% NOTE: when compiling with Portable CLU,
% this program needs to be merged with 'useful.lib' to get log()
%
% pclu -merge $CLUHOME/lib/useful.lib -compile entropy.clu

shannon = proc (s: string) returns (real)
    % find the frequency of each character
    freq: array[int] := array[int]$fill(0, 256, 0)
    for c: char in string$chars(s) do
        i: int := char$c2i(c)
        freq[i] := freq[i] + 1
    end

    % calculate the component for each character
    h: real := 0.0
    rlen: real := real$i2r(string$size(s))
    for i: int in array[int]$indexes(freq) do
        if freq[i] ~= 0 then
            f: real := real$i2r(freq[i]) / rlen
            h := h - f * log(f) / log(2.0)
        end
    end
    return (h)
end shannon

start_up = proc ()
    po: stream := stream$primary_output()
    stream$putl(po, f_form(shannon("1223334444"), 1, 6))
end start_up