day 4

1 files changed, 59 insertions, 0 deletions

diff --git a/04/2.scm b/04/2.scm
new file mode 100644
index 0000000..fbd5694
--- /dev/null
+++ b/04/2.scm
@@ -0,0 +1,59 @@
+(import (chicken string))
+(import (chicken io))
+(import (chicken sort))
+(import matchable)
+(import srfi-1)
+(import regex)
+
+(define input
+  (with-input-from-file "input"
+    (lambda ()
+      (let loop ((ret '()))
+        (let ((line (read-line)))
+          (if (eof-object? line)
+              (reverse ret)
+              (loop (cons line ret))))))))
+
+(define (transpose in)
+  (define cols (string-length (car in)))
+  (let loop ((i 0) (ret '()))
+    (if (>= i cols)
+        (reverse ret)
+        (loop (+ i 1) (cons (list->string (map (lambda (x) (string-ref x i))
+                                                in))
+                            ret)))))
+
+(define (flip in)
+  (define (flip-row row)
+    (list->string (reverse (string->list row))))
+  (map flip-row in))
+
+(define (char-at in x y)
+  (string-ref (list-ref in y) x))
+
+(define (check-xmas mat x y)
+  (and (equal? #\M (char-at mat x y))
+       (equal? #\M (char-at mat x (+ 2 y)))
+       (equal? #\S (char-at mat (+ x 2) y))
+       (equal? #\S (char-at mat (+ x 2) (+ y 2)))
+       (equal? #\A (char-at mat (+ x 1) (+ y 1)))))
+
+(define (count-mat mat)
+  (define count 0)
+  (define width (string-length (car mat)))
+  (define height (length mat))
+  (do ((x 0 (+ 1 x)))
+      ((>= x (- width 2)) #f)
+    (do ((y 0 (+ 1 y)))
+        ((>= y (- height 2)) #f)
+      (if (check-xmas mat x y)
+          (set! count (+ 1 count))
+          #f)))
+  count)
+
+(define mat-list
+  (list input (flip input)
+        (transpose input) (flip (transpose input))))
+
+(display (apply + (map count-mat mat-list)))
+(newline)