Add test for FingerTree#asList().subList().

reactfx/src/test/java/org/reactfx/util/FingerTreeTest.java

@@ -0,0 +1,44 @@
+package org.reactfx.util;
+
+import static org.junit.Assert.*;
+
+import java.util.Arrays;
+import java.util.List;
+import java.util.Random;
+
+import org.junit.Test;
+
+public class FingerTreeTest {
+
+    /**
+     * Returns a random int, with higher probability for larger numbers.
+     */
+    private static int progressiveInt(Random rnd, int bound) {
+        double d = rnd.nextDouble();
+        d = d*d*d;
+        int i = (int) Math.floor(d * bound);
+        return bound - 1 - i;
+    }
+
+    @Test
+    public void testSubList() {
+        final int n = 50000;
+
+        Integer[] arr = new Integer[n];
+        for(int i=0; i<n; ++i) arr[i] = i;
+
+        List<Integer> list = Arrays.asList(arr);
+        List<Integer> treeList = FingerTree.mkTree(list).asList();
+        assertEquals(list, treeList);
+
+        Random rnd = new Random(12345);
+        while(list.size() > 0) {
+            int len = progressiveInt(rnd, list.size() + 1);
+            int offset = rnd.nextInt(list.size() - len + 1);
+            list = list.subList(offset, offset + len);
+            treeList = treeList.subList(offset, offset + len);
+            assertEquals(list, treeList);
+        }
+    }
+
+}