Solve Problem 3.

1 files changed, 38 insertions, 0 deletions

diff --git a/problem-3.scm b/problem-3.scm
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+(use-modules (srfi srfi-1))
+
+(define (prime-sieve n)
+  (define (mark-multiples! bits x)
+    (define (mark y)
+      (when (< y n)
+        (bitvector-set! bits y #t)
+        (mark (+ y y))))
+    (mark x))
+  (define (sieve m primes bits)
+    (cond ((= m n)
+           primes)
+          ((bitvector-ref bits m)
+           (sieve (1+ m) primes bits))
+          (else
+           (mark-multiples! bits m)
+           (sieve (1+ m) (cons m primes) bits))))
+  (reverse (sieve 2 '() (make-bitvector n))))
+
+(define (divisible? m n)
+  (zero? (modulo m n)))
+
+(define (integer x)
+  (inexact->exact (floor x)))
+
+(define (factors n)
+  (define (factor n primes factors)
+    (cond ((null? primes)
+           factors)
+          ((divisible? n (car primes))
+           (factor (/ n (car primes))
+                   (cdr primes)
+                   (cons (car primes) factors)))
+          (else
+           (factor n (cdr primes) factors))))
+  (factor n (prime-sieve (integer (1+ (sqrt n)))) '()))
+
+(car (factors 600851475143))