From e8e40984335b70d2ae58386ce8ee43069c81d0c9 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <code@jdkaplan.dev>
Date: Thu, 5 Dec 2024 21:36:30 -0800
Subject: [PATCH] 2024 Day 6 Part 1

---
 2024/src/aoc.zig   |   6 +-
 2024/src/day06.zig | 160 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 165 insertions(+), 1 deletion(-)
 create mode 100644 2024/src/day06.zig

diff --git a/2024/src/aoc.zig b/2024/src/aoc.zig
index 4845736..df1bb5d 100644
--- a/2024/src/aoc.zig
+++ b/2024/src/aoc.zig
@@ -56,8 +56,12 @@ pub fn AutoHashSet(comptime T: type) type {
             return self.map.contains(v);
         }
 
-        pub fn valueIterator(self: Self) Map.KeyIterator {
+        pub fn iterator(self: Self) Map.KeyIterator {
             return self.map.keyIterator();
         }
+
+        pub fn count(self: Self) usize {
+            return self.map.count();
+        }
     };
 }
diff --git a/2024/src/day06.zig b/2024/src/day06.zig
new file mode 100644
index 0000000..acd847b
--- /dev/null
+++ b/2024/src/day06.zig
@@ -0,0 +1,160 @@
+const std = @import("std");
+const aoc = @import("aoc.zig");
+
+const Allocator = std.mem.Allocator;
+const ArrayList = std.ArrayList;
+
+pub fn main() !void {
+    var gpa: std.heap.GeneralPurposeAllocator(.{}) = .{};
+    defer _ = gpa.deinit();
+    const allocator = gpa.allocator();
+
+    const file = try std.fs.cwd().openFile("input/day06.txt", .{});
+    defer file.close();
+
+    const text = try file.readToEndAlloc(allocator, comptime 1 << 30);
+    defer allocator.free(text);
+
+    var lab = try Lab.parse(allocator, text);
+    defer lab.deinit();
+
+    const stdout_file = std.io.getStdOut().writer();
+    var bw = std.io.bufferedWriter(stdout_file);
+    const stdout = bw.writer();
+
+    try stdout.print("{d}\n", .{try part1(allocator, lab)});
+    try bw.flush();
+}
+
+const Coords = struct {
+    r: i32,
+    c: i32,
+
+    fn plus(self: Coords, delta: Delta) Coords {
+        return .{
+            .r = self.r + delta.dr,
+            .c = self.c + delta.dc,
+        };
+    }
+};
+
+const Delta = struct {
+    dr: i32,
+    dc: i32,
+
+    fn new(dr: i32, dc: i32) Delta {
+        return .{ .dr = dr, .dc = dc };
+    }
+};
+
+const Direction = enum {
+    N,
+    E,
+    S,
+    W,
+
+    fn delta(self: Direction) Delta {
+        return switch (self) {
+            .N => Delta.new(-1, 0),
+            .S => Delta.new(1, 0),
+            .W => Delta.new(0, -1),
+            .E => Delta.new(0, 1),
+        };
+    }
+
+    fn clockwise(self: Direction) Direction {
+        return switch (self) {
+            .N => .E,
+            .E => .S,
+            .S => .W,
+            .W => .N,
+        };
+    }
+};
+
+const Lab = struct {
+    const Self = @This();
+    const Map = std.AutoHashMap(Coords, u8);
+
+    map: Map,
+    start: Coords,
+
+    fn parse(allocator: std.mem.Allocator, text: []u8) !Self {
+        var map = Map.init(allocator);
+        var start: Coords = undefined;
+
+        var it = std.mem.tokenizeScalar(u8, text, '\n');
+        var r: usize = 0;
+        while (it.next()) |row| : (r += 1) {
+            for (row, 0..) |char, c| {
+                const coords = Coords{ .r = @intCast(r), .c = @intCast(c) };
+                try map.put(coords, char);
+
+                if (char == '^') {
+                    start = coords;
+                }
+            }
+        }
+
+        return .{ .map = map, .start = start };
+    }
+
+    fn deinit(self: *Self) void {
+        self.map.deinit();
+    }
+
+    fn iterator(self: *const Self) Map.Iterator {
+        return self.map.iterator();
+    }
+
+    fn get(self: *const Self, k: Coords) ?u8 {
+        return self.map.get(k);
+    }
+};
+
+const Guard = struct {
+    coords: Coords,
+    dir: Direction,
+
+    fn facing(self: Guard) Coords {
+        return self.coords.plus(self.dir.delta());
+    }
+
+    fn move(self: *Guard) void {
+        self.coords = self.facing();
+    }
+
+    fn rotate(self: *Guard) void {
+        self.dir = self.dir.clockwise();
+    }
+};
+
+fn part1(allocator: Allocator, lab: Lab) !usize {
+    var guard = Guard{
+        .coords = lab.start,
+        .dir = .N,
+    };
+
+    var visited = aoc.AutoHashSet(Guard).init(allocator);
+    defer visited.deinit();
+
+    while (!visited.contains(guard)) {
+        try visited.put(guard);
+        const next = lab.get(guard.facing()) orelse break;
+        if (next == '#') {
+            guard.rotate();
+        } else {
+            guard.move();
+        }
+    }
+
+    var uniq = aoc.AutoHashSet(Coords).init(allocator);
+    defer uniq.deinit();
+
+    var it = visited.iterator();
+    while (it.next()) |v| {
+        try uniq.put(v.coords);
+    }
+
+    return uniq.count();
+}