it.unimi.dsi.fastutil.shorts.ShortArrayFrontCodedList Maven / Gradle / Ivy
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/*
* Copyright (C) 2002-2023 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.
*/
package it.unimi.dsi.fastutil.shorts;
import static it.unimi.dsi.fastutil.BigArrays.copyFromBig;
import static it.unimi.dsi.fastutil.BigArrays.copyToBig;
import static it.unimi.dsi.fastutil.BigArrays.grow;
import static it.unimi.dsi.fastutil.BigArrays.trim;
import it.unimi.dsi.fastutil.BigArrays;
import it.unimi.dsi.fastutil.objects.AbstractObjectList;
import it.unimi.dsi.fastutil.objects.ObjectListIterator;
import it.unimi.dsi.fastutil.longs.LongArrays;
import java.io.Serializable;
import java.util.Iterator;
import java.util.Collection;
import java.util.NoSuchElementException;
import java.util.RandomAccess;
/**
* Compact storage of lists of arrays using front-coding (also known as prefix-omission)
* compression.
*
*
* This class stores immutably a list of arrays in a single
* {@linkplain it.unimi.dsi.fastutil.BigArrays big array} using front coding (of course, the
* compression will be reasonable only if the list is sorted lexicographically—see below). It
* implements an immutable type-specific list that returns the i-th array when calling
* {@link #get(int) get(i)}. The returned array may be freely modified.
*
*
* Front-coding (also known as prefix-omission) compression is based on the idea that if the
* i-th and the (i+1)-th array have a common prefix, we might store the length
* of the common prefix, and then the rest of the second array.
*
*
* This approach, of course, requires that once in a while an array is stored entirely. The
* ratio of a front-coded list defines how often this happens (once every {@link #ratio()}
* arrays). A higher ratio means more compression, but means also a longer access time, as more
* arrays have to be probed to build the result. Note that we must build an array every time
* {@link #get(int)} is called, but this class provides also methods that extract one of the stored
* arrays in a given array, reducing garbage collection. See the documentation of the family of
* {@code get()} methods.
*
*
* By setting the ratio to 1 we actually disable front coding: however, we still have a data
* structure storing large list of arrays with a reduced overhead (just one integer per array, plus
* the space required for lengths).
*
*
* Note that the typical usage of front-coded lists is under the form of serialized objects;
* usually, the data that has to be compacted is processed offline, and the resulting structure is
* stored permanently. Since the pointer array is not stored, the serialized format is very small.
*
*
Implementation Details
*
*
* All arrays are stored in a {@linkplain it.unimi.dsi.fastutil.BigArrays big array}. A separate
* array of pointers indexes arrays whose position is a multiple of the ratio: thus, a higher ratio
* means also less pointers.
*
*
* More in detail, an array whose position is a multiple of the ratio is stored as the array length,
* followed by the elements of the array. The array length is coded by a simple variable-length list
* of k-1 bit blocks, where k is the number of bits of the underlying
* primitive type. All other arrays are stored as follows: let {@code common} the length of the
* maximum common prefix between the array and its predecessor. Then we store the array length
* decremented by {@code common}, followed by {@code common}, followed by the array elements whose
* index is greater than or equal to {@code common}. For instance, if we store {@code foo},
* {@code foobar}, {@code football} and {@code fool} in a front-coded character-array list with
* ratio 3, the character array will contain
*
*
* 3 f o o 3 3 b a r 5 3 t b a l l 4 f o o l
*
*/
public class ShortArrayFrontCodedList extends AbstractObjectList implements Serializable, Cloneable, RandomAccess {
private static final long serialVersionUID = 1L;
/** The number of arrays in the list. */
protected final int n;
/** The ratio of this front-coded list. */
protected final int ratio;
/** The big array containing the compressed arrays. */
protected final short[][] array;
/** The pointers to entire arrays in the list. */
protected transient long[] p;
/**
* Creates a new front-coded list containing the arrays returned by the given iterator.
*
* @param arrays an iterator returning arrays.
* @param ratio the desired ratio.
*/
public ShortArrayFrontCodedList(final Iterator arrays, final int ratio) {
if (ratio < 1) throw new IllegalArgumentException("Illegal ratio (" + ratio + ")");
short[][] array = ShortBigArrays.EMPTY_BIG_ARRAY;
long[] p = LongArrays.EMPTY_ARRAY;
short[][] a = new short[2][];
long curSize = 0;
int n = 0, b = 0, length;
while (arrays.hasNext()) {
a[b] = arrays.next();
length = a[b].length;
if (n % ratio == 0) {
p = LongArrays.grow(p, n / ratio + 1);
p[n / ratio] = curSize;
array = grow(array, curSize + count(length) + length, curSize);
curSize += writeInt(array, length, curSize);
copyToBig(a[b], 0, array, curSize, length);
curSize += length;
} else {
final int minLength = Math.min(a[1 - b].length, length);
int common;
for (common = 0; common < minLength; common++) if (a[0][common] != a[1][common]) break;
length -= common;
array = grow(array, curSize + count(length) + count(common) + length, curSize);
curSize += writeInt(array, length, curSize);
curSize += writeInt(array, common, curSize);
copyToBig(a[b], common, array, curSize, length);
curSize += length;
}
b = 1 - b;
n++;
}
this.n = n;
this.ratio = ratio;
this.array = trim(array, curSize);
this.p = LongArrays.trim(p, (n + ratio - 1) / ratio);
}
/**
* Creates a new front-coded list containing the arrays in the given collection.
*
* @param c a collection containing arrays.
* @param ratio the desired ratio.
*/
public ShortArrayFrontCodedList(final Collection c, final int ratio) {
this(c.iterator(), ratio);
}
/* The following (rather messy) methods implements the encoding of arbitrary integers inside a big array.
* Unfortunately, we have to specify different codes for almost every type. */
/**
* Reads a coded length.
*
* @param a the data big array.
* @param pos the starting position.
* @return the length coded at {@code pos}.
*/
static int readInt(final short a[][], long pos) {
final short s0 = BigArrays.get(a, pos);
return s0 >= 0 ? s0 : s0 << 16 | (BigArrays.get(a, pos + 1) & 0xFFFF);
}
/**
* Computes the number of elements coding a given length.
*
* @param length the length to be coded.
* @return the number of elements coding {@code length}.
*/
static int count(final int length) {
return length < (1 << 15) ? 1 : 2;
}
/**
* Writes a length.
*
* @param a the data array.
* @param length the length to be written.
* @param pos the starting position.
* @return the number of elements coding {@code length}.
*/
static int writeInt(final short a[][], int length, long pos) {
if (length < (1 << 15)) {
BigArrays.set(a, pos, (short)length);
return 1;
}
BigArrays.set(a, pos++, (short)(-(length >>> 16) - 1));
BigArrays.set(a, pos, (short)(length & 0xFFFF));
return 2;
}
/**
* Returns the ratio of this list.
*
* @return the ratio of this list.
*/
public int ratio() {
return ratio;
}
/**
* Computes the length of the array at the given index.
*
*
* This private version of {@link #arrayLength(int)} does not check its argument.
*
* @param array the data array.
* @param index an index.
* @return the length of the {@code index}-th array.
*/
private int length(final int index) {
final short[][] array = this.array;
final int delta = index % ratio; // The index into the p array, and the delta inside the block.
long pos = p[index / ratio]; // The position into the array of the first entire word before the index-th.
int length = readInt(array, pos);
if (delta == 0) return length;
// First of all, we recover the array length and the maximum amount of copied elements.
int common;
pos += count(length) + length;
length = readInt(array, pos);
common = readInt(array, pos + count(length));
for (int i = 0; i < delta - 1; i++) {
pos += count(length) + count(common) + length;
length = readInt(array, pos);
common = readInt(array, pos + count(length));
}
return length + common;
}
/**
* Computes the length of the array at the given index.
*
* @param index an index.
* @return the length of the {@code index}-th array.
*/
public int arrayLength(final int index) {
ensureRestrictedIndex(index);
return length(index);
}
/**
* Extracts the array at the given index.
*
* @param index an index.
* @param a the array that will store the result (we assume that it can hold the result).
* @param offset an offset into {@code a} where elements will be store.
* @param length a maximum number of elements to store in {@code a}.
* @return the length of the extracted array.
*/
private int extract(final int index, final short a[], final int offset, final int length) {
final int delta = index % ratio; // The delta inside the block.
final long startPos = p[index / ratio]; // The position into the array of the first entire word before the
// index-th.
long pos, prevArrayPos;
int arrayLength = readInt(array, pos = startPos), currLen = 0, actualCommon;
if (delta == 0) {
pos = p[index / ratio] + count(arrayLength);
copyFromBig(array, pos, a, offset, Math.min(length, arrayLength));
return arrayLength;
}
int common = 0;
for (int i = 0; i < delta; i++) {
prevArrayPos = pos + count(arrayLength) + (i != 0 ? count(common) : 0);
pos = prevArrayPos + arrayLength;
arrayLength = readInt(array, pos);
common = readInt(array, pos + count(arrayLength));
actualCommon = Math.min(common, length);
if (actualCommon <= currLen) currLen = actualCommon;
else {
copyFromBig(array, prevArrayPos, a, currLen + offset, actualCommon - currLen);
currLen = actualCommon;
}
}
if (currLen < length) copyFromBig(array, pos + count(arrayLength) + count(common), a, currLen + offset, Math.min(arrayLength, length - currLen));
return arrayLength + common;
}
/**
* {@inheritDoc}
*
* @implSpec This implementation delegates to {@link #getArray(int)}.
*/
@Override
public short[] get(final int index) {
return getArray(index);
}
/**
* Returns an array stored in this front-coded list.
*
* @param index an index.
* @return the corresponding array stored in this front-coded list.
*/
public short[] getArray(final int index) {
ensureRestrictedIndex(index);
final int length = length(index);
final short a[] = new short[length];
extract(index, a, 0, length);
return a;
}
/**
* Stores in the given array elements from an array stored in this front-coded list.
*
* @param index an index.
* @param a the array that will store the result.
* @param offset an offset into {@code a} where elements will be store.
* @param length a maximum number of elements to store in {@code a}.
* @return if {@code a} can hold the extracted elements, the number of extracted elements;
* otherwise, the number of remaining elements with the sign changed.
*/
public int get(final int index, final short[] a, final int offset, final int length) {
ensureRestrictedIndex(index);
ShortArrays.ensureOffsetLength(a, offset, length);
final int arrayLength = extract(index, a, offset, length);
if (length >= arrayLength) return arrayLength;
return length - arrayLength;
}
/**
* Stores in the given array an array stored in this front-coded list.
*
* @param index an index.
* @param a the array that will store the content of the result (we assume that it can hold the
* result).
* @return if {@code a} can hold the extracted elements, the number of extracted elements;
* otherwise, the number of remaining elements with the sign changed.
*/
public int get(final int index, final short[] a) {
return get(index, a, 0, a.length);
}
@Override
public int size() {
return n;
}
@Override
public ObjectListIterator listIterator(final int start) {
ensureIndex(start);
return new ObjectListIterator() {
short s[] = ShortArrays.EMPTY_ARRAY;
int i = 0;
long pos = 0;
boolean inSync; // Whether the current value in a is the string just before the next to be produced.
{
if (start != 0) {
if (start == n) i = start; // If we start at the end, we do nothing.
else {
pos = p[start / ratio];
int j = start % ratio;
i = start - j;
while (j-- != 0) next();
}
}
}
@Override
public boolean hasNext() {
return i < n;
}
@Override
public boolean hasPrevious() {
return i > 0;
}
@Override
public int previousIndex() {
return i - 1;
}
@Override
public int nextIndex() {
return i;
}
@Override
public short[] next() {
int length, common;
if (!hasNext()) throw new NoSuchElementException();
if (i % ratio == 0) {
pos = p[i / ratio];
length = readInt(array, pos);
s = ShortArrays.ensureCapacity(s, length, 0);
copyFromBig(array, pos + count(length), s, 0, length);
pos += length + count(length);
inSync = true;
} else {
if (inSync) {
length = readInt(array, pos);
common = readInt(array, pos + count(length));
s = ShortArrays.ensureCapacity(s, length + common, common);
copyFromBig(array, pos + count(length) + count(common), s, common, length);
pos += count(length) + count(common) + length;
length += common;
} else {
s = ShortArrays.ensureCapacity(s, length = length(i), 0);
extract(i, s, 0, length);
}
}
i++;
return ShortArrays.copy(s, 0, length);
}
@Override
public short[] previous() {
if (!hasPrevious()) throw new NoSuchElementException();
inSync = false;
return getArray(--i);
}
};
}
/**
* Returns a copy of this list.
*
* @return a copy of this list.
*/
@Override
public ShortArrayFrontCodedList clone() {
return this;
}
@Override
public String toString() {
final StringBuffer s = new StringBuffer();
s.append("[");
for (int i = 0; i < n; i++) {
if (i != 0) s.append(", ");
s.append(ShortArrayList.wrap(getArray(i)).toString());
}
s.append("]");
return s.toString();
}
/**
* Computes the pointer array using the currently set ratio, number of elements and underlying
* array.
*
* @return the computed pointer array.
*/
protected long[] rebuildPointerArray() {
final long[] p = new long[(n + ratio - 1) / ratio];
final short a[][] = array;
int length, count;
long pos = 0;
for (int i = 0, j = 0, skip = ratio - 1; i < n; i++) {
length = readInt(a, pos);
count = count(length);
if (++skip == ratio) {
skip = 0;
p[j++] = pos;
pos += count + length;
} else pos += count + count(readInt(a, pos + count)) + length;
}
return p;
}
private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
// Rebuild pointer array
p = rebuildPointerArray();
}
}