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/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/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

@@ -35,3 +35,31 @@ pub fn splitLines(buffer: []const u8) mem.SplitIterator(u8, .scalar) {
 pub fn splitSpace(buffer: []const u8) mem.SplitIterator(u8, .scalar) {
     return split(buffer, ' ');
 }
+
+pub fn allocGrid(comptime T: type, alloc: mem.Allocator, w: usize, h: usize, init: T) ![][]T {
+    var grid = try alloc.alloc([]T, h);
+    for (0..h) |i| {
+        grid[i] = try alloc.alloc(T, w);
+        @memset(grid[i], init);
+    }
+    return grid;
+}
+
+pub fn toGridOwned(alloc: mem.Allocator, buffer: []const u8, sep: u8) ![][]u8 {
+    if (buffer.len == 0) return error.Empty;
+    const w = mem.indexOfScalar(u8, buffer, sep) orelse unreachable;
+    var h = mem.count(u8, buffer, &[_]u8{sep});
+    if (buffer[buffer.len - 1] != sep) h += 1;
+
+    var grid = try alloc.alloc([]u8, h);
+
+    var it = split(buffer, sep);
+    var i: usize = 0;
+    while (it.next()) |line| {
+        grid[i] = try alloc.alloc(u8, w);
+        @memcpy(grid[i], line);
+        i += 1;
+    }
+
+    return grid;
+}