net: Add (net ricketyspace sicp one twentyfour).

* net/ricketyspace/sicp/one/twentyfour.scm: New module.

1 files changed, 63 insertions, 0 deletions

diff --git a/net/ricketyspace/sicp/one/twentyfour.scm b/net/ricketyspace/sicp/one/twentyfour.scm
new file mode 100644
index 0000000..58e4fcf
--- /dev/null
+++ b/net/ricketyspace/sicp/one/twentyfour.scm
@@ -0,0 +1,63 @@
+;;;; Under Creative Commons Attribution-ShareAlike 4.0
+;;;; International. See
+;;;; <https://creativecommons.org/licenses/by-sa/4.0/>.
+
+(define-module (net ricketyspace sicp one twentyfour)
+  #:use-module (srfi srfi-1)
+  #:export (three-primes-gt-f
+            three-primes-gt-1000-f
+            three-primes-gt-10000-f
+            three-primes-gt-100000-f
+            three-primes-gt-1000000-f))
+
+(define (square x)
+  (* x x))
+
+(define (expmod base exp m)
+  (cond ((= exp 0) 1)
+        ((even? exp)
+         (remainder (square (expmod base (/ exp 2) m))
+                    m))
+        (else
+         (remainder (* base (expmod base (- exp 1) m))
+                    m))))
+
+(define (fermat-test n)
+  (define (try-it a)
+    (= (expmod a n n) a))
+  (try-it (+ 1 (random (- n 1)))))
+
+(define (fast-prime? n times)
+  (cond ((= times 0) #t)
+        ((fermat-test n) (fast-prime? n (- times 1)))
+        (else #f)))
+
+(define (odd-and-prime n)
+  (and (odd? n) (fast-prime? n (quotient n 2))))
+
+(define (search-for-primes start end)
+  (cond ((> start end) '())
+        ((odd-and-prime start)
+         (cons start (search-for-primes (1+ start) end)))
+        (else (search-for-primes (1+ start) end))))
+
+(define (three-primes-gt-f n)
+  (three-primes-gt-iter n 10 '()))
+
+(define (three-primes-gt-iter n e l)
+  (cond ((> (length l) 3) (take l 3))
+        (else (three-primes-gt-iter n
+                                    (+ e 10)
+                                    (search-for-primes n e)))))
+
+(define (three-primes-gt-1000-f)
+  (three-primes-gt 1000))
+
+(define (three-primes-gt-10000-f)
+  (three-primes-gt 10000))
+
+(define (three-primes-gt-100000-f)
+  (three-primes-gt 100000))
+
+(define (three-primes-gt-1000000-f)
+  (three-primes-gt 1000000))