(ns clojure-solution.core
  (:require [loom.graph :as loom])
  (:gen-class))

(defn make-graph [graph line]
  (let [[_ x y] (re-find #"Step (.) must be finished before step (.) can begin\." line)]
    (assoc graph x (conj (get graph x #{}) y))))

(defn read-file [path]
  (with-open [rdr (clojure.java.io/reader path)]
    (reduce make-graph {} (line-seq rdr))))

(defn get-min [dg]
  (->>
    (for [node (loom/nodes dg) :when (= (loom/in-degree dg node) 0)] node)
    (apply min-key #(int (char (first %))))))

(defn solve [graph]
  (loop [order [] dg graph]
    (if (empty? (loom/nodes dg))
      order
      (let [m (get-min dg)]
        (recur (conj order m) (loom/remove-nodes dg m))))))


(defn part1 [graph]
  (let [dg (loom/digraph graph)]
    (solve dg)))

(defn -main
  [& args]
  (time (println (part1 (read-file "resources/input.txt")))))