diff --git a/day6.zig b/day6.zig
new file mode 100644
index 0000000..92e5212
--- /dev/null
+++ b/day6.zig
@@ -0,0 +1,456 @@
+const std = @import("std");
+
+const utils = @import("utils.zig");
+
+pub fn main() !void {
+    var gpa = std.heap.GeneralPurposeAllocator(.{}){};
+    const alloc = gpa.allocator();
+
+    const filename = "inputs/day6.txt";
+
+    {
+        const file_reader = try utils.FileReader.init(alloc, filename);
+        defer file_reader.deinit();
+
+        const result = try part1(alloc, file_reader.reader());
+
+        try std.io.getStdOut().writer().print("Day 6, part 1: {}\n", .{result});
+    }
+
+    {
+        const file_reader = try utils.FileReader.init(alloc, filename);
+        defer file_reader.deinit();
+
+        const result = try part2(alloc, file_reader.reader());
+
+        try std.io.getStdOut().writer().print("Day 6, part 1: {}\n", .{result});
+    }
+}
+
+const Orientation = enum {
+    up,
+    right,
+    down,
+    left,
+};
+
+const MapElem = enum {
+    empty,
+    obstacle,
+    guard,
+    visited,
+};
+
+const GuardPos = struct {
+    x: i32,
+    y: i32,
+    orientation: Orientation,
+};
+
+const Map = [][]MapElem;
+
+const MapState = struct {
+    alloc: std.mem.Allocator,
+
+    size_x: usize,
+    size_y: usize,
+
+    map: Map,
+
+    guard_pos: GuardPos,
+
+    pub fn format(self: @This(), comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype) !void {
+        _ = fmt;
+        _ = options;
+
+        for (self.map) |line| {
+            for (line) |elem| {
+                const c: u8 = switch (elem) {
+                    .empty => '.',
+                    .guard => switch (self.guard_pos.orientation) {
+                        .up => '^',
+                        .down => 'v',
+                        .left => '<',
+                        .right => '>',
+                    },
+                    .obstacle => '#',
+                    .visited => 'X',
+                };
+
+                try writer.print("{c}", .{c});
+            }
+
+            try writer.print("\n", .{});
+        }
+    }
+
+    fn deinit(self: MapState) void {
+        for (self.map) |map_line| {
+            self.alloc.free(map_line);
+        }
+
+        self.alloc.free(self.map);
+    }
+
+    fn copyFrom(self: *MapState, other: MapState) void {
+        for (self.map, other.map) |*map_line, map_line_orig| {
+            @memcpy(map_line.*, map_line_orig);
+        }
+    }
+
+    fn clone(self: MapState, alloc: std.mem.Allocator) !MapState {
+        const map_orig = self.map;
+
+        const map = try alloc.alloc([]MapElem, map_orig.len);
+
+        for (map, map_orig) |*map_line, map_line_orig| {
+            map_line.* = try alloc.alloc(MapElem, map_line_orig.len);
+        }
+
+        var new_map_state = MapState{
+            .alloc = alloc,
+            .size_x = self.size_x,
+            .size_y = self.size_y,
+            .guard_pos = self.guard_pos,
+            .map = map,
+        };
+
+        new_map_state.copyFrom(self);
+
+        return new_map_state;
+    }
+
+    fn parse(alloc: std.mem.Allocator, reader: anytype) !MapState {
+        var line_reader = utils.lineReader(alloc, reader);
+        defer line_reader.deinit();
+
+        var map = std.ArrayList([]MapElem).init(alloc);
+
+        var guard_pos: GuardPos = undefined;
+
+        var x: usize = 0;
+        while (try line_reader.next()) |line| : (x += 1) {
+            const map_line = try alloc.alloc(MapElem, line.len);
+
+            for (0.., line, map_line) |y, c, *map_elem| {
+                map_elem.* = switch (c) {
+                    '.' => .empty,
+                    '#' => .obstacle,
+                    '^' => blk: {
+                        guard_pos = GuardPos{
+                            .x = @intCast(x),
+                            .y = @intCast(y),
+                            .orientation = .up,
+                        };
+
+                        break :blk .guard;
+                    },
+                    else => unreachable,
+                };
+            }
+
+            try map.append(map_line);
+        }
+
+        const map_slice = try map.toOwnedSlice();
+        map.deinit();
+
+        const size_x = map_slice.len;
+        const size_y = map_slice[0].len;
+
+        for (map_slice) |map_line| {
+            std.debug.assert(map_line.len == size_y);
+        }
+
+        return .{
+            .alloc = alloc,
+            .size_x = size_x,
+            .size_y = size_y,
+            .map = map_slice,
+            .guard_pos = guard_pos,
+        };
+    }
+
+    fn get(self: MapState, x: i32, y: i32) MapElem {
+        std.debug.assert(0 <= x and x < (self.size_x));
+        std.debug.assert(0 <= y and y < (self.size_y));
+
+        return self.map[@intCast(x)][@intCast(y)];
+    }
+
+    fn set(self: *MapState, x: i32, y: i32, elem: MapElem) void {
+        std.debug.assert(0 <= x and x < (self.size_x));
+        std.debug.assert(0 <= y and y < (self.size_y));
+
+        self.map[@intCast(x)][@intCast(y)] = elem;
+    }
+
+    fn turn(self: *MapState) void {
+        self.guard_pos.orientation = switch (self.guard_pos.orientation) {
+            .up => .right,
+            .right => .down,
+            .down => .left,
+            .left => .up,
+        };
+    }
+
+    fn nextPos(self: MapState) ?struct { x: i32, y: i32 } {
+        const guard_pos = self.guard_pos;
+
+        const x = guard_pos.x;
+        const y = guard_pos.y;
+
+        var new_x: i32 = undefined;
+        var new_y: i32 = undefined;
+
+        switch (guard_pos.orientation) {
+            .up => {
+                new_x = x - 1;
+                new_y = y;
+
+                if (new_x < 0) {
+                    return null;
+                }
+            },
+            .right => {
+                new_x = x;
+                new_y = y + 1;
+
+                if (new_y >= self.size_y) {
+                    return null;
+                }
+            },
+            .down => {
+                new_x = x + 1;
+                new_y = y;
+
+                if (new_x >= self.size_x) {
+                    return null;
+                }
+            },
+            .left => {
+                new_x = x;
+                new_y = y - 1;
+
+                if (new_y < 0) {
+                    return null;
+                }
+            },
+        }
+
+        return .{ .x = new_x, .y = new_y };
+    }
+
+    fn advance(self: *MapState) enum { not_done, done } {
+        const guard_pos = &self.guard_pos;
+
+        const x = guard_pos.x;
+        const y = guard_pos.y;
+
+        if (self.get(x, y) != .guard) {
+            std.debug.print("x: {}    y: {}\n", .{ x, y });
+            std.debug.print("map on failure:\n{}\n", .{self});
+        }
+
+        std.debug.assert(self.get(x, y) == .guard);
+
+        const new_pos = self.nextPos() orelse {
+            // walked out of the map
+            std.debug.assert(self.get(x, y) == .guard);
+
+            self.set(x, y, .visited);
+
+            return .done;
+        };
+
+        const new_x = new_pos.x;
+        const new_y = new_pos.y;
+
+        if (self.get(new_x, new_y) == .obstacle) {
+            // found obstacle: turn and advance
+            self.turn();
+            return advance(self);
+        }
+
+        std.debug.assert(self.get(x, y) == .guard);
+        self.set(x, y, .visited);
+
+        guard_pos.x = new_x;
+        guard_pos.y = new_y;
+
+        self.set(new_x, new_y, .guard);
+
+        return .not_done;
+    }
+
+    fn countVisited(self: MapState) u64 {
+        var count: u64 = 0;
+
+        for (self.map) |line| {
+            for (line) |elem| {
+                std.debug.assert(elem != .guard);
+
+                if (elem == .visited) {
+                    count += 1;
+                }
+            }
+        }
+
+        return count;
+    }
+};
+
+fn part1(alloc: std.mem.Allocator, reader: anytype) !u64 {
+    var map_arena = std.heap.ArenaAllocator.init(alloc);
+    defer map_arena.deinit();
+
+    var map_state = try MapState.parse(map_arena.allocator(), reader);
+
+    while (map_state.advance() != .done) {}
+
+    const visited_count = map_state.countVisited();
+
+    return visited_count;
+}
+
+fn part2(alloc: std.mem.Allocator, reader: anytype) !u64 {
+    var map_arena = std.heap.ArenaAllocator.init(alloc);
+    defer map_arena.deinit();
+
+    const map_state_orig = try MapState.parse(map_arena.allocator(), reader);
+
+    var map_state_iter = try map_state_orig.clone(map_arena.allocator());
+
+    var tmp_arena = std.heap.ArenaAllocator.init(alloc);
+    defer tmp_arena.deinit();
+
+    var num_loops: u64 = 0;
+
+    var map_state_all_obstacles = try map_state_orig.clone(alloc);
+    defer map_state_all_obstacles.deinit();
+
+    var guard_pos_set = std.AutoHashMap(GuardPos, void).init(alloc);
+    defer guard_pos_set.deinit();
+
+    while (map_state_iter.advance() != .done) {
+
+        // current guard pos after last step
+        const cur_guard_pos = map_state_iter.guard_pos;
+
+        const cur_x = cur_guard_pos.x;
+        const cur_y = cur_guard_pos.y;
+
+        if (cur_x == map_state_orig.guard_pos.x and cur_y == map_state_orig.guard_pos.y) {
+            // don't put obstacle at initial position!
+            continue;
+        }
+
+        if (map_state_all_obstacles.get(cur_x, cur_y) == .obstacle) {
+            // we already tried this one
+            continue;
+        }
+
+        // clone original map state
+        var map_state_modified = try map_state_orig.clone(tmp_arena.allocator());
+
+        // put a new obstacle at the current position of the guard
+        // since obstacles only influence the path of the guard if she would step on,
+        // conversely the only relevant positions for new obstacles are the tiles she actually
+        // steps on
+        map_state_modified.set(cur_x, cur_y, .obstacle);
+        map_state_all_obstacles.set(cur_x, cur_y, .obstacle);
+
+        guard_pos_set.clearRetainingCapacity();
+
+        try guard_pos_set.put(map_state_modified.guard_pos, {});
+
+        while (map_state_modified.advance() != .done) {
+            // std.debug.print("modified map:\n{}\n", .{map_state_modified});
+
+            const guard_pos = map_state_modified.guard_pos;
+
+            if (guard_pos_set.contains(guard_pos)) {
+                num_loops += 1;
+                break;
+            }
+
+            try guard_pos_set.put(map_state_modified.guard_pos, {});
+        }
+
+        _ = tmp_arena.reset(.retain_capacity);
+    }
+
+    return num_loops;
+}
+
+test "part1 example" {
+    const alloc = std.testing.allocator;
+
+    const example =
+        \\....#.....
+        \\.........#
+        \\..........
+        \\..#.......
+        \\.......#..
+        \\..........
+        \\.#..^.....
+        \\........#.
+        \\#.........
+        \\......#...
+    ;
+
+    var stream = std.io.fixedBufferStream(example);
+
+    const result = try part1(alloc, stream.reader());
+
+    try std.testing.expect(result == 41);
+}
+
+test "part1 input" {
+    const alloc = std.testing.allocator;
+
+    const filename = "inputs/day6.txt";
+
+    const file_reader = try utils.FileReader.init(alloc, filename);
+    defer file_reader.deinit();
+
+    const result = try part1(alloc, file_reader.reader());
+
+    try std.testing.expect(result == 4722);
+}
+
+test "part2 example" {
+    const alloc = std.testing.allocator;
+
+    const example =
+        \\....#.....
+        \\.........#
+        \\..........
+        \\..#.......
+        \\.......#..
+        \\..........
+        \\.#..^.....
+        \\........#.
+        \\#.........
+        \\......#...
+    ;
+
+    var stream = std.io.fixedBufferStream(example);
+
+    const result = try part2(alloc, stream.reader());
+
+    try std.testing.expect(result == 6);
+}
+
+test "part2 input" {
+    const alloc = std.testing.allocator;
+
+    const filename = "inputs/day6.txt";
+
+    const file_reader = try utils.FileReader.init(alloc, filename);
+    defer file_reader.deinit();
+
+    const result = try part2(alloc, file_reader.reader());
+
+    try std.testing.expect(result == 1602);
+}
diff --git a/inputs/day6.txt b/inputs/day6.txt
new file mode 100644
index 0000000..3c8989d
--- /dev/null
+++ b/inputs/day6.txt
@@ -0,0 +1,130 @@
+..#.....................#............#....#...........#...............................#.....#.....................................
+.................#............................#.............##..#...............................#....#..#.........#...............
+.............................................##.........................................#....................#...............#...#
+...##....................#...............#.....#....................#......##..............#............................#....#....
+.................................#..#..............#...........#.............#..#........................#............#...........
+.................#.........................#...........................#........................#........................#........
+...#....................................#....#.........#................#...............#.....#................#.....#............
+.............#.#..........................................................#...#..................#....#.......#.....#...#.........
+.....#........................#....##...#............................................................#............................
+..................#...#.........................................#..................#.............................#.#........#.....
+......#...............................#.#..........##..#...........#.........................#.......#....................#.......
+.................................#..............#...............#........#..........#.........#............#.........#.......#...#
+..#..#............#...#.....................#...............#............#.#........................#........#...............#....
+..............#..#.#........................#.........#...........##.......#..........#.....................#..............#...#..
+..........................#.................##...................................#....#.........#.................................
+..................#..#........#....#...............#..............#.......#.......#......................................#..#...#.
+..........#.#.................#.........#...................#.......................##......#....................#.......#........
+..........#.........#...#......................................................................#.....#............#........#......
+...##.............#...........................................................#..#....................##.#...#.#..#.........#..#..
+.....#......#.............................#.......#...#...#...............#.....#............#........#..#.#......................
+........#................#.........#.......................................................................#..........#...........
+.......#........................................#..........#...............................#..#...................................
+..................##.......#.......#.....#............#.......#...#.........#....##...............#..#.............#..........#...
+.......#...........#...........#...........................#.............#.........#..#........................#.##...............
+.....................................................................#........#....#...........................................#..
+....#...........#......#.........#....................................#.................................#......#..................
+...#........................................#..#...............................#.....................#.#......#...................
+..#.....#.................................#.................#....#............................................#......#............
+...................................................##...........#.......#.............#.................#.......#.................
+..........................................................##..............#....#....#.............#...............................
+........................................................#.....................................#................................#..
+..............##....#..#..........#...........#......#..#....#.......#................#............................#..#...#.......
+..........................#....................#.................................................#................................
+#................................................#.....................#..#...............##........#...#...#.........#...........
+...........#.................................#..#....................#.#.......#..........................#.......#.......#.......
+..............................#......#......#.................#.................#.................#.#.............#..#.#..........
+.......................#....................#....................................#.....#.#.......#............................#...
+...............................................................................................#.#................................
+#..................#.....#...............................#..............#...#.....................................................
+..........................................................#.................................................#.....................
+..........#.........................#.....................................................#.........#..........#................#.
+........#..............#..#......................................#.........#............#.................................#.......
+.......#.........................................................#.........#................#...............................#.....
+...........#............................................#...#.............................#.....^................................#
+................#.............#...#..............................#..................................#.............................
+.........................................................................................................#........................
+.................................................#........#.......................#.#.....#......................#................
+.........#.........#...#....................................................#.....................................................
+##.##..#...................................#......................................................................................
+.................#...........................................................................................#........#...........
+.........................................................................#........................#....#......................#...
+..............#......................#........#..........................#.........#..............................................
+.......................#............................................................................##............................
+.....#.................#........#...#...............#.............................#......................#........................
+....#...................................................................................#............##...........................
+................#............................#............................................#.......................................
+.......................................................................................................#......#...................
+......................................................................................#...#.......................................
+...............................................#........#..........#............................#..............#.......#..........
+..............#.....................#........#..#............................#...#..#.#...................................#.......
+..........#...............................#............................#...........................................#..............
+....#............#.#....#......................................................................#..................................
+.......................#.......................#................................#..............#.....................#.........#..
+...........#.........................#.............#.#........#........#..........................................#...............
+.......#......................#....................#...............................#......................#...........#...........
+............#.....................................................................................................................
+....#.......#............................................................#.........#.............#..........#....#..........#.....
+.......#.........................................................................#..................#..#....#.......#.............
+............................#....#.......#......#.......#.....................#.................#.................................
+.......................................................#.............#..........................#.................................
+....................................#.....................................##......................#............................#..
+..............#...............#....................#.......................#................#......................#..............
+....................#...........................#...................................................................#.............
+...#..........................#...........................#............................................................#..........
+..............#.................#.............................#..#.......#.......................#.##.............................
+....................#.....#...#..............................................#......#...........#.................#..#....#.......
+#......................................#.....................#..#..#.#...........#.............#.................................#
+.....................#...........................................................#..........#............##..........#............
+.......#..................................#...............#..........#....#...................................................#...
+.............#.................#...........................#..............#.................................................#.....
+...............................#......................................##...................................#.#......#..........#..
+.................##....................#.................................................#......................#................#
+........#....................................#..........#.........................#..................#......................#.....
+...#....#...........................#..............................#.#..........................#.................................
+.#...................................#..#......#.................................................................#................
+.................................#...........#................#.#........................#....................................##..
+#..#.....##...............#................................................................#....#......#.........................#
+.#....#......................................................#.........................#.......................................#..
+.#..............................................#.................................................................#...............
+...............#..........................................#...#...............................#.#..#.#........#...................
+..............#................................#.......#..................................#.........................#........#....
+.......................##..................................................#......................................................
+..............................#...............................##..................................................................
+#....................#...........................................................#.............................#..................
+#..................................................................#...#..........................................................
+#..#........#...................................#..#..........##..............#...................................................
+.......................#.....................#....................................................................................
+.....................#...........#.............................................................#........................#....#....
+...............#............................................................................##...#..........................#.....
+...#......................................#.......................................................................................
+....#...........................#.........#.................................................#..........#...#........#....#........
+......#....#.#............................#..........................................#........................................#...
+...........#...#.............................................#......................#..................#.....................#....
+...#..........#........#..........#.......#..............#.............................#.......................#................#.
+...........#...............#...........................................................#..........................................
+..#..........##.................................................#...#...............#.......................#...............##....
+.......................#..............#.............#......................................................#..................#...
+..............#..#........##..........#...................................................#.............##....................#...
+...................#..................................................#................#......#.....#.........#.#...#.............
+..................#.................................................#.................................................#.........#.
+......................................#.#........#.....#...#..........#..............#.........................#...#..............
+.................................#..............#..#.....................#................#.............#..#..#...................
+.........#.......#.#..............#..#...............#.................#..................................#..#....................
+.............................#....#.......##..................................................................................##..
+.....................................................................................#........#.......#......#....................
+........#...........#..........#..........#............................#...............#........#.#...........................#...
+.#.................#...#..............#...#...................#........#...............#..........................................
+.........#............................................##........#......#.....................#.......................#............
+.......................................#..........#..#......#........#..........#..........................#....................#.
+....#......#..........................#....................#.......................................#.....###.......#..............
+.......#..................#....#.......................................................................#..........................
+....#........................#.............................................................#.......#..........................#...
+......................#.................................#..........#................#..#.........................#................
+...........#....................................................#..............#...........................#......................
+.....#................#........#...#....#........#............#...............#.......................#..#.........#..............
+#..................................................#...........................#....#...........#.................................
+....#.#.....................................#.#...............#................#....................................#.............
+............#....................#.#.#...............................................#...................##.......................
+...##..............#........................#.............#.......#...........#.#.##..............................#....##.........
+......#...........#....................................................#.........................##...............................