#lang sicp

;; Library functions
(define (accumulate op initial sequence)
  (if (null? sequence)
      initial
      (op (car sequence)
          (accumulate op
                      initial
                      (cdr sequence)))))
(define (filter predicate sequence)
  (cond ((null? sequence) nil)
        ((predicate (car sequence))
         (cons (car sequence)
               (filter predicate
                       (cdr sequence))))
        (else  (filter predicate
                       (cdr sequence)))))
(define (smallest-divisor n)
  (find-divisor n 2))
(define (square n) (* n n))
(define (find-divisor n test-divisor)
  (cond ((> (square test-divisor) n)
         n)
        ((divides? test-divisor n)
         test-divisor)
        (else (find-divisor
               n
               (+ test-divisor 1)))))
(define (divides? a b)
  (= (remainder b a) 0))
(define (prime? n)
  (= n (smallest-divisor n)))



(define (enumerate-interval low high)
  (if (> low high)
      nil
      (cons low 
            (enumerate-interval 
             (+ low 1) 
             high))))

(define (sum-primes-1 a b)
  (define (iter count accum)
    (cond ((> count b) accum)
          ((prime? count)
           (iter (+ count 1)
                 (+ count accum)))
          (else (iter (+ count 1) accum))))
  (iter a 0))

(define (sum-primes-2 a b)
  (accumulate
   +
   0
   (filter prime? (enumerate-interval a b))))


(sum-primes-1 1 1000)
(sum-primes-2 1 1000)