day_3.clj

(ns advent-2019.day-3
  (:require [clojure.set :as set]))

(def directions {"U" [0 -1]
                 "D" [0 1]
                 "L" [-1 0]
                 "R" [1 0]})

(defn input-item-to-list [input-item]
  (let [[input-again direction count] (re-matches #"(.)(\d*)" input-item)
        count (Integer. count)]
    (repeat count
            (directions direction))))

(defn input-items-to-list [input-items]
  (mapcat input-item-to-list input-items))

;;copy, put as strings, e.g. (def w1input (input-items-to-list ["R1003","U476","L876","U147"]))
;;This does require manipulation of the input data, but that a choice for speed.
(def w1input (input-items-to-list []))
(def w2input (input-items-to-list []))

(defn positions
  "Given a set of STEPS (like [[0 1] [-1 0]]), return all positions taken by those steps.

  The result from the example is [[0 0] [0 1] [-1 1]]."
  [steps]
  (into #{}
        (reductions (partial map +)
                    [0 0]
                    steps)))

(def w1positions (positions w1input))
(def w2positions (positions w2input))

(def common-positions
  (set/intersection w1positions w2positions))

(defn manhattan-distance
  ([position]
   (manhattan-distance [0 0] position))
  ([[p1x p1y :as position1]
    [p2x p2y :as position2]]
   (+ (Math/abs (- p2x p1x))
      (Math/abs (- p2y p1y)))))

(def p1-solution
  (apply min (map manhattan-distance (disj common-positions [0 0]))))