net: Add (net ricketyspace sicp one twentyeight).

* net/ricketyspace/sicp/one/twentyeight.scm: New file.

1 files changed, 34 insertions, 0 deletions

diff --git a/net/ricketyspace/sicp/one/twentyeight.scm b/net/ricketyspace/sicp/one/twentyeight.scm
new file mode 100644
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--- /dev/null
+++ b/net/ricketyspace/sicp/one/twentyeight.scm
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+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one twentyeight)
+  #:use-module (srfi srfi-1)
+  #:export (miller-rabin-test prime?))
+
+(define (sqmod x m)
+  "Return x^2 if `x^2 mod m` is not equal to `1 mod m`
+and x != m - 1 and x != 1; 0 otherwise."
+  (let ((square (* x x)))
+    (cond ((and  (= (remainder square m) 1) ; 1 mod m = 1
+                 (not (= x (1- m)))
+                 (not (= x 1)))
+           0)
+          (else square))))
+
+(define (expmod base exp m)
+  (cond ((= exp 0) 1)
+        ((even? exp)
+         (remainder (sqmod (expmod base (/ exp 2) m) m)
+                    m))
+        (else
+         (remainder (* base (expmod base (- exp 1) m))
+                    m))))
+
+(define (miller-rabin-test n)
+  (define (pass? a)
+    (= (expmod a (1- n) n) 1))
+  (fold (lambda (a p) (and (pass? a) p)) #t (iota (1- n) 1)))
+
+(define (prime? n)
+  (if (miller-rabin-test n) #t #f))