//
// Crude test - adjust the PC clock N seconds per day.
//

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>

char Usage[] =
    "Usage: perday [seconds per day]\n"
    "\n"
    "Examples:\n"
    "    perday 1\n"
    "    perday -12\n"
    "    perday 4.32\n"
    "    perday 2000  [ just for demo ]\n";

void ShowSystemTime (char *s, PSYSTEMTIME st, char *t);

void main (int argc, char *argv[])
{
    int bump_milliseconds;
    double seconds_per_day;
    long sleep_time;
    SYSTEMTIME st1, st2, st3;

    if (argc < 2) {
        fprintf(stderr, Usage);
        exit(1);
    }

    sscanf(argv[1], "%lf", &seconds_per_day);

    printf("%.3lf seconds per day is %.3f ppm\n",
           seconds_per_day, 1e6 * seconds_per_day / 86400.0);

    //
    // Wake up every now and then and adjust the PC clock
    // by 1/4 second.
    //
    // Adjusting by more than one second may cause discontinuous
    // time stamps for some apps and adjusting less than 50 ms
    // may be null due to the finite resolution of the PC clock.
    // So 1/4 second seems fair.
    //
    // Also try to adjust in between seconds to reduce the chance
    // of rollover.
    //
    // Note: This tool is not reliable on a heavily loaded system.
    //

    bump_milliseconds = 250;
    sleep_time = (long) (86400.0 /  seconds_per_day / (1e3 / bump_milliseconds));
    if (sleep_time < 0) {
        sleep_time *= -1;
        bump_milliseconds *= -1;
    }

    printf("Will adjust PC clock about %d ms every %ld seconds\n", bump_milliseconds, sleep_time);

    while (1) {
        while (1) {
            GetSystemTime(&st1);
            if (st1.wMilliseconds >= 400 && st1.wMilliseconds <= 600) {
                st2 = st1;
                st2.wMilliseconds += bump_milliseconds;
                SetSystemTime(&st2);
                GetSystemTime(&st3);
                break;
            }
            Sleep(10);
        }

        ShowSystemTime("was ", &st1, "\n");
        ShowSystemTime("set ", &st2, "\n");
        ShowSystemTime("now ", &st3, "\n");
        printf("sleeping...\n");

        Sleep(sleep_time * 1000);
    }

    exit(0);
}

void ShowSystemTime (char *s, PSYSTEMTIME st, char *t)
{
    printf("%s%.4d-%.2d-%.2d %.2d:%.2d:%.2d.%.3d UTC%s",
           s,
           st->wYear,
           st->wMonth,
           st->wDay,
           st->wHour,
           st->wMinute,
           st->wSecond,
           st->wMilliseconds,
           t);
    return;
}