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;;;;
;;;; License: CC0-1.0
;;;;
(define-module (net ricketyspace sicp two fortytwo)
#:export (safe?))
(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 (position-row pos)
(car pos))
(define (position-col pos)
(car (cdr pos)))
(define (abs-diff pos-a pos-b)
(cons (abs (- (position-row pos-a) (position-row pos-b)))
(abs (- (position-col pos-a) (position-col pos-b)))))
(define (safe? k positions)
(define (kth-position)
(let ((kpos (filter (lambda (pos)
(= (position-col pos) k))
positions)))
(if (not (null? kpos)) (car kpos) #f)))
(define (same-row?)
(let* ((kpos (kth-position))
(in-same-row (filter (lambda (pos)
(and (not (equal? pos kpos))
(= (position-row pos)
(position-row kpos))))
positions)))
(if (null? in-same-row) #f #t)))
(define (same-diagonal?)
(let* ((kpos (kth-position))
(in-same-diag (filter (lambda (pos)
(let ((diff (abs-diff pos kpos)))
(and (not (equal? pos kpos))
(= (car diff) (cdr diff)))))
positions)))
(if (null? in-same-diag) #f #t)))
(not (or (same-row?) (same-diagonal?))))
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