function encode(sequence s)
sequence out = {}
integer prev_char,count = 1
    if length(s) then
        prev_char = s[1]
        for i=2 to length(s) do
            if s[i]!=prev_char then
                out &= {count,prev_char}
                prev_char = s[i]
                count = 1
            else
                count += 1
            end if
        end for
        out &= {count,prev_char}
    end if
    return out
end function

function decode(sequence s)
sequence out = {}
    for i=1 to length(s) by 2 do
        out &= repeat(s[i+1],s[i])
    end for
    return out
end function

sequence s = encode("WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW")
pp(s)
?decode(s)