diff --git a/src/day16.zig b/src/day16.zig
new file mode 100644
index 0000000..3ad469c
--- /dev/null
+++ b/src/day16.zig
@@ -0,0 +1,159 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day16.txt");
+    const sln = try solve(util.gpa, input);
+    std.debug.print("{d}\n", .{sln.a});
+    std.debug.print("{d}\n", .{sln.b});
+}
+
+const Solution = struct {
+    a: usize,
+    b: usize,
+};
+
+const Direction = enum {
+    up,
+    right,
+    down,
+    left,
+
+    fn x(self: @This()) isize {
+        return switch (self) {
+            .up, .down => 0,
+            .left => -1,
+            .right => 1,
+        };
+    }
+
+    fn y(self: @This()) isize {
+        return switch (self) {
+            .left, .right => 0,
+            .up => -1,
+            .down => 1,
+        };
+    }
+
+    fn rotate(self: @This(), nineties: usize) Direction {
+        return @enumFromInt((@intFromEnum(self) + nineties) % std.enums.values(@This()).len);
+    }
+};
+
+const Point = struct {
+    x: isize,
+    y: isize,
+    dir: Direction,
+};
+
+var states: std.AutoArrayHashMap(Point, void) = undefined;
+
+fn run(alloc: mem.Allocator, grid: [][]u8, visited: [][]bool, init_x: isize, init_y: isize, init_dir: Direction) !void {
+    var x = init_x;
+    var y = init_y;
+    var dir = init_dir;
+
+    while (0 <= x and x < grid[0].len - 1 and 0 <= y and y < grid.len - 1) {
+        visited[@intCast(y)][@intCast(x)] = true;
+        var looped = try states.getOrPut(.{ .x = x, .y = y, .dir = dir });
+        if (looped.found_existing) break;
+        // std.debug.print("-----\n", .{});
+        // std.debug.print("{} {} {}\n", .{ x, y, dir });
+        // std.time.sleep(1_000_000_000);
+        switch (grid[@intCast(y)][@intCast(x)]) {
+            '.' => {
+                x += dir.x();
+                y += dir.y();
+            },
+            '|' => {
+                switch (dir) {
+                    .up, .down => y += dir.y(),
+                    .left, .right => {
+                        try run(alloc, grid, visited, x, y - 1, .up);
+                        try run(alloc, grid, visited, x, y + 1, .down);
+                    },
+                }
+            },
+            '-' => {
+                switch (dir) {
+                    .left, .right => x += dir.x(),
+                    .up, .down => {
+                        try run(alloc, grid, visited, x - 1, y, .left);
+                        try run(alloc, grid, visited, x + 1, y, .right);
+                    },
+                }
+            },
+            '/' => {
+                switch (dir) {
+                    .left, .right => dir = dir.rotate(3),
+                    .up, .down => dir = dir.rotate(1),
+                }
+                x += dir.x();
+                y += dir.y();
+            },
+            '\\' => {
+                switch (dir) {
+                    .left, .right => dir = dir.rotate(1),
+                    .up, .down => dir = dir.rotate(3),
+                }
+                x += dir.x();
+                y += dir.y();
+            },
+            else => unreachable,
+        }
+    }
+}
+
+fn solve(alloc: mem.Allocator, comptime input: []const u8) !Solution {
+    const trimmed = mem.trimRight(u8, input, "\n\r");
+    const w = mem.indexOfScalar(u8, trimmed, '\n') orelse unreachable;
+    const h = mem.count(u8, trimmed, "\n");
+
+    var visited = try util.allocGrid(bool, alloc, w, h, false);
+    const grid = try util.toGridOwned(alloc, trimmed, '\n');
+
+    states = std.AutoArrayHashMap(Point, void).init(alloc);
+    defer states.deinit();
+    try run(alloc, grid, visited, 0, 0, .right);
+
+    var energized: usize = 0;
+    for (visited) |row| {
+        for (row) |cell| {
+            if (cell) energized += 1;
+        }
+    }
+
+    for (grid) |row| {
+        alloc.free(row);
+    }
+    for (visited) |row| {
+        alloc.free(row);
+    }
+    alloc.free(grid);
+    alloc.free(visited);
+    return .{ .a = energized, .b = 0 };
+}
+
+const sample =
+    \\.|...\....
+    \\|.-.\.....
+    \\.....|-...
+    \\........|.
+    \\..........
+    \\.........\
+    \\..../.\\..
+    \\.-.-/..|..
+    \\.|....-|.\
+    \\..//.|....
+;
+
+test "silver" {
+    const sln = try solve(std.testing.allocator, sample);
+    try std.testing.expectEqual(@as(usize, 46), sln.a);
+}
+
+test "gold" {
+    // const sln = try solve(std.testing.allocator, input);
+    // try std.testing.expectEqual(@as(usize, 0), sln.b);
+}
diff --git a/src/util.zig b/src/util.zig
index d262933..808a9ea 100644
--- a/src/util.zig
+++ b/src/util.zig
@@ -35,3 +35,31 @@ pub fn splitLines(buffer: []const u8) mem.SplitIterator(u8, .scalar) {
 pub fn splitSpace(buffer: []const u8) mem.SplitIterator(u8, .scalar) {
     return split(buffer, ' ');
 }
+
+pub fn allocGrid(comptime T: type, alloc: mem.Allocator, w: usize, h: usize, init: T) ![][]T {
+    var grid = try alloc.alloc([]T, h);
+    for (0..h) |i| {
+        grid[i] = try alloc.alloc(T, w);
+        @memset(grid[i], init);
+    }
+    return grid;
+}
+
+pub fn toGridOwned(alloc: mem.Allocator, buffer: []const u8, sep: u8) ![][]u8 {
+    if (buffer.len == 0) return error.Empty;
+    const w = mem.indexOfScalar(u8, buffer, sep) orelse unreachable;
+    var h = mem.count(u8, buffer, &[_]u8{sep});
+    if (buffer[buffer.len - 1] != sep) h += 1;
+
+    var grid = try alloc.alloc([]u8, h);
+
+    var it = split(buffer, sep);
+    var i: usize = 0;
+    while (it.next()) |line| {
+        grid[i] = try alloc.alloc(u8, w);
+        @memcpy(grid[i], line);
+        i += 1;
+    }
+
+    return grid;
+}