From 7f35208133d37b08259a7e205de36522ff837775 Mon Sep 17 00:00:00 2001
From: Jordan Doyle <jordan@doyle.la>
Date: Sat, 07 Dec 2024 21:26:25 +0000
Subject: [PATCH] Add 2024 day 6

---
 README      |   4 ++--
 2024/06.zig | 122 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 124 insertions(+), 2 deletions(-)

diff --git a/README b/README
index 05114d6..10906c9 100644
--- a/README
+++ a/README
@@ -19,8 +19,8 @@
 |        3 | Perl     | |        3 | Haskell  | |        3 | Clojure  |
 |        4 | Rust     | |        4 | Haskell  | |        4 | Jsonnet  |
 |        5 | PHP      | |        5 | Rust     | |        5 | HCL/TF   |
-+---------------------+ |        6 | Haskell  | |        6 | Apache2  |
-                        |        7 | Haskell  | +---------------------+
+|        6 | Zig      | |        6 | Haskell  | |        6 | Apache2  |
++---------------------+ |        7 | Haskell  | +---------------------+
                         |        8 | Haskell  |
                         |        9 | Haskell  |
                         |       10 | Haskell  |
diff --git a/2024/06.zig b/2024/06.zig
new file mode 100644
index 0000000..fe35b3d 100644
--- /dev/null
+++ a/2024/06.zig
@@ -1,0 +1,122 @@
+const std = @import("std");
+
+const Vec = struct {
+    x: usize,
+    y: usize,
+};
+
+const Direction = enum {
+    Up,
+    Down,
+    Left,
+    Right,
+
+    fn turn(self: Direction) Direction {
+        return switch (self) {
+            .Up => .Right,
+            .Right => .Down,
+            .Down => .Left,
+            .Left => .Up,
+        };
+    }
+
+    fn step(self: Direction, player: *Vec) void {
+        switch (self) {
+            .Up => player.y -= 1,
+            .Right => player.x += 1,
+            .Down => player.y += 1,
+            .Left => player.x -= 1,
+        }
+    }
+
+    fn stepBack(self: Direction, player: *Vec) void {
+        switch (self) {
+            .Up => player.y += 1,
+            .Right => player.x -= 1,
+            .Down => player.y -= 1,
+            .Left => player.x += 1,
+        }
+    }
+};
+
+pub fn main() !void {
+    const in = std.io.getStdIn().reader();
+
+    var msg_buf: [200]u8 = undefined;
+
+    var blocks = std.ArrayList(Vec).init(std.heap.page_allocator);
+    var player = Vec{ .x = 0, .y = 0 };
+    var gridBounds = Vec{ .x = 0, .y = 0 };
+
+    var y: usize = 0;
+    while (try in.readUntilDelimiterOrEof(&msg_buf, '\n')) |line| : (y += 1) {
+        for (line, 0..) |char, x| {
+            gridBounds = Vec{ .x = x, .y = y };
+
+            switch (char) {
+                '#' => try blocks.append(.{ .x = x, .y = y }),
+                '^' => player = .{ .x = x, .y = y },
+                else => continue,
+            }
+        }
+    }
+
+    var part1 = try findExitPath(player, gridBounds, blocks);
+    std.debug.print("{d}\n", .{part1.?.count()});
+
+    var part2: usize = 0;
+    var iter = part1.?.iterator();
+    while (iter.next()) |data| {
+        if (data.key_ptr.y == player.y and data.key_ptr.x == player.x) {
+            continue;
+        }
+
+        try blocks.append(.{ .x = data.key_ptr.x, .y = data.key_ptr.y });
+
+        if (try findExitPath(player, gridBounds, blocks) == null) {
+            part2 += 1;
+        }
+
+        _ = blocks.pop();
+    }
+
+    std.debug.print("{d}", .{part2});
+}
+
+fn findExitPath(initialPlayer: Vec, gridBounds: Vec, blocks: std.ArrayList(Vec)) !?std.AutoHashMap(Vec, u32) {
+    var seenPositions = std.AutoHashMap(Vec, u32).init(std.heap.page_allocator);
+    var direction = Direction.Up;
+    var player = initialPlayer;
+
+    while (player.x <= gridBounds.x and player.y <= gridBounds.y and player.x > 0 and player.y > 0) {
+        direction.step(&player);
+
+        if (containsVec(blocks.items, player)) {
+            direction.stepBack(&player);
+            direction = direction.turn();
+        } else {
+            const res = seenPositions.get(player);
+            if (res) |value| {
+                if (value == 100) {
+                    // assume we're stuck in a loop
+                    return null;
+                }
+                try seenPositions.put(player, value + 1);
+            } else {
+                try seenPositions.put(player, 1);
+            }
+        }
+    }
+
+    return seenPositions;
+}
+
+fn containsVec(items: []Vec, vec: Vec) bool {
+    for (items) |elem| {
+        if (elem.x == vec.x and elem.y == vec.y) {
+            return true;
+        }
+    }
+
+    return false;
+}
--
rgit 0.1.5