diff --git a/src/day12.zig b/src/day12.zig
index e9b20c9..ddee97c 100644
--- a/src/day12.zig
+++ b/src/day12.zig
@@ -14,21 +14,117 @@ const Solution = struct {
     b: usize,
 };
 
-fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
-    _ = alloc;
+fn hash(alloc: mem.Allocator, pattern: []const u8, groups: []const u8, curr_group: u8) ![]const u8 {
+    var a = try std.ArrayList(u8).initCapacity(alloc, pattern.len + groups.len + curr_group);
+    try a.appendSlice(pattern);
+    try a.appendSlice(groups);
+    try a.append(curr_group);
+    return try a.toOwnedSlice();
+}
 
+// Mystery records to whether they fit.
+var memo: std.StringHashMap(u32) = undefined;
+
+fn copy(comptime T: type, alloc: mem.Allocator, s: []const T) ![]T {
+    var a = try std.ArrayList(T).initCapacity(alloc, s.len);
+    try a.insertSlice(0, s);
+    return try a.toOwnedSlice();
+}
+
+fn solvePattern(alloc: mem.Allocator, pattern: []const u8, groups: []const u8, curr_group: u8) !u32 {
+    std.debug.print("{s}, {any}, {}\n", .{ pattern, groups, curr_group });
+
+    const h = try hash(alloc, pattern, groups, curr_group);
+    if (memo.get(h)) |m| {
+        alloc.free(h);
+        std.debug.print("==> short-circuit {}\n", .{m});
+        return m;
+    }
+
+    if (pattern.len == 0) {
+        std.debug.print("=> done ok: {}\n", .{groups.len == 0});
+        return if (groups.len == 0) 1 else 0;
+    }
+
+    const res: u32 = b: {
+        switch (pattern[0]) {
+            '#' => {
+                // Too many #
+                if (groups.len == 0 or curr_group >= groups[0]) {
+                    std.debug.print("== bad fit\n", .{});
+                    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 #
+                        std.debug.print("== bad fit\n", .{});
+                        break :b 0;
+                    } else {
+                        // Group done
+                        std.debug.print("=> group {} ok\n", .{groups[0]});
+                        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 copy(u8, alloc, pattern);
+                defer alloc.free(p1);
+                const p2 = try copy(u8, alloc, pattern);
+                defer alloc.free(p2);
+
+                p1[0] = '#';
+                p2[0] = '.';
+
+                std.debug.print("=> test ?\n", .{});
+                break :b try solvePattern(alloc, p1, groups, curr_group) +
+                    try solvePattern(alloc, p2, groups, curr_group);
+            },
+            else => unreachable,
+        }
+    };
+
+    std.debug.print("<= {s}, {any}, {} = {}\n", .{ pattern, groups, curr_group, res });
+    try memo.put(h, res);
+
+    return res;
+}
+
+fn solve(alloc: mem.Allocator, input: []const u8) !Solution {
+    memo = std.StringHashMap(u32).init(alloc);
+    defer memo.deinit();
+
+    var sum: u32 = 0;
     var it = util.splitLines(input);
     while (it.next()) |line| {
         if (line.len == 0) continue;
 
         var lit = util.splitSpace(line);
-        const record = lit.next().?;
-        _ = record;
-        const groups = lit.next().?;
-        _ = groups;
+        const pattern = lit.next().?;
+        const raw_groups = lit.next().?;
+        const groups = try util.parseIntsScalar(u8, alloc, raw_groups, .{ .sep = ',' });
+        defer alloc.free(groups);
+
+        std.debug.print("-----------------------------------------------\n", .{});
+        sum += try solvePattern(alloc, pattern, groups, 0);
+        std.debug.print("{}\n", .{sum});
     }
 
-    return .{ .a = 0, .b = 0 };
+    var mkeys = memo.keyIterator();
+    while (mkeys.next()) |k| {
+        alloc.free(k.*);
+    }
+
+    return .{ .a = sum, .b = 0 };
 }
 
 test "silver" {
@@ -46,8 +142,13 @@ test "silver" {
 
 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, 0), sln.b);
+    try std.testing.expectEqual(@as(usize, 525152), sln.b);
 }