cracking.p_02_05

(ns cracking.p_02_05
  (:require [clojure.test :refer :all]))

(defn different? [[a b]] (not (= a b)))

(defn first-elem [[a b]] a)

; This is a solution using "Floyd's cycle-finding algorithm"
(defn detect-loop [coll]
  (let [tortoise (drop 1 coll)
        hare (drop 2 coll)
        steps (map vector tortoise (take-nth 2 hare))
        met (drop-while different? steps)
        reset (map vector coll (map first-elem met))]
    (first-elem (first (drop-while different? reset)))))

(def circular (concat [\a \b] (cycle [\c \d \e])))

(deftest a-test
  (testing
    (is (= (detect-loop circular) \c))))