Circular primes implementation and tests

1 files changed, 41 insertions, 0 deletions

diff --git a/circular-primes/cp.scm b/circular-primes/cp.scm
new file mode 100644
index 0000000..1f4105c
--- /dev/null
+++ b/circular-primes/cp.scm
@@ -0,0 +1,41 @@
+(define-module (cp)
+  #:use-module (soe)
+  #:use-module  (srfi srfi-1)
+  #:export (generate-circular-primes-to
+            number->circular-list
+            list-rotate))
+
+
+(define (generate-circular-primes-to n)
+  "Returns the 'circular primes' under N."
+  (let loop ((primes (generate-primes-to n))
+             (circular-primes '()))
+    (if (null? primes)
+        (sort circular-primes <)
+        (let* ((circle (number->circular-list (car primes)))
+               (prev-num-primes (length primes))
+               (primes (lset-difference eqv? primes circle)))
+          (loop primes
+                ;; primes contains no duplicates, so:
+                (if (eq? (length primes) (- prev-num-primes (length circle)))
+                    (append circle circular-primes)
+                    circular-primes))))))
+
+(define (number->circular-list n)
+  "Given number N, returns a list of all rotated
+numbers of N, i.e.:
+
+123 -> '(312 231 123)"
+  (let ((lst (string->list (number->string n))))
+    (let loop ((accum (list lst))
+               (next (list-rotate lst)))
+      (if (equal? next lst)
+          (map (lambda (x) (string->number (list->string x)))
+               accum)
+          (loop (cons next accum)
+                (list-rotate next))))))
+
+(define (list-rotate lst)
+  (if (null? lst)
+      '()
+      (append (cdr lst) (list (car lst)))))