RemoteFactoryPassCode

#include <stdio.h>
#include <time.h>
#include <math.h>
/*
   Premise: This program can be built cross platform, to support generation
    of the system 69 factory passcodes to be provided to technicians for a
    predefined date of service operations. Where our hosts are in remote locations,
    and internet / cellular connectivity may not be available. Service techs
    may build the code to run on a predefined service day, which a code will
    be generated. No other day codes are provided, and the program will not
    generate a code for any day other than the allowed day.

    --Service Techs generate the binary to provide to remote techs using an internal code generator

    --Remote Techs download the binary provided to access the factory configuration page, for the specified service day.

   Manual Compilation:
   Linux: $gcc -O2 -Wall -Wextra -s -DALLOWED_YEAR=2025 -DALLOWED_MONTH=12 -DALLOWED_DAY=19 day.c -lm -o day
   MsOS:  $x86_64-w64-mingw32-gcc -O2 -Wall -Wextra -s -DALLOWED_YEAR=2025 -DALLOWED_MONTH=12 -DALLOWED_DAY=19 -lm day.c -o day

*/

    #ifndef ALLOWED_YEAR
    #error "ALLOWED_YEAR must be defined at compile time"
    #endif
    #ifndef ALLOWED_MONTH
    #error "ALLOWED_MONTH must be defined at compile time"
    #endif
    #ifndef ALLOWED_DAY
    #error "ALLOWED_DAY must be defined at compile time"
    #endif

static void get_local_time(struct tm *out){
    time_t now = time(NULL);
    #if defined(_WIN32)
        localtime_s(out, &now);
    #else
        localtime_r(&now, out);
    #endif
}

static int date_is_allowed(const struct tm *lt){
    return (lt->tm_year + 1900 == ALLOWED_YEAR &&
            lt->tm_mon + 1     == ALLOWED_MONTH &&
            lt->tm_mday        == ALLOWED_DAY);
}

/* Generate Daily Factory Passcode  */
static int gp(const struct tm *lt){
    int month = lt->tm_mon + 1;
    int day   = lt->tm_mday;

    double a   = (month + day) * 6.9;
    double sum = log(a);
    return (int)(sum * 420);
}

int main(void){
    struct tm lt;
    get_local_time(&lt);

    if (!date_is_allowed(&lt)) {
        fprintf(stderr,"This utility is only valid on: %02d-%02d-%04d\n", ALLOWED_DAY, ALLOWED_MONTH, ALLOWED_YEAR);
        return 1;
    }
    printf("Factory Passcode for Today ONLY:\n");
    int pass = gp(&lt);
    printf("%d\n", pass);
    #if defined(_WIN32)
        printf("\nPress ENTER to exit...");
        fflush(stdout);
        getchar();
    #endif
    return 0;
}