Extract RingBuffer class from NetdEventListenerService

This patch takes out the ring buffer array added for NFLOG wakeup packet
events logging and extract it into its own class for reuse. This new
RingBuffer class has the two minimal useful functions append() and
toArray().

Bug: 65164242
Bug: 65700460
Test: runtest frameworks-net, with new unit test
Change-Id: Ib94d79a93f4e99661b7d0fac67117b91d57af980

tests/net/java/com/android/internal/util/RingBufferTest.java

@@ -0,0 +1,145 @@
+/*
+ * Copyright (C) 2017 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.internal.util;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.fail;
+
+import android.support.test.filters.SmallTest;
+import android.support.test.runner.AndroidJUnit4;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import java.util.Arrays;
+import java.util.Objects;
+
+
+@SmallTest
+@RunWith(AndroidJUnit4.class)
+public class RingBufferTest {
+
+    @Test
+    public void testEmptyRingBuffer() {
+        RingBuffer<String> buffer = new RingBuffer<>(String.class, 100);
+
+        assertArraysEqual(new String[0], buffer.toArray());
+    }
+
+    @Test
+    public void testIncorrectConstructorArguments() {
+        try {
+            RingBuffer<String> buffer = new RingBuffer<>(String.class, -10);
+            fail("Should not be able to create a negative capacity RingBuffer");
+        } catch (IllegalArgumentException expected) {
+        }
+
+        try {
+            RingBuffer<String> buffer = new RingBuffer<>(String.class, 0);
+            fail("Should not be able to create a 0 capacity RingBuffer");
+        } catch (IllegalArgumentException expected) {
+        }
+    }
+
+    @Test
+    public void testRingBufferWithNoWrapping() {
+        RingBuffer<String> buffer = new RingBuffer<>(String.class, 100);
+
+        buffer.append("a");
+        buffer.append("b");
+        buffer.append("c");
+        buffer.append("d");
+        buffer.append("e");
+
+        String[] expected = {"a", "b", "c", "d", "e"};
+        assertArraysEqual(expected, buffer.toArray());
+    }
+
+    @Test
+    public void testRingBufferWithCapacity1() {
+        RingBuffer<String> buffer = new RingBuffer<>(String.class, 1);
+
+        buffer.append("a");
+        assertArraysEqual(new String[]{"a"}, buffer.toArray());
+
+        buffer.append("b");
+        assertArraysEqual(new String[]{"b"}, buffer.toArray());
+
+        buffer.append("c");
+        assertArraysEqual(new String[]{"c"}, buffer.toArray());
+
+        buffer.append("d");
+        assertArraysEqual(new String[]{"d"}, buffer.toArray());
+
+        buffer.append("e");
+        assertArraysEqual(new String[]{"e"}, buffer.toArray());
+    }
+
+    @Test
+    public void testRingBufferWithWrapping() {
+        int capacity = 100;
+        RingBuffer<String> buffer = new RingBuffer<>(String.class, capacity);
+
+        buffer.append("a");
+        buffer.append("b");
+        buffer.append("c");
+        buffer.append("d");
+        buffer.append("e");
+
+        String[] expected1 = {"a", "b", "c", "d", "e"};
+        assertArraysEqual(expected1, buffer.toArray());
+
+        String[] expected2 = new String[capacity];
+        int firstIndex = 0;
+        int lastIndex = capacity - 1;
+
+        expected2[firstIndex] = "e";
+        for (int i = 1; i < capacity; i++) {
+            buffer.append("x");
+            expected2[i] = "x";
+        }
+        assertArraysEqual(expected2, buffer.toArray());
+
+        buffer.append("x");
+        expected2[firstIndex] = "x";
+        assertArraysEqual(expected2, buffer.toArray());
+
+        for (int i = 0; i < 10; i++) {
+            for (String s : expected2) {
+                buffer.append(s);
+            }
+        }
+        assertArraysEqual(expected2, buffer.toArray());
+
+        buffer.append("a");
+        expected2[lastIndex] = "a";
+        assertArraysEqual(expected2, buffer.toArray());
+    }
+
+    static <T> void assertArraysEqual(T[] expected, T[] got) {
+        if (expected.length != got.length) {
+            fail(Arrays.toString(expected) + " and " + Arrays.toString(got)
+                    + " did not have the same length");
+        }
+
+        for (int i = 0; i < expected.length; i++) {
+            if (!Objects.equals(expected[i], got[i])) {
+                fail(Arrays.toString(expected) + " and " + Arrays.toString(got)
+                        + " were not equal");
+            }
+        }
+    }
+}