--- Module for creating a thread pool, based on Lua Lanes.
-- @module lqc.threading.thread_pool
-- @alias ThreadPool
local MsgProcessor = require 'lqc.threading.msg_processor'
local map = require 'lqc.helpers.map'
local lanes = require('lanes').configure {
    with_timers = false,
}

--- Checks if x is a positive integer (excluding 0)
-- @param x value to be checked
-- @return true if x is a non-zero positive integer; otherwise false.
local function is_positive_integer(x)
    return type(x) == 'number' and x % 1 == 0 and x > 0
end

--- Checks if the thread pool args are valid.
-- @return nil; raises an error if invalid args are passed in.
local function check_threadpool_args(num_threads)
    if not is_positive_integer(num_threads) then
        error 'num_threads should be an integer > 0'
    end
end

--- Creates and starts a thread.
-- @param func Function the thread should run after startup
-- @return a new thread object
local function make_thread(func)
    return lanes.gen('*', func)()
end

local ThreadPool = {}
local ThreadPool_mt = {
    __index = ThreadPool,
}

--- Creates a new thread pool with a specific number of threads
-- @param num_threads Amount of the threads the pool should have
-- @return thread pool with a specific number of threads
function ThreadPool.new(num_threads)
    check_threadpool_args(num_threads)
    local linda = lanes.linda()
    local thread_pool = {
        threads = {},
        linda = linda,
        numjobs = 0,
    }

    for _ = 1, num_threads do
        table.insert(thread_pool.threads, make_thread(MsgProcessor.new(linda)))
    end
    return setmetatable(thread_pool, ThreadPool_mt)
end

--- Schedules a task to a thread in the thread pool
-- @param task A function that should be run on the thread
function ThreadPool:schedule(task)
    self.numjobs = self.numjobs + 1
    self.linda:send(nil, MsgProcessor.TASK_TAG, task)
end

--- Stops all threads in the threadpool. Blocks until all threads are finished
-- @return a table containing all results (in no specific order)
function ThreadPool:join()
    map(self.threads, function()
        self:schedule(MsgProcessor.STOP_VALUE)
    end)
    map(self.threads, function(thread)
        thread:join()
    end)

    local results = {}
    for _ = 1, self.numjobs - #self.threads do -- don't count stop job at end
        local _, result = self.linda:receive(nil, MsgProcessor.RESULT_TAG)
        if result ~= MsgProcessor.VOID_RESULT then
            table.insert(results, result)
        end
    end
    return results
end

return ThreadPool