aoc2021_day_5.clj

(ns io.github.galuque.aoc-2021.day-5
  (:require [clojure.java.io :as io]
            [clojure.string :as str]
            [clojure.set :as set]))

(def input (-> "day_5/input.txt"
               io/resource
               slurp))

(def sample-input "0,9 -> 5,9
8,0 -> 0,8
9,4 -> 3,4
2,2 -> 2,1
7,0 -> 7,4
6,4 -> 2,0
0,9 -> 2,9
3,4 -> 1,4
0,0 -> 8,8
5,5 -> 8,2
")

(defn parse-point [s]
  (mapv parse-long (str/split s #",")))

(defn parse-line [line]
  (mapv parse-point line))

(defn parse-input [input]
  (->> input
       (re-seq #"\d+,\d+ -> \d+,\d+")
       (mapv (comp
              (partial parse-line)
              (partial re-seq #"\d+,\d+")))))

(defn horizontal-line? [line]
  (let [[_ y1 _ y2] (flatten line)]
    (= y1 y2)))

(defn vertical-line? [line]
  (let [[x1 _ x2 _] (flatten line)]
    (= x1 x2)))

(defn line->points* [a b common]
  (partition 2 (interleave (repeat common) (range a (inc b)))))

(defn diag-range [a b]
  (if (< a b)
    (range a (inc b))
    (range a (dec b) -1 )))

(defn line->points [line]
  (let [[x1 y1 x2 y2] (flatten line)]
    (cond
      (vertical-line? line)
      (if (< y1 y2)
        (line->points* y1 y2 x1)
        (line->points* y2 y1 x1))

      (horizontal-line? line)
      (if (< x1 x2)
        (map reverse (line->points* x1 x2 y1))
        (map reverse (line->points* x2 x1 y1)))

      :else
      (partition 2
                 (interleave (diag-range x1 x2)
                             (diag-range y1 y2))))))

(defn intersections [line1 line2]
  (set/intersection (set (line->points line1))
                    (set (line->points line2))))

(defn solution [lines]
  (let [inters (for [line1 lines
                     line2 lines
                     :when (not= line1 line2)]
                 (intersections line1 line2))]
    (->> inters
         (filter not-empty)
         frequencies
         (filter #(-> % val (>= 2)))
         (map first)
         (apply set/union)
         count)))

(defn part-1 [input]
  (let [lines (filterv (some-fn vertical-line?
                                horizontal-line?)
                       input)]
    (solution lines)))

(comment

  (part-1 (parse-input sample-input))
  ;; => 5

  (part-1 (parse-input input))
  ;; => 6710

  )

(def part-2 solution)

(comment

  (part-2 (parse-input sample-input))
  ;; => 12
  
  (part-2 (parse-input input))
  ;; => 20121
  )