Advent of Code 2023, day 6

aoc2023-day6.clj

(ns day6
  (:require [clojure.string :as str]))

(defn parse [task]
  (->> (line-seq (io/reader "2023/day6.txt"))
       (map #(map parse-long
                  (re-seq #"\d+" (case task
                                   1 %
                                   2 (str/replace % #" " "")))))))

(defn winning-options [time distance]
  ;; a^2 - Ta + D = 0
  (let [d (- (* time time) (* 4 distance))
        x1 (long (/ (- time (Math/sqrt d)) 2))
        ;; Second root is symmetrical
        x2 (- time x1)]
    (dec (- x2 x1))))

(defn solve [task]
  (reduce * (apply map winning-options (parse task))))