add factor-alist and export it (chibi math prime)

a more useful version of factor

lib/chibi/math/prime-test.sld

@@ -90,6 +90,11 @@
       (test '(3 3 3 5 7) (factor 945))
       (test-error (factor 0))
 
+      (test '() (factor-alist 1))
+      (test '((2 . 3) (3 . 2)) (factor-alist 72))
+      (test '((3 . 3) (5 . 1) (7 . 1)) (factor-alist 945))
+      (test-error (factor-alist 0))
+
       (test 0 (aliquot 1))
       (test 975 (aliquot 945))
       (test-error (aliquot 0))

lib/chibi/math/prime.scm

@@ -226,6 +226,15 @@
                           (lp (+ i 2) (+ ii (* (+ i 1) 4))
                               n (put res i k)))))))))))))
 
+;;> Returns the factorization of \var{n} as a list of
+;;> elements of the form \scheme{(\var{p} . \var{k})},
+;;> where \var{p} is a prime factor
+;;> and \var{k} is its multiplicity.
+(define factor-alist
+  (let ((rfactor (make-factorizer '()
+                  (lambda (l p k) (cons (cons p k) l)))))
+    (lambda (n) (reverse (rfactor n)))))
+
 ;;> Returns the factorization of \var{n} as a monotonically
 ;;> increasing list of primes.
 (define (factor n)

lib/chibi/math/prime.sld

@@ -5,7 +5,8 @@
    ((library (srfi 151)) (import (srfi 151)))
    ((library (srfi 33)) (import (srfi 33)))
    (else (import (srfi 60))))
-  (export prime? nth-prime prime-above prime-below factor perfect?
+  (export prime? nth-prime prime-above prime-below
+          factor factor-alist perfect?
           totient aliquot
           provable-prime? probable-prime?
           random-prime random-prime-distinct-from