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;;;; License: CC0-1.0
(define-module (net ricketyspace sicp two forty)
#:use-module (net ricketyspace sicp one twentyseven)
#:export (unique-pairs
prime-sum-pairs))
(define (accumulate op initial sequence)
(if (null? sequence)
initial
(op (car sequence)
(accumulate op initial (cdr sequence)))))
(define (enumerate-interval low high)
(if (> low high)
'()
(cons low (enumerate-interval (+ low 1) high))))
(define (prime-sum? pair)
(prime? (+ (car pair) (cadr pair))))
(define (make-pair-sum pair)
(list (car pair) (cadr pair) (+ (car pair) (cadr pair))))
(define (flatmap proc seq)
(accumulate append '() (map proc seq)))
(define (unique-pairs n)
(flatmap (lambda (i)
(map (lambda (j) (list i j))
(enumerate-interval 1 (- i 1))))
(enumerate-interval 1 n)))
(define (prime-sum-pairs n)
(map make-pair-sum
(filter prime-sum?
(unique-pairs n))))
;;;
;;; Guile REPL
;;;
;;; scheme@(guile-user)> ,re (net ricketyspace sicp two forty)
;;; scheme@(guile-user)> (unique-pairs 6)
;;; $8 = ((2 1) (3 1) (3 2) (4 1) (4 2) (4 3) (5 1) (5 2) (5 3) (5 4) (6 1) (6 2) (6 3) (6 4) (6 5))
;;; scheme@(guile-user)> (prime-sum-pairs 6)
;;; $9 = ((2 1 3) (3 2 5) (4 1 5) (4 3 7) (5 2 7) (6 1 7) (6 5 11))
;;;
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