solve advent of code 2023 08

1 files changed, 60 insertions, 0 deletions

diff --git a/advent-of-code/2023/08/2.rkt b/advent-of-code/2023/08/2.rkt
new file mode 100644
index 0000000..35ca47a
--- /dev/null
+++ b/advent-of-code/2023/08/2.rkt
@@ -0,0 +1,60 @@
+#lang racket
+
+(require "../../lib/utils.rkt")
+
+(define lines
+  (call-with-input-file "input"
+    (lambda (fp)
+      (get-lines fp))))
+
+(define instructions (string->list (car lines)))
+
+(define (make-instruction-generator)
+  (define cur instructions)
+  (lambda ()
+    (when (null? cur)
+      (set! cur instructions))
+    (define ret (car cur))
+    (set! cur (cdr cur))
+    ret))
+
+(define next-instruction (make-instruction-generator))
+
+(set! lines (cddr lines))
+
+(define (parse-map str)
+  (define p1 (string-split str "="))
+  (define key (string-trim (car p1)))
+  (define val-str (string-trim (cadr p1)))
+  (define value (list (substring val-str 1 4) (substring val-str 6 9)))
+  (cons key value))
+
+(define the-map (make-hash (map parse-map lines)))
+
+(define (is-start? str)
+  (char=? #\A (string-ref str 2)))
+
+(define (is-end? str)
+  (char=? #\Z (string-ref str 2)))
+
+(define (simulate start)
+  (let loop ((i 0) (cur start))
+    (if (is-end? cur)
+      i
+      (let ()
+        (define instruction (next-instruction))
+        (define (next-point point)
+          (define mapping (hash-ref the-map point))
+          (if (char=? instruction #\L)
+            (car mapping)
+            (cadr mapping)))
+        (loop (+ 1 i) (next-point cur))))))
+
+
+(define starts
+  (let ()
+    (define lst (hash->list the-map))
+    (define keys (map car lst))
+    (filter is-start? keys)))
+
+(apply lcm (map simulate starts))
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