The GCHQ 2015 Christmas puzzle is a Nonogram puzzle, which involves filling in squares on a grid to reveal a picture, guided by constraints on the rows and columns. For a computer, a nice way to solve this problem is using a SAT solver. But humans aren't great at SAT solving, and I was given a print-out of this puzzle while on holiday, with no computer. I'd never encountered such a puzzle before, so working with a friend (and some wine) we came up with an approach, and set about applying it. Alas, robustly applying an algorithm with many steps is not easy for a human, and we eventually ended up with contradictions. On returning from holiday, I automated our approach, and tested it. Our approach worked, and the code is below.

### The Problem

The puzzle is:

It comprises a 25x25 grid, some filled in squares, and alongside each row/column are the number of consecutive squares that must be filled in each line. For example, the 8th row down must have two runs of three filled squares, with a gap in between, and potentially gaps before or after.

### Our Approach

Our approach was to take each line and compute the number of "free" gaps - how many spaces could be inserted with choice. For one row (4 from the bottom) the entire grid is constrained, with no free gaps. Starting with the most constrained lines, we tried to figure out where the pieces could go, based on the existing squares. We quickly realised that *negative* information was important, so tagged each square with "don't know" (left blank), must be filled (we shaded it in) or must be unfilled (we drew a circle in it). For each line in isolation, looking at the constraints, we inferred squares to be filled or unfilled by examining the possible locations of each run.

### The Code

I implemented our approach in Haskell, complete code is available here.

Our constraint system works over a grid where each square is in one of three states. We can encode the grid as `[[Maybe Bool]]`

. The `[[.]]`

is a list of lists, where the outer list is a list of rows, and the inner list is a list of squares. Each of the inner lists must be the same length, and for the GCHQ puzzle, they must all be 25 elements long. For the squares we use `Maybe Bool`

, with `Nothing`

for unknown and `Just`

for known, using `True`

as filled and `False`

as unfilled.

Given the `[[Maybe Bool]]`

grid and the constraints, our approach was to pick a single line, and try to layout the runs, identifying squares that must be `True`

/`False`

. To replicate that process on a computer, I wrote a function `tile`

that given the constraints and the existing line, produces all possible lines that fit. The code is:

```
tile :: [Int] -> [Maybe Bool] -> [[Bool]]
tile [] xs = maybeToList $ xs ~> replicate (length xs) False
tile (c:cs) xs = concat [map (\r -> a ++ b ++ c ++ r) $ tile cs xs
| gap <- [0 .. length xs - (c + sum cs + length cs)]
, (false,xs) <- [splitAt gap xs]
, (true,xs) <- [splitAt c xs]
, (space,xs) <- [splitAt 1 xs]
, Just a <- [false ~> replicate gap False]
, Just b <- [true ~> replicate c True]
, Just c <- [space ~> replicate (length space) False]]
```

The first equation (second line) says that if there are no remaining constraints we set all remaining elements to `False`

. We use the `~>`

operator to check our desired assignment is consistent with the information already in the line:

```
(~>) :: [Maybe Bool] -> [Bool] -> Maybe [Bool]
(~>) xs ys | length xs == length ys &&
and (zipWith (\x y -> maybe True (== y) x) xs ys)
= Just ys
(~>) _ _ = Nothing
```

This function takes a line of the grid (which may have unknowns), and a possible line (which is entirely concrete), and either returns `Nothing`

(inconsistent) or `Just`

the proposed line. We first check the sizes are consistent, then that everything which is concrete (not a `Nothing`

) matches the proposed value.

Returning to the second equation in `tile`

, the idea is to compute how many spaces could occur at this point. Taking the example of a line 25 long, with two runs of size 3, we could have anywhere between 0 and 18 (25-3-3-1) spaces first. For each possible size of gap, we split the line up (the `splitAt`

calls), then constrain each piece to match the existing grid (using `~>`

).

Given a way of returning all possible lines, we then collapse that into a single line, by marking all squares which could be either `True`

or `False`

as `Nothing`

:

```
constrainLine :: [Int] -> [Maybe Bool] -> Maybe [Maybe Bool]
constrainLine cs xs = if null xs2 then Nothing else mapM f $ transpose xs2
where xs2 = tile cs xs
f (x:xs) = Just $ if not x `elem` xs then Nothing else Just x
```

If there are no satisfying assignments for the line, we return `Nothing`

- that implies the constraints are unsatisfiable. Next, we scale up to a side of constraints, by combining all the constraints and lines:

```
constrainSide :: [[Int]] -> [[Maybe Bool]] -> Maybe [[Maybe Bool]]
constrainSide cs xs = sequence $ zipWith constrainLine cs xs
```

Finally, to constrain the entire grid, we constrain one side, then the other. To simplify the code, we just transpose the grid in between, so we can treat the rows and columns identically:

```
constrainGrid :: [[Int]] -> [[Int]] -> [[Maybe Bool]] -> Maybe [[Maybe Bool]]
constrainGrid rows cols xs =
fmap transpose . constrainSide cols .
transpose =<< constrainSide rows xs
```

To constrain the whole problem we apply `constrainGrid`

repeatedly, until it returns `Nothing`

(the problem is unsatisfiable), we have a complete solution (problem solved), or nothing changes. If nothing changes then there might be two solutions, or our approach might not be powerful enough without using search.

### The Result

After four iterations we end up with a fully constrained answer. To see the progress, after one iteration we have:

```
..XXXXX...X.OO..X.XXXXX..
..OOOOX.X.O.....O.XOOOO..
..XXX.X....O...OX.X.XX...
X.XXX.X....XXXXXX.X.XX...
X.XXX.X..XXXX..XX.X.XX..X
X.OOOOX...XO...OO.XOOO..X
XXXXXXXOXOXOXOXOXOXXXXXXX
..OOO.OO..XOOOX.XOOOOO.O.
..XX..XX.OXOXOXXXOXO...X.
..XO..OO....OXX.O.O....X.
..X...X......X..X......O.
..O...O......XO.X........
..XX..X.X....O.OO.X......
..OXX.O.X....XXXX.X......
..XX..XXXXX..O.OO........
..X...O.X..O..O.X...O....
..X...X.X.OXO.O.X...X....
..OOO.O.X..O..O.X...X..X.
X.XXXXX.......O.X...X..X.
X.OOO.X.....XOO.X...X..X.
X.XXX.X.....XXOOX...X...O
XOXXXOXOXXXOXXXXXXXXXXOXX
..XXX.X.....XXXXX..XXXX.O
..OOOOX......OOOO...O.X..
..XXXXX......XOXX.O.X.X..
```

Here a `.`

stands for `Nothing`

. After four iterations we reach the answer in a total of 0.28s:

```
XXXXXXXOXXXOOOXOXOXXXXXXX
XOOOOOXOXXOXXOOOOOXOOOOOX
XOXXXOXOOOOOXXXOXOXOXXXOX
XOXXXOXOXOOXXXXXXOXOXXXOX
XOXXXOXOOXXXXXOXXOXOXXXOX
XOOOOOXOOXXOOOOOOOXOOOOOX
XXXXXXXOXOXOXOXOXOXXXXXXX
OOOOOOOOXXXOOOXXXOOOOOOOO
XOXXOXXXOOXOXOXXXOXOOXOXX
XOXOOOOOOXXXOXXOOOOXOOOXO
OXXXXOXOXXXXOXXOXOOOOXXOO
OXOXOOOXOOOXOXOXXXXOXOXXX
OOXXOOXOXOXOOOOOOXXOXXXXX
OOOXXXOXXOXXOXXXXXXOXXXOX
XOXXXXXXXXXOXOXOOXXOOOOXO
OXXOXOOXXOOOXXOXXXOOOOOXO
XXXOXOXOXOOXOOOOXXXXXOXOO
OOOOOOOOXOOOXXOXXOOOXXXXX
XXXXXXXOXOOXXOOOXOXOXOXXX
XOOOOOXOXXOOXOOXXOOOXXOXO
XOXXXOXOOOXXXXOOXXXXXOOXO
XOXXXOXOXXXOXXXXXXXXXXOXX
XOXXXOXOXOOXXXXXXOXXXXXXO
XOOOOOXOOXXOOOOOOXOXOXXOO
XXXXXXXOXXOOOXOXXOOOXXXXX
```

**Update:** On the third attempt, my friend managed to solve it manually using our technique, showing it does work for humans too.