diff --git a/src/day15.zig b/src/day15.zig
new file mode 100644
index 0000000..866ab54
--- /dev/null
+++ b/src/day15.zig
@@ -0,0 +1,107 @@
+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 += c;
+        v *= 17;
+        v %= 256;
+    }
+    return v;
+}
+
+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]?std.StringArrayHashMap(u8) = [_]?std.StringArrayHashMap(u8){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);
+
+        // Split init instead of using orelse because references.
+        if (boxes[label_hash] == null) {
+            boxes[label_hash] = std.StringArrayHashMap(u8).init(alloc);
+        }
+        var box = &(boxes[label_hash] orelse unreachable);
+
+        switch (line[op]) {
+            '-' => _ = box.orderedRemove(label),
+            '=' => {
+                const focus = try std.fmt.parseInt(u8, line[op + 1 ..], 10);
+                try box.put(label, focus);
+            },
+            else => unreachable,
+        }
+
+        // std.debug.print("{s}\n", .{ line });
+        // for (boxes, 0..) |bx, n| {
+        //     if (bx) |b| {
+        //         std.debug.print("box {}: {any} = {any}\n", .{ n, b.keys(), b.values() });
+        //     }
+        // }
+        // std.debug.print("---\n", .{});
+    }
+
+    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| {
+            const label = entry.key_ptr.*;
+            _ = label;
+            const focal_len = entry.value_ptr.*;
+            const focus_power = box_n * lens_n * focal_len;
+            // std.debug.print("{s}: box {} * slot {} * focal length {} = {}\n", .{ label, box_n, lens_n, focal_len, focus_power });
+            focus += focus_power;
+            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);
+}