Teaching and learning resources • Re: Advent of Code 2024

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/*  Advent of Code 2024 Day 16 Reindeer Maze    Written 2025 by Eric Olson */#include <limits.h>#include <pico/stdlib.h>#include <stdio.h>#include <stdlib.h>#include <string.h>static uint32_t tic_start;static const char* maze[] = {#include "day16.txt"};static void tic() { tic_start = time_us_32(); }static double toc() { return (time_us_32() - tic_start) / 1e6; }static const int dirs[4][2] = {{0, 1}, {-1, 0}, {0, -1}, {1, 0}};static int isvertex(int i, int j) {    if (maze[i][j] == 'S') return 1;    if (maze[i][j] == 'E') return 1;    if (maze[i][j] == '#') return 0;    int r = 0;    for (char d = 0; d < 4; d++) {        int di = dirs[d][0], dj = dirs[d][1];        if (maze[i + di][j + dj] != '#') r++;    }    if (r != 2) return 1;    return 0;}static void vcount(int* pm, int* pstart, int* pstop) {    int m = 0;    int start, stop;    for (int i = 0; i < 141; i++)        for (int j = 0; j < 141; j++)            if (isvertex(i, j)) {                if (maze[i][j] == 'S')                    start = m;                else if (maze[i][j] == 'E')                    stop = m;                m++;            }    *pm = m;    *pstart = start;    *pstop = stop;    return;}static void ntoij(int* pi, int* pj, int n) {    int m = 0;    for (int i = 0; i < 141; i++)        for (int j = 0; j < 141; j++)            if (isvertex(i, j)) {                if (m == n) {                    *pi = i;                    *pj = j;                    return;                }                m++;            }    printf("Tried to retrieve %d of %d vertices!\n", n, m);    exit(1);}static int ijton(int hi, int hj) {    int m = 0;    for (int i = 0; i < 141; i++)        for (int j = 0; j < 141; j++)            if (isvertex(i, j)) {                if (i == hi && j == hj) return m;                m++;            }    printf("Tried to retrieve (%d,%d) but not a vertice!\n", hi, hj);    exit(1);}typedef struct {    int w[4], e[4];    char de[4];} vertspec;static char dijtod(int di, int dj) {    for (int d = 0; d < 4; d++)        if (dirs[d][0] == di && dirs[d][1] == dj) return d;    printf("Unknown direction (%d,%d)\n", di, dj);    exit(1);}static int follow(int* pn, char* pd, int i, int j, int di, int dj) {    int w = 0, e = 0;    i += di;    j += dj;    w++;    while (!isvertex(i, j)) {        if (maze[i + di][j + dj] != '#') {            i += di;            j += dj;            w++;            continue;        }        int ti = dj;        int tj = -di;        if (maze[i + ti][j + tj] != '#') {            di = ti;            dj = tj;            w += 1000;            continue;        }        ti = -dj;        tj = di;        if (maze[i + ti][j + tj] != '#') {            di = ti;            dj = tj;            w += 1000;            continue;        }    }    *pn = ijton(i, j);    *pd = dijtod(di, dj);    return w;}static void mkgraph(int n, vertspec vn[]) {    for (int k = 0; k < n; k++) {        int i, j;        ntoij(&i, &j, k);        for (int d = 0; d < 4; d++) {            int di = dirs[d][0], dj = dirs[d][1];            if (maze[i + di][j + dj] != '#') {                int e;                char de;                int w = follow(&e, &de, i, j, di, dj);                vn[k].w[d] = w;                vn[k].e[d] = e;                vn[k].de[d] = de;            }        }    }}static void mkpath(int n, int pn[][4], vertspec vn[], int start) {    for (int i = 0; i < n; i++)        for (int d = 0; d < 4; d++) pn[i][d] = INT_MAX;    pn[start][0] = 0;    int done = 0;    while (!done) {        done = 1;        for (int i = 0; i < n; i++) {            for (int d = 0; d < 4; d++) {                int t = pn[i][d];                if (t < INT_MAX) {                    if (vn[i].w[d] > 0) {                        int tn = t + vn[i].w[d];                        if (tn < pn[vn[i].e[d]][vn[i].de[d]]) {                            pn[vn[i].e[d]][vn[i].de[d]] = tn;                            done = 0;                        }                    }                    int dl = (d + 1) % 4;                    if (vn[i].w[dl] > 0) {                        int tn = t + 1000;                        if (tn < pn[i][dl]) {                            pn[i][dl] = tn;                            done = 0;                        }                    }                    int dr = (d + 3) % 4;                    if (vn[i].w[dr] > 0) {                        int tn = t + 1000;                        if (tn < pn[i][dr]) {                            pn[i][dr] = tn;                            done = 0;                        }                    }                }            }        }    }    return;}static void parts(int* pp1, int* pp2, int n, vertspec vn[], int pn[][4], int stop) {    int p1 = INT_MAX, dmin = 0;    for (int d = 0; d < 4; d++) {        int t = pn[stop][d];        if (t < p1) {            p1 = t;            dmin = d;        }    }    char sni[n][4], sno[n][4], cv[n];    bzero(sni, n * 4 * sizeof(char));    bzero(sno, n * 4 * sizeof(char));    bzero(cv, n * sizeof(char));    sni[stop][dmin] = 1;    cv[stop] = 1;    int p2 = 0;    int done = 0;    while (!done) {        done = 1;        for (int i = 0; i < n; i++) {            for (int d = 0; d < 4; d++) {                if (vn[i].w[d] > 0 && !sno[i][d])                    if (sni[vn[i].e[d]][vn[i].de[d]])                        if (pn[i][d] + vn[i].w[d] == pn[vn[i].e[d]][vn[i].de[d]]) {                            p2 += vn[i].w[d] % 1000 - 1;                            sno[i][d] = 1;                            sni[i][d] = 1;                            cv[i] = 1;                            done = 0;                        }                int dl = (d + 3) % 4, dr = (d + 1) % 4;                if (vn[i].w[dr] > 0 && !sni[i][dl]) {                    if (sni[i][d]) {                        if (pn[i][dl] + 1000 == pn[i][d]) {                            sni[i][dl] = 1;                            done = 0;                        }                    }                }                if (vn[i].w[dl] > 0 && !sni[i][dr]) {                    if (sni[i][d]) {                        if (pn[i][dr] + 1000 == pn[i][d]) {                            sni[i][dr] = 1;                            done = 0;                        }                    }                }            }        }    }    for (int i = 0; i < n; i++)        if (cv[i]) p2++;    *pp1 = p1;    *pp2 = p2;    return;}int main() {    stdio_init_all();    printf("Advent of Code 2024 Day 16 Reindeer Maze\n\n");    tic();    int n, start, stop;    vcount(&n, &start, &stop);    vertspec vn[n];    bzero(vn, sizeof(vertspec) * n);    mkgraph(n, vn);    int pn[n][4];    mkpath(n, pn, vn, start);    int p1, p2;    parts(&p1, &p2, n, vn, pn, stop);    printf("Part 1 The lowest score a Reindeer could get is %d\n", p1);    printf("Part 2 The best viewing paths consist of %d tiles\n", p2);    printf("\nTotal execution time %g seconds.\n", toc());}

Code:

Advent of Code 2024 Day 16 Reindeer MazePart 1 The lowest score a Reindeer could get is 85432Part 2 The best viewing paths consist of 465 tilesTotal execution time 8.11771 seconds.