AoC 2024 day 7

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

struct Equation { uint64_t target; int size; int nums[12]; } typedef Equation;

uint64_t combine_numbers(uint64_t num1, uint64_t num2) {
    int digits = 0, multiplier = 1;
    uint64_t temp = num2;
    do { temp /= 10; digits++; } while (temp > 0); // Find the number of digits in num2
    for (int i = 0; i < digits; i++) { multiplier *= 10; }; // Multiply num1 by 10^digits and add num2

    return num1 * multiplier + num2;
}

// Recursive function to find the correct combination
int find_combination(Equation *eq, int index, uint64_t cur_total, int p2) {
    if (index == eq->size) {
        if (cur_total == eq->target) { return 1; }; // Found a solution
        return 0; // No solution
    };
    // Add the next number with a '+' operator
    uint64_t next_total = cur_total + eq->nums[index];
    if (find_combination(eq, index+1, next_total, p2)) { return 1; };

    // Add the next number with a '*' operator
    next_total = (index != 0) ? cur_total * eq->nums[index] : cur_total + eq->nums[index];
    if (find_combination(eq, index+1, next_total, p2)) { return 1;};

    // Add the next number with a '||' operator
    if (index > 0 && p2){
        next_total = combine_numbers(cur_total, eq->nums[index]);
        if (find_combination(eq, index+1, next_total, p2)) { return 1;};
    };

    return 0; // No solution
}

int main() {
    FILE *fptr = fopen("/media/int_drive/Development/c/AoC/2024/07/input.txt", "r");
    if (fptr == NULL) { printf("Failed to open the input file"); return 1; };

    // Count number of lines
    int lines = 0;
    char buf[1024];
    while(fgets(buf, sizeof(buf), fptr)){ ++lines; };
    printf("Total number of lines: %d\n", lines);
    rewind(fptr);

    // Parsing lines
    Equation eq_arr[lines];
    int index = 0, scanned = 0;
    while (fgets(buf, sizeof(buf), fptr)) {
        scanned = sscanf(buf, "%lu: %d %d %d %d %d %d %d %d %d %d %d %d", &(eq_arr[index].target),
                 &(eq_arr[index].nums[0]), &(eq_arr[index].nums[1]), &(eq_arr[index].nums[2]),  &(eq_arr[index].nums[3]),
                 &(eq_arr[index].nums[4]), &(eq_arr[index].nums[5]), &(eq_arr[index].nums[6]),  &(eq_arr[index].nums[7]),
                 &(eq_arr[index].nums[8]), &(eq_arr[index].nums[9]), &(eq_arr[index].nums[10]), &(eq_arr[index].nums[11]));
        eq_arr[index++].size = scanned-1;
    };
    fclose(fptr);
    // Check parsing results
    //for (int i = 0; i < lines; ++i){ printf("target: %d | ", eq_arr[i].target); for (int j = 0; j < eq_arr[i].size; ++j){printf("%d ", eq_arr[i].nums[j]); }; putchar('\n'); };

    // PART 1
    uint64_t result = 0;
    int good = 0, bad = 0;
    for (int i = 0; i < lines; ++i){
        int index = 0;
        uint64_t cur_total = 0;
        if (find_combination(&eq_arr[i], index, cur_total, 0)){ result += eq_arr[i].target; ++good;
        } else { ++ bad; };
    };

    printf("Sum of possible equations: %lu (good: %d bad: %d)\n", result, good, bad);

    // PART 2
    result = 0, good = 0, bad = 0;
    for (int i = 0; i < lines; ++i){
        int index = 0;
        uint64_t cur_total = 0;
        if (find_combination(&eq_arr[i], index, cur_total, 1)){ result += eq_arr[i].target; ++good;
        } else { ++ bad; };
    };

    printf("Sum of possible equations: %lu (good: %d bad: %d)\n", result, good, bad);

    return 0;
}