allwpilib/src/support/timestamp.cpp

/*----------------------------------------------------------------------------*/
/* Copyright (c) FIRST 2015. All Rights Reserved.                             */
/* Open Source Software - may be modified and shared by FRC teams. The code   */
/* must be accompanied by the FIRST BSD license file in the root directory of */
/* the project.                                                               */
/*----------------------------------------------------------------------------*/
#include "timestamp.h"

#include <cassert>
#include <exception>

#ifdef _WIN32
#include <windows.h>
#else
#include <time.h>
#endif

// offset in microseconds
static unsigned long long zerotime() {
#ifdef _WIN32
  FILETIME ft;
  unsigned long long tmpres = 0;
  // 100-nanosecond intervals since January 1, 1601 (UTC)
  // which means 0.1 us
  GetSystemTimeAsFileTime(&ft);
  tmpres |= ft.dwHighDateTime;
  tmpres <<= 32;
  tmpres |= ft.dwLowDateTime;
  // January 1st, 1970 - January 1st, 1601 UTC ~ 369 years
  // or 116444736000000000 us
  static const unsigned long long deltaepoch = 116444736000000000ull;
  tmpres -= deltaepoch;
  return tmpres;
#else
  timespec ts;

  clock_gettime(CLOCK_REALTIME, &ts);

  // in 100-ns intervals
  return static_cast<unsigned long long>(ts.tv_sec) * 10000000ull +
         static_cast<unsigned long long>(ts.tv_nsec) / 100u;
#endif
}

static unsigned long long timestamp() {
#ifdef _WIN32
  LARGE_INTEGER li;
  QueryPerformanceCounter(&li);
  // there is an imprecision with the initial value,
  // but what matters is that timestamps are monotonic and consistent
  return static_cast<unsigned long long>(li.QuadPart);
#else
  timespec ts;

  // cannot fail, parameters are correct and we checked earlier we can
  // access the clock
  clock_gettime(CLOCK_MONOTONIC, &ts);
  // in ns
  return static_cast<unsigned long long>(ts.tv_sec) * 1000000000ull +
         static_cast<unsigned long long>(ts.tv_nsec);
#endif
}

static unsigned long long update_frequency() {
#ifdef _WIN32
  LARGE_INTEGER li;
  if (!QueryPerformanceFrequency(&li) || !li.QuadPart) {
    // log something
    std::terminate();
  }
  return static_cast<unsigned long long>(li.QuadPart);
#else
  timespec ts;

  if (clock_getres(CLOCK_MONOTONIC, &ts) < 0) {
    // log error
    std::terminate();
  }

  assert(!ts.tv_sec);

  // this is the precision of the clock, we want the number of updates per
  // second, which is 1 / ts.tv_nsec * 1,000,000,000
  static const unsigned long long billion = 1000000000ull;

  return billion / static_cast<unsigned long long>(ts.tv_nsec);
#endif
}

static const unsigned long long zerotime_val = zerotime();
static const unsigned long long offset_val = timestamp();
static const unsigned long long frequency_val = update_frequency();

unsigned long long NT_Now() {
  assert(offset_val > 0u);
  assert(frequency_val > 0u);
  unsigned long long delta = timestamp() - offset_val;
  // because the frequency is in update per seconds, we have to multiply the
  // delta by 10,000,000
  unsigned long long delta_in_us = delta * 10000000ull / frequency_val;
  return delta_in_us + zerotime_val;
}