Teaching and learning resources • Re: Advent of Code 2025

Code:

$ ./day01-cAdvent of Code 2025 Day 1 Secret Entrance (c)Part 1 The actual password is 997Part 2 Following method 0x434C49434B gives 5978Total execution time 0.000571019 seconds.

Code:

/*  Advent of Code 2025 Day 1 Secret Entrance (c)    Written December 2025 by Eric Olson */#include <stdio.h>#include <stdlib.h>#include <time.h>static struct timespec tic_start;void tic(){    clock_gettime(CLOCK_MONOTONIC_RAW,&tic_start);}double toc(){    struct timespec tic_stop;    clock_gettime(CLOCK_MONOTONIC_RAW,&tic_stop);    double sec=tic_stop.tv_sec-tic_start.tv_sec;    return sec+(tic_stop.tv_nsec-tic_start.tv_nsec)*1.0e-9;}void dowork(){    FILE *fp=fopen("day01.txt","r");    int p1=0,p2=0;    int d=50;    while(!feof(fp)){        char c; int x;        if(fscanf(fp,"%c%d\n",&c,&x)!=2) break;        int r;        switch(c){case 'L':            if(d>0){                d-=x;                r=(100-d)/100;            } else {                d-=x;                r=-d/100;            }            d=(d%100+100)%100;            break;case 'R':            d+=x;            r=d/100;            d%=100;            break;default:            fprintf(stderr,"Unknown direction %c!\n",c);            exit(1);        }        if(d==0) p1++;        p2+=r;    }    fclose(fp);    printf("Part 1 The actual password is %d\n",p1);    printf("Part 2 Following method 0x434C49434B gives %d\n",p2);}int main(){    printf("Advent of Code 2025 Day 1 Secret Entrance (c)\n\n");    tic();    dowork();    double t=toc();    printf("\nTotal execution time %g seconds.\n",t);    exit(0);}