Bug 785945 - Add LRU cache implementation (r=mfinkle)

mobile/android/base/Makefile.in

@@ -32,6 +32,7 @@ UTIL_JAVA_FILES := \
   INISection.java \
   util/EventDispatcher.java \
   util/FloatUtils.java \
+  util/LruCache.java \
   $(NULL)
 
 FENNEC_JAVA_FILES = \

mobile/android/base/util/LruCache.java

@@ -0,0 +1,323 @@
+/*
+ * 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 org.mozilla.gecko.util;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+/**
+ * Static library version of {@link android.util.LruCache}. Used to write apps
+ * that run on API levels prior to 12. When running on API level 12 or above,
+ * this implementation is still used; it does not try to switch to the
+ * framework's implementation. See the framework SDK documentation for a class
+ * overview.
+ */
+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 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 final V get(K key) {
+        if (key == null) {
+            throw new NullPointerException("key == null");
+        }
+
+        V mapValue;
+        synchronized (this) {
+            mapValue = map.get(key);
+            if (mapValue != null) {
+                hitCount++;
+                return mapValue;
+            }
+            missCount++;
+        }
+
+        /*
+         * Attempt to create a value. This may take a long time, and the map
+         * may be different when create() returns. If a conflicting value was
+         * added to the map while create() was working, we leave that value in
+         * the map and release the created value.
+         */
+
+        V createdValue = create(key);
+        if (createdValue == null) {
+            return null;
+        }
+
+        synchronized (this) {
+            createCount++;
+            mapValue = map.put(key, createdValue);
+
+            if (mapValue != null) {
+                // There was a conflict so undo that last put
+                map.put(key, mapValue);
+            } else {
+                size += safeSizeOf(key, createdValue);
+            }
+        }
+
+        if (mapValue != null) {
+            entryRemoved(false, key, createdValue, mapValue);
+            return mapValue;
+        } else {
+            trimToSize(maxSize);
+            return createdValue;
+        }
+    }
+
+    /**
+     * Caches {@code value} for {@code key}. The value is moved to the head of
+     * the queue.
+     *
+     * @return the previous value mapped by {@code key}.
+     */
+    public final V put(K key, V value) {
+        if (key == null || value == null) {
+            throw new NullPointerException("key == null || value == null");
+        }
+
+        V previous;
+        synchronized (this) {
+            putCount++;
+            size += safeSizeOf(key, value);
+            previous = map.put(key, value);
+            if (previous != null) {
+                size -= safeSizeOf(key, previous);
+            }
+        }
+
+        if (previous != null) {
+            entryRemoved(false, key, previous, value);
+        }
+
+        trimToSize(maxSize);
+        return previous;
+    }
+
+    /**
+     * @param maxSize the maximum size of the cache before returning. May be -1
+     *     to evict even 0-sized elements.
+     */
+    private void trimToSize(int maxSize) {
+        while (true) {
+            K key;
+            V value;
+            synchronized (this) {
+                if (size < 0 || (map.isEmpty() && size != 0)) {
+                    throw new IllegalStateException(getClass().getName()
+                            + ".sizeOf() is reporting inconsistent results!");
+                }
+
+                if (size <= maxSize || map.isEmpty()) {
+                    break;
+                }
+
+                Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
+                key = toEvict.getKey();
+                value = toEvict.getValue();
+                map.remove(key);
+                size -= safeSizeOf(key, value);
+                evictionCount++;
+            }
+
+            entryRemoved(true, key, value, null);
+        }
+    }
+
+    /**
+     * Removes the entry for {@code key} if it exists.
+     *
+     * @return the previous value mapped by {@code key}.
+     */
+    public final V remove(K key) {
+        if (key == null) {
+            throw new NullPointerException("key == null");
+        }
+
+        V previous;
+        synchronized (this) {
+            previous = map.remove(key);
+            if (previous != null) {
+                size -= safeSizeOf(key, previous);
+            }
+        }
+
+        if (previous != null) {
+            entryRemoved(false, key, previous, null);
+        }
+
+        return previous;
+    }
+
+    /**
+     * Called for entries that have been evicted or removed. This method is
+     * invoked when a value is evicted to make space, removed by a call to
+     * {@link #remove}, or replaced by a call to {@link #put}. The default
+     * implementation does nothing.
+     *
+     * <p>The method is called without synchronization: other threads may
+     * access the cache while this method is executing.
+     *
+     * @param evicted true if the entry is being removed to make space, false
+     *     if the removal was caused by a {@link #put} or {@link #remove}.
+     * @param newValue the new value for {@code key}, if it exists. If non-null,
+     *     this removal was caused by a {@link #put}. Otherwise it was caused by
+     *     an eviction or a {@link #remove}.
+     */
+    protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
+
+    /**
+     * 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.
+     *
+     * <p>The method is called without synchronization: other threads may
+     * access the cache while this method is executing.
+     *
+     * <p>If a value for {@code key} exists in the cache when this method
+     * returns, the created value will be released with {@link #entryRemoved}
+     * and discarded. This can occur when multiple threads request the same key
+     * at the same time (causing multiple values to be created), or when one
+     * thread calls {@link #put} while another is creating a value for the same
+     * key.
+     */
+    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 #entryRemoved} on each removed entry.
+     */
+    public final void evictAll() {
+        trimToSize(-1); // -1 will evict 0-sized elements
+    }
+
+    /**
+     * For caches that do not override {@link #sizeOf}, this returns the number
+     * of entries in the cache. For all other caches, this returns the sum of
+     * the sizes of the entries in this cache.
+     */
+    public synchronized final int size() {
+        return size;
+    }
+
+    /**
+     * For caches that do not override {@link #sizeOf}, this returns the maximum
+     * number of entries in the cache. For all other caches, this returns the
+     * maximum sum of the sizes of the entries in this cache.
+     */
+    public synchronized final int maxSize() {
+        return maxSize;
+    }
+
+    /**
+     * 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);
+    }
+}