September 2017 Update

Task/Caesar-cipher/8th/caesar-cipher.8th

@@ -0,0 +1,23 @@
+\ Ensure the output char is in the correct range:
+: modulate \ char base -- char
+  tuck n:- 26 n:+ 26 n:mod n:+ ;
+
+\ Symmetric Caesar cipher.  Input is text and number of characters to advance
+\ (or retreat, if negative).  That value should be in the range 1..26
+: caesar \ intext key -- outext
+  >r
+  (
+    \ Ignore anything below '.' as punctuation:
+    dup '. n:> if
+      \ Do the conversion
+      dup r@ n:+ swap
+      \ Wrap appropriately
+      'A 'Z between if 'A else 'a then modulate
+    then
+  ) s:map rdrop ;
+
+"The five boxing wizards jump quickly!"
+dup . cr
+1  caesar dup . cr
+-1 caesar . cr
+bye