src.it.unimi.dsi.big.util.SemiExternalGammaBigList Maven / Gradle / Ivy
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package it.unimi.dsi.big.util;
/*
* DSI utilities
*
* Copyright (C) 2007-2020 Sebastiano Vigna
*
* This library is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as published by the Free
* Software Foundation; either version 3 of the License, or (at your option)
* any later version.
*
* This library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License
* for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this program; if not, see This class is a semi-external {@link it.unimi.dsi.fastutil.longs.LongBigList} that
* MG4J uses to access files containing frequencies.
*
*
Instead, this class accesses frequencies in their
* compressed forms, and provides entry points for random access to each long. At construction
* time, entry points are computed with a certain step, which is the number of longs
* accessible from each entry point, or, equivalently, the maximum number of longs that will
* be necessary to read to access a given long.
*
*
Warning: This class is not thread safe, and needs to be synchronised to be used in a
* multithreaded environment.
*
* @author Fabien Campagne
* @author Sebastiano Vigna
* @since 2.0
*/
public class SemiExternalGammaBigList extends AbstractLongBigList {
public final static int DEFAULT_STEP = 128;
/** Position in the offset stream for each random access entry point (one each {@link #step} elements). */
private final long[] position;
/** Stream over the compressed offset information. */
private final InputBitStream ibs;
/** Maximum number of longs to skip. */
private final int step;
/** The number of longs. */
private final long numLongs;
/** Creates a new semi-external list.
*
* @param longs a bit stream containing γ-encoded longs.
* @param step the step used to build random-access entry points, or -1 to get {@link #DEFAULT_STEP}; note that
* a step causing more than 231 slots will be silently increased.
* @param numLongs the overall number of offsets (i.e., the number of terms).
*/
public SemiExternalGammaBigList(final InputBitStream longs, final int step, final long numLongs) throws IOException {
// We guarantee that the default step is such that we cannot cause problems.
this.step = Math.max(step == -1 ? DEFAULT_STEP : step, (int)(numLongs / (1L << 31)));
int slots = (int)((numLongs + this.step - 1) / this.step);
this.position = new long[slots];
this.numLongs = numLongs;
this.ibs = longs;
ibs.position(0);
ibs.readBits(0);
final int lastSlot = position.length - 1;
for (int i = 0; i <= lastSlot; i++) {
position[i] = ibs.readBits();
if (i != lastSlot) ibs.skipGammas(this.step);
}
}
/** Creates a new semi-external list.
*
*
This quick-and-dirty constructor estimates the number of longs by checking
* for an {@link EOFException}.
*
* @param longs a bit stream containing γ-encoded longs.
*/
public SemiExternalGammaBigList(final InputBitStream longs) throws IOException {
this(longs, DEFAULT_STEP, estimateNumberOfLongs(longs));
}
private static int estimateNumberOfLongs(final InputBitStream longs) {
int numLongs = 0;
try {
longs.position(0);
for(;;) {
longs.readLongGamma();
numLongs++;
}
}
catch(EOFException e) {
return numLongs;
}
catch (IOException e) {
throw new RuntimeException(e);
}
}
@Override
public final long getLong(final long index) {
if (index < 0 || index >= numLongs) throw new IndexOutOfBoundsException(Long.toString(index));
final int slotNumber = (int)(index / step);
final int k = (int)(index % step);
try {
ibs.position(position[slotNumber]);
ibs.skipGammas(k);
return ibs.readLongGamma();
}
catch(IOException e) {
throw new RuntimeException(e);
}
}
@Override
public long size64() {
return numLongs;
}
}