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////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// Copyright (c) 2018-2022 Saxonica Limited
// This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0.
// If a copy of the MPL was not distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
// This Source Code Form is "Incompatible With Secondary Licenses", as defined by the Mozilla Public License, v. 2.0.
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
package net.sf.saxon.ma.zeno;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
/**
* An implementation of sequences as a list-of-lists, where the sublists at the
* end of the master list tend to be small, and the sublists at the start tend
* to be larger (or the other way around if the list is built by prepending items
* rather than appending them). The number of sublists is of the order log(N) where
* N is the length of the sequence, giving logarithmic performance or better for
* appending items to either end of the sequence, or for getting the Nth item.
*
* This is an immutable data structure; updating operations create a new
* ZenoChain leaving the original unchanged.
*
* In effect the ZenoChain is a tree with a constant depth of 3, implemented as
* a list of lists.
*
* For a list built by appending to the end, the size of sublists goes as
* follows as the list grows: (1).. (32).. (32,1).. (32,32).. (64,1).. (64,32)..
* (64,32,1).. (64,32,32).. (64,64,1).. (64,64,32).. (128,32,1).. (128,64,1)..
* (128,64,32,1).. For a list of 20,000 items we get 10 sublists with sizes
* (8192, 4096, 4096, 2048, 1024, 256, 128, 64, 64, 32). The exact numbers don't matter,
* the important thing is that the number of sublists is log(N) with shorter
* sublists at the end of the sequence where append/prepend operations take place.
*
* When two lists are concatenated, the two master lists are first concatenated,
* followed by a consolidation to combine short lists now appearing near the middle
* of the structure, to reduce the number of sublists.
*
* @param the type of the items in the list
*/
public class ZenoChain implements Iterable {
// The data structure is implemented as a list of lists
private final ArrayList> masterList;
/**
* Create an empty sequence
*/
public ZenoChain() {
masterList = new ArrayList<>(8);
}
/**
* Private constructor to create a ZenoChain with a given master list
* @param masterList the supplied master list
*/
private ZenoChain(ArrayList> masterList) {
this.masterList = masterList;
}
/**
* Append an item to this list. This is an immutable operation; the original list is unchanged
* @param item the item to be appended
* @return the list that results from the append operation
*/
public ZenoChain add(T item) {
ArrayList> masterList2 = new ArrayList<>(masterList);
// If the list is empty, create a new singleton list
if (masterList2.isEmpty()) {
ArrayList newSegment = new ArrayList<>(32);
newSegment.add(item);
masterList2.add(newSegment);
return new ZenoChain<>(masterList2);
}
int threshold = 32;
int index = masterList2.size() - 1;
// Get the last segment
ArrayList segment = masterList2.get(index);
if (segment.size() < threshold) {
// if the last segment is smaller than the threshold size, copy it,
// add the item to the new copy, and change the master list to
// refer to the new segment.
ArrayList segment2 = new ArrayList<>(32);
segment2.addAll(segment);
segment2.add(item);
masterList2.set(index, segment2);
return new ZenoChain<>(masterList2);
} else {
// if the last segment has reached the threshold size, consider
// combining it with the penultimate segment
while (true) {
index--;
threshold *= 2;
if (index < 0) {
// we've reached the start of the list. No combining of segments
// is possible, so just create a new final segment containing the new item alone
ArrayList newFinalSegment = new ArrayList<>();
newFinalSegment.add(item);
masterList2.add(newFinalSegment);
return new ZenoChain<>(masterList2);
}
ArrayList priorSegment = masterList2.get(index);
if (priorSegment.size() + segment.size() <= threshold) {
// combine two adjacent segments into one
ArrayList combinedSegment = new ArrayList<>(priorSegment.size() + segment.size());
combinedSegment.addAll(priorSegment);
combinedSegment.addAll(segment);
// add the combined segment to the master list, in place of the first of the pair
masterList2.set(index, combinedSegment);
// remove the second of the pair segment
masterList2.remove(index+1);
// create a new final segment containing the new item alone
ArrayList newFinalSegment = new ArrayList<>();
newFinalSegment.add(item);
// and add it to the master list
masterList2.add(newFinalSegment);
return new ZenoChain<>(masterList2);
}
// These two segments couldn't be combined because the total size was too large
// so we now consider merging earlier segments. For example if the segment sizes
// were (64, 64, 32) we will merge the first two to become (128, 32)
segment = priorSegment;
// continue looping
}
}
}
/**
* Prepend an item. This is an immutable operation; the original list is unchanged.
* @param item the item to be added at the start of the sequence
* @return the list resulting from the prepend operation
*/
public ZenoChain prepend(T item) {
ArrayList> masterList2 = new ArrayList<>(masterList);
// If the list is empty, create a new singleton list
if (masterList2.isEmpty()) {
return add(item);
}
int threshold = 32;
int index = 0;
ArrayList segment = masterList2.get(index);
// If the first segment is small enough, extend it by creating a copy
// with one extra item at the start
if (segment.size() < threshold) {
ArrayList segment2 = new ArrayList<>(32);
segment2.add(item);
segment2.addAll(segment);
masterList2.set(index, segment2);
return new ZenoChain<>(masterList2);
} else {
// Starting with the first two segments, see if there are two adjacent segments that
// can be concatenated into a single segment without exceeding a threshold size. The
// threshold size increases the further you are from the start of the sequence,
while (true) {
index++;
threshold *= 2;
if (index >= masterList2.size()) {
// We've got to the end without finding two segments to concatenate.
// Simply add a new singleton segment at the start.
ArrayList newInitialSegment = new ArrayList<>();
newInitialSegment.add(item);
masterList2.add(0, newInitialSegment);
return new ZenoChain<>(masterList2);
}
ArrayList nextSegment = masterList2.get(index);
// Try joining this segment and the next segment
if (nextSegment.size() + segment.size() <= threshold) {
ArrayList combinedSegment = new ArrayList<>();
combinedSegment.addAll(segment);
combinedSegment.addAll(nextSegment);
masterList2.set(index, combinedSegment);
masterList2.remove(index - 1);
// Now add a new singleton segment at the start
ArrayList newInitialSegment = new ArrayList<>();
newInitialSegment.add(item);
masterList2.add(0, newInitialSegment);
return new ZenoChain<>(masterList2);
}
// Continue looking for a pair of adjacent segments to combine
segment = nextSegment;
}
}
}
/**
* Append a sequence of items. This is an immutable operation; the original list is unchanged.
* @param items the sequence of items to be appended
* @return the concatenated sequence
*/
public ZenoChain addAll(Iterable items) {
ZenoChain result = this;
for (T item : items) {
result = result.add(item);
}
return result;
}
/**
* Concatenate two ZenoChains to form a new ZenoChain, leaving the original operands unchanged
* @param other the ZenoChain to be concatenated after this one
* @return a new ZenoChain whose elements are the elements of this ZenoChain followed by
* the elements of the other ZenoChain.
*/
public ZenoChain concat(ZenoChain other) {
ArrayList> newMaster = new ArrayList<>(masterList.size() + other.masterList.size());
newMaster.addAll(masterList);
newMaster.addAll(other.masterList);
return new ZenoChain(newMaster).reorganize();
}
/**
* Internal method to reduce fragmentation in a ZenoChain
* @return a new ZenoChain with identical contents to the supplied ZenoChain,
* but with better segmentation.
*/
private ZenoChain reorganize() {
// Useful after concatenating multiple chains, to reduce the number of segments.
// Starting from the right, if we find a segment that is smaller than both its
// neighbours, merge it with its left-hand neighbour.
for (int i=masterList.size()-2; i>=1; i--) {
int priorSize = masterList.get(i-1).size();
int segSize = masterList.get(i).size();
int nextSize = masterList.get(i+1).size();
if (segSize <= priorSize && segSize <= nextSize) {
ArrayList combinedSegment = new ArrayList<>(priorSize + segSize);
combinedSegment.addAll(masterList.get(i-1));
combinedSegment.addAll(masterList.get(i));
masterList.set(i-1, combinedSegment);
masterList.remove(i);
}
}
return new ZenoChain(masterList);
}
/**
* Get the item at position n, zero-based
* @param n the requested index
* @return the item at position n
* @throws IndexOutOfBoundsException if n is negative or beyond the end of the list
*/
public T get(int n) {
if (n < 0) {
throw new IndexOutOfBoundsException("Index " + n + " is negative");
}
int offset = 0;
for (ArrayList segment : masterList) {
if (offset + segment.size() > n) {
return segment.get(n - offset);
}
offset += segment.size();
}
throw new IndexOutOfBoundsException("Index " + n + " is too large");
}
/**
* Get a sublist of this list.
* Note that unlike {@link List#subList(int, int)}, the returned list is not "backed" by the
* original list; changes to the returned list will not affect the original list in any way.
* (This is inevitable, since both lists are immutable).
* @param start the zero-based start position of the required sublist, inclusive
* @param end the zero-based end position of the required sublist, exclusive
* @return the sublist, as a new ZenoChain
* @throws IndexOutOfBoundsException under the same conditions as for {@link List#subList(int, int)}:
* (start < 0 || end > size || start > end)
*/
public ZenoChain subList(int start, int end) {
// The implementation approach is as follows. We always create a new master list.
// Segments of the original list that are fully in the range of the sublist are
// referenced from the new master list directly, without copying. Segments that
// overlap the requested start and end points are partially "copied", as required,
// using the Java {@List.sublist()} mechanism - which does not actually create a
// copy.
if (start < 0 || start > end) {
throw new IndexOutOfBoundsException("start position for subList is out of range");
}
ArrayList> newMaster = new ArrayList<>();
int offset = 0;
int remainingLength = end - start;
boolean active = false;
// Process all the segments, with different treatment for (a) segments before the
// start position, (b) segments overlapping the start position, (c) segments wholly
// included in the sublist, (d) segments overlapping the end position, (e) segments
// beyond the end position.
for (ArrayList segment : masterList) {
if (active) {
if (remainingLength > segment.size()) {
// Segment is wholly included
remainingLength -= segment.size();
newMaster.add(segment); // No need to copy the segment, because it's immutable
} else {
// Segment spans the end position
newMaster.add(new ArrayList(segment.subList(0, remainingLength)));
return new ZenoChain(newMaster);
}
} else if (offset + segment.size() > start) {
// segment spans the start position
int localStart = start - offset;
if (remainingLength > segment.size() - localStart) {
// we copy this segment to the end
if (start == 1 && segment.size() > 128) {
// special case for tail() - break a long first segment to reduce the cost next time.
// This assumes it's likely tail() will be called again on the sublist
newMaster.add(new ArrayList(segment.subList(localStart, localStart + 64)));
newMaster.add(new ArrayList(segment.subList(localStart + 64, segment.size())));
} else {
newMaster.add(new ArrayList(segment.subList(localStart, segment.size())));
}
remainingLength -= (segment.size() - localStart);
active = true;
} else {
// segment spans both the start and end positions
newMaster.add(new ArrayList(segment.subList(localStart, localStart + remainingLength)));
return new ZenoChain(newMaster);
}
} else if (remainingLength == 0) {
break; // do nothing; we're past the end position.
}
offset += segment.size();
}
if (remainingLength > 0) {
throw new IndexOutOfBoundsException("end position for subList is out of range");
}
return new ZenoChain(newMaster);
}
/**
* Get the size of the list
* @return the size of the list
*/
public int size() {
int total = 0;
for (ArrayList segment : masterList) {
total += segment.size();
}
return total;
}
/**
* Ask if the list is empty
* @return true if the size is zero
*/
public boolean isEmpty() {
return masterList.isEmpty()
|| (masterList.size()==1 && masterList.get(0).isEmpty());
}
/**
* Ask if the list is a singleton
*
* @return true if the size is one
*/
public boolean isSingleton() {
return masterList.size() == 1 && masterList.get(0).size() == 1;
}
/**
* Iterate over the items
* @return a Java-style iterator over the items
*/
public Iterator iterator() {
return new ZenoChainIterator(masterList);
}
/**
* Get a string representation of the ZenoChain.
* @return the string representation in the form (X,Y,Z,...) where X,
* Y, and Z are the string representations of the elements of the sequence
*/
public String toString() {
StringBuilder sb = new StringBuilder();
for (List segment : masterList) {
sb.append("(");
for (T item : segment) {
sb.append(item).append(",");
}
sb.setCharAt(sb.length()-1, ')');
}
return sb.toString();
}
/**
* Diagnostic display of a {@code ZenoChain}, exposing its internal structure
* @return a string that shows the sizes of the segments, for example (64,32,4) for a
* {@code ZenoChain} of length 100.
*/
public String showMetrics() {
StringBuilder sb = new StringBuilder();
sb.append('(');
for (List segment : masterList) {
sb.append(segment.size()).append(",");
}
sb.setCharAt(sb.length()-1, ')');
return sb.toString();
}
}