Rewrite list mirroring function and add test

1 files changed, 24 insertions, 19 deletions

diff --git a/src/ch/epfl/xblast/Lists.java b/src/ch/epfl/xblast/Lists.java
index f468788..51e76b0 100644
--- a/src/ch/epfl/xblast/Lists.java
+++ b/src/ch/epfl/xblast/Lists.java
@@ -3,6 +3,8 @@ package ch.epfl.xblast;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 
 /**
  * Lists utility class providing common operations on lists.
@@ -13,30 +15,33 @@ import java.util.List;
 public final class Lists {
 
     /**
-     * Return a symmetric version of the list.
+     * Returns a reversed copy of the given list, leaving the original one unmodified.
      *
-     * @param l the input list
-     * @return mirroredList the mirrored list
+     * @param l   the list to reverse
+     * @param <T> the type of the list's elements
+     * @return a reversed copy of the list.
+     */
+    public static <T> List<T> reversed(List<T> l) {
+        List<T> r = new ArrayList<>(l);
+        Collections.reverse(r);
+        return r;
+    }
+
+    /**
+     * Returns a symmetric version of the list, without repeating the last element of the input list.
+     * For instance, mirrored([kay]) will return [kayak].
+     *
+     * @param l   the input list
+     * @param <T> the type of the list's elements
+     * @return the mirrored list
      * @throws IllegalArgumentException if the given list is empty
      */
     public static <T> List<T> mirrored(List<T> l) {
-        if (l.size() == 0) {
-            throw new IllegalArgumentException();
-        }
-
-        List<T> mirroredList = new ArrayList<T>();
-        List<T> rightSide = new ArrayList<T>();
-
-        for (int i = 0; i < l.size() - 1; i++) {
-            rightSide.add(l.get(i));
-        }
-
-        Collections.reverse(rightSide);
-
-        mirroredList.addAll(l);
-        mirroredList.addAll(rightSide);
+        if (l == null || l.size() == 0) throw new IllegalArgumentException();
 
-        return mirroredList;
+        return Stream
+                .concat(l.stream(), Lists.reversed(l).stream().skip(1))
+                .collect(Collectors.toList());
     }
 
 }