Advent of Code 2025 Day 2

December 2, 2025

Day 2 of solving The Advent of Code in Clojure .

The 2nd day had us do some magic with numbers. The quest was to find some silly patterns in the numbers. I chose to use good old regular expressions to do the job, while a colleague of mine chose to use math. Both work really well. The 2nd part of the puzzle is to find repeating patterns, where the 1st only wanted pairs.

(ns day2
  (:require
   [clojure.string :as str]))

(def parse-int Long/parseLong)

(defn parse-ranges [input]
  (map (fn [in]
         (let [[_ start end] (first (re-seq #"(\d+)-(\d+)" in))]
           (filter #(re-matches #"(\d+)\1" (str %))
              (range (parse-int start) (inc (parse-int end))))))
       input))

(defn parse-ranges2 [input]
  (map (fn [in]
         (let [[_ start end] (first (re-seq #"(\d+)-(\d+)" in))]
           (filter #(re-matches #"(\d+)(\1+)" (str %)) 
               (range (parse-int start) (inc (parse-int end))))))
       input))

(def part1 (reduce +
            (->
            (slurp "resources/two.in")
            (str/trim)
            (str/split #",")
            (parse-ranges)
            (flatten))))

(def part2 (reduce + (->
            (slurp "resources/two.in")
            (str/trim)
            (str/split #",")
            (parse-ranges2)
            (flatten))))

(prn part1 part2)