MODULE Entropy;
FROM InOut IMPORT WriteString, WriteLn;
FROM RealInOut IMPORT WriteReal;
FROM Strings IMPORT Length;
FROM MathLib IMPORT ln;

PROCEDURE entropy(s: ARRAY OF CHAR): REAL;
    VAR freq: ARRAY [0..255] OF CARDINAL;
        i, length: CARDINAL;
        h, f: REAL;
BEGIN
    (* the entropy of the empty string is zero *)
    length := Length(s);
    IF length = 0 THEN RETURN 0.0; END;

    (* find the frequency of each character *)
    FOR i := 0 TO 255 DO freq[i] := 0; END;
    FOR i := 0 TO length-1 DO
        INC(freq[ORD(s[i])]);
    END;

    (* calculate the component for each character *)
    h := 0.0;
    FOR i := 0 TO 255 DO
        IF freq[i] # 0 THEN
            f := FLOAT(freq[i]) / FLOAT(length);
            h := h - f * (ln(f) / ln(2.0));
        END;
    END;
    RETURN h;
END entropy;

BEGIN
    WriteReal(entropy("1223334444"), 14);
    WriteLn;
END Entropy.