Merge pull request #2 from rmrt1n/cryptography

added cryptography directory, caesar-cypher and factorial algorithms

Author: abranhe

algorithms/cryptography/caesar-cipher.rkt

@@ -0,0 +1,34 @@
+#lang racket
+
+(provide caesar-cypher)
+
+;; Turn plain text into cypher text with caesar cypher algorithm.
+;; Numbers, symbols and whitespace are not changed.
+;; args:
+;;   str: string to encode
+;;   shift: optional shift length, default is 3
+(define (caesar-cypher str [shift 3])
+  (let* ([shift-uppercase
+          (lambda (c)
+            (integer->char
+             (+ 65 (remainder (+ shift (char->integer c) -65) 26))))]
+         [shift-lowercase
+          (lambda (c)
+            (integer->char
+             (+ 97 (remainder (+ shift (char->integer c) -97) 26))))]
+         [shift
+          (lambda (c)
+            (cond [(and (char-alphabetic? c)
+                        (char-upper-case? c))
+                   (shift-uppercase c)]
+                  [(and (char-alphabetic? c)
+                        (char-lower-case? c))
+                   (shift-lowercase c)]
+                  [else c]))]
+         [char-list (string->list str)])
+    (let loop ([acc '()]
+               [ls char-list])
+      (if (null? ls)
+        (list->string (reverse acc))
+        (loop (cons (shift (car ls)) acc) (cdr ls))))))
+

algorithms/math/factorial.rkt

@@ -0,0 +1,11 @@
+#lang racket
+
+(provide factorial)
+
+(define (factorial n)
+  (let loop ([acc 1]
+             [n n])
+    (if (zero? n)
+      acc
+      (loop (* acc n) (sub1 n)))))
+