WeakIdentityHashMap: Refine capacity computation; Bitfield.Util add '…

…PowerOf2' functions

src/java/com/jogamp/common/util/Bitfield.java

@@ -41,6 +41,14 @@ public interface Bitfield {
      * Bit operation utilities (static).
      */
     public static class Util {
+        /**
+         * Maximum 32bit integer value being of {@link #isPowerOf2(int)}.
+         * <p>
+         * We rely on the JVM spec {@link Integer#SIZE} == 32.
+         * </p>
+         */
+        public static final int MAX_POWER_OF_2 = 1 << ( Integer.SIZE - 2 );
+
         /**
          * Returns the 32 bit mask of n-bits, i.e. n low order 1’s.
          * <p>
@@ -67,6 +75,9 @@ public static int getBitMask(final int n) {
          *   http://tekpool.wordpress.com/category/bit-count/
          *   http://www.hackersdelight.org/
          * </pre>
+         * <p>
+         * We rely on the JVM spec {@link Integer#SIZE} == 32.
+         * </p>
          */
         public static final int bitCount(int n) {
             // Note: Original used 'unsigned int',
@@ -87,6 +98,47 @@ public static final int bitCount(int n) {
             n = n + (n >>> 16);
             return n & 0x3f;
         }
+
+        /**
+         * Returns {@code true} if the given integer is a power of 2
+         * <p>
+         * Source: bithacks: http://www.graphics.stanford.edu/~seander/bithacks.html#DetermineIfPowerOf2
+         * </p>
+         */
+        public static final boolean isPowerOf2(final int n) {
+            return 0<n && 0 == (n & (n - 1));
+        }
+        /**
+         * Returns the next higher power of 2 of 32-bit of given {@code n}
+         * <p>
+         * Source: bithacks: http://www.graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2
+         * </p>
+         * <p>
+         * We rely on the JVM spec {@link Integer#SIZE} == 32.
+         * </p>
+         */
+        public static final int nextPowerOf2(int n) {
+            n--;
+            n |= n >>> 1;
+            n |= n >>> 2;
+            n |= n >>> 4;
+            n |= n >>> 8;
+            n |= n >>> 16;
+            return (n < 0) ? 1 : n + 1; // avoid edge case where n is 0, it would return 0, which isn't a power of 2
+        }
+        /**
+         * If the given {@code n} is not {@link #isPowerOf2(int)} return {@link #nextPowerOf2(int)},
+         * otherwise return {@code n} unchanged.
+         * <pre>
+         * return isPowerOf2(n) ? n : nextPowerOf2(n);
+         * </pre>
+         * <p>
+         * We rely on the JVM spec {@link Integer#SIZE} == 32.
+         * </p>
+         */
+        public static final int roundToPowerOf2(final int n) {
+            return isPowerOf2(n) ? n : nextPowerOf2(n);
+        }
     }
     /**
      * Simple {@link Bitfield} factory for returning the efficient implementation.

src/java/com/jogamp/common/util/WeakIdentityHashMap.java

@@ -89,15 +89,64 @@ public WeakIdentityHashMap() {
    * Usable slots before resize are {@code capacity * loadFactor}.
    * </p>
    * <p>
-   * Capacity for n-slots w/o resize would be {@code Math.ceil(n/loadFactor)}.
+   * Capacity for n-slots w/o resize would be {@code (float)n/loadFactor + 1.0f}, see {@link #capacityForRequiredSize(int, float[])}.
    * </p>
    * @param initialCapacity default value would be 16, i.e. 12 slots @ 0.75f loadFactor before resize
    * @param loadFactor default value would be 0.75f
+   * @see #capacityForRequiredSize(int, float[])
+   * @see #createWithRequiredSize(int, float)
    */
   public WeakIdentityHashMap(final int initialCapacity, final float loadFactor) {
       backingStore = new HashMap<>(initialCapacity, loadFactor);
   }
 
+  /**
+   * Static creation method using {@link #capacityForRequiredSize(int, float[])}
+   * to instantiate a new {@link WeakIdentityHashMap} via {@link #WeakIdentityHashMap(int, float)}.
+   *
+   * @param requiredSize the user desired n-slots before resize
+   * @param loadFactor given loadFactor, which might be increased a little to avoid next PowerOf2 bloat
+   * @return the new {@link WeakIdentityHashMap} instance
+   */
+  @SuppressWarnings("rawtypes")
+  public static WeakIdentityHashMap<?, ?> createWithRequiredSize(final int requiredSize, final float loadFactor) {
+      final float[] lf = { loadFactor };
+      final int icap = capacityForRequiredSize(requiredSize, lf);
+      return new WeakIdentityHashMap(icap, lf[0]);
+  }
+
+  /**
+   * Returns the [initial] capacity using the given {@code loadFactor}
+   * and {@code requiredSize}.
+   * <p>
+   * General calculation is {@code (float)requiredSize/loadFactor + 1.0f}, using {@code loadFactor := 0.75f}.
+   * </p>
+   * <p>
+   * In case above computed capacity is {@link Bitfield.Util#isPowerOf2(int)},
+   * the given {@code loadFactor} will be increased to avoid next PowerOf2 table size initialization.
+   * </p>
+   * @param requiredSize the user desired n-slots before resize
+   * @param loadFactor given loadFactor, which might be increased a little to avoid next PowerOf2 bloat
+   * @return the [initial] capacity to be used for {@link #WeakIdentityHashMap(int, float)}
+   */
+  public static int capacityForRequiredSize(final int requiredSize, final float[] loadFactor) {
+      if( requiredSize >= Bitfield.Util.MAX_POWER_OF_2 ) {
+          return Integer.MAX_VALUE;
+      }
+      float lf = loadFactor[0];
+      int c0 = (int)( requiredSize/lf + 1.0f );
+      if( !Bitfield.Util.isPowerOf2(c0) || 0.86f <= lf ) {
+          return c0;
+      }
+      do {
+          lf += 0.01f;
+          c0 = (int)( requiredSize/lf + 1.0f );
+      } while( Bitfield.Util.isPowerOf2(c0) && 0.86f > lf );
+
+      loadFactor[0] = lf;
+      return c0;
+  }
+
   @Override
   public void clear() {
     backingStore.clear();
@@ -192,7 +241,9 @@ public boolean isEmpty() {
   public void putAll(final Map<? extends K, ? extends V> t) {
     final int n = t.size();
     if ( 0 < n ) {
-        final Map<IdentityWeakReference<K>, V> t2 = new HashMap<>((int)Math.ceil(n/0.75), 0.75f);
+        final float[] lf = { 0.75f };
+        final int icap = capacityForRequiredSize(n, lf);
+        final Map<IdentityWeakReference<K>, V> t2 = new HashMap<>(icap, lf[0]);
         for (final Map.Entry<? extends K, ? extends V> e : t.entrySet()) {
           t2.put(new IdentityWeakReference<K>(e.getKey(), queue), e.getValue());
         }