day10?

jesus

.gitea/workflows/build.yml

@@ -12,5 +12,7 @@ jobs:
         uses: https://github.com/cachix/install-nix-action@v23
         with:
           github_access_token: ${{ secrets.INPUT_GITHUB_ACCESS_TOKEN }}
+      - name: Stub data files
+        run: touch src/data/day{01..31}.txt
       - name: Build package
         run: nix build

src/day01.zig

@@ -1,16 +1,17 @@
 const std = @import("std");
 const util = @import("util.zig");
 
+const numbers = [_][]const u8{ "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };
+
 pub fn main() !void {
     const input = @embedFile("data/day01.txt");
-    const sln = try solve(input);
+    std.debug.print("{d}\n", .{try solve(input, &[_][]u8{})});
-    std.debug.print("Solution: {d}", .{sln});
+    std.debug.print("{d}\n", .{try solve(input, &numbers)});
 }
 
-const numbers = [_][]const u8{ "zero", "one", "two", "three", "four", "five", "six", "seven", "eight", "nine" };
 const zero = @as(u8, '0');
 
-fn solve(input: []const u8) !u32 {
+fn solve(input: []const u8, words: []const []const u8) !u32 {
     var sum: u32 = 0;
 
     var it = std.mem.splitAny(u8, input, "\n");
@@ -27,9 +28,9 @@ fn solve(input: []const u8) !u32 {
             if (line[offset] >= '0' and line[offset] <= '9') {
                 n = line[offset];
             } else {
-                inline for (numbers, 0..) |word, value| {
+                for (words, 0..) |word, value| {
                     if (std.mem.startsWith(u8, line[offset..], word)) {
-                        n = value + zero;
+                        n = @as(u8, @intCast(value)) + zero;
                         break;
                     }
                 }
@@ -49,7 +50,17 @@ fn solve(input: []const u8) !u32 {
     return sum;
 }
 
-test "sample" {
+test "silver" {
+    const input =
+        \\1abc2
+        \\pqr3stu8vwx
+        \\a1b2c3d4e5f
+        \\treb7uchet
+    ;
+    try std.testing.expectEqual(@as(u32, 142), try solve(input, &[_][]u8{}));
+}
+
+test "gold" {
     const input =
         \\two1nine
         \\eightwothree
@@ -59,6 +70,5 @@ test "sample" {
         \\zoneight234
         \\7pqrstsixteen
     ;
-    const sln = try solve(input);
+    try std.testing.expectEqual(@as(u32, 281), try solve(input, &numbers));
-    try std.testing.expectEqual(@as(u32, 281), sln);
 }

src/day02.zig

@@ -0,0 +1,94 @@
+const std = @import("std");
+
+pub fn main() !void {
+    const input = @embedFile("data/day02.txt");
+    const target = Game{ .red = 12, .green = 13, .blue = 14 };
+    const sln = try solve(input, target);
+
+    std.debug.print("{d}\n", .{sln.possible});
+    std.debug.print("{d}\n", .{sln.powers});
+}
+
+const Day02Error = error{
+    MissingColon,
+};
+
+const Game = struct {
+    red: u32 = 0,
+    blue: u32 = 0,
+    green: u32 = 0,
+
+    fn parse(line: []const u8) !@This() {
+        var self = @This(){};
+        var n: u32 = 0;
+
+        const colon = std.mem.indexOfScalar(u8, line, ':') orelse return Day02Error.MissingColon;
+        var it = std.mem.splitAny(u8, line[colon + 1 ..], ",; ");
+        while (it.next()) |item| {
+            if (item.len == 0) {
+                continue;
+            }
+
+            switch (item[0]) {
+                'r' => self.red = @max(self.red, n),
+                'g' => self.green = @max(self.green, n),
+                'b' => self.blue = @max(self.blue, n),
+                '0'...'9' => n = try std.fmt.parseInt(u8, item, 10),
+                else => unreachable,
+            }
+        }
+
+        return self;
+    }
+
+    fn possible(self: @This(), other: @This()) bool {
+        return self.red <= other.red and self.green <= other.green and self.blue <= other.blue;
+    }
+};
+
+const Solution = struct {
+    possible: u32,
+    powers: u32,
+};
+
+fn solve(input: []const u8, target: Game) !Solution {
+    var possible: u32 = 0;
+    var powers: u32 = 0;
+
+    var index: u32 = 1;
+    var line_it = std.mem.splitScalar(u8, input, '\n');
+    while (line_it.next()) |line| : (index += 1) {
+        if (line.len == 0) {
+            continue;
+        }
+
+        const game = try Game.parse(line);
+        if (game.possible(target)) {
+            possible += index;
+        }
+        powers += game.red * game.blue * game.green;
+    }
+
+    return Solution{ .possible = possible, .powers = powers };
+}
+
+fn solveSample() !Solution {
+    const input =
+        \\Game 1: 3 blue, 4 red; 1 red, 2 green, 6 blue; 2 green
+        \\Game 2: 1 blue, 2 green; 3 green, 4 blue, 1 red; 1 green, 1 blue
+        \\Game 3: 8 green, 6 blue, 20 red; 5 blue, 4 red, 13 green; 5 green, 1 red
+        \\Game 4: 1 green, 3 red, 6 blue; 3 green, 6 red; 3 green, 15 blue, 14 red
+        \\Game 5: 6 red, 1 blue, 3 green; 2 blue, 1 red, 2 green
+    ;
+    return solve(input, .{ .red = 12, .blue = 14, .green = 13 });
+}
+
+test "silver" {
+    const sln = try solveSample();
+    try std.testing.expectEqual(@as(u32, 8), sln.possible);
+}
+
+test "gold" {
+    const sln = try solveSample();
+    try std.testing.expectEqual(@as(u32, 2286), sln.powers);
+}

src/day03.zig

@@ -0,0 +1,145 @@
+const std = @import("std");
+const util = @import("util.zig");
+
+pub fn main() !void {
+    const input = @embedFile("data/day03.txt");
+    const sln = try solve(util.gpa, input);
+    std.debug.print("{d}\n", .{sln.sum});
+    std.debug.print("{d}\n", .{sln.gears});
+}
+
+const Part = struct {
+    value: usize,
+    line: usize,
+    start: usize,
+    end: usize,
+};
+
+const Symbol = struct { char: u8, line: usize, offset: usize };
+
+const Solution = struct {
+    sum: usize,
+    gears: usize,
+};
+
+fn absDiff(comptime T: type, a: T, b: T) T {
+    return @max(a, b) - @min(a, b);
+}
+
+fn solve(alloc: std.mem.Allocator, input: []const u8) !Solution {
+    var parts = std.ArrayList(Part).init(alloc);
+    var symbols = std.ArrayList(Symbol).init(alloc);
+
+    defer parts.deinit();
+    defer symbols.deinit();
+
+    // Accumulate parts and symbols.
+    var it = std.mem.splitScalar(u8, input, '\n');
+    var lnum: usize = 0;
+    while (it.next()) |line| : (lnum += 1) {
+        var current: Part = undefined;
+        var in_part = false;
+
+        var offset: usize = 0;
+        for (line) |char| {
+            switch (char) {
+                '0'...'9' => {
+                    if (!in_part) {
+                        current.line = lnum;
+                        current.start = offset;
+                        current.value = 0;
+                        in_part = true;
+                    }
+                    current.value = current.value * 10 + (char - '0');
+                },
+                '.' => {
+                    if (in_part) {
+                        current.end = offset - 1;
+                        try parts.append(current);
+                    }
+                    in_part = false;
+                },
+                else => {
+                    if (in_part) {
+                        current.end = offset - 1;
+                        try parts.append(current);
+                    }
+                    in_part = false;
+                    try symbols.append(.{ .char = char, .line = lnum, .offset = offset });
+                },
+            }
+            offset += 1;
+        }
+
+        // Handle number at the end of a line.
+        if (in_part) {
+            current.end = offset - 1;
+            try parts.append(current);
+        }
+    }
+
+    // Locate actual parts.
+    var sum: usize = 0;
+    var gear_sum: usize = 0;
+    var gears = std.AutoHashMap(Symbol, Part).init(alloc);
+    defer gears.deinit();
+
+    ploop: for (parts.items) |part| {
+        // std.debug.print("part {d}x{d}..{d}\n", .{ part.line, part.start, part.end });
+        for (symbols.items) |symbol| {
+            // std.debug.print("symbol {d}x{d}\n", .{ symbol.line, symbol.offset });
+            const inOffset = part.start <= symbol.offset + 1 and part.end + 1 >= symbol.offset;
+            const inLine = absDiff(usize, part.line, symbol.line) <= 1;
+            if (inOffset and inLine) {
+                // std.debug.print("part {d}x({d}..{d})={d}\n", .{ part.line, part.start, part.end, part.value });
+                sum += part.value;
+
+                // Detect gears.
+                if (symbol.char == '*') {
+                    if (gears.get(symbol)) |other_gear| {
+                        gear_sum += part.value * other_gear.value;
+                        _ = gears.remove(symbol);
+                    } else {
+                        try gears.put(symbol, part);
+                    }
+                }
+
+                continue :ploop;
+            }
+        }
+    }
+
+    return .{ .sum = sum, .gears = gear_sum };
+}
+
+test "silver" {
+    const input =
+        \\467..114..
+        \\...*......
+        \\..35..633.
+        \\......#...
+        \\617*......
+        \\.....+.58.
+        \\..592.....
+        \\......755.
+        \\...$.*....
+        \\664..598..
+    ;
+    try std.testing.expectEqual(@as(usize, 4361), (try solve(std.testing.allocator, input)).sum);
+}
+
+test "gold" {
+    const input =
+        \\467..114..
+        \\...*......
+        \\..35..633.
+        \\......#...
+        \\617*......
+        \\.....+.58.
+        \\..592.....
+        \\......755.
+        \\...$.*....
+        \\.664.598..
+    ;
+    try std.testing.expectEqual(@as(usize, 467835), (try solve(std.testing.allocator, input)).gears);
+}

src/day04.zig

@@ -0,0 +1,99 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day04.txt");
+    const sln = try solve(util.gpa, input);
+    std.debug.print("{d}\n", .{sln.value});
+    std.debug.print("{d}\n", .{sln.copies});
+}
+
+const Solution = struct {
+    value: usize,
+    copies: usize,
+};
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var value: usize = 0;
+    var card_counts = std.AutoHashMap(usize, usize).init(alloc);
+    defer card_counts.deinit();
+
+    var card: usize = 0;
+    var it = util.splitLines(input);
+    while (it.next()) |line| : (card += 1) {
+        if (line.len == 0) continue;
+
+        const sep = mem.indexOfScalar(u8, line, ':').?;
+        var line_it = util.split(line[sep + 1 ..], '|');
+        const winning = try util.parseIntsScalar(usize, alloc, line_it.next().?, .{});
+        defer alloc.free(winning);
+        const mine = try util.parseIntsScalar(usize, alloc, line_it.next().?, .{});
+        defer alloc.free(mine);
+
+        var matches: usize = 0;
+        for (mine) |n| {
+            if (mem.containsAtLeast(usize, winning, 1, &[_]usize{n})) {
+                matches += 1;
+            }
+        }
+        if (matches > 0) {
+            value += std.math.pow(usize, 2, matches - 1);
+        }
+
+        // Initialise with 1 copy of current card
+        const copies = blk: {
+            const entry = try card_counts.getOrPut(card);
+            if (entry.found_existing) {
+                entry.value_ptr.* += 1;
+            } else {
+                entry.value_ptr.* = 1;
+            }
+            break :blk entry.value_ptr.*;
+        };
+
+        // 1 additional card for each match, multiplied by copies of this card
+        for (1..matches + 1) |offset| {
+            const winning_card_entry = try card_counts.getOrPut(card + offset);
+            if (winning_card_entry.found_existing) {
+                winning_card_entry.value_ptr.* += copies;
+            } else {
+                winning_card_entry.value_ptr.* = copies;
+            }
+        }
+    }
+
+    var copy_count: usize = 0;
+    var copy_it = card_counts.valueIterator();
+    while (copy_it.next()) |v| {
+        copy_count += v.*;
+    }
+
+    return .{ .value = value, .copies = copy_count };
+}
+
+test "silver" {
+    const input =
+        \\Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+        \\Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+        \\Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+        \\Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+        \\Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+        \\Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 13), sln.value);
+}
+
+test "gold" {
+    const input =
+        \\Card 1: 41 48 83 86 17 | 83 86  6 31 17  9 48 53
+        \\Card 2: 13 32 20 16 61 | 61 30 68 82 17 32 24 19
+        \\Card 3:  1 21 53 59 44 | 69 82 63 72 16 21 14  1
+        \\Card 4: 41 92 73 84 69 | 59 84 76 51 58  5 54 83
+        \\Card 5: 87 83 26 28 32 | 88 30 70 12 93 22 82 36
+        \\Card 6: 31 18 13 56 72 | 74 77 10 23 35 67 36 11
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 30), sln.copies);
+}

src/day05.zig

@@ -0,0 +1,223 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day05.txt");
+    const sln = try solve(util.gpa, input);
+    std.debug.print("{d}\n", .{sln});
+    std.debug.print("{d}\n", .{sln});
+}
+
+fn Range(comptime T: type) type {
+    return struct {
+        start: T,
+        end: T,
+
+        const Self = @This();
+
+        /// Returns the intersection of two ranges.
+        pub fn intersect(self: Self, other: Self) ?Self {
+            const a = if (self.start < other.start) self else other;
+            const b = if (self.start < other.start) other else self;
+
+            if (a.start == a.end or b.start == b.end) return null; // Points don't intersect.
+
+            if (a.end < b.start) return null; // No intersection.
+            if (a.end > b.end) return b; // A is a superset of B.
+            return .{ .start = b.start, .end = a.end };
+        }
+
+        /// Returns the union of two ranges.
+        pub fn join(self: Self, other: Self) ?Self {
+            if (self.intersect(other) == null) return null; // Can't union disjoint ranges.
+            // Now guaranteed that they overlap.
+            return .{ .start = @min(self.start, other.start), .end = @max(self.end, other.end) };
+        }
+    };
+}
+
+const Map = struct {
+    offset: isize,
+    source: Range(usize),
+};
+
+const Component = struct {
+    stage: usize,
+    range: Range(usize),
+};
+
+fn mergeRanges(ranges: *std.ArrayList(Component)) void {
+    var i: usize = 0;
+    while (i < ranges.items.len) : (i += 1) {
+        var item = ranges.items[i];
+        var j = i + 1;
+        while (j < ranges.items.len) {
+            const next = ranges.items[j];
+            if (item.range.join(next.range)) |merged| {
+                const merged_item = .{ .stage = item.stage, .range = merged };
+                item = merged_item;
+                ranges.items[i] = merged_item;
+                _ = ranges.swapRemove(j);
+            } else {
+                j += 1;
+            }
+        }
+    }
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !usize {
+    var it = util.splitLines(input);
+    const seed_line = it.next().?;
+    const seeds = try util.parseIntsScalar(usize, alloc, seed_line[mem.indexOfScalar(u8, seed_line, ':').? + 1 ..], .{});
+    defer alloc.free(seeds);
+
+    var ranges = std.ArrayList(Component).init(alloc);
+    defer ranges.deinit();
+
+    {
+        var index: usize = 0;
+        while (index < seeds.len) : (index += 2) {
+            try ranges.append(.{
+                .stage = 0,
+                .range = .{
+                    .start = seeds[index],
+                    .end = seeds[index] + seeds[index + 1],
+                },
+            });
+        }
+    }
+
+    var current_stage: usize = 0;
+    while (it.next()) |line| {
+        if (line.len == 0) {
+            for (ranges.items, 0..) |item, index| {
+                if (item.stage != current_stage) {
+                    ranges.items[index] = .{ .stage = current_stage, .range = item.range };
+                }
+            }
+            mergeRanges(&ranges);
+        } else if (mem.endsWith(u8, line, "map:")) {
+            current_stage += 1;
+        } else {
+            const map_line = try util.parseIntsScalar(usize, alloc, line, .{});
+            defer alloc.free(map_line);
+            const map = Map{
+                .offset = @as(isize, @intCast(map_line[0])) - @as(isize, @intCast(map_line[1])),
+                .source = .{ .start = map_line[1], .end = map_line[1] + map_line[2] },
+            };
+
+            var i: usize = ranges.items.len;
+            while (i != 0) : (i -= 1) {
+                if (ranges.items[i - 1].stage == current_stage) continue;
+                const item = ranges.swapRemove(i - 1);
+
+                if (item.range.intersect(.{ .start = 0, .end = map.source.start })) |section| {
+                    // Non-transitioning section.
+                    try ranges.append(.{ .stage = item.stage, .range = section });
+                }
+                if (item.range.intersect(map.source)) |section| {
+                    // Transitioning section.
+                    try ranges.append(.{
+                        .stage = current_stage,
+                        .range = .{
+                            .start = @intCast(@as(isize, @intCast(section.start)) + map.offset),
+                            .end = @intCast(@as(isize, @intCast(section.end)) + map.offset),
+                        },
+                    });
+                }
+                if (item.range.intersect(.{ .start = map.source.end, .end = std.math.maxInt(usize) })) |section| {
+                    // Non-transitioning section.
+                    try ranges.append(.{ .stage = item.stage, .range = section });
+                }
+            }
+        }
+    }
+
+    var min_loc: usize = std.math.maxInt(usize);
+    for (ranges.items) |item| {
+        min_loc = @min(min_loc, item.range.start);
+    }
+    return min_loc;
+}
+
+test "silver" {
+    const input =
+        \\seeds: 79 14 55 13
+        \\
+        \\seed-to-soil map:
+        \\50 98 2
+        \\52 50 48
+        \\
+        \\soil-to-fertilizer map:
+        \\0 15 37
+        \\37 52 2
+        \\39 0 15
+        \\
+        \\fertilizer-to-water map:
+        \\49 53 8
+        \\0 11 42
+        \\42 0 7
+        \\57 7 4
+        \\
+        \\water-to-light map:
+        \\88 18 7
+        \\18 25 70
+        \\
+        \\light-to-temperature map:
+        \\45 77 23
+        \\81 45 19
+        \\68 64 13
+        \\
+        \\temperature-to-humidity map:
+        \\0 69 1
+        \\1 0 69
+        \\
+        \\humidity-to-location map:
+        \\60 56 37
+        \\56 93 4
+    ;
+    _ = input;
+    // const sln = try solve(std.testing.allocator, input);
+    // try std.testing.expectEqual(@as(usize, 35), sln);
+}
+
+test "gold" {
+    const input =
+        \\seeds: 79 14 55 13
+        \\
+        \\seed-to-soil map:
+        \\50 98 2
+        \\52 50 48
+        \\
+        \\soil-to-fertilizer map:
+        \\0 15 37
+        \\37 52 2
+        \\39 0 15
+        \\
+        \\fertilizer-to-water map:
+        \\49 53 8
+        \\0 11 42
+        \\42 0 7
+        \\57 7 4
+        \\
+        \\water-to-light map:
+        \\88 18 7
+        \\18 25 70
+        \\
+        \\light-to-temperature map:
+        \\45 77 23
+        \\81 45 19
+        \\68 64 13
+        \\
+        \\temperature-to-humidity map:
+        \\0 69 1
+        \\1 0 69
+        \\
+        \\humidity-to-location map:
+        \\60 56 37
+        \\56 93 4
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 46), sln);
+}

src/day06.zig

@@ -0,0 +1,73 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day06.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,
+};
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var result: usize = 1;
+    {
+        var it = util.splitLines(input);
+        const times = try util.parseIntsScalar(usize, alloc, it.next().?[10..], .{});
+        const distances = try util.parseIntsScalar(usize, alloc, it.next().?[10..], .{});
+        defer alloc.free(times);
+        defer alloc.free(distances);
+
+        for (times, distances) |time, minDistance| {
+            var winning: usize = 0;
+            for (0..time - 1) |speed| {
+                const distance = speed * (time - speed);
+                if (distance > minDistance) winning += 1;
+            }
+            result *= winning;
+        }
+    }
+
+    var result2: usize = 1;
+    {
+        var it = util.splitLines(input);
+        var td: [2]usize = undefined;
+        for (0..td.len) |i| {
+            const line = try mem.replaceOwned(u8, alloc, it.next().?[10..], " ", "");
+            defer alloc.free(line);
+            td[i] = try std.fmt.parseInt(usize, line, 10);
+        }
+
+        var winning2: usize = 0;
+        for (0..td[0] - 1) |speed| {
+            const distance = speed * (td[0] - speed);
+            if (distance > td[1]) winning2 += 1;
+        }
+        result2 *= winning2;
+    }
+
+    return .{ .a = result, .b = result2 };
+}
+
+test "silver" {
+    const input =
+        \\Time:      7  15   30
+        \\Distance:  9  40  200
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 288), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\Time:      7  15   30
+        \\Distance:  9  40  200
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 71503), sln.b);
+}

src/day07.zig

@@ -0,0 +1,132 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day07.txt");
+    const part1 = try solve(util.gpa, input, jack);
+    const part2 = try solve(util.gpa, input, joker);
+    std.debug.print("{d}\n", .{part1});
+    std.debug.print("{d}\n", .{part2});
+}
+
+const Cards = [5]u8;
+
+const Score = enum(u8) {
+    fiveKind,
+    fourKind,
+    fullHouse,
+    threeKind,
+    twoPair,
+    onePair,
+    high,
+};
+
+const Hand = struct {
+    cards: [5]u8,
+    bid: u32,
+    score: Score,
+};
+
+const jack = 10;
+const joker = 0;
+
+fn fixCards(cards: *[5]u8, jValue: u8) void {
+    for (cards, 0..) |card, i| {
+        cards[i] = switch (card) {
+            'A' => 13,
+            'K' => 12,
+            'Q' => 11,
+            'J' => jValue,
+            'T' => 9,
+            '2'...'9' => card - '1',
+            else => unreachable,
+        };
+    }
+}
+
+fn u8gt(context: void, a: u8, b: u8) bool {
+    _ = context;
+    return a > b;
+}
+
+fn scoreHand(cards: Cards) Score {
+    var jokers: usize = 0;
+    var dups: [14]u8 = undefined;
+    @memset(&dups, 0);
+    for (cards) |a| {
+        if (a == joker) {
+            jokers += 1;
+        } else {
+            dups[a] += 1;
+        }
+    }
+    mem.sort(u8, &dups, {}, u8gt);
+    return switch (dups[0] + jokers) {
+        5 => .fiveKind,
+        4 => .fourKind,
+        3 => if (dups[1] == 2) .fullHouse else .threeKind,
+        2 => if (dups[1] == 2) .twoPair else .onePair,
+        else => .high,
+    };
+}
+
+fn compareHand(context: void, a: Hand, b: Hand) bool {
+    _ = context;
+    if (a.score != b.score) {
+        return @intFromEnum(a.score) > @intFromEnum(b.score);
+    } else {
+        for (a.cards, b.cards) |cardA, cardB| {
+            if (cardA != cardB) return cardA < cardB;
+        }
+    }
+    unreachable;
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8, jValue: u8) !u32 {
+    var hands = try std.ArrayList(Hand).initCapacity(alloc, 1001);
+    defer hands.deinit();
+
+    var it = util.splitLines(input);
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        var cards = line[0..5].*;
+        fixCards(&cards, jValue);
+        const bid = try std.fmt.parseInt(u32, line[6..], 10);
+        const score = scoreHand(cards);
+        try hands.append(.{ .cards = cards, .bid = bid, .score = score });
+    }
+
+    mem.sort(Hand, hands.items, {}, compareHand);
+
+    var winnings: u32 = 0;
+    for (hands.items, 1..) |hand, rank| {
+        winnings += hand.bid * @as(u32, @intCast(rank));
+    }
+
+    return winnings;
+}
+
+test "silver" {
+    const input =
+        \\32T3K 765
+        \\T55J5 684
+        \\KK677 28
+        \\KTJJT 220
+        \\QQQJA 483
+    ;
+    const sln = try solve(std.testing.allocator, input, jack);
+    try std.testing.expectEqual(@as(usize, 6440), sln);
+}
+
+test "gold" {
+    const input =
+        \\32T3K 765
+        \\T55J5 684
+        \\KK677 28
+        \\KTJJT 220
+        \\QQQJA 483
+    ;
+    const sln = try solve(std.testing.allocator, input, joker);
+    try std.testing.expectEqual(@as(usize, 5905), sln);
+}

src/day08.zig

@@ -0,0 +1,104 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day08.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 Step = struct {
+    left: [3]u8,
+    right: [3]u8,
+};
+
+fn gcd(a: usize, b: usize) usize {
+    return if (b == 0) a else gcd(b, a % b);
+}
+
+fn lcm(ns: []u32) usize {
+    var acc: usize = ns[0];
+    for (ns[1..]) |n| {
+        const nsize = @as(usize, n);
+        acc = acc / gcd(acc, nsize) * nsize;
+    }
+    return acc;
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var states = std.AutoHashMap([3]u8, Step).init(alloc);
+    defer states.deinit();
+
+    var it = util.splitLines(input);
+    const steps = it.next().?;
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        try states.put(line[0..3].*, .{ .left = line[7..10].*, .right = line[12..15].* });
+    }
+
+    var a_nodes = std.AutoHashMap([3]u8, Step).init(alloc);
+    defer a_nodes.deinit();
+    var state_it = states.iterator();
+    while (state_it.next()) |entry| {
+        if (entry.key_ptr[2] == 'A') {
+            try a_nodes.put(entry.key_ptr.*, entry.value_ptr.*);
+        }
+    }
+
+    var all_steps = std.ArrayList(u32).init(alloc);
+    defer all_steps.deinit();
+    var a_it = a_nodes.iterator();
+    while (a_it.next()) |start| {
+        std.debug.print("{s}\n", .{start.key_ptr});
+        var position: [3]u8 = start.key_ptr.*;
+        var count: u32 = 0;
+        find: while (position[2] != 'Z') : (count += 1) {
+            const curr_state = states.get(position) orelse break :find; // ???????
+            position = if (steps[count % steps.len] == 'L') curr_state.left else curr_state.right;
+        }
+        try all_steps.append(count);
+    }
+
+    return .{ .a = 2, .b = lcm(all_steps.items) };
+}
+
+test "silver" {
+    const input =
+        \\RL
+        \\
+        \\AAA = (BBB, CCC)
+        \\BBB = (DDD, EEE)
+        \\CCC = (ZZZ, GGG)
+        \\DDD = (DDD, DDD)
+        \\EEE = (EEE, EEE)
+        \\GGG = (GGG, GGG)
+        \\ZZZ = (ZZZ, ZZZ)
+    ;
+    _ = input;
+    // const sln = try solve(std.testing.allocator, input);
+    // try std.testing.expectEqual(@as(usize, 2), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\LR
+        \\
+        \\11A = (11B, XXX)
+        \\11B = (XXX, 11Z)
+        \\11Z = (11B, XXX)
+        \\22A = (22B, XXX)
+        \\22B = (22C, 22C)
+        \\22C = (22Z, 22Z)
+        \\22Z = (22B, 22B)
+        \\XXX = (XXX, XXX)
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 6), sln.b);
+}

src/day09.zig

@@ -0,0 +1,80 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day09.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: isize,
+    b: isize,
+};
+
+fn drv(alloc: mem.Allocator, seq: []const isize) ![]const isize {
+    var list = try std.ArrayList(isize).initCapacity(alloc, seq.len - 1);
+    for (0..seq.len - 1) |i| {
+        try list.append(seq[i + 1] - seq[i]);
+    }
+    return list.toOwnedSlice();
+}
+
+fn zeroed(seq: []const isize) bool {
+    for (seq) |n| {
+        if (n != 0) return false;
+    }
+    return true;
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var it = util.splitLines(input);
+    var prev_sum: isize = 0;
+    var next_sum: isize = 0;
+
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        var seqs = std.ArrayList([]const isize).init(alloc);
+        defer seqs.deinit();
+        // Find zero derivation
+        try seqs.append(try util.parseIntsScalar(isize, alloc, line, .{}));
+        while (!zeroed(seqs.items[seqs.items.len - 1])) {
+            try seqs.append(try drv(alloc, seqs.items[seqs.items.len - 1]));
+        }
+        // Extrapolate
+        var next: isize = 0;
+        var prev: isize = 0;
+        for (0..seqs.items.len) |i| {
+            const seq = seqs.items[seqs.items.len - i - 1];
+            prev = seq[0] - prev;
+            next += seq[seq.len - 1];
+            alloc.free(seq);
+        }
+        prev_sum += prev;
+        next_sum += next;
+    }
+
+    return .{ .a = next_sum, .b = prev_sum };
+}
+
+test "silver" {
+    const input =
+        \\0 3 6 9 12 15
+        \\1 3 6 10 15 21
+        \\10 13 16 21 30 45
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(isize, 114), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\0 3 6 9 12 15
+        \\1 3 6 10 15 21
+        \\10 13 16 21 30 45
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(isize, 2), sln.b);
+}

src/day10.zig

@@ -0,0 +1,170 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+const assert = std.debug.assert;
+
+pub fn main() !void {
+    const input = @embedFile("data/day10.txt");
+    const sln = try solve(util.gpa, input);
+    try printPipes(std.io.getStdOut(), 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(u2) {
+    up,
+    down,
+    left,
+    right,
+};
+
+fn Walker(comptime T: type) type {
+    return struct {
+        r: T,
+        c: T,
+        facing: Direction,
+
+        fn init(r: T, c: T) @This() {
+            return .{ .r = r, .c = c, .facing = .up };
+        }
+
+        fn walk(self: *@This(), grid: [][]u8) void {
+            self.walkSpace(grid[self.r][self.c]);
+        }
+
+        fn walkSpace(self: *@This(), char: u8) void {
+            switch (char) {
+                'S' => {},
+                '|' => assert(self.facing == .up or self.facing == .down),
+                '-' => assert(self.facing == .left or self.facing == .right),
+                'L' => switch (self.facing) {
+                    .down => self.turn(.right),
+                    .left => self.turn(.up),
+                    else => unreachable,
+                },
+                'J' => switch (self.facing) {
+                    .down => self.turn(.left),
+                    .right => self.turn(.up),
+                    else => unreachable,
+                },
+                '7' => switch (self.facing) {
+                    .up => self.turn(.left),
+                    .right => self.turn(.down),
+                    else => unreachable,
+                },
+                'F' => switch (self.facing) {
+                    .up => self.turn(.right),
+                    .left => self.turn(.down),
+                    else => unreachable,
+                },
+                else => unreachable,
+            }
+            self.fwd();
+        }
+
+        fn move(self: *@This(), row: i32, col: i32) void {
+            self.*.r += row;
+            self.*.c += col;
+        }
+
+        fn fwd(self: *@This()) void {
+            switch (self.facing) {
+                .up => self.move(-1, 0),
+                .down => self.move(1, 0),
+                .left => self.move(0, -1),
+                .right => self.move(0, 1),
+            }
+        }
+
+        fn turn(self: *@This(), dir: Direction) void {
+            self.*.facing = dir;
+        }
+    };
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    _ = alloc;
+    var grid: [][]u8 = undefined; // try alloc.alloc(u8, mem.count(u8, input, &[_]u8{'\n'}));
+
+    var s_found = false;
+    var sr: u32 = 0;
+    var sc: u32 = 0;
+
+    var it = util.splitLines(input);
+    var curr_row: u32 = 0;
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        // grid[curr_row] = line;
+
+        if (!s_found) {
+            if (mem.indexOfScalar(u8, line, 'S')) |col| {
+                _ = col;
+                sr = curr_row;
+                // sc = col;
+                s_found = true;
+            }
+        }
+    }
+
+    // Find actual starting position.
+    var walker = Walker(u32).init(sr, sc);
+    if (mem.indexOfScalar(u8, "|7F", grid[sr - 1][sc])) |_| {
+        walker.facing = .up;
+    } else if (mem.indexOfScalar(u8, "|JL", grid[sr + 1][sc])) |_| {
+        walker.facing = .down;
+    } else if (mem.indexOfScalar(u8, "-LF", grid[sr][sc - 1])) |_| {
+        walker.facing = .left;
+    } else if (mem.indexOfScalar(u8, "-J7", grid[sr][sc + 1])) |_| {
+        walker.facing = .right;
+    }
+
+    var steps: u32 = 1;
+    while (true) {
+        steps += 1;
+        if (grid[walker.r][walker.c] == 'S') break;
+    }
+
+    return .{ .a = steps / 2, .b = 0 };
+}
+
+fn printPipes(writer: anytype, input: []const u8) !void {
+    for (input) |char| {
+        _ = switch (char) {
+            '|' => try writer.write("│"),
+            '-' => try writer.write("─"),
+            'L' => try writer.write("└"),
+            'J' => try writer.write("┘"),
+            '7' => try writer.write("┐"),
+            'F' => try writer.write("┌"),
+            '.' => try writer.write(" "),
+            'S' => try writer.write("o"),
+            '\n' => try writer.write("\n"),
+            else => unreachable,
+        };
+    }
+}
+
+test "silver" {
+    const input =
+        \\-L|F7
+        \\7S-7|
+        \\L|7||
+        \\-L-J|
+        \\L|-JF
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 6), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 0), sln.b);
+}

src/day11.zig

@@ -0,0 +1,155 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day11.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,
+};
+
+fn Point(comptime T: type) type {
+    return struct { x: T, y: T };
+}
+
+fn findGaps(comptime T: type, alloc: mem.Allocator, occupied: std.AutoHashMap(T, void)) ![]T {
+    var it = occupied.keyIterator();
+    var sorted_occupied = try std.ArrayList(T).initCapacity(alloc, occupied.count());
+    defer sorted_occupied.deinit();
+    while (it.next()) |item| {
+        try sorted_occupied.append(item.*);
+    }
+    mem.sort(T, sorted_occupied.items, {}, std.sort.asc(T));
+
+    var gaps = std.ArrayList(T).init(alloc);
+    errdefer gaps.deinit();
+
+    var gap: T = 0;
+    var i: usize = 0;
+    const max_occ = if (sorted_occupied.items.len != 0) std.mem.max(T, sorted_occupied.items) else 0;
+    while (gap < max_occ and i < sorted_occupied.items.len) : (gap += 1) {
+        if (sorted_occupied.items[i] != gap) {
+            try gaps.append(gap);
+        } else {
+            i += 1;
+        }
+    }
+
+    return gaps.toOwnedSlice();
+}
+
+fn expand(comptime T: type, alloc: mem.Allocator, galaxies: []Point(T), scale: T) !void {
+    var seen_cols = std.AutoHashMap(T, void).init(alloc);
+    defer seen_cols.deinit();
+    var seen_rows = std.AutoHashMap(T, void).init(alloc);
+    defer seen_rows.deinit();
+
+    for (galaxies) |galaxy| {
+        try seen_cols.put(galaxy.x, {});
+        try seen_rows.put(galaxy.y, {});
+    }
+
+    var gap_cols = try findGaps(T, alloc, seen_cols);
+    defer alloc.free(gap_cols);
+    var gap_rows = try findGaps(T, alloc, seen_rows);
+    defer alloc.free(gap_rows);
+
+    for (0..galaxies.len) |i| {
+        const galaxy = galaxies[i];
+
+        var col_offset: T = 0;
+        for (gap_cols) |gap| {
+            if (gap > galaxy.x) break;
+            col_offset += scale - 1;
+        }
+
+        var row_offset: T = 0;
+        for (gap_rows) |gap| {
+            if (gap > galaxy.y) break;
+            row_offset += scale - 1;
+        }
+
+        galaxies[i] = Point(T){ .x = galaxy.x + col_offset, .y = galaxy.y + row_offset };
+    }
+}
+
+fn absDiff(comptime T: type, a: T, b: T) T {
+    return @max(a, b) - @min(a, b);
+}
+
+fn solvePart(alloc: mem.Allocator, input: []const u8, scale: usize) !usize {
+    var galaxies = std.ArrayList(Point(usize)).init(alloc);
+    defer galaxies.deinit();
+
+    var it = util.splitLines(input);
+    var y: usize = 0;
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        var x: usize = 0;
+        for (line) |char| {
+            switch (char) {
+                '.' => {},
+                '#' => try galaxies.append(Point(usize){ .x = x, .y = y }),
+                else => unreachable,
+            }
+            x += 1;
+        }
+        y += 1;
+    }
+
+    try expand(usize, alloc, galaxies.items, scale);
+
+    var distance: usize = 0;
+    for (galaxies.items, 0..) |a, i| {
+        for (galaxies.items[i + 1 .. galaxies.items.len]) |b| {
+            const d = absDiff(usize, a.x, b.x) + absDiff(usize, a.y, b.y);
+            distance += d;
+        }
+    }
+
+    return distance;
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    return .{ .a = try solvePart(alloc, input, 2), .b = try solvePart(alloc, input, 1_000_000) };
+}
+
+test "silver" {
+    const input =
+        \\...#......
+        \\.......#..
+        \\#.........
+        \\..........
+        \\......#...
+        \\.#........
+        \\.........#
+        \\..........
+        \\.......#..
+        \\#...#.....
+    ;
+    const sln = try solvePart(std.testing.allocator, input, 2);
+    try std.testing.expectEqual(@as(usize, 374), sln);
+}
+
+test "gold" {
+    const input =
+        \\...#......
+        \\.......#..
+        \\#.........
+        \\..........
+        \\......#...
+        \\.#........
+        \\.........#
+        \\..........
+        \\.......#..
+        \\#...#.....
+    ;
+    try std.testing.expectEqual(@as(usize, 1030), try solvePart(std.testing.allocator, input, 10));
+    try std.testing.expectEqual(@as(usize, 8410), try solvePart(std.testing.allocator, input, 100));
+}

src/day12.zig

@@ -0,0 +1,163 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day12.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,
+};
+
+fn hash(alloc: mem.Allocator, pattern: []const u8, groups: []const u8, curr_group: u8) ![]u8 {
+    var a = try std.ArrayList(u8).initCapacity(alloc, pattern.len + groups.len + 1);
+    try a.appendSlice(pattern);
+    try a.appendSlice(groups);
+    try a.append(curr_group);
+    return try a.toOwnedSlice();
+}
+
+var memo: std.StringHashMap(usize) = undefined;
+
+fn clone(comptime T: type, alloc: mem.Allocator, s: []const T) ![]T {
+    var a = try alloc.alloc(T, s.len);
+    @memcpy(a, s);
+    return a;
+}
+
+fn solvePattern(alloc: mem.Allocator, pattern: []const u8, groups: []const u8, curr_group: u8) !usize {
+    const h = try hash(alloc, pattern, groups, curr_group);
+    if (memo.get(h)) |m| {
+        alloc.free(h);
+        return m;
+    }
+
+    if (pattern.len == 0) {
+        return if (groups.len == 0) 1 else 0;
+    }
+
+    const res: usize = b: {
+        switch (pattern[0]) {
+            '#' => {
+                // Too many #
+                if (groups.len == 0 or curr_group >= groups[0]) {
+                    break :b 0;
+                }
+                // Special-case last #
+                if (pattern.len == 1 and groups.len == 1 and groups[0] == curr_group + 1) {
+                    break :b 1;
+                }
+                break :b try solvePattern(alloc, pattern[1..], groups, curr_group + 1);
+            },
+            '.' => {
+                if (curr_group > 0) {
+                    if (groups.len == 0 or curr_group != groups[0]) {
+                        // Too many #
+                        break :b 0;
+                    } else {
+                        // Group done
+                        break :b try solvePattern(alloc, pattern[1..], groups[1..], 0);
+                    }
+                } else {
+                    // Not growing a group, skip.
+                    break :b try solvePattern(alloc, pattern[1..], groups, 0);
+                }
+            },
+            '?' => {
+                const p1 = try clone(u8, alloc, pattern);
+                defer alloc.free(p1);
+                const p2 = try clone(u8, alloc, pattern);
+                defer alloc.free(p2);
+
+                p1[0] = '#';
+                p2[0] = '.';
+
+                break :b try solvePattern(alloc, p1, groups, curr_group) +
+                    try solvePattern(alloc, p2, groups, curr_group);
+            },
+            else => unreachable,
+        }
+    };
+
+    try memo.put(h, res);
+
+    return res;
+}
+
+fn unfold(alloc: mem.Allocator, input: []const u8, sep: u8) ![]const u8 {
+    var buf = try std.ArrayList(u8).initCapacity(alloc, input.len * 5 + 5);
+    for (0..5) |i| {
+        if (i > 0 and sep > 0) try buf.append(sep);
+        try buf.appendSlice(input);
+    }
+    return try buf.toOwnedSlice();
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    memo = std.StringHashMap(usize).init(alloc);
+    defer memo.deinit();
+
+    var sum1: usize = 0;
+    var sum2: usize = 0;
+
+    var it = util.splitLines(input);
+    var i: usize = 1;
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        std.debug.print("line {} ", .{i});
+
+        var lit = util.splitSpace(line);
+        const pattern = lit.next().?;
+        const raw_groups = lit.next().?;
+        const groups = try util.parseIntsScalar(u8, alloc, raw_groups, .{ .sep = ',' });
+        defer alloc.free(groups);
+
+        const unfolded_pattern = try unfold(alloc, pattern, '?');
+        const unfolded_groups = try unfold(alloc, groups, 0);
+        defer alloc.free(unfolded_pattern);
+        defer alloc.free(unfolded_groups);
+
+        sum1 += try solvePattern(alloc, pattern, groups, 0);
+        sum2 += try solvePattern(alloc, unfolded_pattern, unfolded_groups, 0);
+        std.debug.print("{} {}\n", .{ sum1, sum2 });
+        i += 1;
+    }
+
+    var mkeys = memo.keyIterator();
+    while (mkeys.next()) |k| {
+        alloc.free(k.*);
+    }
+
+    return .{ .a = sum1, .b = sum2 };
+}
+
+test "silver" {
+    const input =
+        \\???.### 1,1,3
+        \\.??..??...?##. 1,1,3
+        \\?#?#?#?#?#?#?#? 1,3,1,6
+        \\????.#...#... 4,1,1
+        \\????.######..#####. 1,6,5
+        \\?###???????? 3,2,1
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 21), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\???.### 1,1,3
+        \\.??..??...?##. 1,1,3
+        \\?#?#?#?#?#?#?#? 1,3,1,6
+        \\????.#...#... 4,1,1
+        \\????.######..#####. 1,6,5
+        \\?###???????? 3,2,1
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 525152), sln.b);
+}

src/day13.zig

@@ -0,0 +1,136 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day13.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,
+};
+
+fn transpose(alloc: mem.Allocator, grid: []const []const u8) ![]const []const u8 {
+    std.debug.assert(grid.len > 0 and grid[0].len > 0);
+    var g = try alloc.alloc([]u8, grid[0].len);
+    for (0..grid[0].len) |i| {
+        g[i] = try alloc.alloc(u8, grid.len);
+    }
+    for (0..grid.len) |i| {
+        for (0..grid[i].len) |j| {
+            g[j][i] = grid[i][j];
+        }
+    }
+    return g;
+}
+
+fn findReflection(input: []const []const u8, err_tolerance: usize) ?usize {
+    if (input.len == 0) return 0;
+
+    const h = input.len;
+    const w = input[0].len;
+
+    for (0..h - 1) |scan| {
+        var offset: usize = 0;
+        var col: usize = 0;
+        var errors: usize = 0;
+        while (true) {
+            if (input[scan - offset][col] != input[scan + offset + 1][col]) {
+                errors += 1;
+            }
+            if (errors > err_tolerance + 1) {
+                break;
+            }
+            col = (col + 1) % w;
+            if (col == 0) offset += 1;
+            if (scan < offset or scan + offset + 1 >= h) {
+                if (errors == err_tolerance) return scan + 1;
+                break;
+            }
+        }
+    }
+
+    return null;
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var curr_pattern = std.ArrayList([]const u8).init(alloc);
+    defer curr_pattern.deinit();
+
+    var it = util.splitLines(input);
+    var sums = [2]usize{ 0, 0 };
+    while (it.next()) |line| {
+        if (line.len == 0) {
+            const pattern = try curr_pattern.toOwnedSlice();
+            defer alloc.free(pattern);
+            for (0..2) |et| {
+                if (findReflection(pattern, et)) |score| {
+                    sums[et] += 100 * score;
+                } else {
+                    const t = try transpose(alloc, pattern);
+                    sums[et] += findReflection(t, et) orelse unreachable;
+
+                    // Ew.
+                    for (t) |r| {
+                        alloc.free(r);
+                    }
+                    alloc.free(t);
+                }
+            }
+        } else {
+            try curr_pattern.append(line);
+        }
+    }
+
+    return .{ .a = sums[0], .b = sums[1] };
+}
+
+test "silver" {
+    const input =
+        \\#.##..##.
+        \\..#.##.#.
+        \\##......#
+        \\##......#
+        \\..#.##.#.
+        \\..##..##.
+        \\#.#.##.#.
+        \\
+        \\#...##..#
+        \\#....#..#
+        \\..##..###
+        \\#####.##.
+        \\#####.##.
+        \\..##..###
+        \\#....#..#
+        \\
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 405), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\#.##..##.
+        \\..#.##.#.
+        \\##......#
+        \\##......#
+        \\..#.##.#.
+        \\..##..##.
+        \\#.#.##.#.
+        \\
+        \\#...##..#
+        \\#....#..#
+        \\..##..###
+        \\#####.##.
+        \\#####.##.
+        \\..##..###
+        \\#....#..#
+        \\
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 400), sln.b);
+}

src/day14.zig

@@ -0,0 +1,94 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day14.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,
+};
+
+fn northWeight(alloc: mem.Allocator, grid: [][]u8) !usize {
+    var next_slot = try alloc.alloc(u32, grid[0].len);
+    defer alloc.free(next_slot);
+    @memset(next_slot, 0);
+
+    var weight: usize = 0;
+    for (grid, 0..) |line, row| {
+        for (line, 0..) |char, col| {
+            switch (char) {
+                'O' => {
+                    weight += grid.len - next_slot[col];
+                    next_slot[col] += 1;
+                },
+                '#' => next_slot[col] = @as(u32, @intCast(row)) + 1,
+                else => {},
+            }
+        }
+    }
+    return weight;
+}
+
+fn spin(grid: [][]u8, n: usize) void {
+    _ = grid;
+    for (0..n) |_| {}
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    var height = mem.count(u8, input, &[_]u8{'\n'});
+    if (input[input.len - 1] != '\n') height += 1;
+
+    var grid = try alloc.alloc([]u8, height);
+    defer alloc.free(grid);
+
+    var it = util.splitLines(input);
+    var row: u32 = 0;
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        grid[row] = try alloc.alloc(u8, line.len);
+        @memcpy(grid[row], line);
+        row += 1;
+    }
+
+    const w = try northWeight(alloc, grid);
+    spin(grid, 1000000000);
+    const v = try northWeight(alloc, grid);
+    _ = v;
+
+    for (grid) |line| {
+        alloc.free(line);
+    }
+    return .{ .a = w, .b = 0 };
+}
+
+test "silver" {
+    const input =
+        \\O....#....
+        \\O.OO#....#
+        \\.....##...
+        \\OO.#O....O
+        \\.O.....O#.
+        \\O.#..O.#.#
+        \\..O..#O..O
+        \\.......O..
+        \\#....###..
+        \\#OO..#....
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 136), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\
+    ;
+    _ = input;
+    // const sln = try solve(std.testing.allocator, input);
+    // try std.testing.expectEqual(@as(usize, 0), sln.b);
+}

src/day15.zig

@@ -0,0 +1,91 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/day15.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,
+};
+
+fn hash(input: []const u8) u32 {
+    var v: u32 = 0;
+    for (input) |c| {
+        v = ((v + c) * 17) % 256;
+    }
+    return v;
+}
+
+const SAHM = std.StringArrayHashMap(u8);
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    const trimmed = mem.trimRight(u8, input, "\n\r");
+
+    var hash_sum: u32 = 0;
+    var it = util.split(trimmed, ',');
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        hash_sum += hash(line);
+    }
+
+    var boxes: [256]?SAHM = [_]?SAHM{null} ** 256;
+    it = util.split(trimmed, ',');
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+        const op = mem.indexOfAny(u8, line, "-=") orelse unreachable;
+        const label = line[0..op];
+        const label_hash = hash(label);
+
+        var box = &(boxes[label_hash] orelse init: {
+            var b = SAHM.init(alloc);
+            boxes[label_hash] = b;
+            break :init b;
+        });
+
+        switch (line[op]) {
+            '-' => _ = box.orderedRemove(label),
+            '=' => try box.put(label, try std.fmt.parseInt(u8, line[op + 1 ..], 10)),
+            else => unreachable,
+        }
+    }
+
+    var focus: usize = 0;
+    for (boxes, 1..) |maybe_box, box_n| {
+        var box = maybe_box orelse continue;
+        var box_it = box.iterator();
+        var lens_n: usize = 1;
+        while (box_it.next()) |entry| {
+            focus += box_n * lens_n * entry.value_ptr.*;
+            lens_n += 1;
+        }
+        box.deinit();
+    }
+
+    return .{ .a = hash_sum, .b = focus };
+}
+
+test "sample hash" {
+    try std.testing.expectEqual(@as(usize, 52), hash("HASH"));
+}
+
+test "silver" {
+    const input =
+        \\rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 1320), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\rn=1,cm-,qp=3,cm=2,qp-,pc=4,ot=9,ab=5,pc-,pc=6,ot=7
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 145), sln.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);
+}

src/util.zig

@@ -1,4 +1,65 @@
 const std = @import("std");
+const mem = std.mem;
 
-const gpa_impl = std.heap.GeneralPurposeAllocator(.{});
+var gpa_impl = std.heap.GeneralPurposeAllocator(.{}){};
+/// General purpose allocator.
 pub const gpa = gpa_impl.allocator();
+
+pub const ParseIntsOptions = struct {
+    base: u8 = 10,
+    sep: u8 = ' ',
+};
+
+/// Parses separated integers. Sequential separators are treated as one. Caller owns memory.
+pub fn parseIntsScalar(comptime T: type, alloc: std.mem.Allocator, input: []const u8, opts: ParseIntsOptions) ![]T {
+    var items = std.ArrayList(T).init(alloc);
+    errdefer items.deinit();
+
+    var it = split(input, opts.sep);
+    while (it.next()) |i| {
+        if (i.len == 0) continue;
+        try items.append(try std.fmt.parseInt(T, i, opts.base));
+    }
+    return try items.toOwnedSlice();
+}
+
+/// Shortcut to splitScalar with a byte buffer.
+pub fn split(buffer: []const u8, sep: u8) mem.SplitIterator(u8, .scalar) {
+    return mem.splitScalar(u8, buffer, sep);
+}
+
+pub fn splitLines(buffer: []const u8) mem.SplitIterator(u8, .scalar) {
+    return split(buffer, '\n');
+}
+
+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;
+}

template.zig

@@ -0,0 +1,42 @@
+const std = @import("std");
+const util = @import("util.zig");
+const mem = std.mem;
+
+pub fn main() !void {
+    const input = @embedFile("data/dayXX.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,
+};
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    _ = alloc;
+
+    var it = util.splitLines(input);
+    while (it.next()) |line| {
+        if (line.len == 0) continue;
+    }
+
+    return .{ .a = 0, .b = 0 };
+}
+
+test "silver" {
+    const input =
+        \\
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 0), sln.a);
+}
+
+test "gold" {
+    const input =
+        \\
+    ;
+    const sln = try solve(std.testing.allocator, input);
+    try std.testing.expectEqual(@as(usize, 0), sln.b);
+}