Grow ArrayCollection geometrically when inserting at an index

src/main/java/com/reandroid/utils/collection/ArrayCollection.java

@@ -1026,7 +1026,12 @@ private void slideLeft(int position, int amount){
     private void slideRight(int position, int amount){
         boolean locked = mLocked;
         this.mLocked = true;
-        ensureCapacity(amount);
+        if(availableCapacity() < amount){
+            // Grow by the usual step rather than by exactly 'amount', otherwise
+            // inserting n items one by one reallocates and copies the array n times
+            int grow = calculateGrow();
+            ensureCapacity(grow > amount ? grow : amount);
+        }
         Object[] elements = this.mElements;
         int size = this.size;
         int i = size - 1;
@@ -1095,9 +1100,10 @@ public void ensureCapacity(int capacity) {
             return;
         }
         int size = this.size;
-        int length = size + capacity;
-        Object[] update = getNewArray(length);
         Object[] elements = this.mElements;
+        // 'capacity' is what is still missing on top of the free slots
+        int length = elements.length + capacity;
+        Object[] update = getNewArray(length);
         if(elements.length == 0 || size == 0){
             this.mElements = update;
             return;

src/test/java/com/reandroid/utils/collection/ArrayCollectionTest.java

@@ -0,0 +1,35 @@
+package com.reandroid.utils.collection;
+
+import org.junit.Assert;
+import org.junit.Test;
+
+public class ArrayCollectionTest {
+
+    @Test
+    public void testInsertAtIndexThenSetSize() {
+        ArrayCollection<Integer> collection = new ArrayCollection<>();
+        for (int i = 0; i < 1000; i++) {
+            collection.add(i / 2, i);
+        }
+        Assert.assertEquals(1000, collection.size());
+        // insertions leave spare capacity behind, growing must still fit
+        collection.setSize(1500);
+        Assert.assertEquals(1500, collection.size());
+        collection.ensureCapacity(10);
+        Assert.assertTrue(collection.availableCapacity() >= 10);
+        int free = collection.availableCapacity();
+        collection.ensureCapacity(free + 5);
+        Assert.assertTrue(collection.availableCapacity() >= free + 5);
+    }
+
+    @Test
+    public void testInsertOrder() {
+        ArrayCollection<Integer> collection = new ArrayCollection<>();
+        for (int i = 0; i < 100; i++) {
+            collection.add(0, i);
+        }
+        for (int i = 0; i < 100; i++) {
+            Assert.assertEquals(Integer.valueOf(99 - i), collection.get(i));
+        }
+    }
+}