1 files changed, 62 insertions, 0 deletions
diff --git a/day25.scm b/day25.scm
new file mode 100644
index 0000000..57cdb9d
--- /dev/null
+++ b/day25.scm
@@ -0,0 +1,62 @@
+(define-module (day25)
+  #:use-module (ice-9 rdelim)
+  #:use-module (rnrs io ports)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-43))
+
+(define +block-chars+ (char-set #\# #\.))
+
+(define (read-key-or-lock port topc expect-end)
+  (define vec (make-vector 5 0))
+  (let iter ((i 0))
+    (unless (= i 5)
+      (let ((line (read-line port)))
+        (when (eof-object? line)
+          (error "Unexpected EOF in the middle of a key."))
+        (unless (string-every +block-chars+ line)
+          (error "Lines in keys/locks must be '#' or '.'." line))
+        (let jiter ((j 0))
+          (unless (= j 5)
+            (when (char=? topc (string-ref line j))
+              (unless (= i (vector-ref vec j))
+                (error
+                 (string-append "Columns must be all '#' followed by all '.' "
+                                "or vice versa; cannot alternate.")
+                 vec line i j))
+              (vector-set! vec j (+ i 1)))
+            (jiter (+ j 1)))))
+      (iter (+ i 1))))
+  (define end-line (read-line port))
+  (unless (equal? end-line expect-end)
+    (error "Expected key/lock end line to be ~a; got ~a." expect-end end-line))
+  (define blank (read-line port))
+  (unless (or (eof-object? blank) (string=? "" blank))
+    (error "Expected blank line or end-of-file." blank))
+  vec)
+
+(define (read-key port)
+  (define line (read-line port))
+  (cond ((or (eof-object? line) (string=? "" line))
+         (values (eof-object) (eof-object)))
+        ((string=? line ".....")
+         (values 'key (read-key-or-lock port #\. "#####")))
+        ((string=? line "#####")
+         (values 'lock (read-key-or-lock port #\# ".....")))
+        (else (error "Expected key ('.....') or lock ('#####')." line))))
+
+(define (read-input port)
+  (let iter ((keys '()) (locks '()))
+    (define-values (type vec) (read-key port))
+    (cond ((eof-object? type) (values keys locks))
+          ((eq? type 'key) (iter (cons vec keys) locks))
+          (else (iter keys (cons vec locks))))))
+
+(define (part1 port)
+  (define-values (keys locks) (read-input port))
+  (fold (lambda (key cnt)
+          (fold (lambda (lock cnt)
+                  (if (vector-every >= key lock) (+ cnt 1) cnt)) cnt locks))
+        0 keys))
+
+(define (part2 port)
+  "yay!")