day 06

day-06/input.txt

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day-06/main.c

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+#include "aoc.h"
+
+typedef enum Direction {
+    Up, Right, Down, Left
+} Direction;
+
+static const int offsets[4][2] = {
+    [Up]    = { 0, -1},
+    [Right] = { 1,  0},
+    [Down]  = { 0,  1},
+    [Left]  = {-1,  0}
+};
+
+i32 traverse(Grid *grid, int x, int y, Direction facing, Arena arena) {
+    u8 *visited = ARENA_ALLOC_ARRAY(&arena, u8, grid->width * grid->height);
+
+    i32 result = 0;
+    for (;;) {
+        int index = y * grid->width + x;
+        if (visited[index] & (1 << facing)) {
+            // loop detected
+            result = -1;
+            break;
+        }
+        else if (visited[index] == 0) {
+            result++;
+        }
+        visited[index] |= (1 << facing);
+
+        int nx = x + offsets[facing][0];
+        int ny = y + offsets[facing][1];
+        if (nx >= 0 && ny >= 0 && nx < grid->width && ny < grid->height) {
+            i32 next_index = ny * grid->width + nx;
+            if (grid->grid[next_index] == '#') {
+                facing = (facing + 1) % 4;
+            }
+            else {
+                x = nx;
+                y = ny;
+            }
+        }
+        else {
+            break;
+        }
+    }
+    return result;
+}
+
+int main(int argc, char **argv) {
+    Arena *arena = make_arena(Megabytes(1));
+
+    str input = read_file(arena, argv[1]);
+    Grid grid = parse_grid(input, arena);
+
+    int start_x = -1, start_y = -1;
+    for (int y = 0; y < grid.height; y++) {
+        for (int x = 0; x < grid.width; x++) {
+            if (grid_at(&grid, x, y) == '^') {
+                start_x = x;
+                start_y = y;
+                grid.grid[y * grid.width + x] = ':';
+            }
+        }
+    }
+
+    i32 part_1 = traverse(&grid, start_x, start_y, Up, *arena);
+
+    i32 part_2 = 0;
+    for (int y = 0; y < grid.height; y++) {
+        for (int x = 0; x < grid.width; x++) {
+            if (x == start_x && y == start_y) {
+                continue;
+            }
+            int index = y * grid.width + x;
+            u8 saved;
+            if ((saved = grid.grid[index]) != '#') {
+                grid.grid[index] = '#';
+                i32 visited = traverse(&grid, start_x, start_y, Up, *arena);
+                part_2 += (visited == -1);
+                grid.grid[index] = saved;
+            }
+        }
+    }
+
+    printf("%d\n", part_1);
+    printf("%d\n", part_2);
+} 

day-06/test.txt

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+....#.....
+.........#
+..........
+..#.......
+.......#..
+..........
+.#..^.....
+........#.
+#.........
+......#...