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It is a library to compress and uncompress arrays of integers very fast. The assumption is that most (but not all) values in your array use less than 32 bits.

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/**
 * This is code is released under the
 * Apache License Version 2.0 http://www.apache.org/licenses/.
 *
 * (c) Daniel Lemire, http://lemire.me/en/
 */

package me.lemire.integercompression;


/**
 * Interface describing a standard CODEC to compress integers. This is a
 * variation on the IntegerCODEC interface meant to be used for random access.
 * 
 * The main difference is that we must specify the number of integers we wish to
 * decode. This information should be stored elsewhere.
 * 
 * This interface was designed by the Terrier team for their search engine.
 * 
 * @author Daniel Lemire
 * 
 */
public interface SkippableIntegerCODEC {
    /**
     * Compress data from an array to another array.
     * 
     * Both inpos and outpos are modified to represent how much data was read
     * and written to if 12 ints (inlength = 12) are compressed to 3 ints, then
     * inpos will be incremented by 12 while outpos will be incremented by 3 we
     * use IntWrapper to pass the values by reference.
     * 
     * @param in
     *            input array
     * @param inpos
     *            location in the input array
     * @param inlength
     *            how many integers to compress
     * @param out
     *            output array
     * @param outpos
     *            where to write in the output array
     */
    public void headlessCompress(int[] in, IntWrapper inpos, int inlength, int[] out,
            IntWrapper outpos);

    /**
     * Uncompress data from an array to another array.
     * 
     * Both inpos and outpos parameters are modified to indicate new positions
     * after read/write.
     * 
     * @param in
     *            array containing data in compressed form
     * @param inpos
     *            where to start reading in the array
     * @param inlength
     *            length of the compressed data (ignored by some schemes)
     * @param out
     *            array where to write the compressed output
     * @param outpos
     *            where to write the compressed output in out
     * @param num
     *            number of integers we want to decode, the actual number of integers decoded can be less
     */
    public void headlessUncompress(int[] in, IntWrapper inpos, int inlength, int[] out,
            IntWrapper outpos, int num);

}