Initial data commit

Task/Entropy/Delphi/entropy.delphi

@@ -0,0 +1,60 @@
+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
+  Result := '';
+  for n := 1 to length(str) do
+    if (PosEx(str[n], str, n) > 0) and (PosEx(str[n], Result, 1) = 0) then
+      Result := Result + 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(Result, 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(Result[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);
+  Result := 0.0;
+  for i := 1 to length(us) do
+    Result := Result - pf[i] / l * log2(pf[i] / l);
+end;
+
+begin
+  Writeln('Entropy of "', strng, '" is ', entropy(strng): 2: 5, ' bits.');
+  readln;
+end.