Data commit

Task/Entropy/ALGOL-W/entropy.alg

@@ -0,0 +1,54 @@
+begin
+    % calculates the shannon entropy of a string                          %
+    % strings are fixed length in algol W and the length is part of the   %
+    % type, so we declare the string parameter to be the longest possible %
+    % string length (256 characters) and have a second parameter to       %
+    % specify how much is actually used                                   %
+    real procedure shannon_entropy ( string(256) value s
+                                   ; integer     value stringLength
+                                   );
+    begin
+
+        real    probability, entropy;
+
+        % algol W assumes there are 256 possible characters %
+        integer MAX_CHAR;
+                MAX_CHAR := 256;
+
+        % declarations must preceed statements, so we start a new         %
+        % block here so we can use MAX_CHAR as an array bound             %
+        begin
+
+            % increment an integer variable                               %
+            procedure incI ( integer value result a ) ; a := a + 1;
+
+            integer array charCount( 1 :: MAX_CHAR );
+
+            % count the occurances of each character in s                 %
+            for charPos := 1 until MAX_CHAR do charCount( charPos ) := 0;
+            for sPos := 0 until stringLength - 1 do incI( charCount( decode( s( sPos | 1 ) ) ) );
+
+            % calculate the entropy, we use log base 10 and then convert  %
+            % to log base 2 after calculating the sum                     %
+
+            entropy := 0.0;
+            for charPos := 1 until MAX_CHAR do
+            begin
+                if charCount( charPos ) not = 0
+                then begin
+                    % have a character that occurs in the string          %
+                    probability := charCount( charPos ) / stringLength;
+                    entropy     := entropy - ( probability * log( probability ) )
+                end
+            end charPos
+
+        end;
+
+        entropy / log( 2 )
+    end shannon_entropy ;
+
+    % test the shannon entropy routine                                    %
+    r_format := "A"; r_w := 12; r_d := 6; % set output to fixed format    %
+    write( shannon_entropy( "1223334444", 10 ) )
+
+end.