Java Array Size Get arraySize(final int expected, final float f)

Description

Returns the least power of two smaller than or equal to 2³⁰ and larger than or equal to Math.ceil( expected / f ).

License

Apache License

Parameter

Parameter	Description
expected	the expected number of elements in a hash table.
f	the load factor.

Exception

Parameter	Description
IllegalArgumentException	if the necessary size is larger than 2<sup>30</sup>.

Return

the minimum possible size for a backing array.

Declaration

public static int arraySize(final int expected, final float f) 

Method Source Code

//package com.java2s;
/*/*www  .  j av a  2 s.  c  o m*/
 * Copyright (C) 2002-2011 Sebastiano Vigna
 *
 * 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.
 */

public class Main {
    /** Returns the least power of two smaller than or equal to 2<sup>30</sup> and larger than or equal to <code>Math.ceil( expected / f )</code>.
     *
     * @param expected the expected number of elements in a hash table.
     * @param f the load factor.
     * @return the minimum possible size for a backing array.
     * @throws IllegalArgumentException if the necessary size is larger than 2<sup>30</sup>.
     */
    public static int arraySize(final int expected, final float f) {
        final long s = nextPowerOfTwo((long) Math.ceil(expected / f));
        if (s > (1 << 30))
            throw new IllegalArgumentException(
                    "Too large (" + expected + " expected elements with load factor " + f + ")");
        return (int) s;
    }

    /** Return the least power of two greater than or equal to the specified value.
     *
     * <p>Note that this function will return 1 when the argument is 0.
     *
     * @param x an integer smaller than or equal to 2<sup>30</sup>.
     * @return the least power of two greater than or equal to the specified value.
     */
    public static int nextPowerOfTwo(int x) {
        if (x == 0)
            return 1;
        x--;
        x |= x >> 1;
        x |= x >> 2;
        x |= x >> 4;
        x |= x >> 8;
        return (x | x >> 16) + 1;
    }

    /** Return the least power of two greater than or equal to the specified value.
     *
     * <p>Note that this function will return 1 when the argument is 0.
     *
     * @param x a long integer smaller than or equal to 2<sup>62</sup>.
     * @return the least power of two greater than or equal to the specified value.
     */
    public static long nextPowerOfTwo(long x) {
        if (x == 0)
            return 1;
        x--;
        x |= x >> 1;
        x |= x >> 2;
        x |= x >> 4;
        x |= x >> 8;
        x |= x >> 16;
        return (x | x >> 32) + 1;
    }
}

Related

1. arraySize(final int expected, final float f)
2. arraySizeOf(int[] array)