adventofcode2024/day9/c/day9.c

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

#define char2i(c) (c - 48)
#define MAX_DISK_SIZE 1024*1024

int main() {
    char c;

    uint32_t *disk = calloc(MAX_DISK_SIZE, sizeof(disk[0]));
    int disk_size = 0;
    uint32_t file_count = 0;
    int is_file = 1;

    while ((c = getchar()) != EOF) {
        int n = char2i(c);
        uint32_t data = is_file ? file_count++ : UINT32_MAX;

        for (int i = 0; i < n; i++) {
            disk[disk_size++] = data;
        }

        is_file = !is_file;
    }

    // Create copy
    uint32_t *disk_copy = calloc(disk_size, sizeof(disk_copy[0]));
    memcpy(disk_copy, disk, disk_size * sizeof(disk[0]));

    int last_empty = 0;
    int inserted_back = 0;
    for (;;) {
        // Find next empty space
        while (disk_copy[last_empty] != UINT32_MAX && last_empty < disk_size - inserted_back - 1) last_empty++;

        if (last_empty >= disk_size - inserted_back - 1) {
            break;
        }

        disk_copy[last_empty] = disk_copy[disk_size - inserted_back - 1];
        disk_copy[disk_size - inserted_back - 1] = UINT32_MAX;
        inserted_back++;
    }

    uint64_t checksum = 0;

    for (int i = 0; i < disk_size; i++) {
        if (disk_copy[i] != UINT32_MAX) {
            checksum += disk_copy[i] * i;
        }
    }

    free(disk_copy);

    printf("%lu\n", checksum);

    // Part two
    for (int i = file_count - 1; i >= 0; i--) {
        // Get file size inefficiently
        int file_size = 0;
        int file_begin = 0;
        for (int j = 0; j < disk_size; j++) {
            if (disk[j] == i) {
                if (file_size == 0) {
                    file_begin = j;
                }
                file_size++;
            }
        }

        // Find empty space
        for (int j = 0; j < disk_size; j++) {
            // We only place files left
            if (disk[j] == i) {
                break;
            }

            if (disk[j] == UINT32_MAX) {
                // Let's see if we have enough space
                int fit = 1;
                for (int k = 0; k < file_size; k++) {
                    if (disk[j + k] != UINT32_MAX) {
                        fit = 0;
                        break;
                    }
                }

                if (fit) {
                    for (int k = 0; k < file_size; k++) {
                        disk[j + k] = i;
                        disk[file_begin + k] = UINT32_MAX;
                    }
                    break;
                }
            }
        }
    }

    checksum = 0;

    for (int i = 0; i < disk_size; i++) {
        if (disk[i] != UINT32_MAX) {
            checksum += disk[i] * i;
        }
    }

    free(disk);

    printf("%lu\n", checksum);
}