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;;;; License: CC0-1.0
;;;;
;;;; ___________________________________
;;;; < let's make some distinct bastards >
;;;; -----------------------------------
;;;; \
;;;; \
;;;; ,;;;;;;;,
;;;; ;;;;;;;;;;;,
;;;; ;;;;;'_____;'
;;;; ;;;(/))))|((\
;;;; _;;((((((|))))
;;;; / |_\\\\\\\\\\\\
;;;; .--~( \ ~))))))))))))
;;;; / \ `\-(((((((((((\\
;;;; | | `\ ) |\ /|)
;;;; | | `. _/ \_____/ |
;;;; | , `\~ /
;;;; | \ \ /
;;;; | `. `\| /
;;;; | ~- `\ /
;;;; \____~._/~ -_, (\
;;;; |-----|\ \ ';;
;;;; | | :;;;' \
;;;; | / | |
;;;; | | |
;;;;
(define-module (net ricketyspace sicp two fortyone)
#:export (bastard-triplets))
(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 (flatmap proc seq)
(accumulate append '() (map proc seq)))
(define (bastard-triplets n s)
(define (spawn-triplet i n)
(flatmap (lambda (j)
(map (lambda (k) (list i j k))
(enumerate-interval 1 n)))
(enumerate-interval 1 n)))
(define (spawn-triplets n)
(flatmap (lambda (i) (spawn-triplet i n))
(enumerate-interval 1 n)))
(define (distinct-sum-equals-s? triplet)
(let ((i (car triplet))
(j (cadr triplet))
(k (caddr triplet)))
(and
;; check if i, j, k are distinct.
(cond ((= i j) #f)
((= i k) #f)
((= j k) #f)
(else #t))
;; check if i + j + k = s
(= (+ i j k) s))))
(filter distinct-sum-equals-s? (spawn-triplets n)))
;;;
;;; Guile REPL
;;;
;;; scheme@(guile-user)> ,re (net ricketyspace sicp two fortyone)
;;; scheme@(guile-user)> (bastard-triplets 10 6)
;;; $8 = ((1 2 3) (1 3 2) (2 1 3) (2 3 1) (3 1 2) (3 2 1))
;;;
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