    if not called then
      reject(promise, reason)
    end
  end
end

run = function(promise)
  if promise.state == State.PENDING then return end

  do_async(function()
    -- drain promise.queue while allowing pushes from within callbacks
    local q = promise.queue
    local i = 0
    while i < #q do
      i = i + 1
      local obj = q[i]
      local success, result = pcall(function()
        local success = obj.fulfill or passthrough
        local failure = obj.reject or errorthrough
        local callback = promise.state == State.FULFILLED and success or failure
        return callback(promise.value)
      end)

      if not success then
        reject(obj.promise, result)
      else
        resolve(obj.promise, result)
      end
    end
    for j = 1, i do
      q[j] = nil
    end
  end)
end

function Promise.new(callback)
  local instance = {
    queue = {}
  }
  setmetatable(instance, Promise.mt)

  if callback then
    callback(
      function(value)
        resolve(instance, value)
      end,
      function(reason)
        reject(instance, reason)
      end
    )
  end

  return instance
end

function Promise:catch(callback)
  return self:next(nil, callback)
end

function Promise:resolve(value)
  fulfill(self, value)
end

function Promise:reject(reason)
  reject(self, reason)
end

function Promise.update()
  while true do
    local async = table.remove(queue, 1)

    if not async then
      break
    end

    async()
  end
end

-- resolve when all promises complete
function Promise.all(...)
  local promises = {...}
  local results = {}
  local state = State.FULFILLED
  local remaining = #promises

  local promise = Promise.new()

  local check_finished = function()
    if remaining > 0 then
      return
    end
    transition(promise, state, results)
  end

  for i,p in ipairs(promises) do
    p:next(
      function(value)
        results[i] = value
        remaining = remaining - 1
        check_finished()
      end,
      function(value)
        results[i] = value
        remaining = remaining - 1
        state = State.REJECTED
        check_finished()
      end
    )
  end

  check_finished()

  return promise
end

-- resolve with first promise to complete
function Promise.race(...)
  local promises = {...}
  local promise = Promise.new()

  Promise.all(...):next(nil, function(value)
    reject(promise, value)
  end)

  local success = function(value)
    fulfill(promise, value)
  end

  for _,p in ipairs(promises) do
    p:next(success)
  end

  return promise
end

return Promise