day 16

aoc/aoc.h

@@ -28,6 +28,10 @@ grid_at(Grid *grid, i32 x, i32 y) {
     return grid->grid[y * grid->width + x];
 }
 
+static inline bool
+grid_contains(Grid *grid, i32 x, i32 y) {
+    return x >= 0 && y >= 0 && x < grid->width && y < grid->height;
+}
 
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
 #include <stdio.h>
@@ -53,6 +57,12 @@ typedef double f64;
 #define MIN(a, b) ((a) < (b) ? (a) : (b))
 #define CLAMP(x, lo, hi) ((x) < (lo) ? (lo) : (x) > (hi) ? (hi) : (x))
 
+#define MIN3(a, b, c) MIN(MIN(a, b), c)
+#define MAX3(a, b, c) MAX(MAX(a, b), c)
+
+#define MIN4(a, b, c, d) MIN(MIN(a, b), MIN(c, d))
+#define MAX4(a, b, c, d) MAX(MAX(a, b), MAX(c, d))
+
 #define Kilobytes(x) ((u64) (x) << 10)
 #define Megabytes(x) ((u64) (x) << 20)
 #define Gigabytes(x) ((u64) (x) << 30)

day-16/input.txt

@@ -0,0 +1,141 @@
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+###.#.#######.###.#.#.#####.#.#.#.#####.###.#####.#.#.#.#####.#.#.###.#.###.###.#.#.#.#.#.#####.#.#.#.#.###.#.#######.#######.#####.#.#.#.#.#
+#...#.......#...#.#.......#...#.#.#...#.#.#.......#.#...........#...#.#...........#.#...#.......#.#.#.#...#.#.#.....#.#...#...#.#...........#
+#.###.#########.###.#####.#####.#.###.#.#.#.#######.###.#####.#####.#.###.#.###.#.#.#.###.###.#.#.#.#.#.###.#.###.###.###.#.###.#.#########.#
+#.#...............#...................#.#...#.......#.........#.....#...#...#.......#.....#...#...#.#.......#.#...#...#...#.#...#...#.......#
+#.#.###.###.#.###.###.#.###.#.#.#####.#.#####.#######.#.#####.#.#####.#####.###.#.###########.#.###.#.#######.#.###.###.#.#.#.#.###.###.###.#
+#.#...#.#...#...#.....#.....#.#...#.#.#...#...#.......#.#...#.#.#.....#...#.#...........#...#.#.#.#...#.....................#.#...#...#...#.#
+#.#####.#.#################.#.###.#.#.###.#.#####.#####.#.#.#.#.#.#####.#.#.#.#.#.#####.#.#.#.#.#.#.###.###.#.#.#######.###.#.###.###.###.#.#
+#.......#.................#.#...#...#.#.#.#...#...#.....#.#...#.#.#.....#...#...#...#...#.#.#.#.#.......#...#.#.#.....#.#...#.#.....#...#.#.#
+#.#############.###.#######.###.###.#.#.#.###.#.#####.#.#.###.#.#.#.#####.#.###.#.###.###.#.###.###########.#.#.#.###.#.#.###.#.#######.#.#.#
+#.#.........#.#.#...#.......#.#...#.#...#...#.#.#.....#...#.....#.#.#.....#.....#...#.....#...#.#.............................#.....#...#.#.#
+#.#######.#.#.#.#####.#######.###.#####.###.#.#.#.#####.#######.#.#.#######.#######.#####.###.#.#.#.#.###.#######.#.###.#.#.#.#####.#.###.#.#
+#S........#...............................#.....#...............#.#.........................#.....#...#...........#.......#.......#.......#.#
+#############################################################################################################################################

day-16/main.c

@@ -0,0 +1,172 @@
+#include "aoc.h"
+#include "limits.h"
+
+typedef struct {
+    i32 x, y;
+} Vec2i;
+
+typedef enum {
+    Right,
+    Down,
+    Left,
+    Up,
+} Orientation;
+
+static inline Orientation
+turn(Orientation orientation, i32 delta) {
+    return (orientation + delta) % 4;
+}
+
+static const Vec2i OFFSET[] = {
+    [Right] = { .x = 1, .y = 0 },
+    [Down]  = { .x = 0, .y = 1 },
+    [Left]  = { .x = -1, .y = 0 },
+    [Up]    = { .x = 0, .y = -1 },
+};
+
+typedef struct {
+    i32 x, y;
+    Orientation orientation;
+} Tile;
+
+typedef struct TileQueue {
+    Tile *tiles;
+    isize len;
+    isize read, write;
+} TileQueue;
+
+static inline bool
+tile_queue_empty(TileQueue *queue) {
+    return queue->read == queue->write;
+}
+
+static Tile
+tile_queue_pop(TileQueue *queue) {
+    ASSERT(!tile_queue_empty(queue));
+    Tile result = queue->tiles[queue->read];
+    queue->read = (queue->read + 1) % queue->len;
+    return result;
+}
+
+static void
+tile_queue_push(TileQueue *queue, i32 x, i32 y, Orientation orientation) {
+    isize next = (queue->write + 1) % queue->len;
+    ASSERT(next != queue->read);
+    queue->tiles[queue->write] = (Tile) { .x = x, .y = y, .orientation = orientation };
+    queue->write = next;
+}
+
+typedef struct {
+    i32 distance;
+    DYNAMIC_ARRAY(Tile) predecessors;
+} Cell;
+
+static void
+solve(Arena *arena, Grid *grid) {
+    i32 start_x = -1, start_y = -1;
+    i32 end_x = -1, end_y = -1;
+    for (i32 y = 0; y < grid->height; y++) {
+        for (i32 x = 0; x < grid->width; x++) {
+            if (grid->grid[y * grid->width + x] == 'E') {
+                end_x = x;
+                end_y = y;
+            }
+            else if (grid->grid[y * grid->width + x] == 'S') {
+                start_x = x;
+                start_y = y;
+            }
+        }
+    }
+    ASSERT(start_x != -1 && start_y != -1);
+    ASSERT(end_x != -1 && end_y != -1);
+
+    Cell *cells = ARENA_ALLOC_ARRAY(arena, Cell, 4 * grid->width * grid->height);
+    for (isize i = 0; i < 4 * grid->width * grid->height; i++) {
+        cells[i].distance = INT_MAX;
+    }
+    // `cells` is a 3d array indexed as cells[4 * (y * width + x) + orientation]
+#define CELL(x, y, o) cells[4 * ((y) * grid->width + (x)) + (o)]
+#define DISTANCE(x, y, o) CELL(x, y, o).distance
+
+    Orientation start_orientation = Right;
+    DISTANCE(start_x, start_y, start_orientation) = 0;
+    isize queue_size = 2048;
+    TileQueue queue = {
+        .tiles = ARENA_ALLOC_ARRAY(arena, Tile, queue_size),
+        .len = queue_size,
+    };
+    tile_queue_push(&queue, start_x, start_y, start_orientation);
+
+    while (!tile_queue_empty(&queue)) {
+        // walk straight
+        Tile current = tile_queue_pop(&queue);
+        i32 x = current.x;
+        i32 y = current.y;
+        Orientation orientation = current.orientation;
+        i32 dist = DISTANCE(current.x, current.y, current.orientation);
+        if (grid->grid[y * grid->width + x] == '#') {
+            continue;
+        }
+        i32 nx = current.x + OFFSET[current.orientation].x;
+        i32 ny = current.y + OFFSET[current.orientation].y;
+#define TRY(x_, y_, o_, delta) do { \
+    i32 candidate = DISTANCE(x_, y_, o_); \
+    if (dist + delta <= candidate) { \
+        DISTANCE(x_, y_, o_) = dist + delta; \
+        Cell *cell = &CELL(x_, y_, o_); \
+        if (dist + delta < candidate) { \
+            tile_queue_push(&queue, x_, y_, o_); \
+            cell->predecessors.len = 0; \
+        } \
+        *push(&cell->predecessors, arena) = (Tile) { .x = x, .y = y, .orientation = orientation }; \
+    } \
+} while (0)
+        TRY(nx, ny, orientation, 1);
+        // turn 90deg cw
+        TRY(x, y, turn(orientation, 1), 1000);
+        // turn 180deg cw
+        TRY(x, y, turn(orientation, 2), 2000);
+        // turn 90deg ccw
+        TRY(x, y, turn(orientation, 3), 1000);
+#undef TRY
+    }
+
+    i32 part_1 = MIN4(DISTANCE(end_x, end_y, Right), DISTANCE(end_x, end_y, Down), DISTANCE(end_x, end_y, Left), DISTANCE(end_x, end_y, Up));
+    
+    // Reconstruct best path
+    u8 *best_path = ARENA_ALLOC_ARRAY(arena, u8, grid->width * grid->height);
+    queue.write = 0;
+    queue.read = 0;
+    tile_queue_push(&queue, end_x, end_y, Right);
+    tile_queue_push(&queue, end_x, end_y, Down);
+    tile_queue_push(&queue, end_x, end_y, Left);
+    tile_queue_push(&queue, end_x, end_y, Up);
+    while (!tile_queue_empty(&queue)) {
+        Tile tile = tile_queue_pop(&queue);
+        best_path[tile.y * grid->width + tile.x] = 1;
+        Cell *cell = &CELL(tile.x, tile.y, tile.orientation);
+        for (isize i = 0; i < cell->predecessors.len; i++) {
+            Tile pred = cell->predecessors.data[i];
+            tile_queue_push(&queue, pred.x, pred.y, pred.orientation);
+        }
+    }
+
+    i32 part_2 = 0;
+    for (i32 y = 0; y < grid->height; y++) {
+        for (i32 x = 0; x < grid->width; x++) {
+            part_2 += (i32) best_path[y * grid->width + x];
+        }
+    }
+
+    printf("%d\n", part_1);
+    printf("%d\n", part_2);
+#undef DISTANCE
+#undef CELL
+}
+
+int main(int argc, char **argv) {
+    Arena *arena = make_arena(Megabytes(16));
+    Grid input = parse_grid(read_file(arena, argv[1]), arena);
+
+    solve(arena, &input);
+}

day-16/test.txt

@@ -0,0 +1,15 @@
+###############
+#.......#....E#
+#.#.###.#.###.#
+#.....#.#...#.#
+#.###.#####.#.#
+#.#.#.......#.#
+#.#.#####.###.#
+#...........#.#
+###.#.#####.#.#
+#...#.....#.#.#
+#.#.#.###.#.#.#
+#.....#...#.#.#
+#.###.#.#.#.#.#
+#S..#.....#...#
+###############

day-16/test2.txt

@@ -0,0 +1,4 @@
+##########
+#S...#E..#
+#........#
+##########