Start of day 1.

01/01.rkt

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+#lang racket
+(require "../lib/aoc-files.rkt")
+
+;; https://adventofcode.com/2025/day/1
+
+#|
+The attached document (your puzzle input) contains a sequence of rotations, one per line, which tell you how to open the safe. A rotation starts with an L or R which indicates whether the rotation should be to the left (toward lower numbers) or to the right (toward higher numbers). Then, the rotation has a distance value which indicates how many clicks the dial should be rotated in that direction.
+
+So, if the dial were pointing at 11, a rotation of R8 would cause the dial to point at 19. After that, a rotation of L19 would cause it to point at 0.
+
+Because the dial is a circle, turning the dial left from 0 one click makes it point at 99. Similarly, turning the dial right from 99 one click makes it point at 0.
+
+So, if the dial were pointing at 5, a rotation of L10 would cause it to point at 95. After that, a rotation of R5 could cause it to point at 0.
+
+The dial starts by pointing at 50.
+
+You could follow the instructions, but your recent required official North Pole secret entrance security training seminar taught you that the safe is actually a decoy. The actual password is the number of times the dial is left pointing at 0 after any rotation in the sequence.
+|#
+
+(define (day-01r1 ls)
+  (define (add0 n) n)
+  (define (day-01h ls ptr count)
+    (cond
+      [(empty? ls) count]
+      [else
+       (define operator (if (string-prefix? (first ls) "L") - +))
+       (define dist (string->number (string-trim (first ls)
+                                                 (~a (string-ref (first ls) 0)))))
+       (define landing (operator ptr (modulo dist 100)))
+       (define result (cond
+                       [(< landing 0) (+ landing 100)]
+                       [(>= landing 100) (remainder landing 100)]
+                       [else landing]))
+       (if (or (> result 100) (< result 0))
+           (error (format "Out of bounds: ~a~n" result))
+           (day-01h (rest ls) result (if (zero? result) (add1 count) count)))]))
+  (day-01h ls 50 0))
+
+(define test-input (read-input 1 #:round 'test))
+(day-01r1 test-input)
+(define r1-input (read-input 1 #:round 'r1));
+(day-01r1 r1-input)
+
+(define (day-01r2 ls #:incr [incr (lambda (x) 1)])
+  (define (add0 n) n)
+  (define (day-01h ls ptr count)
+    (cond
+      [(empty? ls) count]
+      [else
+       (define operator (if (string-prefix? (first ls) "L") - +))
+       (define dist (string->number (string-trim (first ls)
+                                                 (~a (string-ref (first ls) 0)))))
+       (define landing (operator ptr (modulo dist 100)))
+       (define result (cond
+                       [(< landing 0) (cons (+ landing 100) (incr dist))]
+                       [(>= landing 100) (cons (remainder landing 100)  (incr dist))]
+                       [(= landing 0) (cons 0  (incr dist))]
+                       [else (cons landing 0)]))
+       (if (or (> (car result) 100) (< (car result) 0))
+           (error (format "Out of bounds: ~a~n" result))
+           (day-01h (rest ls) (car result) (if (zero? (car result)) (+ (cdr result) (add1 count)) count)))]))
+  (day-01h ls 50 0))
+
+(day-01r2 test-input)
+(day-01r2 r1-input)
+(day-01r2 test-input #:incr (lambda (dist) (modulo dist 100)))
+(day-01r2 r1-input #:incr (lambda (dist) (modulo dist 100)))