5 files changed, 631 insertions, 0 deletions

diff --git a/src/24/input b/src/24/input
new file mode 100644
index 0000000..8b15285
--- /dev/null
+++ b/src/24/input
@@ -0,0 +1,377 @@
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diff --git a/src/24/input-test b/src/24/input-test
new file mode 100644
index 0000000..3dc2f67
--- /dev/null
+++ b/src/24/input-test
@@ -0,0 +1,20 @@
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diff --git a/src/24/main.zig b/src/24/main.zig
new file mode 100644
index 0000000..d32abfb
--- /dev/null
+++ b/src/24/main.zig
@@ -0,0 +1,125 @@
+const std = @import("std");
+const aoc = @import("aoc");
+
+const Color = enum { BLACK, WHITE };
+const Dir = enum { E, SE, SW, W, NW, NE };
+const dirs = [_]aoc.Pos{
+    aoc.Pos{ .x = 2, .y = 0 },
+    aoc.Pos{ .x = 1, .y = -1 },
+    aoc.Pos{ .x = -1, .y = -1 },
+    aoc.Pos{ .x = -2, .y = 0 },
+    aoc.Pos{ .x = -1, .y = 1 },
+    aoc.Pos{ .x = 1, .y = 1 },
+};
+const tokens = [_][]const u8{
+    "e",
+    "se",
+    "sw",
+    "w",
+    "nw",
+    "ne",
+};
+const Tile = struct {
+    color: Color,
+};
+
+fn parseInput(tiles: *std.AutoHashMap(aoc.Pos, Tile), input: []const u8) !void {
+    var lineit = std.mem.tokenize(u8, input, "\n");
+    while (lineit.next()) |line| {
+        var pos = aoc.Pos{ .x = 0, .y = 0 };
+
+        var i: usize = 0;
+        while (i < line.len) {
+            var dir = for (tokens) |tok, ti| {
+                if (i + tok.len > line.len) continue;
+                if (std.mem.eql(u8, tok, line[i .. i + tok.len])) {
+                    i += tok.len;
+                    break @intToEnum(Dir, @intCast(u3, ti));
+                }
+            } else return aoc.Error.InvalidInput;
+            if (aoc.debug) std.debug.print("{} ", .{dir});
+            pos = pos.add(dirs[@enumToInt(dir)]);
+        }
+        if (aoc.debug) std.debug.print("=> {} {}\n", .{ pos.x, pos.y });
+
+        var tile = tiles.getEntry(pos);
+        if (tile != null) {
+            tile.?.value_ptr.color = if (tile.?.value_ptr.color == Color.WHITE) Color.BLACK else Color.WHITE;
+        } else {
+            try tiles.put(pos, Tile{ .color = Color.BLACK });
+        }
+    }
+}
+
+fn applyRule(pos: aoc.Pos, before: *std.AutoHashMap(aoc.Pos, Tile), after: *std.AutoHashMap(aoc.Pos, Tile)) !void {
+    if (after.contains(pos)) return;
+    const old_tile = before.get(pos);
+    const old_color = if (old_tile == null) Color.WHITE else old_tile.?.color;
+
+    var adj: u32 = 0;
+    for (dirs) |d| {
+        const tile = before.get(pos.add(d));
+        if (tile != null and tile.?.color == Color.BLACK) adj += 1;
+    }
+
+    if (adj == 2 and old_color == Color.WHITE) {
+        try after.put(pos, Tile{ .color = Color.BLACK });
+    } else if ((adj == 0 or adj > 2) and old_color == Color.BLACK) {
+        try after.put(pos, Tile{ .color = Color.WHITE });
+    } else {
+        try after.put(pos, Tile{ .color = old_color });
+    }
+}
+
+fn doRound(allocator: std.mem.Allocator, tiles: *std.AutoHashMap(aoc.Pos, Tile)) !void {
+    var result = std.AutoHashMap(aoc.Pos, Tile).init(allocator);
+    defer result.deinit();
+
+    var mapit = tiles.iterator();
+    while (mapit.next()) |kv| {
+        for (dirs) |d| {
+            try applyRule(kv.key_ptr.add(d), tiles, &result);
+        }
+        try applyRule(kv.key_ptr.*, tiles, &result);
+    }
+
+    std.mem.swap(std.AutoHashMap(aoc.Pos, Tile), &result, tiles);
+}
+
+fn countBlack(tiles: *std.AutoHashMap(aoc.Pos, Tile)) u32 {
+    var count: u32 = 0;
+    var mapit = tiles.iterator();
+    while (mapit.next()) |kv| {
+        if (kv.value_ptr.color == Color.BLACK) count += 1;
+    }
+    return count;
+}
+
+fn part1(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
+    _ = args;
+
+    var tiles = std.AutoHashMap(aoc.Pos, Tile).init(allocator);
+    defer tiles.deinit();
+
+    try parseInput(&tiles, input);
+
+    return try std.fmt.allocPrint(allocator, "{}", .{countBlack(&tiles)});
+}
+
+fn part2(allocator: std.mem.Allocator, input: []u8, args: [][]u8) !?[]u8 {
+    _ = args;
+
+    var tiles = std.AutoHashMap(aoc.Pos, Tile).init(allocator);
+    defer tiles.deinit();
+
+    try parseInput(&tiles, input);
+
+    var round: usize = 0;
+    while (round < 100) : (round += 1) {
+        try doRound(allocator, &tiles);
+    }
+
+    return try std.fmt.allocPrint(allocator, "{}", .{countBlack(&tiles)});
+}
+
+pub const main = aoc.main(part1, part2, .{ "317", "3804" });
diff --git a/src/24/part1 b/src/24/part1
new file mode 100644
index 0000000..8ad7455
--- /dev/null
+++ b/src/24/part1
@@ -0,0 +1,61 @@
+--- Day 24: Lobby Layout ---
+
+Your raft makes it to the tropical island; it turns out that the small crab was an excellent
+navigator. You make your way to the resort.
+
+As you enter the lobby, you discover a small problem: the floor is being renovated. You can't even
+reach the check-in desk until they've finished installing the new tile floor.
+
+The tiles are all hexagonal; they need to be arranged in a hex grid with a very
+specific color pattern. Not in the mood to wait, you offer to help figure out the pattern.
+
+The tiles are all white on one side and black on the other. They start
+with the white side facing up. The lobby is large enough to fit whatever pattern might need to
+appear there.
+
+A member of the renovation crew gives you a list of the tiles that need to be flipped
+over (your puzzle input). Each line in the list identifies a single tile that needs to be
+flipped by giving a series of steps starting from a reference tile in the very center
+of the room. (Every line starts from the same reference tile.)
+
+Because the tiles are hexagonal, every tile has six neighbors: east, southeast,
+southwest, west, northwest, and northeast. These directions are given in your list, respectively, as
+e, se, sw, w, nw, and ne. A tile is identified by a series of these directions with no
+delimiters; for example, esenee identifies the tile you land on if you start at the reference
+tile and then move one tile east, one tile southeast, one tile northeast, and one tile east.
+
+Each time a tile is identified, it flips from white to black or from black to white. Tiles might be
+flipped more than once. For example, a line like esew flips a tile immediately adjacent to the
+reference tile, and a line like nwwswee flips the reference tile itself.
+
+Here is a larger example:
+
+sesenwnenenewseeswwswswwnenewsewsw
+neeenesenwnwwswnenewnwwsewnenwseswesw
+seswneswswsenwwnwse
+nwnwneseeswswnenewneswwnewseswneseene
+swweswneswnenwsewnwneneseenw
+eesenwseswswnenwswnwnwsewwnwsene
+sewnenenenesenwsewnenwwwse
+wenwwweseeeweswwwnwwe
+wsweesenenewnwwnwsenewsenwwsesesenwne
+neeswseenwwswnwswswnw
+nenwswwsewswnenenewsenwsenwnesesenew
+enewnwewneswsewnwswenweswnenwsenwsw
+sweneswneswneneenwnewenewwneswswnese
+swwesenesewenwneswnwwneseswwne
+enesenwswwswneneswsenwnewswseenwsese
+wnwnesenesenenwwnenwsewesewsesesew
+nenewswnwewswnenesenwnesewesw
+eneswnwswnwsenenwnwnwwseeswneewsenese
+neswnwewnwnwseenwseesewsenwsweewe
+wseweeenwnesenwwwswnew
+
+In the above example, 10 tiles are flipped once (to black), and 5 more are flipped twice (to black,
+then back to white). After all of these instructions have been followed, a total of 10
+tiles are black.
+
+Go through the renovation crew's list and determine which tiles they need to flip. After all of the
+instructions have been followed, how many tiles are left with the black side up?
+
+
diff --git a/src/24/part2 b/src/24/part2
new file mode 100644
index 0000000..784cf49
--- /dev/null
+++ b/src/24/part2
@@ -0,0 +1,48 @@
+--- Part Two ---
+
+The tile floor in the lobby is meant to be a living art exhibit. Every day, the tiles are all
+flipped according to the following rules:
+
+
+ - Any black tile with zero or more than 2 black tiles
+immediately adjacent to it is flipped to white.
+ - Any white tile with exactly 2 black tiles immediately adjacent to it is
+flipped to black.
+
+
+Here, tiles immediately adjacent means the six tiles directly touching the tile in
+question.
+
+The rules are applied simultaneously to every tile; put another way, it is first
+determined which tiles need to be flipped, then they are all flipped at the same time.
+
+In the above example, the number of black tiles that are facing up after the given number of days
+has passed is as follows:
+
+Day 1: 15
+Day 2: 12
+Day 3: 25
+Day 4: 14
+Day 5: 23
+Day 6: 28
+Day 7: 41
+Day 8: 37
+Day 9: 49
+Day 10: 37
+
+Day 20: 132
+Day 30: 259
+Day 40: 406
+Day 50: 566
+Day 60: 788
+Day 70: 1106
+Day 80: 1373
+Day 90: 1844
+Day 100: 2208
+
+After executing this process a total of 100 times, there would be 2208 black tiles
+facing up.
+
+How many tiles will be black after 100 days?
+
+