1 files changed, 54 insertions, 0 deletions
diff --git a/perfectly-spherical-houses-in-a-vacuum/pshiav.scm b/perfectly-spherical-houses-in-a-vacuum/pshiav.scm
new file mode 100644
index 0000000..d117cce
--- /dev/null
+++ b/perfectly-spherical-houses-in-a-vacuum/pshiav.scm
@@ -0,0 +1,54 @@
+(define-module (pshiav)
+  #:use-module (srfi srfi-1)
+  #:export (pshiav
+	    add-coordinates
+	    direction->pair))
+
+(define (pshiav str num)
+  "Given string of cardinal directions STR,
+calculates the number of visited places on
+a grid.
+
+NUM specifies the number of visitors. Will
+always be at least one."
+  (set! num (max num 1))
+  (let ((visitors (map (lambda (x)
+			 (add-coordinates))
+		       (iota num))))
+    (let loop ((i 0) (lst (map direction->pair
+			       (string->list str))))
+      (if (null? lst)
+	  (length (delete-duplicates
+		   (apply append (map (lambda (visitor)
+					(visitor '(0 . 0)))
+				      visitors))))
+	  (begin
+	    ((list-ref visitors i) (car lst))
+	    (loop (modulo (1+ i) (length visitors))
+		  (cdr lst)))))))
+
+(define (add-coordinates)
+  (let ((hist '((0 . 0))))
+    (lambda dir
+      "Given cons pair DIR, increments the previous
+position, and returns HIST, a history of visited
+positions."
+      (set! hist (cons
+		  (cons (+ (caar dir)
+			   (caar hist))
+			(+ (cdar dir)
+			   (cdar hist)))
+		  hist))
+      hist)))
+
+
+(define (direction->pair char)
+  "Given a character representing one of the four
+cardinal directions, return a pair representing
+a change in coordinates."
+  (cond
+   ((char=? char #\^) '(0 . 1))
+   ((char=? char #\>) '(1 . 0))
+   ((char=? char #\v) '(0 . -1))
+   ((char=? char #\<) '(-1 . 0))
+   (#t '(0 . 0))))