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-rw-r--r--net/ricketyspace/sicp/two/fortytwo.scm40
1 files changed, 38 insertions, 2 deletions
diff --git a/net/ricketyspace/sicp/two/fortytwo.scm b/net/ricketyspace/sicp/two/fortytwo.scm
index e284591..af5b500 100644
--- a/net/ricketyspace/sicp/two/fortytwo.scm
+++ b/net/ricketyspace/sicp/two/fortytwo.scm
@@ -4,7 +4,8 @@
(define-module (net ricketyspace sicp two fortytwo)
#:export (safe?
- adjoin-position))
+ adjoin-position
+ queens))
(define (accumulate op initial sequence)
(if (null? sequence)
@@ -24,7 +25,7 @@
(car pos))
(define (position-col pos)
- (car (cdr pos)))
+ (cdr pos))
(define (abs-diff pos-a pos-b)
(cons (abs (- (position-row pos-a) (position-row pos-b)))
@@ -58,3 +59,38 @@
(define (adjoin-position new-row k rest-of-queens)
(let ((queen-at-k (cons new-row k)))
(append rest-of-queens (list queen-at-k))))
+
+(define empty-board '())
+
+(define (queens board-size)
+ (define (queen-cols k)
+ (if (= k 0)
+ (list empty-board)
+ (filter
+ (lambda (positions) (safe? k positions))
+ (flatmap
+ (lambda (rest-of-queens)
+ (map (lambda (new-row)
+ (adjoin-position new-row k rest-of-queens))
+ (enumerate-interval 1 board-size)))
+ (queen-cols (- k 1))))))
+ (queen-cols board-size))
+
+;;;
+;;; Guile REPL
+;;;
+;;; scheme@(guile-user)> ,use (net ricketyspace sicp two fortytwo)
+;;; scheme@(guile-user)> (queens 8)
+;;; $2 = (((1 . 1) (5 . 2) (8 . 3) (6 . 4) (3 . 5) (7 . 6) (2 . 7) (4 . 8)) ((1 . 1) (6 . 2) (8 . 3) (3 . 4) (7 . 5) (4 . 6) (2 . 7) (5 . 8)) ((1 . 1) (7 . 2) (4 . 3) (6 . 4) (8 . 5) (2 . 6) (5 . 7) (3 . 8)) ((1 . 1) (7 . 2) (5 . 3) (8 . 4) (2 . 5) (4 . 6) (6 . 7) (3 . 8)) ((2 . 1) (4 . 2) (6 . 3) (8 . 4) (3 . 5) (1 . 6) (7 . 7) (5 . 8)) ((2 . 1) (5 . 2) (7 . 3) (1 . 4) (3 . 5) (8 . 6) (6 . 7) (4 . 8)) ((2 . 1) (5 . 2) (7 . 3) (4 . 4) (1 . 5) (8 . 6) (6 . 7) (3 . 8)) ((2 . 1) (6 . 2) (1 . 3) (7 . 4) (4 . 5) (8 . 6) (3 . 7) (5 . 8)) ((2 . 1) (6 . 2) (8 . 3) (3 . 4) (1 . 5) (4 . 6) (7 . 7) (5 . 8)) ((2 . 1) (7 . 2) (3 . 3) (6 . 4) (8 . 5) (5 . 6) (1 . 7) (4 . 8)) ((2 . 1) (7 . 2) (5 . 3) (8 . 4) (1 . 5) (4 . 6) (6 . 7) (3 . 8)) ((2 . 1) (8 . 2) (6 . 3) (1 . 4) (3 . 5) (5 . 6) (7 . 7) (4 . 8)) ((3 . 1) (1 . 2) (7 . 3) (5 . 4) (8 . 5) (2 . 6) (4 . 7) (6 . 8)) ((3 . 1) (5 . 2) (2 . 3) (8 . 4) (1 . 5) (7 . 6) (4 . 7) (6 . 8)) ((3 . 1) (5 . 2) (2 . 3) (8 . 4) (6 . 5) (4 . 6) (7 . 7) (1 . 8)) ((3 . 1) (5 . 2) (7 . 3) (1 . 4) (4 . 5) (2 . 6) (8 . 7) (6 . 8)) ((3 . 1) (5 . 2) (8 . 3) (4 . 4) (1 . 5) (7 . 6) (2 . 7) (6 . 8)) ((3 . 1) (6 . 2) (2 . 3) (5 . 4) (8 . 5) (1 . 6) (7 . 7) (4 . 8)) ((3 . 1) (6 . 2) (2 . 3) (7 . 4) (1 . 5) (4 . 6) (8 . 7) (5 . 8)) ((3 . 1) (6 . 2) (2 . 3) (7 . 4) (5 . 5) (1 . 6) (8 . 7) (4 . 8)) ((3 . 1) (6 . 2) (4 . 3) (1 . 4) (8 . 5) (5 . 6) (7 . 7) (2 . 8)) ((3 . 1) (6 . 2) (4 . 3) (2 . 4) (8 . 5) (5 . 6) (7 . 7) (1 . 8)) ((3 . 1) (6 . 2) (8 . 3) (1 . 4) (4 . 5) (7 . 6) (5 . 7) (2 . 8)) ((3 . 1) (6 . 2) (8 . 3) (1 . 4) (5 . 5) (7 . 6) (2 . 7) (4 . 8)) ((3 . 1) (6 . 2) (8 . 3) (2 . 4) (4 . 5) (1 . 6) (7 . 7) (5 . 8)) ((3 . 1) (7 . 2) (2 . 3) (8 . 4) (5 . 5) (1 . 6) (4 . 7) (6 . 8)) ((3 . 1) (7 . 2) (2 . 3) (8 . 4) (6 . 5) (4 . 6) (1 . 7) (5 . 8)) ((3 . 1) (8 . 2) (4 . 3) (7 . 4) (1 . 5) (6 . 6) (2 . 7) (5 . 8)) ((4 . 1) (1 . 2) (5 . 3) (8 . 4) (2 . 5) (7 . 6) (3 . 7) (6 . 8)) ((4 . 1) (1 . 2) (5 . 3) (8 . 4) (6 . 5) (3 . 6) (7 . 7) (2 . 8)) ((4 . 1) (2 . 2) (5 . 3) (8 . 4) (6 . 5) (1 . 6) (3 . 7) (7 . 8)) ((4 . 1) (2 . 2) (7 . 3) (3 . 4) (6 . 5) (8 . 6) (1 . 7) (5 . 8)) ((4 . 1) (2 . 2) (7 . 3) (3 . 4) (6 . 5) (8 . 6) (5 . 7) (1 . 8)) ((4 . 1) (2 . 2) (7 . 3) (5 . 4) (1 . 5) (8 . 6) (6 . 7) (3 . 8)) ((4 . 1) (2 . 2) (8 . 3) (5 . 4) (7 . 5) (1 . 6) (3 . 7) (6 . 8)) ((4 . 1) (2 . 2) (8 . 3) (6 . 4) (1 . 5) (3 . 6) (5 . 7) (7 . 8)) ((4 . 1) (6 . 2) (1 . 3) (5 . 4) (2 . 5) (8 . 6) (3 . 7) (7 . 8)) ((4 . 1) (6 . 2) (8 . 3) (2 . 4) (7 . 5) (1 . 6) (3 . 7) (5 . 8)) ((4 . 1) (6 . 2) (8 . 3) (3 . 4) (1 . 5) (7 . 6) (5 . 7) (2 . 8)) ((4 . 1) (7 . 2) (1 . 3) (8 . 4) (5 . 5) (2 . 6) (6 . 7) (3 . 8)) ((4 . 1) (7 . 2) (3 . 3) (8 . 4) (2 . 5) (5 . 6) (1 . 7) (6 . 8)) ((4 . 1) (7 . 2) (5 . 3) (2 . 4) (6 . 5) (1 . 6) (3 . 7) (8 . 8)) ((4 . 1) (7 . 2) (5 . 3) (3 . 4) (1 . 5) (6 . 6) (8 . 7) (2 . 8)) ((4 . 1) (8 . 2) (1 . 3) (3 . 4) (6 . 5) (2 . 6) (7 . 7) (5 . 8)) ((4 . 1) (8 . 2) (1 . 3) (5 . 4) (7 . 5) (2 . 6) (6 . 7) (3 . 8)) ((4 . 1) (8 . 2) (5 . 3) (3 . 4) (1 . 5) (7 . 6) (2 . 7) (6 . 8)) ((5 . 1) (1 . 2) (4 . 3) (6 . 4) (8 . 5) (2 . 6) (7 . 7) (3 . 8)) ((5 . 1) (1 . 2) (8 . 3) (4 . 4) (2 . 5) (7 . 6) (3 . 7) (6 . 8)) ((5 . 1) (1 . 2) (8 . 3) (6 . 4) (3 . 5) (7 . 6) (2 . 7) (4 . 8)) ((5 . 1) (2 . 2) (4 . 3) (6 . 4) (8 . 5) (3 . 6) (1 . 7) (7 . 8)) ((5 . 1) (2 . 2) (4 . 3) (7 . 4) (3 . 5) (8 . 6) (6 . 7) (1 . 8)) ((5 . 1) (2 . 2) (6 . 3) (1 . 4) (7 . 5) (4 . 6) (8 . 7) (3 . 8)) ((5 . 1) (2 . 2) (8 . 3) (1 . 4) (4 . 5) (7 . 6) (3 . 7) (6 . 8)) ((5 . 1) (3 . 2) (1 . 3) (6 . 4) (8 . 5) (2 . 6) (4 . 7) (7 . 8)) ((5 . 1) (3 . 2) (1 . 3) (7 . 4) (2 . 5) (8 . 6) (6 . 7) (4 . 8)) ((5 . 1) (3 . 2) (8 . 3) (4 . 4) (7 . 5) (1 . 6) (6 . 7) (2 . 8)) ((5 . 1) (7 . 2) (1 . 3) (3 . 4) (8 . 5) (6 . 6) (4 . 7) (2 . 8)) ((5 . 1) (7 . 2) (1 . 3) (4 . 4) (2 . 5) (8 . 6) (6 . 7) (3 . 8)) ((5 . 1) (7 . 2) (2 . 3) (4 . 4) (8 . 5) (1 . 6) (3 . 7) (6 . 8)) ((5 . 1) (7 . 2) (2 . 3) (6 . 4) (3 . 5) (1 . 6) (4 . 7) (8 . 8)) ((5 . 1) (7 . 2) (2 . 3) (6 . 4) (3 . 5) (1 . 6) (8 . 7) (4 . 8)) ((5 . 1) (7 . 2) (4 . 3) (1 . 4) (3 . 5) (8 . 6) (6 . 7) (2 . 8)) ((5 . 1) (8 . 2) (4 . 3) (1 . 4) (3 . 5) (6 . 6) (2 . 7) (7 . 8)) ((5 . 1) (8 . 2) (4 . 3) (1 . 4) (7 . 5) (2 . 6) (6 . 7) (3 . 8)) ((6 . 1) (1 . 2) (5 . 3) (2 . 4) (8 . 5) (3 . 6) (7 . 7) (4 . 8)) ((6 . 1) (2 . 2) (7 . 3) (1 . 4) (3 . 5) (5 . 6) (8 . 7) (4 . 8)) ((6 . 1) (2 . 2) (7 . 3) (1 . 4) (4 . 5) (8 . 6) (5 . 7) (3 . 8)) ((6 . 1) (3 . 2) (1 . 3) (7 . 4) (5 . 5) (8 . 6) (2 . 7) (4 . 8)) ((6 . 1) (3 . 2) (1 . 3) (8 . 4) (4 . 5) (2 . 6) (7 . 7) (5 . 8)) ((6 . 1) (3 . 2) (1 . 3) (8 . 4) (5 . 5) (2 . 6) (4 . 7) (7 . 8)) ((6 . 1) (3 . 2) (5 . 3) (7 . 4) (1 . 5) (4 . 6) (2 . 7) (8 . 8)) ((6 . 1) (3 . 2) (5 . 3) (8 . 4) (1 . 5) (4 . 6) (2 . 7) (7 . 8)) ((6 . 1) (3 . 2) (7 . 3) (2 . 4) (4 . 5) (8 . 6) (1 . 7) (5 . 8)) ((6 . 1) (3 . 2) (7 . 3) (2 . 4) (8 . 5) (5 . 6) (1 . 7) (4 . 8)) ((6 . 1) (3 . 2) (7 . 3) (4 . 4) (1 . 5) (8 . 6) (2 . 7) (5 . 8)) ((6 . 1) (4 . 2) (1 . 3) (5 . 4) (8 . 5) (2 . 6) (7 . 7) (3 . 8)) ((6 . 1) (4 . 2) (2 . 3) (8 . 4) (5 . 5) (7 . 6) (1 . 7) (3 . 8)) ((6 . 1) (4 . 2) (7 . 3) (1 . 4) (3 . 5) (5 . 6) (2 . 7) (8 . 8)) ((6 . 1) (4 . 2) (7 . 3) (1 . 4) (8 . 5) (2 . 6) (5 . 7) (3 . 8)) ((6 . 1) (8 . 2) (2 . 3) (4 . 4) (1 . 5) (7 . 6) (5 . 7) (3 . 8)) ((7 . 1) (1 . 2) (3 . 3) (8 . 4) (6 . 5) (4 . 6) (2 . 7) (5 . 8)) ((7 . 1) (2 . 2) (4 . 3) (1 . 4) (8 . 5) (5 . 6) (3 . 7) (6 . 8)) ((7 . 1) (2 . 2) (6 . 3) (3 . 4) (1 . 5) (4 . 6) (8 . 7) (5 . 8)) ((7 . 1) (3 . 2) (1 . 3) (6 . 4) (8 . 5) (5 . 6) (2 . 7) (4 . 8)) ((7 . 1) (3 . 2) (8 . 3) (2 . 4) (5 . 5) (1 . 6) (6 . 7) (4 . 8)) ((7 . 1) (4 . 2) (2 . 3) (5 . 4) (8 . 5) (1 . 6) (3 . 7) (6 . 8)) ((7 . 1) (4 . 2) (2 . 3) (8 . 4) (6 . 5) (1 . 6) (3 . 7) (5 . 8)) ((7 . 1) (5 . 2) (3 . 3) (1 . 4) (6 . 5) (8 . 6) (2 . 7) (4 . 8)) ((8 . 1) (2 . 2) (4 . 3) (1 . 4) (7 . 5) (5 . 6) (3 . 7) (6 . 8)) ((8 . 1) (2 . 2) (5 . 3) (3 . 4) (1 . 5) (7 . 6) (4 . 7) (6 . 8)) ((8 . 1) (3 . 2) (1 . 3) (6 . 4) (2 . 5) (5 . 6) (7 . 7) (4 . 8)) ((8 . 1) (4 . 2) (1 . 3) (3 . 4) (6 . 5) (2 . 6) (7 . 7) (5 . 8)))
+;;; scheme@(guile-user)> (length $2)
+;;; $3 = 92
+;;; scheme@(guile-user)> (list-ref $2 (random 93))
+;;; $4 = ((5 . 1) (2 . 2) (6 . 3) (1 . 4) (7 . 5) (4 . 6) (8 . 7) (3 . 8))
+;;; scheme@(guile-user)> (list-ref $2 (random 93))
+;;; $5 = ((4 . 1) (6 . 2) (1 . 3) (5 . 4) (2 . 5) (8 . 6) (3 . 7) (7 . 8))
+;;; scheme@(guile-user)> (list-ref $2 (random 93))
+;;; $6 = ((7 . 1) (2 . 2) (4 . 3) (1 . 4) (8 . 5) (5 . 6) (3 . 7) (6 . 8))
+;;; scheme@(guile-user)> (list-ref $2 (random 93))
+;;; $7 = ((6 . 1) (3 . 2) (1 . 3) (8 . 4) (5 . 5) (2 . 6) (4 . 7) (7 . 8))
+;;; scheme@(guile-user)> (list-ref $2 (random 93))
+;;; $8 = ((4 . 1) (8 . 2) (5 . 3) (3 . 4) (1 . 5) (7 . 6) (2 . 7) (6 . 8))