Data commit

Task/Entropy/Modula-2/entropy.mod2

@@ -0,0 +1,36 @@
+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.