adventofcode2024/day7/c/day7.c

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <omp.h>

#define CHARS_MAX 1024
#define MAX_EQUATIONS 32*1024
#define MAX_EQUATION_VALUES 128

typedef struct equation {
    uint64_t result;
    uint64_t *values;
    int values_count;
} equation_t;

int solvable(equation_t, int);

int main() {
    char *p, *buf, c;

    buf = calloc(CHARS_MAX, sizeof(char));
    p = buf;

    equation_t *equations = calloc(MAX_EQUATIONS, sizeof(equations[0]));
    int equation_count = 0;

    while ((c = getchar()) != EOF) {
        *p++ = c;
        if (c != '\n') {
            continue;
        }
        p = buf;

        sscanf(p, "%lu", &equations[equation_count].result);
        while (*p++ != ':');

        equations[equation_count].values = calloc(MAX_EQUATION_VALUES, sizeof(equations[0].values[0]));
        do {
            p++;
            sscanf(p, "%lu", &equations[equation_count].values[equations[equation_count].values_count]);
            equations[equation_count].values_count++;
            while(*p != ' ' && *p != '\n') p++;
        } while (*p != '\n');

        memset(buf, 0, CHARS_MAX);
        p = buf;
        equation_count++;
    }

    free(buf);

    uint64_t count = 0;
    uint64_t count2 = 0;
#pragma omp parallel for
    for (int i = 0; i < equation_count; i++) {
        if (solvable(equations[i], 0))
#pragma omp critical
        {
            count += equations[i].result;
        }
        if (solvable(equations[i], 1))
#pragma omp critical
        {
            count2 += equations[i].result;
        }
    }

    for (int i = 0; i < equation_count; i++) {
        free(equations[i].values);
    }
    free(equations);

    printf("%lu\n", count);
    printf("%lu\n", count2);
}

int solvable(equation_t equation, int concat_allowed) {
    uint64_t sum = equation.values[0] + equation.values[1];
    uint64_t product = equation.values[0] * equation.values[1];
    uint64_t concat;
    // Concat
    char buf[128];
    sprintf(buf, "%lu%lu", equation.values[0], equation.values[1]);
    sscanf(buf, "%lu", &concat);
    if (equation.values_count == 2) {
        return (sum == equation.result) || (product == equation.result) || (concat == equation.result);
    }

    uint64_t *values_tmp = calloc(equation.values_count - 1, sizeof(values_tmp[0]));

    // Add
    values_tmp[0] = sum;
    memcpy(values_tmp + 1, equation.values + 2, (equation.values_count - 2) * sizeof(values_tmp[0]));
    equation_t tmp = {
        .result = equation.result,
        .values_count = equation.values_count - 1,
        .values = values_tmp,
    };

    int solvable_sum = solvable(tmp, concat_allowed);

    // Multiply
    values_tmp[0] = product;
    int solvable_product = solvable(tmp, concat_allowed);

    // Concat
    values_tmp[0] = concat;
    int solvable_concat = concat_allowed ? solvable(tmp, 1) : 0;

    free(values_tmp);
    return solvable_sum || solvable_product || solvable_concat;
}