adventofcode2024/day20/c/day20.c

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

#define MAX_DIMENSION 1024
#define QUEUE_MAX 32*1024
#define abs(a) ((a) < 0 ? (-(a)) : (a))

const int directions[][2] = {
    {0, 1},
    {1, 0},
    {0, -1},
    {-1, 0}
};

#define directions_len (sizeof(directions)/sizeof(directions[0]))

typedef enum tile {
    EMPTY,
    WALL,
} tile_t;

int ospf(tile_t **map, int rows, int colums, int start[2], int end[2]);

int main() {
    char c;

    tile_t **map = calloc(MAX_DIMENSION, sizeof(map[0]));
    int rows = 0, columns, i = 0;
    map[0] = calloc(MAX_DIMENSION, sizeof(map[0][0]));

    int start[2], end[2];
    while ((c = getchar()) != EOF) {
        switch(c) {
            case '#':
                map[rows][i] = WALL;
                break;
            case '.':
                map[rows][i] = EMPTY;
                break;
            case 'S':
                map[rows][i] = EMPTY;
                start[0] = rows;
                start[1] = i;
                break;
            case 'E':
                map[rows][i] = EMPTY;
                end[0] = rows;
                end[1] = i;
                break;
        }

        i++;
        if (c != '\n') {
            continue;
        }

        columns = i - 1;
        i = 0;
        rows++;
        map[rows] = calloc(MAX_DIMENSION, sizeof(map[0][0]));
    }

    int without_cheats = 0;

    // Shortest path search
    uint32_t **costs = calloc(rows, sizeof(costs[0]));
    for (int i = 0; i < rows; i++) {
        costs[i] = calloc(columns, sizeof(costs[0][0]));
        memset(costs[i], 255, columns * sizeof(costs[0][0]));
    }

    int (*queue)[2] = calloc(QUEUE_MAX, sizeof(queue[0]));
    int queue_num = 1;
    // Start at 0,0
    queue[0][0] = start[0];
    queue[0][1] = start[1];
    costs[start[0]][start[1]] = 0;

    while (queue_num > 0) {
        // Pop first
        int x = queue[0][0];
        int y = queue[0][1];
        uint32_t cost = costs[x][y];

        if (x == end[0] && y == end[1]) {
            without_cheats = cost;
            break;
        }

        for (int i = 0; i < queue_num - 1; i++) {
            queue[i][0] = queue[i+1][0];
            queue[i][1] = queue[i+1][1];
        }
        queue_num--;

        // Check each direction
        for (int i = 0; i < directions_len; i++) {
            int dx = directions[i][0];
            int dy = directions[i][1];

            int next_x = x + dx;
            int next_y = y + dy;

            // Outside of map, into wall, or better path exists
            if (next_x < 0 || next_y < 0 || next_x >= rows || next_y >= columns || map[next_x][next_y] || costs[next_x][next_y] <= cost + 1) {
                continue;
            }

            // Insert into queue
            uint32_t next_cost = cost + 1;
            costs[next_x][next_y] = next_cost;
            int insert_idx;
            for (insert_idx = 0; insert_idx < queue_num; insert_idx++) {
                int qx = queue[insert_idx][0];
                int qy = queue[insert_idx][1];
                if (costs[qx][qy] > next_cost) {
                    break;
                }
            }

            for (int j = queue_num; j > insert_idx; j--) {
                queue[j][0] = queue[j-1][0];
                queue[j][1] = queue[j-1][1];
            }
            queue[insert_idx][0] = next_x;
            queue[insert_idx][1] = next_y;
            queue_num++;
        }
    }

    int good_cheats = 0;
    int good_cheat_limit = 100;

    // Find where to cheat
    for (i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++) {
            if (map[i][j] != EMPTY) {
                continue;
            }
            // Try each direction and check if there's a wall
            for (int k = 0; k < directions_len; k++) {
                int dx = directions[k][0];
                int dy = directions[k][1];
                int x = i + dx;
                int y = j + dy;

                if (x < 0 || y < 0 || x >= rows || y >= columns || map[x][y] != WALL) {
                    continue;
                }

                // Now check whether we can get back to the track to shorten out time
                for (int l = 0; l < directions_len; l++) {
                    int dx = directions[l][0];
                    int dy = directions[l][1];
                    int xx = x + dx;
                    int yy = y + dy;

                    if (xx < 0 || yy < 0 || xx >= rows || yy >= columns || map[xx][yy] == WALL) {
                        continue;
                    }

                    // Not good cheat enough
                    if (costs[xx][yy] < costs[i][j] + good_cheat_limit + 2) {
                        continue;
                    }

                    good_cheats++;
                }
            }
        }
    }

    int good_cheats2 = 0;
    int good_cheat_limit2 = 100;

    // Find where to cheat
    for (i = 0; i < rows; i++) {
        for (int j = 0; j < columns; j++) {
            if (map[i][j] != EMPTY) {
                continue;
            }

            // Check submap, we can get to anywhere inside a 20 radius "circle" by activating the cheat here
            for (int k = i-20; k <= i+20; k++) {
                for (int l = j-(20-abs(k-i)); l <= j+(20-abs(k-i)); l++) {
                    if (k < 0 || l < 0 || k >= rows || l >= columns) {
                        continue;
                    }

                    // We don't want to get into a wall
                    if (map[k][l] == WALL) {
                        continue;
                    }

                    // Not good cheat enough
                    if (costs[k][l] < costs[i][j] + good_cheat_limit2 + abs(k-i) + abs(l-j)) {
                        continue;
                    }

                    good_cheats2++;
                }
            }
        }
    }

    printf("%i\n", good_cheats);
    printf("%i\n", good_cheats2);

    for (i = 0; i < rows; i++) {
        free(costs[i]);
    }
    free(costs);
    for (i = 0; i < rows + 1; i++) {
        free(map[i]);
    }
    free(map);
}