using system;

encode s = strcat $ map (sprintf "%d%s") $ encode $ chars s with
  encode [] = [];
  encode xs@(x:_) = (#takewhile (==x) xs,x) : encode (dropwhile (==x) xs);
end;

decode s = strcat [c | n,c = parse s; i = 1..n] with
  parse s::string = regexg item "([0-9]+)(.)" REG_EXTENDED s 0;
  item info = val (reg 1 info!1), reg 2 info!1;
end;

let s = "WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW";
let r = encode s; // "12W1B12W3B24W1B14W"
decode r;