advent/2024/01.c

#include "stdio.h"
#include "stdlib.h"
#include "string.h"


int cmp(const void *a, const void *b) {
    int *_a = (int *)a;
    int *_b = (int *)b;
    return *_a - *_b;
}


int count_occurrences(int num, int arr[], int len) {
    int total = 0;
    for (int i = 0; i < len; i++) {
        if (arr[i] == num) {
            total++;
        }
    }
    return total;
}


int part1(int nums_l[], int nums_r[], int len) {
    int sum = 0;
    for (int i = 0; i < len; i++) {
        int diff = nums_l[i] - nums_r[i];
        if (diff < 0) {
            diff = diff * -1;
        }
        sum += diff;
    }
    return sum;
}


int part2(int nums_l[], int nums_r[], int len) {
    int score = 0;
    for (int i = 0; i < len; i++) {
        score += nums_l[i] * count_occurrences(nums_l[i], nums_r, len);
    }
    return score;
}


int main() {
    FILE *file = fopen("data/01.txt", "r");
    char *data = (char *)malloc(16384);
    size_t data_len = fread(data, 1, 16384, file);

    int nums_l[1000];
    int nums_r[1000];

    int nlines = 0;
    while (1) {
        char* line = strsep(&data, "\n");
        int left = strtol(line, &line, 10);
        int right = strtol(line, NULL, 10);

        // if `strtol` fails, it apparently just returns 0, how helpful
        if (left == 0 && right == 0) {
            break;
        }

        nums_l[nlines] = left;
        nums_r[nlines] = right;

        nlines++;
    }

    qsort(nums_l, nlines, 4, cmp);
    qsort(nums_r, nlines, 4, cmp);

    int solution_1 = part1(nums_l, nums_r, nlines);
    printf("Part 1: %i\n", solution_1);

    int solution_2 = part2(nums_l, nums_r, nlines);
    printf("Part 2: %i\n", solution_2);
}