Cryptarithm

A recent post to the Puzzles and Logic Problems alias at work: The problem below is an example of a cryptarithm – a basic math problem made more difficult by obscuring each digit with a letter or other symbol.

In this problem, there are 10 unique letters. As an initial hint, S = 1. Each letter represents a unique integer ranging from 0-9, so given the hint that S = 1, no other letter is equal to ‘1’.

With some stolen code for the permutations function, my solution turned out pretty succinct in F# and reads nearly like English:

let

rec permutations list taken = // credit to Tomas Petricek for this
    seq { if Set.count taken = List.length list then yield [] else
          for e in list do
            if not (Set.mem e taken) then
              for p in permutations list (Set.add e taken) do
                yield e :: p }

let

s = 1 // given

let validate attempt =
    let decimal list = let digits d (a, m) = (a + d * m, m * 10)
                       List.fold_right digits list (0, 1) |> fst
    let a::b::r::e::l::o::m::h::v::_ = attempt
    let barrel = decimal [b;a;r;r;e;l]
    let brooms = decimal [b;r;o;o;m;s]
    let shovels = decimal [s;h;o;v;e;l;s]
    barrel + brooms = shovels

let print = Seq.iter2 (printfn "%c = %i") "SABRELOMHV"

let attempts = permutations [0..9] (Set.singleton s)
let solution = s :: Seq.find validate attempts
print solution