adventofcode/src/c/2024/day20/part2.c

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

#define BOARD_SIZE 141

#define CHEAT_LENGTH 20

#define IN_BOUND(x, y) (x < BOARD_SIZE && x >= 0 && y < BOARD_SIZE && y >= 0)
#define MIN(a,b) (((a)<(b))?(a):(b))

struct coordinate {

    int x, y;

};


struct coordinate queue[10000];
int queue_len;
int queue_start;

char map[BOARD_SIZE][BOARD_SIZE];
int values[BOARD_SIZE][BOARD_SIZE];

struct coordinate cheat_deltas[10000];
int num_cheat_deltas;


int queue_pop(struct coordinate *coord) {

    if (queue_len == 0) {
        return -1;
    }

    memcpy(coord, &queue[queue_start], sizeof(struct coordinate));

    queue_start++;
    queue_len--;

    return 0;

}


void queue_push(struct coordinate *coord) {

    
    //printf("%d, %da\n", coord->x, coord->y);

    memcpy(&queue[queue_start + queue_len], coord, sizeof(struct coordinate));
    queue_len++;

}


void check_and_push(struct coordinate *coord, int depth) {

    //printf("%d\n", depth);

    if (!IN_BOUND(coord->x, coord->y) || map[coord->x][coord->y] == '#') {
        return;
    }

    if (values[coord->x][coord->y] == 100000) {
        //printf("%d\n", depth);
        values[coord->x][coord->y] = depth + 1;
        queue_push(coord);

    }


}

int dfs(struct coordinate coord) {

    queue_push(&coord);
    values[coord.x][coord.y] = 0;

    while (!queue_pop(&coord)) {

        //printf("%d, %d!", coord.x, coord.y);

        int value = values[coord.x][coord.y];

        //printf("%d, %d: %d\n", coord.x, coord.y, values[coord.x][coord.y]);

        coord.x++;
        check_and_push(&coord, value);
        coord.x -= 2;
        check_and_push(&coord, value);
        coord.x++;

        coord.y++;
        check_and_push(&coord, value);
        coord.y -= 2;
        check_and_push(&coord, value);
        coord.y++;


    }

    return 0;
}

int cheats = 0;

int test_cheat(int x, int y, int value, int dist) {
    int v;
    if (IN_BOUND(x, y) && (map[x][y] == '.' || map[x][y] == 'E') && (v = values[x][y] - value) > dist) {
        v -= dist;
        //printf("cheat: %d\n", v, x, y);

        if (v >= 100) {
            cheats++;
        }

        return v;
    }   
    return 0;
}

int test_next(int x, int y, int value) {
    //printf("%d, %d\n", value, values[x][y]);
    return (IN_BOUND(x, y) && map[x][y] == '.' && values[x][y] - value == 1);
}


int find_cheat(int x, int y) {

    printf("%d, %d ", x, y);
    printf("%c\n", map[x][y]);

    if (!IN_BOUND(x, y)) {
        return 0;
    }


    int value = values[x][y];
    int v;

    for (int i = 0; i < num_cheat_deltas; i++) {
        //printf("%d, %d\n", x + cheat_deltas[i].x, y + cheat_deltas[i].y);
        test_cheat(x + cheat_deltas[i].x, y + cheat_deltas[i].y, value, abs(cheat_deltas[i].x) + abs(cheat_deltas[i].y));
    }

    if (test_next(x + 1, y, value)) {
        return find_cheat(x + 1, y);
    }
    if (test_next(x - 1, y, value)) {
        return find_cheat(x - 1, y);
    }
    if (test_next(x, y + 1, value)) {
        return find_cheat(x, y + 1);
    }
    if (test_next(x, y - 1, value)) {
        return find_cheat(x, y - 1);
    }

    return 0;

}



int main() {

    // calc cheats
    
    for (int x = -CHEAT_LENGTH; x <= CHEAT_LENGTH; x++) {
        for (int y = -CHEAT_LENGTH; y <= CHEAT_LENGTH; y++) {

            if (abs(x) + abs(y) <= CHEAT_LENGTH) {
                //printf("%d, %d\n", x, y);

                cheat_deltas[num_cheat_deltas].x = x;
                cheat_deltas[num_cheat_deltas].y = y;
                num_cheat_deltas++;
                
            }
        }
    }

    //exit(0);

    char *line_buf = NULL;
    size_t buffer_size = 100;

    for (int i = 0; i < BOARD_SIZE; i++) {
        for (int j = 0; j < BOARD_SIZE; j++) {
            map[i][j] = '.';
            values[i][j] = 100000;
        }
    }

    struct coordinate coords[10000];
    int num_cords = 0;

    
    struct coordinate start, end;

    int y = 0;
    while (getline(&line_buf, &buffer_size, stdin) > 0) {
        printf(line_buf);
        for (int x = 0; x < BOARD_SIZE; x++) {
            map[x][y] = line_buf[x];

            if (line_buf[x] == 'S') {
                start.x = x;
                start.y = y;
            }

            if (line_buf[x] == 'E') {
                end.x = x;
                end.y = y;
            }
        }
        y++;
    }

    dfs(start);
    find_cheat(start.x, start.y);
    printf("res: %d", cheats);
}