aoc/src/2024/05.c

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "lfinput.h"

#define RULES_SZ 1176

struct Rule {
    int a;
    int b;
};

static int part_one(int *pages, size_t sz, struct Rule *rules) {
    int midpoint = pages[sz / 2];

    for (size_t ridx = 0; ridx < RULES_SZ; ++ridx) {
        int a = -1;
        int b = 500;

        for (int pidx = 0; pidx < sz; ++pidx) {
            if (pages[pidx] == rules[ridx].a) {
                a = pidx;
            }
            if (pages[pidx] == rules[ridx].b) {
                b = pidx;
            }
            if (a > b) {
                midpoint = 0;
            }
        }
    }
    return midpoint;
}

int rule_matrix[100][100] = {0};

static int sort(const void *a, const void *b) {
    int ia = *(int*)a;
    int ib = *(int*)b;
    if (rule_matrix[ia][ib]) {
        return -1;
    }
    if (rule_matrix[ib][ia]) {
        return 1;
    }
    return 0;
}

static int part_two(int *pages, size_t sz, struct Rule *rules) {
    int s = 0;
    int sorted[100];
    memcpy(sorted, pages, sizeof(int) * sz);
    qsort(sorted, sz, sizeof(int), sort);
    if (memcmp(pages, sorted, sz * sizeof(int)) != 0) {
        s += sorted[sz / 2];
    }
    return s;
}

void advent2024day05(void) {
    size_t sz = 0;
    char **input = get_lines("input/2024/05", &sz);
    char *errstr = NULL;
    struct Rule rules[RULES_SZ];
    int p1 = 0, p2 = 0;

    size_t i = 0;
    for (; strlen(input[i]) == 5; ++i) {
        sscanf(input[i], "%d|%d", &rules[i].a, &rules[i].b);
        rule_matrix[rules[i].a][rules[i].b] = 1;
    }

    for (; i < sz; ++i) {
        size_t sz_sp = 0;
        char **sp = split(input[i], &sz_sp, ",");
        int *pages = malloc(sizeof(int) * sz_sp);
        for (size_t j = 0; j < sz_sp; ++j) {
            pages[j] = (int) strtonum(sp[j], INT_MIN, INT_MAX, &errstr);
        }

        p1 += part_one(pages, sz_sp, rules);
        p2 += part_two(pages, sz_sp, rules);

        free(pages);
        free(sp);
    }

    printf("%d\n", p1);
    printf("%d\n", p2);

    del_lines(input);
}