replace Task 2's algorithm

I found this algorithm by examining Simon Proctor's answer and it is
much more in the style of functional programming, which fits Clojure
better.

1 files changed, 13 insertions, 15 deletions

diff --git a/challenge-075/tyler-wardhaugh/clojure/src/tw/weekly/ch_2.clj b/challenge-075/tyler-wardhaugh/clojure/src/tw/weekly/ch_2.clj
index 72a4e05269..d7884efb29 100644
--- a/challenge-075/tyler-wardhaugh/clojure/src/tw/weekly/ch_2.clj
+++ b/challenge-075/tyler-wardhaugh/clojure/src/tw/weekly/ch_2.clj
@@ -1,27 +1,25 @@
 (ns tw.weekly.ch-2
   (:require [clojure.edn :as edn]))
 
-; algorithm source: https://stackoverflow.com/a/35931960
-;   Answer by Spectral (https://stackoverflow.com/users/2558887/spectral)
-;   (modified from Python)
+; algorithm source: https://github.com/manwar/perlweeklychallenge-club/blob/master/challenge-075/simon-proctor/raku/ch-2.raku
+;   Challenge answer by Simon Proctor
+;   (modified from Raku)
 
 (defn largest-rectangle-histogram
   "Compute the area of the largest possible rectangle within a histogram."
   [hist]
-  (let [decorated (into [] (concat [-1] hist [-1]))
-         stack (atom (list [0 (first decorated)]))
-         max-area (atom 0)]
-     (reduce-kv (fn [ans k v]
-                  (while (< v (-> @stack peek last))
-                    (let [head (-> (swap-vals! stack pop) first peek last)
-                          area (* head (- k (-> @stack peek first) 1))]
-                      (swap! max-area max area)))
-                  (swap! stack conj [k v])
-                  (max ans @max-area))
-                0 decorated)))
+  (let [hist (vec hist)
+        n (count hist)
+        ranges (vec (for [x (range n), y (range n) :when (<= x y)]
+                      [x y]))]
+    (reduce (fn [ans [s e]]
+              (let [height (apply min (map hist (range s (inc e))))
+                    area (* height (inc (- e s)))]
+                (max ans area)))
+            0 ranges)))
 
 (defn -main
   "Run Task 2 with a list of integers, defaulting to the sample given in the task description."
   [& args]
   (let [A (if (not-empty args) (map edn/read-string args) [2, 1, 4, 5, 3, 7])]
-    (prn (largest-rectangle-histogram A))))
+    (println (largest-rectangle-histogram A))))