1 files changed, 34 insertions, 0 deletions
diff --git a/01/lib.scm b/01/lib.scm
new file mode 100644
index 0000000..fcdf642
--- /dev/null
+++ b/01/lib.scm
@@ -0,0 +1,34 @@
+(library (lib)
+  (export solve-part1 solve-part2)
+  (import (chezscheme))
+
+  ; Rotate contents of `ilist' to the right `times' times
+  (define (list-rotate ilist times)
+    (let loop ((xs ilist) (n times))
+      (if (= n 0)
+          xs
+          (loop (append (cdr xs) (list (car xs))) (- n 1)))))
+
+  (define (solve-puzzle chars chars-rot)
+    (fold-left
+      (lambda (accum c1 c2)
+        ; Does `k'th char in list match with `k'th char in rotated list?
+        (if (char=? c1 c2)
+            ; Assume `c1' is a digit, convert it to integer in [0,9]
+            (+ accum (string->number (string c1)))
+            accum))
+      0 chars chars-rot))
+
+  ; Compare each char to its right neighbour in circular list
+  (define (solve-part1 str)
+    (let* ((chars (string->list str))
+           (chars-rot (list-rotate chars 1)))
+      (solve-puzzle chars chars-rot)))
+
+  ; Compare each char to char halfway across circular list
+  (define (solve-part2 str)
+    (let* ((chars (string->list str))
+           (chars-rot (list-rotate chars (div (length chars) 2))))
+      (solve-puzzle chars chars-rot)))
+
+)