-- A timer that uses the rdtsc instruction to read the CPU timestamp counter.
--
-- Requires DynAsm with Lua mode: https://github.com/luapower/dynasm
--
-- Use it like this:
--
-- local tsc_timer = require('tsc_timer')
-- local t = tsc_timer.start()
-- ... do some things ...
-- t:stop()
-- print(t:value())
--
-- value() returns the estimated number of milliseconds and the number of ticks elapsed
--
local dynasm = require('dynasm')
local ffi = require('ffi')

local rdtsc = dynasm.loadstring [[
local ffi = require('ffi')
local dasm = require('dasm')

|.arch x64
|.actionlist actions

local Dst = dasm.new(actions)
| rdtsc
| shl rdx, 32
| or rax, rdx
| ret

local code = Dst:build()

return function()
  local _ = code
  return ffi.cast('uint64_t __cdecl (*)()', code)()
end
]]()

ffi.cdef [[
int usleep(uint32_t usec);
]]

local _M = {}
local mt = { __index = _M }

function _M.initialize()
  if _M.ticks_per_sec ~= nil then return end
  local start_time = rdtsc()
  ffi.C.usleep(200000)
  local end_time = rdtsc() - start_time
  _M.ticks_per_sec = end_time * 5
end

function _M.start()
  if _M.ticks_per_second == nil then _M.initialize() end
  return setmetatable({ start_time = rdtsc() }, mt)
end

function _M.stop(self)
  local end_time = rdtsc()
  if self.start_time == nil then error("timer has not been started") end
  if self.stop_time ~= nil then error("timer has already been stopped") end
  _M.end_time = end_time
  return _M.value(self)
end

function _M.value(self)
  return tonumber(self.end_time - self.start_time) / tonumber(self.ticks_per_sec) * 1000, tonumber(self.end_time - self.start_time)
end

return _M