advent-of-code-2024/day-18/main.c

#include "aoc.h"

typedef struct {
    i32 x, y;
} Vec2i;

static Vec2i
parse_vec2i(str line, Arena tmp) {
    str left, right;
    str_split(line, STR(","), &left, &right);
    Vec2i result = {
        .x = parse_i64(str_trim(left), tmp),
        .y = parse_i64(str_trim(right), tmp),
    };
    return result;
}

static Vec2i
vec2i(i32 x, i32 y) {
    Vec2i result = { .x = x, .y = y };
    return result;
}

static void
simulate_falling(Vec2i *bytes, isize bytes_len, u8 *grid, i32 width, i32 height) {
    for (isize i = 0; i < bytes_len; i++) {
        Vec2i byte = bytes[i];
        grid[byte.y * width + byte.x] = 1;
    }
}

static void
dump_grid(u8 *grid, i32 width, i32 height) {
    for (i32 y = 0; y < height; y++) {
        for (i32 x = 0; x < width; x++) {
            u8 cell = grid[y * width + x];
            if (cell) {
                printf("#");
            } else {
                printf(".");
            }
        }
        printf("\n");
    }
}

typedef struct {
    Vec2i *points;
    i32 write, read;
    isize size;
} Queue;

static inline i32
queue_next(Queue *queue, i32 index) {
    return (index + 1) % queue->size;
}

static inline void
queue_push(Queue *queue, Vec2i pos) {
    ASSERT(queue_next(queue, queue->write) != queue->read);
    queue->points[queue->write] = pos;
    queue->write = queue_next(queue, queue->write);
}

static inline Vec2i
queue_pop(Queue *queue) {
    ASSERT(queue->read != queue->write);
    Vec2i result = queue->points[queue->read];
    queue->read = queue_next(queue, queue->read);
    return result;
}

static inline bool
queue_empty(Queue *queue) {
    return queue->write == queue->read;
}

static i32
solve(Arena temp, u8 *grid, i32 width, i32 height) {
    i32 end_x = width - 1, end_y = height - 1;
    i32 x = 0, y = 0;
    Queue queue = { .size = 1024 };
    queue.points = ARENA_ALLOC_ARRAY(&temp, Vec2i, queue.size);
    i32 *distance = ARENA_ALLOC_ARRAY(&temp, i32, width * height);
    for (i32 i = 0; i < width * height; i++) {
        distance[i] = INT32_MAX;
    }
    distance[y * width + x] = 0;
    queue_push(&queue, vec2i(x, y));
    static const Vec2i offsets[] = {
        { .x = 0, .y = -1 },
        { .x = 0, .y = 1 },
        { .x = -1, .y = 0 },
        { .x = 1, .y = 0 },
    };
    while (!queue_empty(&queue)) {
        Vec2i point = queue_pop(&queue);
        i32 dist = distance[point.y * width + point.x];
        ASSERT(dist < INT32_MAX);
        for (i32 i = 0; i < 4; i++) {
            Vec2i offset = offsets[i];
            i32 nx = point.x + offset.x;
            i32 ny = point.y + offset.y;
            if (nx >= 0 && ny >= 0 && nx < width && ny < height
                && !grid[ny * width + nx]
                && dist + 1 < distance[ny * width + nx]) 
            {
                distance[ny * width + nx] = dist + 1;
                queue_push(&queue, vec2i(nx, ny));
            }
        }
    }
    return distance[end_y * width + end_x];
}

int main(int argc, char **argv) {
    Arena *arena = make_arena(Megabytes(1));
    Tokens lines = read_lines(arena, argv[1]);

    Vec2i *bytes = ARENA_ALLOC_ARRAY(arena, Vec2i, lines.len);
    bool is_test = true;
    for (isize i = 0; i < lines.len; i++) {
        bytes[i] = parse_vec2i(lines.tokens[i], *arena);
        if (bytes[i].x > 6 || bytes[i].y > 6) {
            is_test = false;
        }
    }

    i32 width, height;
    i32 part_1_limit;
    if (is_test) {
        width = 7;
        height = 7;
        part_1_limit = 12;
    }
    else {
        width = 71;
        height = 71;
        part_1_limit = 1024;
    }
    u8 *grid = ARENA_ALLOC_ARRAY(arena, u8, width * height);

    // part 1
    simulate_falling(bytes, part_1_limit, grid, width, height);
    /* dump_grid(grid, width, height); */
    i32 part_1 = solve(*arena, grid, width, height);
    printf("%d\n", part_1);

    // part 2
    memset(grid, 0, width * height);
    for (isize cutoff = 0; cutoff < lines.len; cutoff++) {
        simulate_falling(bytes + cutoff, 1, grid, width, height);
        i32 result = solve(*arena, grid, width, height);
        if (result == INT32_MAX) {
            printf("%d,%d\n", bytes[cutoff].x, bytes[cutoff].y);
            break;
        }
    }
}