sem.lua

local sem_t = {}
sem_t.__index = sem_t

function sem_t:wait()
  if self.value == 0 then -- cannot decrement, wait for increment
    local thread = coroutine.running()
    table.insert(self.threads, thread)
    coroutine.yield()
  end

  self.value = self.value - 1
end

function sem_t:trywait()
  if self.value == 0 then
    return false
  end

  self.value = self.value - 1
  return true
end

function sem_t:post()
  self.value = self.value + 1

  if self.threads[1] then -- wake a thread if any are waiting
    local thread = table.remove(self.threads)
    local success, err = coroutine.resume(thread)
    if not success then
      error(debug.traceback(thread, err), 0)
    end
  end
end

local function sem_t.new(value)
  return setmetatable({ value = value or 0, threads = {} }, sem_t)
end