Just another update

Task/Entropy/Pascal/entropy.pascal

@@ -0,0 +1,50 @@
+PROGRAM entropytest;
+
+USES StrUtils, Math;
+
+TYPE FArray = ARRAY of CARDINAL;
+
+VAR	 strng: STRING = '1223334444';
+	
+// list unique characters in a string
+FUNCTION uniquechars(str: STRING): STRING;
+	VAR n: CARDINAL;
+	BEGIN
+		uniquechars := '';
+		FOR n := 1 TO length(str) DO
+			IF (PosEx(str[n],str,n)>0)
+				AND (PosEx(str[n],uniquechars,1)=0)
+					THEN uniquechars += str[n];
+	END;
+	
+// obtain a list of character-frequencies for a string
+//  given a string containing its unique characters
+FUNCTION frequencies(str,ustr: STRING): FArray;
+	VAR u,s,p,o: CARDINAL;
+	BEGIN
+		SetLength(frequencies, Length(ustr)+1);
+		p := 0;
+		FOR u := 1 TO length(ustr) DO
+			FOR s := 1 TO length(str) DO BEGIN
+				o := p;	p := PosEx(ustr[u],str,s);
+				IF (p>o) THEN INC(frequencies[u]);
+			END;
+	END;
+
+// Obtain the Shannon entropy of a string
+FUNCTION entropy(s: STRING): EXTENDED;
+	VAR pf : FArray;
+		us : STRING;
+		i,l: CARDINAL;
+	BEGIN
+		us := uniquechars(s);
+		pf := frequencies(s,us);
+		l  := length(s);
+		entropy := 0.0;
+		FOR i := 1 TO length(us) DO
+			entropy -= pf[i]/l * log2(pf[i]/l);
+	END;
+
+BEGIN
+	Writeln('Entropy of "',strng,'" is ',entropy(strng):2:5, ' bits.');
+END.