function log2(atom v)
    return log(v)/log(2)
end function

function entropy(sequence s)
sequence symbols = {},
         counts = {}
    integer N = length(s)
    for i=1 to N do
        object si = s[i]
        integer k = find(si,symbols)
        if k=0 then
            symbols = append(symbols,si)
            counts = append(counts,1)
        else
            counts[k] += 1
        end if
    end for
    atom H = 0
    integer n = length(counts)
    for i=1 to n do
        atom ci = counts[i]/N
        H -= ci*log2(ci)
    end for
    return H
end function

?entropy("1223334444")