New LRU cache class.

Change-Id: I0e6ea1e489c684b876aebd5857c6f16a21048a8d
http://b/3184897

1 files changed, 249 insertions, 0 deletions

diff --git a/core/java/android/util/LruCache.java b/core/java/android/util/LruCache.java
new file mode 100644
index 0000000..9291299
--- /dev/null
+++ b/core/java/android/util/LruCache.java
@@ -0,0 +1,249 @@
+/*
+ * Copyright (C) 2011 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 android.util;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+/**
+ * A cache that holds strong references to a limited number of values. Each time
+ * a value is accessed, it is moved to the head of a queue. When a value is
+ * added to a full cache, the value at the end of that queue is evicted and may
+ * become eligible for garbage collection.
+ *
+ * <p>If your cached values hold resources that need to be explicitly released,
+ * override {@link #entryEvicted}. This method is only invoked when values are
+ * evicted. Values replaced by calls to {@link #put} must be released manually.
+ *
+ * <p>If a cache miss should be computed on demand for the corresponding keys,
+ * override {@link #create}. This simplifies the calling code, allowing it to
+ * assume a value will always be returned, even when there's a cache miss.
+ *
+ * <p>By default, the cache size is measured in the number of entries. Override
+ * {@link #sizeOf} to size the cache in different units. For, this cache is
+ * limited to 4MiB of bitmaps:
+ * <pre>   {@code
+ * int cacheSize = 4 * 1024 * 1024; // 4MiB
+ * LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {
+ *     @Override protected int sizeOf(String key, Bitmap value) {
+ *         return value.getByteCount();
+ *     }
+ * }}</pre>
+ */
+public class LruCache<K, V> {
+    private final LinkedHashMap<K, V> map;
+
+    /** Size of this cache in units. Not necessarily the number of elements. */
+    private int size;
+    private final int maxSize;
+
+    private int putCount;
+    private int createCount;
+    private int evictionCount;
+    private int hitCount;
+    private int missCount;
+
+    /**
+     * @param maxSize for caches that do not override {@link #sizeOf}, this is
+     *     the maximum number of entries in the cache. For all other caches,
+     *     this is the maximum sum of the sizes of the entries in this cache.
+     */
+    public LruCache(int maxSize) {
+        if (maxSize <= 0) {
+            throw new IllegalArgumentException("maxSize <= 0");
+        }
+        this.maxSize = maxSize;
+        this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
+    }
+
+    /**
+     * Returns the value for {@code key} if it exists in the cache or can be
+     * created by {@code #create}. If a value was returned, it is moved to the
+     * head of the queue. This returns null if a value is not cached and cannot
+     * be created.
+     */
+    public synchronized final V get(K key) {
+        if (key == null) {
+            throw new NullPointerException();
+        }
+
+        V result = map.get(key);
+        if (result != null) {
+            hitCount++;
+            return result;
+        }
+
+        missCount++;
+
+        // TODO: release the lock while calling this potentially slow user code
+        result = create(key);
+
+        if (result != null) {
+            createCount++;
+            size += safeSizeOf(key, result);
+            map.put(key, result);
+            trimToSize(maxSize);
+        }
+        return result;
+    }
+
+    /**
+     * Caches {@code value} for {@code key}. The value is moved to the head of
+     * the queue.
+     *
+     * @return the previous value mapped by {@code key}. Although that entry is
+     *     no longer cached, it has not been passed to {@link #entryEvicted}.
+     */
+    public synchronized final V put(K key, V value) {
+        if (key == null || value == null) {
+            throw new NullPointerException();
+        }
+
+        putCount++;
+        size += safeSizeOf(key, value);
+        V previous = map.put(key, value);
+        if (previous != null) {
+            size -= safeSizeOf(key, previous);
+        }
+        trimToSize(maxSize);
+        return previous;
+    }
+
+    private void trimToSize(int maxSize) {
+        while (size > maxSize) {
+            Map.Entry<K, V> toEvict = map.eldest();
+            if (toEvict == null) {
+                break; // map is empty; if size is not 0 then throw an error below
+            }
+
+            K key = toEvict.getKey();
+            V value = toEvict.getValue();
+            map.remove(key);
+            size -= safeSizeOf(key, value);
+            evictionCount++;
+
+            // TODO: release the lock while calling this potentially slow user code
+            entryEvicted(key, value);
+        }
+
+        if (size < 0 || (map.isEmpty() && size != 0)) {
+            throw new IllegalStateException(getClass().getName()
+                    + ".sizeOf() is reporting inconsistent results!");
+        }
+    }
+
+    /**
+     * Called for entries that have reached the tail of the least recently used
+     * queue and are be removed. The default implementation does nothing.
+     */
+    protected void entryEvicted(K key, V value) {}
+
+    /**
+     * Called after a cache miss to compute a value for the corresponding key.
+     * Returns the computed value or null if no value can be computed. The
+     * default implementation returns null.
+     */
+    protected V create(K key) {
+        return null;
+    }
+
+    private int safeSizeOf(K key, V value) {
+        int result = sizeOf(key, value);
+        if (result < 0) {
+            throw new IllegalStateException("Negative size: " + key + "=" + value);
+        }
+        return result;
+    }
+
+    /**
+     * Returns the size of the entry for {@code key} and {@code value} in
+     * user-defined units.  The default implementation returns 1 so that size
+     * is the number of entries and max size is the maximum number of entries.
+     *
+     * <p>An entry's size must not change while it is in the cache.
+     */
+    protected int sizeOf(K key, V value) {
+        return 1;
+    }
+
+    /**
+     * Clear the cache, calling {@link #entryEvicted} on each removed entry.
+     */
+    public synchronized final void evictAll() {
+        trimToSize(-1); // -1 will evict 0-sized elements
+    }
+
+    /**
+     * For caches that do not override {@link #sizeOf}, this is the number of
+     * entries in the cache. For all other caches, this is the sum of the sizes
+     * of the entries in this cache.
+     */
+    public synchronized final int size() {
+        return size;
+    }
+
+    /**
+     * Returns the number of times {@link #get} returned a value.
+     */
+    public synchronized final int hitCount() {
+        return hitCount;
+    }
+
+    /**
+     * Returns the number of times {@link #get} returned null or required a new
+     * value to be created.
+     */
+    public synchronized final int missCount() {
+        return missCount;
+    }
+
+    /**
+     * Returns the number of times {@link #create(Object)} returned a value.
+     */
+    public synchronized final int createCount() {
+        return createCount;
+    }
+
+    /**
+     * Returns the number of times {@link #put} was called.
+     */
+    public synchronized final int putCount() {
+        return putCount;
+    }
+
+    /**
+     * Returns the number of values that have been evicted.
+     */
+    public synchronized final int evictionCount() {
+        return evictionCount;
+    }
+
+    /**
+     * Returns a copy of the current contents of the cache, ordered from least
+     * recently accessed to most recently accessed.
+     */
+    public synchronized final Map<K, V> snapshot() {
+        return new LinkedHashMap<K, V>(map);
+    }
+
+    @Override public synchronized final String toString() {
+        int accesses = hitCount + missCount;
+        int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
+        return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
+                maxSize, hitCount, missCount, hitPercent);
+    }
+}