1 files changed, 54 insertions, 0 deletions
diff --git a/net/ricketyspace/sicp/two/fortytwo.scm b/net/ricketyspace/sicp/two/fortytwo.scm
new file mode 100644
index 0000000..7ccf00e
--- /dev/null
+++ b/net/ricketyspace/sicp/two/fortytwo.scm
@@ -0,0 +1,54 @@
+;;;;
+;;;; 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?))))