get Random Index from bitset - Java java.util

Java examples for java.util:BitSet

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Description

get Random Index from bitset

Demo Code


//package com.java2s;
import java.util.Random;

public class Main {
    public static void main(String[] argv) throws Exception {
        int bitset = 2;
        System.out.println(getRandomIndex(bitset));
    }/*from   w  w  w . j  a  v a2  s. c o  m*/

    /** maximum index of a bitset (minimum index is 0) */
    public static final int MAX_INDEX = 31;
    /** int with all bits set */
    public static final int INT_MASK = 0xffffffff;
    private static Random random = new Random();

    public static int getRandomIndex(int bitset) {
        int cardinality = cardinality(bitset);
        int nr = random.nextInt(cardinality);
        for (int i = nextSetBit(bitset, 0), j = 0; i >= 0; i = nextSetBit(
                bitset, i + 1))
            if (j++ == nr)
                return i;
        return -1;
    }

    /**
     * Returns the number of bits set to <tt>true</tt> in the bitset.
     * 
     * @param  bitset
     *         the bitset.
     * @return the number of bits set to <tt>true</tt> in the bitset.
     */
    public static int cardinality(int bitset) {
        return Integer.bitCount(bitset);
    }

    /**
     * Returns the index of the first bit that is set to <code>true</code> that
     * occurs on or after the specified starting index. If no such bit exists
     * then -1 is returned.
     * 
     * To iterate over the <code>true</code> bits in a bitset, use the following
     * loop:
     * 
     * <pre>
     * for (int i = nextSetBit(bitset, 0); i &gt;= 0; i = nextSetBit(bitset, i + 1))
     *     ; // do something
     * </pre>
     * 
     * @param  bitset
     *         a bitset.
     * @param  fromIndex
     *         the bit index after which the first bit that is set to
     *         <code>true</code> is returned.
     * @return the index of the first bit that is set to <code>true</code> that
     *         occurs on or after the specified starting index
     * @throws IndexOutOfBoundsException
     *         if the specified index is negative
     */
    public static int nextSetBit(int bitset, int fromIndex)
            throws IndexOutOfBoundsException {
        checkIndexRange(fromIndex, Integer.MAX_VALUE);
        if (fromIndex > MAX_INDEX)
            return -1;
        bitset &= (INT_MASK << fromIndex);
        if (bitset != 0)
            return Integer.numberOfTrailingZeros(bitset);
        return -1;
    }

    /**
     * Throws an {@link IndexOutOfBoundsException} if the given bit index is
     * negative or if it the second argument is <code>true</code> also if the
     * index is greater than {@link #MAX_INDEX}.
     * 
     * @param  index
     * @param  checkMax
     * @throws IndexOutOfBoundsException
     */
    private static void checkIndexRange(int index, int maxIndex)
            throws IndexOutOfBoundsException {
        if (index < 0 || index > maxIndex)
            throw new IndexOutOfBoundsException("bitIndex [0..."
                    + MAX_INDEX + "]: " + index);
    }
}

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