Initial data commit

Task/Caesar-cipher/SparForte/caesar-cipher.sparforte

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+#!/usr/local/bin/spar
+pragma annotate( summary, "caesar" )
+              @( description, "Implement a Caesar cipher, both encoding and ")
+              @( description, "decoding.  The key is an integer from 1 to " )
+              @( description, "25.  This cipher rotates (either towards left ")
+              @( description, "or right) the letters of the alphabet (A to " )
+              @( description, "Z). " )
+              @( see_also, "http://rosettacode.org/wiki/Caesar_cipher" )
+              @( author, "Ken O. Burtch" );
+pragma license( unrestricted );
+
+pragma restriction( no_external_commands );
+
+procedure caesar is
+
+   type cipher_value is new natural;
+
+   function to_cipher_value(c: character; offset: character) return cipher_value is
+      cv : integer;
+   begin
+      cv := ( numerics.pos(c)-numerics.pos(offset) ) mod 26;
+      return cipher_value(cv);
+   end to_cipher_value;
+
+   function to_character(cv: cipher_value; offset: character) return character is
+   begin
+      return strings.val( integer(cv) + numerics.pos(offset) );
+   end to_character;
+
+   function encrypt( plain: string; key: cipher_value) return string is
+      cv : cipher_value;
+      cipher_char : character;
+      cipher_text : string;
+      plain_char  : character;
+   begin
+      for i in 1..strings.length( plain ) loop
+         plain_char := strings.element( plain, i);
+         if plain_char >= 'A' and plain_char <= 'Z' then
+            cv := ( to_cipher_value( plain_char, 'A')+key ) mod 26;
+            cipher_char := to_character( cv, 'A' );
+         elsif plain_char >= 'a' and plain_char <= 'z' then
+            cv := ( to_cipher_value( plain_char, 'a')+key ) mod 26;
+            cipher_char := to_character( cv, 'a' );
+         else
+            cipher_char := plain_char;
+         end if;
+         cipher_text := strings.overwrite( @, i, string( cipher_char ) );
+      end loop;
+      return cipher_text;
+   end encrypt;
+
+   text:  string := get_line;
+   key: constant cipher_value := 3; -- Default key from "Commentarii de Bello Gallico"
+
+begin
+   put_line("Plaintext ------------>" & text);
+   text := encrypt(text, key);
+   put_line("Ciphertext ----------->" & text);
+   text := encrypt(text, -key);
+   put_line("Decrypted Ciphertext ->" & text);
+end caesar;