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1339 lines (1179 loc) · 44.7 KB
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// Copyright (c)2023 Jython Developers.
// Licensed to PSF under a contributor agreement.
// Copyright (c) Corporation for National Research Initiatives
package org.python.core;
import java.lang.invoke.MethodHandles;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.StringJoiner;
import java.util.function.Function;
import java.util.function.Supplier;
import org.python.base.InterpreterError;
import org.python.core.PyObjectUtil.NoConversion;
import org.python.core.PySlice.Indices;
import org.python.core.PyType.Spec;
/**
* The Python {@code list} object that is also a Java
* {@code List<Object>}. Operations in both APIs are synchronised
* for safety against concurrent threads.
* <p>
* It is {@code synchronized} so that competing threads should be
* able to access it with roughly the same protection against
* concurrent modification that CPython offers. There are also
* necessary safeguards during {@code sort()} to detect modification
* from within the current thread as a side effect of comparison.
* Java brings its own safeguard within iterators against structural
* concurrent modification.
*
* @implNote The design follows that in Jython 2 with a private Java
* list member to which operations are delegated directly or
* indirectly. In the present design, the indirect delegation is
* through a private delegate member where in the former design
* behaviour was inherited.
*/
// @ExposedType(name = "list", base = PyObject.class, doc = BuiltinDocs.list_doc)
public class PyList implements List<Object>, CraftedPyObject {
public static final PyType TYPE =
PyType.fromSpec(new Spec("list", MethodHandles.lookup()));
/** The Python type of this instance. */
protected final PyType type;
/** Storage for the actual list elements (as a list). */
private final List<Object> list;
/** Implementation help for sequence methods. */
private final ListDelegate delegate = new ListDelegate();
/**
* Synchronisation prevents modification by competing threads during
* an operation, but does not defend against actions within that
* operation by the same thread. One context where this occurs is
* {@code sort()}, where the implementation of {@code __lt__} on
* elements can, in principle, mutate the list being sorted. This
* variable is cleared at the beginning of {@code sort()} and set by
* any operation that modifies the list.
*/
private boolean changed = false;
/**
* Fundamental constructor, specifying actual type and the list that
* will become the storage object.
*
* @param type actual type
* @param list storage object
*/
private PyList(PyType type, List<Object> list) {
this.type = type;
this.list = list;
}
/**
* Construct a Python {@code list} object, specifying actual type
* and initial capacity.
*
* @param type actual type
* @param initialCapacity capacity
*/
public PyList(PyType type, int initialCapacity) {
this(type, new ArrayList<>(initialCapacity));
}
/**
* Construct a Python {@code list} object, specifying initial
* capacity.
*
* @param initialCapacity capacity
*/
public PyList(int initialCapacity) { this(TYPE, new ArrayList<>(initialCapacity)); }
/**
* Construct an empty Python {@code list} object, specifying actual
* type.
*
* @param type actual type
*/
public PyList(PyType type) { this(type, 0); }
/** Construct an empty Python {@code list} object. */
public PyList() { this(TYPE, 0); }
/**
* Construct a Python {@code list} object, specifying actual type
* and initial contents. The contents will be a (shallow) copy of
* the collection.
*
* @param type actual type
* @param c initial contents
*/
public PyList(PyType type, Collection<?> c) {
this(type, c.size());
addAll(c);
}
/**
* Construct a Python {@code list} object, specifying initial
* contents. The contents will be a (shallow) copy of the
* collection.
*
* @param c initial contents
*/
public PyList(Collection<?> c) { this(TYPE, c); }
/**
* Construct a {@code list} with initial contents from an array
* slice.
*
* @param a the array
* @param start of slice
* @param count of elements to take
*/
PyList(Object[] a, int start, int count) {
this(TYPE, count);
int stop = start + count;
for (int i = start; i < stop; i++) { add(a[i]); }
}
/**
* Return a Python {@code list} object, specifying initial
* contents.
*
* @param elements initial element values
* @return list of elements
*/
public static PyList of(Object... elements) { return new PyList(List.of(elements)); }
@Override
public PyType getType() { return type; }
// Special methods -----------------------------------------------
// @ExposedNew
// @ExposedMethod(doc = BuiltinDocs.list___init___doc)
// @formatter:off
/*
void __init__(Object[] args, String[] kwds) {
ArgParser ap = new ArgParser("list", args, kwds, new String[]{"sequence"}, 0);
Object seq = ap.getPyObject(0, null);
clear();
if (seq == null) {
return;
}
/* PyListDerived should be iterated over and not plain copied for cases where someone subclasses list
and overrides __iter__
* /
if (seq instanceof PyListDerived) {
for (Object item : seq.asIterable()) {
append(item);
}
} else if (seq instanceof PyList) {
list.addAll(((PyList) seq).list); // don't convert
} else if (seq instanceof PyList) {
list.addAll((PyTuple) seq);
} else {
for (Object item : seq.asIterable()) {
append(item);
}
}
}
*/
// @formatter:on
// @ExposedMethod(doc = BuiltinDocs.list___len___doc)
synchronized int __len__() {
return size();
}
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___ne___doc)
synchronized Object __ne__(Object o) {
return delegate.cmp(o, Comparison.NE);
}
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___eq___doc)
synchronized Object __eq__(Object o) {
return delegate.cmp(o, Comparison.EQ);
}
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___lt___doc)
synchronized Object __lt__(Object o) {
return delegate.cmp(o, Comparison.LT);
}
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___le___doc)
synchronized Object __le__(Object o) {
return delegate.cmp(o, Comparison.LE);
}
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___gt___doc)
synchronized Object __gt__(Object o) {
return delegate.cmp(o, Comparison.GT);
}
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___ge___doc)
synchronized Object __ge__(Object o) {
return delegate.cmp(o, Comparison.GE);
}
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___imul___doc)
// @formatter:off
/*
synchronized Object __imul__(Object o) {
if (!o.isIndex()) {
return null;
}
int count = o.asIndex(Py.OverflowError);
int size = size();
if (size == 0 || count == 1) {
return this;
}
if (count < 1) {
clear();
return this;
}
if (size > Integer.MAX_VALUE / count) {
throw new MemoryError("");
}
int newSize = size * count;
if (list instanceof ArrayList) {
((ArrayList<Object>) list).ensureCapacity(newSize);
}
List<Object> oldList = new ArrayList<Object>(list);
for (int i = 1; i < count; i++) {
list.addAll(oldList);
}
gListAllocatedStatus = list.size(); // now omit?
return this;
}
*/
// @formatter:on
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___mul___doc)
synchronized Object __mul__(Object n) throws Throwable { return delegate.__mul__(n); }
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___rmul___doc)
synchronized Object __rmul__(Object n) throws Throwable { return delegate.__mul__(n); }
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___add___doc)
synchronized Object __add__(Object o) throws Throwable { return delegate.__add__(o); }
// @ExposedMethod(type = MethodType.BINARY)
synchronized Object __radd__(Object o) throws Throwable { return delegate.__radd__(o); }
// @ExposedMethod(doc = BuiltinDocs.list___contains___doc)
synchronized boolean __contains__(Object o) throws Throwable {
return delegate.__contains__(o);
}
// @ExposedMethod(doc = BuiltinDocs.list___delitem___doc)
synchronized void __delitem__(Object index) throws Throwable {
changed = true;
delegate.__delitem__(index);
}
// @ExposedMethod(doc = BuiltinDocs.list___setitem___doc)
synchronized void __setitem__(Object index, Object value) throws Throwable {
changed = true;
delegate.__setitem__(index, value);
}
// @ExposedMethod(doc = BuiltinDocs.list___getitem___doc)
synchronized Object __getitem__(Object index) throws Throwable {
return delegate.__getitem__(index);
}
// @formatter:off
/*
// @ExposedMethod(doc = BuiltinDocs.list___iter___doc)
Object __iter__() {
return new PyListIterator(this);
}
// @ExposedMethod(doc = BuiltinDocs.list___reversed___doc)
synchronized PyIterator __reversed__() {
return new PyReversedIterator(this);
}
*/
// @formatter:on
@Override
public String toString() {
// XXX Use repr for elements and guard against recursive references
StringJoiner sj = new StringJoiner(", ", "[", "]");
for (Object v : list) { sj.add(v.toString()); }
return sj.toString();
}
// XXX object.__repr__ (calls toString()) should be enough.
// Retaining this code to indicate toString() additions needed.
// @formatter:off
/*
// @ExposedMethod(names = "__repr__")
final synchronized String __repr__() {
ThreadState ts = Py.getThreadState();
if (!ts.enterRepr(this)) {
return "[...]";
}
StringBuilder buf = new StringBuilder("[");
int length = size();
int i = 0;
for (Object item : list) {
buf.append(item.__repr__().toString());
if (i < length - 1) {
buf.append(", ");
}
i++;
}
buf.append("]");
ts.exitRepr(this);
return buf.toString();
}
*/
// @formatter:on
/**
* Add a single element to the end of list.
*
* @param o the element to add.
*/
// @ExposedMethod(doc = BuiltinDocs.list_append_doc)
final synchronized void list_append(Object o) {
changed = true;
list.add(o);
}
/**
* Remove all items from the list (same as {@code del s[:]})
*/
// @ExposedMethod in Python 3
final synchronized void list_clear() {
changed = true;
list.clear();
}
/**
* Return the number elements in the list that are Python-equal to
* the argument.
*
* @param v the value to test for.
* @return the number of occurrences.
* @throws Throwable from the implementation of {@code __eq__}
*/
// @ExposedMethod(doc = BuiltinDocs.list_count_doc)
final synchronized int list_count(Object v) throws Throwable { return delegate.count(v); }
/**
* Return smallest index where an element in the list Python-equals the argument.
*
* @param v the value to look for.
* @param start first index to test
* @param stop first index not to test
* @return index of the occurrence
* @throws ValueError if {@code v} not found
* @throws TypeError from bad {@code start} and {@code stop} types
* @throws Throwable from errors other than indexing
*/
// @ExposedMethod(defaults = {"null", "null"}, doc = BuiltinDocs.list_index_doc)
final synchronized int list_index(Object v, Object start, Object stop) throws TypeError, Throwable {
return delegate.index(v, start, stop);
}
/**
* Insert the argument element into the list at the specified index.
* Same as {@code s[index:index] = [o] if index >= 0}.
*
* @param index the position where the element will be inserted.
* @param o the element to insert.
* @throws TypeError from bad {@code index} type
* @throws Throwable from other conversion errors
*/
// @ExposedMethod(doc = BuiltinDocs.list_insert_doc)
final synchronized void list_insert(Object index, Object o) throws TypeError, Throwable {
changed = true;
delegate.insert(index, o);
}
/**
* Remove from the list the first element that is Python-equal to
* the argument. Same as {@code del s[s.index(x)]}.
*
* @param v the element to search for and remove.
* @throws Throwable from the implementation of {@code __eq__}
*/
// @ExposedMethod(doc = BuiltinDocs.list_remove_doc)
final synchronized void list_remove(Object v) throws Throwable {
int i = find(v);
if (i >= 0) {
changed = true;
list.remove(i);
} else {
throw new ValueError("%s.remove(x): x not in list", getType().name);
}
}
/**
* Return the index of {@code v} in {@link #list} or -1 if not
* found.
*
* @param v the element to search for and remove.
* @return the index of {@code v} or -1 if not found.
* @throws Throwable from the implementation of {@code __eq__}
*/
private int find(Object v) throws Throwable {
int n = list.size();
for (int i = 0; i < n; i++) {
if (Abstract.richCompareBool(v, list.get(i), Comparison.EQ)) { return i; }
}
return -1;
}
/**
* Reverses the items of s in place. The reverse() methods modify the list in place for economy
* of space when reversing a large list. It doesn't return the reversed list to remind you of
* this side effect.
*/
// @ExposedMethod(doc = BuiltinDocs.list_reverse_doc)
final synchronized void reverse() {
Collections.reverse(list);
changed = true;
}
/**
* Remove and return a specified element from the list.
*
* @param n the index of the element to remove and return.
* @return the popped item
*/
// @ExposedMethod(defaults = "-1", doc = BuiltinDocs.list_pop_doc)
final synchronized Object list_pop(int n) {
int size = size();
if (size == 0) {
throw new IndexError("pop from empty list");
} else {
if (n < 0) { n += size; }
if (n < 0 || n >= size) { throw new IndexError("pop index out of range"); }
changed = true;
return list.remove(n);
}
}
/**
* Append the elements in the argument sequence to the end of the
* list, {@code s[len(s):len(s)] = o}.
*
* @param o the sequence of items to append to the list.
* @throws Throwable from attempting to get an iterator on {@code o}
*/
// @ExposedMethod(doc = BuiltinDocs.list_extend_doc)
final synchronized void list_extend(Object o) throws Throwable {
list_extend(o, null);
}
/**
* Append the elements in the argument sequence to the end of the
* list, {@code s[len(s):len(s)] = o}.
*
* @param <E> the type of exception to throw
* @param o the sequence of items to append to the list.
* @param exc a supplier (e.g. lambda expression) for the exception
* to throw if an iterator cannot be formed (or {@code null} for
* a default {@code TypeError})
* @throws E to throw if an iterator cannot be formed
* @throws Throwable from the implementation of {@code o}.
*/
final <E extends PyException> void list_extend(Object o, Supplier<E> exc) throws E, Throwable {
changed = true;
list.addAll(PySequence.fastList(o, exc));
}
// @ExposedMethod(type = MethodType.BINARY, doc = BuiltinDocs.list___iadd___doc)
synchronized Object __iadd__(Object o) throws Throwable {
changed = true;
list_extend(o);
return this;
}
/**
* Sort the items of the list in place, using only < comparisons
* between items. Exceptions are not suppressed — if any
* comparison operations fail, the entire sort operation will fail
* (and the list will likely be left in a partially modified state).
*
* @param key specifies a function of one argument that is used to
* extract a comparison key from each list element, e.g.
* {@code key=str.lower}. The default value is {@code None}.
* @param reverse the list elements are sorted as if each comparison
* were reversed.
* @throws Throwable from object comparison
*/
// @ExposedMethod(doc = BuiltinDocs.list_sort_doc)
final synchronized void sort(Function<Object, Object> key, boolean reverse)
throws Throwable {
// Python: sort(*, key=None, reverse=False)
if (key == null) {
sortOnValue(reverse);
} else {
sortOnKey(key, reverse);
}
}
private synchronized void sortOnValue(boolean reverse) throws Throwable {
// We shall sort on values using this (Python) comparator
final ListElementComparator<Object> cmp = new ListElementComparator<Object>(reverse) {
@Override
boolean lessThan(Object o1, Object o2) throws Throwable {
return Abstract.richCompareBool(o1, o2, Comparison.LT);
}
};
// Now sort the list, failing on any change
changed = false;
Collections.sort(list, cmp);
if (cmp.raisedException()) { throw cmp.getRaisedException(); }
changed = true;
}
/**
* During {@link PyList#sortOnKey(Function, boolean)}, we actually
* sort a list of these key-value objects.
*/
private static class KV {
private final Object key;
private final Object value;
KV(Object key, Object value) {
this.key = key;
this.value = value;
}
}
private synchronized void sortOnKey(Function<Object, Object> keyfunc, boolean reverse)
throws Throwable {
// Make a copy of the list as key-value pairs in kvList
int size = list.size();
final ArrayList<KV> kvList = new ArrayList<KV>(size);
for (Object value : list) {
Object k = keyfunc.apply(value);
kvList.add(new KV(k, value));
}
// We shall sort kvList comparing keys
final ListElementComparator<KV> cmp = new ListElementComparator<KV>(reverse) {
@Override
boolean lessThan(KV o1, KV o2) throws Throwable {
return Abstract.richCompareBool(o1.key, o2.key, Comparison.LT);
}
};
// Now sort the kvList, failing on any change
changed = false;
Collections.sort(kvList, cmp);
if (cmp.raisedException()) { throw cmp.getRaisedException(); }
// Copy values from kvList (sorted on key)
assert kvList.size() == size;
for (int i = 0; i < size; i++) { list.set(i, kvList.get(i).value); }
changed = true;
}
public PyTuple __getnewargs__() { return new PyTuple(new PyTuple(list)); }
// List interface ------------------------------------------------
@Override
public synchronized void add(int index, Object element) {
changed = true;
list.add(index, element);
}
@Override
public synchronized boolean add(Object o) {
changed = true;
return list.add(o);
}
@Override
public synchronized boolean addAll(int index, Collection<?> c) {
changed = true;
return list.addAll(index, c);
}
@Override
public boolean addAll(Collection<?> c) {
changed = true;
return addAll(0, c);
}
@Override
public synchronized void clear() {
changed = true;
list.clear();
}
@Override
public synchronized boolean contains(Object o) {
try {
// Use the Python definition of equality (which may throw)
return delegate.__contains__(o);
} catch (Throwable e) {
return false;
}
}
@Override
public synchronized boolean containsAll(Collection<?> c) {
try {
for (Object o : c) {
// Use the Python definition of equality (which may throw)
if (!delegate.__contains__(o)) { return false; }
}
return true;
} catch (Throwable e) {
return false;
}
}
@Override
public synchronized Object get(int index) {
return list.get(index);
}
@Override
public synchronized int indexOf(Object o) {
return list.indexOf(o);
}
@Override
public synchronized boolean isEmpty() {
return list.isEmpty();
}
@Override
public Iterator<Object> iterator() {
return new Iterator<Object>() {
private final Iterator<Object> iter = list.iterator();
@Override
public boolean hasNext() { return iter.hasNext(); }
@Override
public Object next() {
synchronized (PyList.this) {
return iter.next();
}
}
@Override
public void remove() {
synchronized (PyList.this) {
changed = true;
iter.remove();
}
}
};
}
@Override
public synchronized int lastIndexOf(Object o) {
return list.lastIndexOf(o);
}
@Override
public ListIterator<Object> listIterator() {
return listIterator(0);
}
@Override
public ListIterator<Object> listIterator(final int index) {
return new ListIterator<Object>() {
private final ListIterator<Object> iter = list.listIterator(index);
@Override
public boolean hasNext() { return iter.hasNext(); }
@Override
public Object next() {
synchronized (PyList.this) {
return iter.next();
}
}
@Override
public boolean hasPrevious() { return iter.hasPrevious(); }
@Override
public Object previous() {
synchronized (PyList.this) {
return iter.previous();
}
}
@Override
public int nextIndex() { return iter.nextIndex(); }
@Override
public int previousIndex() { return iter.previousIndex(); }
@Override
public void remove() {
synchronized (PyList.this) {
changed = true;
iter.remove();
}
}
@Override
public void set(Object o) {
synchronized (PyList.this) {
changed = true;
iter.set(o);
}
}
@Override
public void add(Object o) {
synchronized (PyList.this) {
changed = true;
iter.add(o);
}
}
};
}
@Override
public synchronized Object remove(int index) {
changed = true;
return list.remove(index);
}
@Override
public synchronized boolean removeAll(Collection<?> c) {
// Make a list of indices at which to remove an item
List<Integer> erasures = new ArrayList<>(c.size());
int n = list.size();
for (int i = 0; i < n; i++) {
Object item = list.get(i);
for (Object o : c) {
// Use the Python definition of equality (which may throw)
try {
if (Abstract.richCompareBool(o, item, Comparison.EQ)) {
erasures.add(i);
break;
}
} catch (Throwable e) {
// Treat as non-match
}
}
}
// Do the removal
return erase(erasures);
}
@Override
public synchronized boolean retainAll(Collection<?> c) {
// Make a list of indices at which to remove an item
List<Integer> erasures = new ArrayList<>(c.size());
int n = list.size();
for (int i = 0; i < n; i++) {
Object item = list.get(i);
boolean retain = false;
for (Object o : c) {
// Use the Python definition of equality (which may throw)
try {
if (Abstract.richCompareBool(o, item, Comparison.EQ)) {
retain = true;
break;
}
} catch (Throwable e) {
// Treat as non-match
}
}
if (!retain) { erasures.add(i); }
}
// Do the removal
return erase(erasures);
}
@Override
public synchronized Object set(int index, Object element) {
changed = true;
return list.set(index, element);
}
@Override
public synchronized int size() {
return list.size();
}
@Override
public synchronized List<Object> subList(int fromIndex, int toIndex) {
/*
* XXX There is a difficulty here in our management of concurrency.
* The sub-list is an *unsynchronised* view on this PyList's private
* list variable, so it bypasses the synchronisation in the PyList
* wrapper. Here, and in Jython 2, we wrap this view in a new
* PyList. Although that is synchronised (on itself), it is not
* synchronised on this PyList and concurrent access is possible
* through the two objects. Compare
* java.util.Collections.SynchronizedRandomAccessList<E>, where the
* problem is solved by sharing a mutex.
*/
return new PyList(TYPE, list.subList(fromIndex, toIndex));
}
@Override
public synchronized Object[] toArray() { return list.toArray(); }
@SuppressWarnings("unchecked")
@Override
public synchronized <T> T[] toArray(T[] a) {
Class<T> type = (Class<T>)a.getClass().getComponentType();
if (Object.class.equals(type)) {
// Special-case a request for Object[]
a = list.toArray(a);
} else {
// Ensure we have a space the right size
int size = size();
if (a.length < size) {
a = (T[])Array.newInstance(type, size);
} else if (a.length > size) { a[size] = null; }
// Copy list into the array with conversion to T
for (int i = 0; i < size; i++) { a[i] = Abstract.tojava(list.get(i), type); }
}
return a;
}
@Override
public synchronized boolean remove(Object o) {
changed = true;
return list.remove(o);
}
// Java hash and equals ------------------------------------------
@Override
public int hashCode() { return list.hashCode(); }
@Override
public boolean equals(Object other) {
if (this == other) { return true; }
synchronized (this) {
if (other instanceof List) {
return list.equals(other);
} else {
try {
return Abstract.richCompareBool(other, this, Comparison.EQ);
} catch (Throwable t) {
return false;
}
}
}
}
// Supporting code -----------------------------------------------
/**
* This comparator is used in
* {@link PyList#sort(Function, boolean)}, sub-classed for the type
* of sort.
*
* @param <T> type of element to sort (on practice {@code Object} or
* {@code KV}.
*/
private abstract class ListElementComparator<T> implements Comparator<T> {
private Throwable comparatorException;
private final int less;
/**
* @param reverse whether comparisons will be in reveres sense
*/
ListElementComparator(boolean reverse) { this.less = reverse ? 1 : -1; }
/**
* @return the exception raised by any application of
* {@link #lessThan(Object, Object)}
*/
Throwable getRaisedException() {
return comparatorException;
}
/**
* @return whether an exception was raised by any application of
* {@link #lessThan(Object, Object)}
*/
boolean raisedException() {
return comparatorException != null;
}
/**
* Defines the comparison operation to use.
*
* @param o1 left operand
* @param o2 right operand
* @return true iff o1 is less than o2
* @throws Throwable on errors in the comparison
*/
abstract boolean lessThan(T o1, T o2) throws Throwable;
@Override
public int compare(T o1, T o2) {
try {
// PEP 207: sort should only depend on "less-than"
return lessThan(o1, o2) ? less : (lessThan(o2, o1) ? -less : 0);
} catch (Throwable pye) {
// Stash the exception to rethrow later
comparatorException = pye;
// XXX made up answer may violate contract of compare. Why stash?
return 0;
} finally {
// Detect change occurring during the comparisons
if (changed) { throw new ValueError("list modified during sort"); }
}
}
}
/**
* Accept an index, treating negative values as end-relative, and
* bound it to the sequence range. It is not an error for the index
* value to fall outside the valid range. (It is simply clipped to
* the nearer end.)
*
* @param index as presented
* @return bounded {@code 0 <= index <= list.size()}
*/
private int boundedIndex(int index) {
int L = list.size();
return index < 0 ? Math.max(0, index + L) : Math.min(L, index);
}
/**
* Given an ordered ascending list of indices into {@link #list},
* remove the elements at those indices.
*
* @param erasures to remove
* @return {@code true} if {@code erasures} is not empty
*/
private boolean erase(List<Integer> erasures) {
// Copy list to itself skipping each erasure
if (erasures.isEmpty()) { return false; }
Iterator<Integer> ei = erasures.iterator();
changed = true;
/*
* p, q are pointers into the list. We copy elements from list[q] to
* list[p], for p<q. r is the position of the next erasure. Whenever
* q reaches an erasure, we advance q over it without copy. Up to
* the first erasure, no copies are necessary, so we start like
* this, at he first erasure:
*/
int r = ei.next(), p = r, q = r + 1, n = list.size();
while (ei.hasNext()) {
// r is the position of the next erasure