Merge branch 'main' of https://github.com/hunkyjimpjorps/AdventOfCode

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diff --git a/aoc2018/day-05/day-05.rkt b/aoc2018/day-05/day-05.rkt
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+#lang racket
+
+(require advent-of-code
+         threading)
+
+(define starting-chain
+  (~> (fetch-aoc-input (find-session) 2018 5 #:cache #true) string-trim string->list))
+
+(define (reactive-pair? ch1 ch2)
+  (and (equal? (char-downcase ch1) (char-downcase ch2)) (not (equal? ch1 ch2))))
+
+(define (remove-reactive-pairs chs [acc '()])
+  (match chs
+    [(list* ch1 ch2 rest-chs)
+     #:when (reactive-pair? ch1 ch2)
+     (remove-reactive-pairs rest-chs acc)]
+    [(list* ch rest-chs) (remove-reactive-pairs rest-chs (cons ch acc))]
+    [(list) (reverse acc)]))
+
+(define (keep-removing-reactive-pairs chs)
+  (define chs* (remove-reactive-pairs chs))
+  (if (equal? chs chs*) (length chs) (keep-removing-reactive-pairs chs*)))
+
+;; part 1
+(keep-removing-reactive-pairs starting-chain)
+
+;; part 2
+(~>> (for/list ([letter (in-string "abcdefghijklmnopqrstuvwxyz")])
+       (define tweaked-chain (filter (λ (c) (not (equal? (char-downcase c) letter))) starting-chain))
+       (keep-removing-reactive-pairs tweaked-chain))
+     (apply min))