diff --git a/src/aoc.zig b/src/aoc.zig
index 7cb5388..9109ab3 100644
--- a/src/aoc.zig
+++ b/src/aoc.zig
@@ -6,7 +6,6 @@ pub const Input = struct {
     lines: []const []const u8
 };
 
-// For now only .uint is needed
 pub const Result = union(enum) {
     uint: u64,
     int: i64,
diff --git a/src/day11.zig b/src/day11.zig
index 572e35d..aeaf1aa 100644
--- a/src/day11.zig
+++ b/src/day11.zig
@@ -195,7 +195,7 @@ pub fn part2(input: *aoc.Input) !aoc.Result {
     return .{ .uint = sum_min_distances(galaxies.items) };
 }
 
-const example_input = [_][]const u8{
+const example_input = [_][]u8{
     "...#......",
     ".......#..",
     "#.........",
diff --git a/src/day12.zig b/src/day12.zig
new file mode 100644
index 0000000..9b5a586
--- /dev/null
+++ b/src/day12.zig
@@ -0,0 +1,362 @@
+const std = @import("std");
+const aoc = @import("./aoc.zig");
+const Allocator = std.mem.Allocator;
+const ArrayList = std.ArrayList;
+
+const SpringState = enum {
+    Operational, Damaged, Unknown,
+
+    fn from_char(char: u8) ?SpringState {
+        return switch (char) {
+            '.' => .Operational,
+            '#' => .Damaged,
+            '?' => .Unknown,
+            else => null
+        };
+    }
+
+    fn to_char(self: SpringState) u8 {
+        return switch (self) {
+            .Operational => '.',
+            .Damaged     => '#',
+            .Unknown     => '?',
+        };
+    }
+};
+
+fn print_states(states: []const SpringState) void {
+    for (states) |state| {
+        std.debug.print("{c}", .{state.to_char()});
+    }
+    std.debug.print("\n", .{});
+}
+
+const InputRow = struct {
+    states: []SpringState,
+    damaged_groups: []u32,
+
+    fn print(self: InputRow) void {
+        for (self.states) |state| {
+            std.debug.print("{c}", .{state.to_char()});
+        }
+        std.debug.print(" ", .{});
+        for (0.., self.damaged_groups) |i, group_size| {
+            if (i != 0) {
+                std.debug.print(",", .{});
+            }
+            std.debug.print("{}", .{group_size});
+        }
+        std.debug.print("\n", .{});
+    }
+};
+
+const Input = struct {
+    rows: ArrayList(InputRow),
+
+    fn init(allocator: Allocator) Input {
+        return Input{
+            .rows = ArrayList(InputRow).init(allocator)
+        };
+    }
+
+    fn deinit(self: Input) void {
+        const allocator = self.rows.allocator;
+        for (self.rows.items) |row| {
+            allocator.free(row.states);
+            allocator.free(row.damaged_groups);
+        }
+        self.rows.deinit();
+    }
+
+    fn print(self: Input) void {
+        for (self.rows.items) |row| {
+            row.print();
+        }
+    }
+};
+
+fn parse_input_row(allocator: Allocator, line: []const u8) !InputRow {
+    const space_idx = std.mem.indexOfScalar(u8, line, ' ') orelse return error.InvalidFormat;
+    var states = try allocator.alloc(SpringState, space_idx);
+    errdefer allocator.free(states);
+
+    const commas = std.mem.count(u8, line, ",");
+    var damaged_groups = try allocator.alloc(u32, commas + 1);
+    errdefer allocator.free(damaged_groups);
+
+    for (0.., line[0..space_idx]) |i, char| {
+        states[i] = SpringState.from_char(char) orelse return error.InvalidFormat;
+    }
+
+    var it = std.mem.splitScalar(u8, line[(space_idx+1)..], ',');
+    var idx: usize = 0;
+    while (it.next()) |number| {
+        damaged_groups[idx] = try std.fmt.parseInt(u32, number, 10);
+        idx += 1;
+    }
+
+    return InputRow{
+        .states = states,
+        .damaged_groups = damaged_groups,
+    };
+}
+
+fn parse_input(allocator: Allocator, lines: []const []const u8) !Input {
+    var parsed = Input.init(allocator);
+    errdefer parsed.deinit();
+
+    for (lines) |line| {
+        try parsed.rows.append(try parse_input_row(allocator, line));
+    }
+
+    return parsed;
+}
+
+fn is_arrangement_valid(states: []const SpringState, groups: []const u32) bool {
+    var it = std.mem.tokenizeScalar(SpringState, states, .Operational);
+    var i: usize = 0;
+
+    while (it.next()) |group| {
+        if (groups.len == i) return false;
+        if (group.len != groups[i]) return false;
+        i += 1;
+    }
+
+    if (i != groups.len) {
+        return false;
+    }
+
+    return true;
+}
+
+fn count_valid_arrangements(allocator: Allocator, states: []const SpringState, groups: []const u32) !u32 {
+    var unknown_count: u32 = 0;
+    for (states) |state| {
+        if (state == .Unknown) {
+            unknown_count += 1;
+        }
+    }
+
+    if (unknown_count == 0) {
+        if (is_arrangement_valid(states, groups)) {
+            return 1;
+        } else {
+            return 0;
+        }
+    }
+
+    var unknown_indexes = try allocator.alloc(usize, unknown_count);
+    defer allocator.free(unknown_indexes);
+    var is_unknown_damaged = try allocator.alloc(bool, unknown_count);
+    defer allocator.free(is_unknown_damaged);
+
+    @memset(is_unknown_damaged, false);
+
+    var i: usize = 0;
+    for (0.., states) |index, state| {
+        if (state == .Unknown) {
+            unknown_indexes[i] = index;
+            i += 1;
+        }
+    }
+
+    const arrangement = try allocator.dupe(SpringState, states);
+    defer allocator.free(arrangement);
+    @memcpy(arrangement, states);
+
+    var count: u32 = 0;
+    for (0..std.math.pow(u32, 2, unknown_count)) |_| {
+        for (unknown_indexes, is_unknown_damaged) |index, is_damaged| {
+            arrangement[index] = if (is_damaged) .Damaged else .Operational;
+        }
+
+        if (is_arrangement_valid(arrangement, groups)) {
+            count += 1;
+        }
+
+        const carry = is_unknown_damaged[0];
+        is_unknown_damaged[0] = !is_unknown_damaged[0];
+
+        if (carry) {
+            for (is_unknown_damaged[1..]) |*is_damaged| {
+                const was_set = is_damaged.*;
+                is_damaged.* = !is_damaged.*;
+                if (was_set) break;
+            }
+        }
+    }
+
+    return count;
+}
+
+pub fn part1(input: *aoc.Input) !aoc.Result {
+    const allocator = input.allocator;
+    const parsed = try parse_input(allocator, input.lines);
+    defer parsed.deinit();
+
+    var result: u32 = 0;
+    for (parsed.rows.items) |row| {
+        result += try count_valid_arrangements(allocator, row.states, row.damaged_groups);
+    }
+
+    return .{ .uint = result };
+}
+
+fn unfold_input_row(allocator: Allocator, input_row: InputRow) !InputRow {
+    const multiplier = 5;
+
+    const state_count = input_row.states.len;
+    var unfolded_states = try allocator.alloc(SpringState, (state_count + 1) * multiplier - 1);
+    errdefer allocator.free(unfolded_states);
+
+    const group_count = input_row.damaged_groups.len;
+    var unfolded_groups = try allocator.alloc(u32, group_count * multiplier);
+    errdefer allocator.free(unfolded_groups);
+
+    for (0..multiplier) |i| {
+        if (i != 0) {
+            unfolded_states[i*(state_count+1)-1] = .Unknown;
+        }
+
+        @memcpy(unfolded_states[(i*(state_count+1))..(i*(state_count+1) + state_count)], input_row.states);
+        @memcpy(unfolded_groups[(i*group_count)..((i+1)*group_count)], input_row.damaged_groups);
+    }
+
+    return InputRow{
+        .states = unfolded_states,
+        .damaged_groups = unfolded_groups
+    };
+}
+
+fn unfold_input(allocator: Allocator, input: Input) !Input {
+    var unfolded = Input.init(allocator);
+    errdefer unfolded.deinit();
+
+    for (input.rows.items) |row| {
+        try unfolded.rows.append(try unfold_input_row(allocator, row));
+    }
+
+    return unfolded;
+}
+
+const CacheKey = struct {
+    states: []const SpringState,
+    groups: []const u32,
+};
+const CacheKeyContext = struct {
+    fn splitU32(value: u64) [8]u8 {
+        return .{
+            @truncate(value >> 8*0),
+            @truncate(value >> 8*1),
+            @truncate(value >> 8*2),
+            @truncate(value >> 8*3),
+            @truncate(value >> 8*4),
+            @truncate(value >> 8*5),
+            @truncate(value >> 8*6),
+            @truncate(value >> 8*7),
+        };
+    }
+
+    pub fn hash(_: CacheKeyContext, key: CacheKey) u64 {
+        var h = std.hash.Fnv1a_64.init();
+        h.update(&splitU32(@intFromPtr(key.states.ptr)));
+        h.update(&splitU32(@intCast(key.states.len)));
+        h.update(&splitU32(@intFromPtr(key.groups.ptr)));
+        h.update(&splitU32(@intCast(key.groups.len)));
+        return h.final();
+    }
+
+    pub fn eql(_: CacheKeyContext, a: CacheKey, b: CacheKey) bool {
+        return std.mem.eql(SpringState, a.states, b.states) and std.mem.eql(u32, a.groups, b.groups);
+    }
+};
+const Cache = std.HashMap(CacheKey, u64, CacheKeyContext, 80);
+
+fn count_valid_arrangements_dp(cache: *Cache, states: []const SpringState, groups: []const u32) !u64 {
+    if (states.len == 0) {
+        if (groups.len == 0) {
+            // If there are no springs left AND expect no more groups,
+            // THEN this is a valid arrangment
+            return 1;
+        } else {
+            // If there are no springs left, but expected to have some groups,
+            // THEN this is not a valid arrangment
+            return 0;
+        }
+    }
+
+    if (groups.len == 0) {
+        if (std.mem.indexOfScalar(SpringState, states, .Damaged) != null) {
+            // If we expect to see no more groups, but there are still some damanged springs left
+            // THEN this is not a valid arrangment
+            return 0;
+        } else {
+            // If we expect to see no more groups and we have no more damaged springs
+            // THEN this arrangment is valid
+            return 1;
+        }
+    }
+
+    var cache_key = CacheKey{ .states = states, .groups = groups };
+    if (cache.get(cache_key)) |cached_result| {
+        return cached_result;
+    }
+
+    var result: u64 = 0;
+    if (states[0] == .Operational or states[0] == .Unknown) {
+        result += try count_valid_arrangements_dp(cache, states[1..], groups);
+    }
+
+    const group_size = groups[0];
+    if (
+        (states[0] == .Damaged or states[0] == .Unknown) // If the leading spring is damanged
+    and (group_size <= states.len) // If the number of springs left is higher than the group size
+    and (std.mem.indexOfScalar(SpringState, states[0..group_size], .Operational) == null) // If contains continous group of broken springs
+    ) {
+        // If the damaged group ends with an operational spring
+        if (group_size == states.len) {
+            result += try count_valid_arrangements_dp(cache, states[group_size..], groups[1..]);
+        } else if (states[group_size] != .Damaged) {
+            result += try count_valid_arrangements_dp(cache, states[(group_size+1)..], groups[1..]);
+        }
+    }
+
+    try cache.put(cache_key, result);
+    return result;
+}
+
+pub fn part2(input: *aoc.Input) !aoc.Result {
+    const allocator = input.allocator;
+    const parsed = try parse_input(allocator, input.lines);
+    defer parsed.deinit();
+
+    const unfolded = try unfold_input(allocator, parsed);
+    defer unfolded.deinit();
+
+    var cache = Cache.init(allocator);
+    defer cache.deinit();
+
+    var result: u64 = 0;
+    for (unfolded.rows.items) |row| {
+        result += try count_valid_arrangements_dp(&cache, row.states, row.damaged_groups);
+    }
+
+    return .{ .uint = result };
+}
+
+const example_input = [_][]const u8{
+    "???.### 1,1,3",
+    ".??..??...?##. 1,1,3",
+    "?#?#?#?#?#?#?#? 1,3,1,6",
+    "????.#...#... 4,1,1",
+    "????.######..#####. 1,6,5",
+    "?###???????? 3,2,1",
+};
+
+test "part 1 example" {
+    try aoc.expectAnswerUInt(part1, 21, &example_input);
+}
+
+test "part 2 example" {
+    try aoc.expectAnswerUInt(part2, 525152, &example_input);
+}
diff --git a/src/main.zig b/src/main.zig
index 9884386..8884c14 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -32,6 +32,7 @@ const Days = [_]aoc.Day{
     create_day(@import("day9.zig")),
     create_day(@import("day10.zig")),
     create_day(@import("day11.zig")),
+    create_day(@import("day12.zig")),
 };
 
 fn kilobytes(count: u32) u32 {
@@ -187,4 +188,5 @@ test {
     _ = @import("day9.zig");
     _ = @import("day10.zig");
     _ = @import("day11.zig");
+    _ = @import("day12.zig");
 }