TreePVector.subList() makes fewer calls to TreePVector(), IntTreePMap…

…(), and IntTreePMap.withKeysChangedAbove()

CHANGELOG.md

@@ -6,6 +6,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 ### Changed
+- Faster `TreePVector.subList()` makes fewer calls to `TreePVector()`, `IntTreePMap()`, and `IntTreePMap.withKeysChangedAbove()` suggested by [@Groostav](https://github.com/Groostav)] in [[#74](https://github.com/hrldcpr/pcollections/issues/74)
 - Reformat with [google-java-format](https://github.com/google/google-java-format)
 
 ## [3.0.4] - 2019-07-24

src/main/java/org/pcollections/IntTreePMap.java

@@ -88,6 +88,20 @@ private IntTreePMap<V> withRoot(final IntTree<V> root) {
   }
 
   //// UNINHERITED METHODS OF IntTreePMap ////
+  /**
+   * @param <V>
+   * @param start
+   * @param end
+   * @return this map but with all keys start<=k<end removed
+   */
+  public IntTreePMap<V> minusRange(final int start, final int end) {
+    IntTree<V> root = this.root;
+    for (int i = start; i < end; i++) {
+      root = root.minus(i);
+    }
+    return withRoot(root);
+  }
+
   IntTreePMap<V> withKeysChangedAbove(final int key, final int delta) {
     // TODO check preconditions of changeKeysAbove()
     // TODO make public?

src/main/java/org/pcollections/TreePVector.java

@@ -90,22 +90,14 @@ public Iterator<E> iterator() {
 
   @Override
   public TreePVector<E> subList(int start, int end) {
-    int size = size();
+    final int size = size();
     if (start < 0 || end > size || start > end) throw new IndexOutOfBoundsException();
+    if (start == 0 && end == size) return this;
     if (start == end) return empty();
 
-    TreePVector<E> v = this;
-    while (end < size) { // remove from end
-      v = v.minus(size - 1);
-      size--;
-    }
-    while (start > 0) { // remove from start
-      v = v.minus(0);
-      size--;
-      start--;
-      end--;
-    }
-    return v;
+    // remove from end, then remove before start:
+    return new TreePVector<E>(
+        this.map.minusRange(end, size).minusRange(0, start).withKeysChangedAbove(start, -start));
   }
 
   //// IMPLEMENTED METHODS OF PVector ////