com.google.javascript.rhino.HamtPMap Maven / Gradle / Ivy
/*
*
* ***** BEGIN LICENSE BLOCK *****
* Version: MPL 1.1/GPL 2.0
*
* The contents of this file are subject to the Mozilla Public License Version
* 1.1 (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.mozilla.org/MPL/
*
* Software distributed under the License is distributed on an "AS IS" basis,
* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
* for the specific language governing rights and limitations under the
* License.
*
* The Original Code is Rhino code, released
* May 6, 1999.
*
* The Initial Developer of the Original Code is
* Netscape Communications Corporation.
* Portions created by the Initial Developer are Copyright (C) 1997-1999
* the Initial Developer. All Rights Reserved.
*
* Contributor(s):
* Google Inc.
*
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* the GNU General Public License Version 2 or later (the "GPL"), in which
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package com.google.javascript.rhino;
import static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Function;
import java.io.Serializable;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Collections;
import java.util.Deque;
import java.util.Iterator;
import java.util.Objects;
import javax.annotation.Nullable;
/**
* An immutable sorted map with efficient (persistent) updates.
*
* Uses a hash array mapped trie: http://en.wikipedia.org/wiki/Hash_array_mapped_trie.
*
*
This implementation stores the bare minimum in each node: a key (with its hash), value, mask,
* and children. It is also optimized to take maximum advantage of binary operations on shared
* trees. Specifically, {@link #reconcile} avoids recursing into entire subtrees if they are
* identical objects. Null keys and values are not allowed. The implementation special-cases away
* the EMPTY map as soon as possible, using 'null' instead for all the logic (since EMPTY violates
* the invariant that key and value are non-null). Finally, we maintain an invariant that the
* entry with the smallest hash code is always at the root of the tree, which avoids almost all
* extra tree rebuilding during binary operations.
*/
// TODO(sdh): Consider using tricks from https://bendyworks.com/blog/leveling-clojures-hash-maps
// We need a solid way to profile the results to see if it's actually worth the extra code.
public final class HamtPMap implements PMap, Serializable {
/**
* Number of bits of fan-out at each level. May be anywhere from 1 (a binary tree) to 5 (for a
* fan-out of 32). Anything larger requires using a long for the mask (which is ugly for the
* JavaScript-compiled version), and zero is essentially a linked list. The relevant trade-off is
* that more fan-out means shallower trees, which should lead to quicker look-up times and more
* efficient updates, but less fan-out makes reconcile() and equivalent() more efficient for maps
* that are mostly the same, since bigger common trees can be pruned from the operation.
* Preliminary profiling suggests that for type-checking purposes, all nonzero values are roughly
* comparable.
*/
private static final int BITS = 4;
/** Number of bits to shift off to get the most significant BITS number of bits. */
private static final int BITS_SHIFT = 32 - BITS;
/** Non-null key (exception: empty map has a null key). */
private final K key;
/** Hash of the key, right-shifted by BITS*depth. */
private final int hash;
/** Non-null value (exceptions: (1) empty map, (2) result of pivot, if not found). */
private final V value;
/** Bit mask indicating the children that are present (bitCount(mask) == children.length). */
private final int mask;
/** Non-null array of children. Elements are never reassigned. */
private final HamtPMap[] children;
private static final HamtPMap[] EMPTY_CHILDREN = new HamtPMap[0];
private static final HamtPMap EMPTY =
new HamtPMap<>(null, 0, null, 0, emptyChildren());
private HamtPMap(K key, int hash, V value, int mask, HamtPMap[] children) {
this.key = key;
this.hash = hash;
this.value = value;
this.mask = mask;
this.children = children;
}
/** Returns an empty map. */
@SuppressWarnings("unchecked") // Empty immutable collection is safe to cast.
public static HamtPMap empty() {
return (HamtPMap) EMPTY;
}
/** Returns an empty array of child maps. */
@SuppressWarnings("unchecked") // Empty array is safe to cast.
private static HamtPMap[] emptyChildren() {
return (HamtPMap[]) EMPTY_CHILDREN;
}
@Override
public String toString() {
StringBuilder sb = new StringBuilder().append("{");
if (!isEmpty()) {
appendTo(sb);
}
return sb.append("}").toString();
}
/** Appends this map's contents to a string builder. */
private void appendTo(StringBuilder sb) {
if (sb.length() > 1) {
sb.append(", ");
}
sb.append(key).append(": ").append(value);
for (HamtPMap child : children) {
child.appendTo(sb);
}
}
/** Returns whether this map is empty. */
@Override
public boolean isEmpty() {
return key == null;
}
/** Returns an iterable for a (possibly null) tree. */
@Override
public Iterable values() {
if (isEmpty()) {
return Collections.emptyList();
}
return () -> new Iter<>(this, map -> map.value);
}
/**
* Retrieves the value associated with the given key from the map, or returns null if it is not
* present.
*/
@Override
public V get(K key) {
return !isEmpty() ? get(key, hash(key)) : null;
}
/** Internal recursive implementation of get(K). */
private V get(K key, int hash) {
if (hash == this.hash && key.equals(this.key)) {
return this.value;
}
int bucket = bucket(hash);
int bucketMask = 1 << bucket;
return (mask & bucketMask) != 0 ? children[index(bucketMask)].get(key, shift(hash)) : null;
}
/**
* Returns a new map with the given key-value pair added. If the value is already present, then
* this same map will be returned.
*/
@Override
public HamtPMap plus(K key, V value) {
return !isEmpty()
? plus(key, hash(key), checkNotNull(value))
: new HamtPMap<>(key, hash(key), value, 0, emptyChildren());
}
/** Internal recursive implementation of plus(K, V). */
private HamtPMap plus(K key, int hash, V value) {
if (hash == this.hash && key.equals(this.key)) {
return value.equals(this.value)
? this
: new HamtPMap<>(key, hash, value, mask, children);
}
if (compareUnsigned(hash, this.hash) < 0) {
return replaceRoot(key, hash, value);
}
int bucket = bucket(hash);
hash = shift(hash);
int bucketMask = 1 << bucket;
int index = index(bucketMask);
if ((mask & bucketMask) != 0) {
// already a child, so overwrite
HamtPMap child = children[index];
HamtPMap newChild = child.plus(key, hash, value);
return child == newChild ? this : withChildren(mask, replaceChild(children, index, newChild));
} else {
// insert at index
HamtPMap newChild = new HamtPMap<>(key, hash, value, 0, emptyChildren());
return withChildren(mask | bucketMask, insertChild(children, index, newChild));
}
}
private HamtPMap replaceRoot(K key, int hash, V value) {
int bucket = bucket(this.hash);
int leafHash = shift(this.hash);
int bucketMask = 1 << bucket;
int index = index(bucketMask);
HamtPMap[] newChildren;
if ((mask & bucketMask) != 0) {
newChildren =
replaceChild(children, index, children[index].plus(this.key, leafHash, this.value));
} else {
HamtPMap newChild = new HamtPMap<>(this.key, leafHash, this.value, 0, emptyChildren());
newChildren = insertChild(children, index, newChild);
}
return new HamtPMap<>(key, hash, value, mask | bucketMask, newChildren);
}
/**
* Returns a new map with the given key removed. If the key was not present in the first place,
* then this same map will be returned.
*/
@Override
public HamtPMap minus(K key) {
return !isEmpty() ? minus(key, hash(key), null) : this;
}
/**
* Internal recursive implementation of minus(K). The value of the removed node is returned via
* the 'value' array, if it is non-null.
*/
private HamtPMap minus(K key, int hash, V[] value) {
if (hash == this.hash && key.equals(this.key)) {
HamtPMap result = deleteRoot(mask, children);
if (value != null) {
value[0] = this.value;
}
return result != null ? result : empty();
}
int bucket = bucket(hash);
int bucketMask = 1 << bucket;
if ((mask & bucketMask) == 0) {
// not present, stop looking
return this;
}
hash = shift(hash);
int index = index(bucketMask);
HamtPMap child = children[index];
HamtPMap newChild = child.minus(key, hash, value);
if (newChild == child) {
return this;
} else if (newChild == EMPTY) {
return withChildren(mask & ~bucketMask, deleteChild(children, index));
} else {
return withChildren(mask, replaceChild(children, index, newChild));
}
}
@Override
public HamtPMap reconcile(PMap that, BiFunction joiner) {
HamtPMap result =
reconcile(
!this.isEmpty() ? this : null,
!that.isEmpty() ? (HamtPMap) that : null,
joiner);
return result != null ? result : empty();
}
/** Internal recursive implementation of reconcile(HamtPMap, BiFunction), factoring out empies. */
private static HamtPMap reconcile(
@Nullable HamtPMap t1,
@Nullable HamtPMap t2,
BiFunction joiner) {
if (t1 == t2) {
return t1;
} else if (t1 == null) {
V newValue = joiner.apply(null, t2.value);
HamtPMap[] newChildren = Arrays.copyOf(t2.children, t2.children.length);
for (int i = 0; i < newChildren.length; i++) {
newChildren[i] = reconcile(null, newChildren[i], joiner);
}
return newValue != null
? new HamtPMap<>(t2.key, t2.hash, newValue, t2.mask, newChildren)
: deleteRoot(t2.mask, newChildren);
} else if (t2 == null) {
V newValue = joiner.apply(t1.value, null);
HamtPMap[] newChildren = Arrays.copyOf(t1.children, t1.children.length);
for (int i = 0; i < newChildren.length; i++) {
newChildren[i] = reconcile(newChildren[i], null, joiner);
}
return newValue != null
? new HamtPMap<>(t1.key, t1.hash, newValue, t1.mask, newChildren)
: deleteRoot(t1.mask, newChildren);
}
// Try as hard as possible to return input trees exactly.
boolean sameChildrenAs1 = true;
boolean sameChildrenAs2 = true;
// If the hashes are different, we need to keep the lower one at the top.
int hashCmp = compareUnsigned(t1.hash, t2.hash);
K key = t1.key;
int hash = t1.hash;
if (hashCmp < 0) {
// t1.key is missing from t2
t2 = t2.vacateRoot();
sameChildrenAs2 = false;
} else if (hashCmp > 0) {
// t2.key is missing from t1
t1 = t1.vacateRoot();
sameChildrenAs1 = false;
key = t2.key;
hash = t2.hash;
} else if (!t1.key.equals(t2.key)) {
// Hash collision: try to rearrange t2 to have the same root as t1
t2 = t2.pivot(t1.key, t1.hash);
sameChildrenAs2 = false;
}
// Note: one or the other (but not both) tree may have a null key at root.
V newValue = Objects.equals(t1.value, t2.value) ? t1.value : joiner.apply(t1.value, t2.value);
int newMask = t1.mask | t2.mask;
sameChildrenAs1 &= (newMask == t1.mask);
sameChildrenAs2 &= (newMask == t2.mask);
@SuppressWarnings("unchecked") // only used internally.
HamtPMap[] newChildren =
newMask != 0
? (HamtPMap[]) new HamtPMap[Integer.bitCount(newMask)]
: emptyChildren();
int mask = newMask;
int index = 0;
while (mask != 0) {
int childBit = Integer.lowestOneBit(mask);
mask &= ~childBit;
HamtPMap child1 = t1.getChild(childBit);
HamtPMap child2 = t2.getChild(childBit);
newChildren[index] = reconcile(child1, child2, joiner);
sameChildrenAs1 &= (newChildren[index] == child1);
sameChildrenAs2 &= (newChildren[index] == child2);
if (newChildren[index] != null) {
index++;
} else {
newMask &= ~childBit;
}
}
if (sameChildrenAs1 && t1.value.equals(newValue)) {
return t1;
} else if (sameChildrenAs2 && t2.value.equals(newValue)) {
return t2;
} else if (newValue == null) {
return deleteRoot(newMask, newChildren);
}
return new HamtPMap<>(key, hash, newValue, newMask, newChildren);
}
/**
* Checks equality recursively based on the given equivalence. Short-circuits as soon as a 'false'
* result is found.
*/
@Override
public boolean equivalent(PMap that, BiPredicate equivalence) {
return equivalent(
!this.isEmpty() ? this : null, !that.isEmpty() ? (HamtPMap) that : null, equivalence);
}
/** Internal recursive implementation of equivalent(HamtPMap, BiPredicate). */
private static boolean equivalent(
@Nullable HamtPMap t1,
@Nullable HamtPMap t2,
BiPredicate equivalence) {
if (t1 == t2) {
return true;
} else if (t1 == null || t2 == null) {
return false;
}
if (t1.hash != t2.hash) {
// Due to our invariant, we can safely conclude that there's a discrepancy in the
// keys without any extra work.
return false;
} else if (!t1.key.equals(t2.key)) {
// Hash collision: try to rearrange t2 to have the same root as t1
t2 = t2.pivot(t1.key, t1.hash);
if (t2.key == null) {
// t1.key not found in t2
return false;
}
}
if (!equivalence.test(t1.value, t2.value)) {
return false;
}
int mask = t1.mask | t2.mask;
while (mask != 0) {
int childBit = Integer.lowestOneBit(mask);
mask &= ~childBit;
if (!equivalent(t1.getChild(childBit), t2.getChild(childBit), equivalence)) {
return false;
}
}
return true;
}
/**
* Returns the index into the 'children' array for the given bit, which must have exactly one bit
* set in its binary representation (i.e. must be a power of two).
*/
private int index(int bit) {
return Integer.bitCount(mask & (bit - 1));
}
/**
* Returns the child for the given bit, which must have exactly one bit set. Returns null if there
* is no child for that bit.
*/
private HamtPMap getChild(int bit) {
return (mask & bit) != 0 ? children[index(bit)] : null;
}
/**
* Perform the hash operation. This is done once per object as it enters the map, then never
* again since the result is passed around internally. We reverse the bits since certain types
* (such as small integers and short strings) have hash codes that only vary in the least
* significant bits, but we use the most significant bits for bucketing in order to maintain
* a canonical structure where the key with the smallest hash is always at the root of the
* tree.
*/
private static int hash(Object key) {
return Integer.reverse(key.hashCode());
}
/** Return the current bucket index from the hash. */
private static int bucket(int hash) {
return hash >>> BITS_SHIFT;
}
/** Return a new hash with the next bucket number shifted off. */
private static int shift(int hash) {
return hash << BITS;
}
/** Unshift the bucket number back onto a hash. */
private static int unshift(int hash, int bucket) {
return (hash >>> BITS) | (bucket << BITS_SHIFT);
}
/** Compare two unsigned integers. */
private static int compareUnsigned(int left, int right) {
// NOTE: JDK 7 does not have a built-in operation for this, other than casting to longs.
// In JDK 8 it's just Integer.compareUnsigned(left, right). For now we emulate it
// by shifting the sign bit away, with a fallback second compare only if needed.
int diff = (left >>> 2) - (right >>> 2);
return diff != 0 ? diff : (left & 3) - (right & 3);
}
/**
* Returns a new version of this map with the given key at the root, and the root element moved to
* some deeper node. If the key is not found, then value will be null.
*/
@SuppressWarnings("unchecked")
private HamtPMap pivot(K key, int hash) {
return pivot(key, hash, null, (V[]) new Object[1]);
}
/**
* Internal recursive version of pivot. If parent is null then the result is used for the value in
* the returned map. The value, if found, is stored in the 'result' array as a secondary return.
*/
private HamtPMap pivot(K key, int hash, HamtPMap parent, V[] result) {
int newMask = mask;
HamtPMap[] newChildren = this.children;
if (hash == this.hash && key.equals(this.key)) {
// Found the key: swap out this key/value with the parent and return the result in the holder.
result[0] = this.value;
} else {
// Otherwise, see if the value might be present in a child.
int searchBucket = bucket(hash);
int replacementBucket = bucket(this.hash);
if (searchBucket == replacementBucket) {
int bucketMask = 1 << searchBucket;
if ((mask & bucketMask) != 0) {
// Found a child, call pivot recursively.
int index = index(bucketMask);
HamtPMap child = newChildren[index];
HamtPMap newChild = child.pivot(key, shift(hash), this, result);
newChildren = replaceChild(newChildren, index, newChild);
}
} else {
int searchMask = 1 << searchBucket;
if ((mask & searchMask) != 0) {
int index = index(searchMask);
HamtPMap child = newChildren[index];
HamtPMap newChild = child.minus(key, shift(hash), result);
if (!newChild.isEmpty()) {
newChildren = replaceChild(newChildren, index, newChild);
} else {
newChildren = deleteChild(newChildren, index);
newMask &= ~searchMask;
}
}
int replacementMask = 1 << replacementBucket;
int index = Integer.bitCount(newMask & (replacementMask - 1));
if ((mask & replacementMask) != 0) {
HamtPMap child = newChildren[index];
HamtPMap newChild = child.plus(this.key, shift(this.hash), this.value);
newChildren = replaceChild(newChildren, index, newChild);
} else {
newChildren =
insertChild(
newChildren,
index,
new HamtPMap<>(this.key, shift(this.hash), this.value, 0, emptyChildren()));
newMask |= replacementMask;
}
}
}
return parent != null
? new HamtPMap<>(parent.key, shift(parent.hash), parent.value, newMask, newChildren)
: new HamtPMap<>(key, hash, result[0], newMask, newChildren);
}
/** Moves the root into the appropriate child. */
private HamtPMap vacateRoot() {
int bucket = bucket(this.hash);
int bucketMask = 1 << bucket;
int index = index(bucketMask);
if ((mask & bucketMask) != 0) {
HamtPMap newChild = children[index].plus(this.key, shift(this.hash), this.value);
return new HamtPMap<>(null, 0, null, mask, replaceChild(children, index, newChild));
}
HamtPMap newChild =
new HamtPMap<>(this.key, shift(this.hash), this.value, 0, emptyChildren());
return new HamtPMap<>(null, 0, null, mask | bucketMask, insertChild(children, index, newChild));
}
/** Returns a copy of this node with a different array of children. */
private HamtPMap withChildren(int mask, HamtPMap[] children) {
return mask == this.mask && children == this.children
? this
: new HamtPMap<>(key, hash, value, mask, children);
}
/**
* Returns a new map with the elements from children. One element is removed from one of the
* children and promoted to a root node. If there are no children, returns null.
*/
private static HamtPMap deleteRoot(int mask, HamtPMap[] children) {
if (mask == 0) {
return null;
}
HamtPMap child = children[0];
int hashBits = Integer.numberOfTrailingZeros(mask);
int newHash = unshift(child.hash, hashBits);
HamtPMap newChild = deleteRoot(child.mask, child.children);
if (newChild == null) {
int newMask = mask & ~Integer.lowestOneBit(mask);
return new HamtPMap<>(child.key, newHash, child.value, newMask, deleteChild(children, 0));
} else {
return new HamtPMap<>(
child.key, newHash, child.value, mask, replaceChild(children, 0, newChild));
}
}
/** Returns a new array of children with an additional child inserted at the given index. */
private static HamtPMap[] insertChild(
HamtPMap[] children, int index, HamtPMap child) {
@SuppressWarnings("unchecked") // only used internally.
HamtPMap[] newChildren = (HamtPMap[]) new HamtPMap[children.length + 1];
newChildren[index] = child;
System.arraycopy(children, 0, newChildren, 0, index);
System.arraycopy(children, index, newChildren, index + 1, children.length - index);
return newChildren;
}
/** Returns a new array of children with the child at the given index replaced. */
private static HamtPMap[] replaceChild(
HamtPMap[] children, int index, HamtPMap child) {
HamtPMap[] newChildren = Arrays.copyOf(children, children.length);
newChildren[index] = child;
return newChildren;
}
/** Returns a new array of children with the child at the given index deleted. */
private static HamtPMap[] deleteChild(
HamtPMap[] children, int index) {
if (children.length == 1) {
// Note: index should always be zero.
return emptyChildren();
}
@SuppressWarnings("unchecked") // only used internally.
HamtPMap[] newChildren = (HamtPMap[]) new HamtPMap[children.length - 1];
System.arraycopy(children, 0, newChildren, 0, index);
System.arraycopy(children, index + 1, newChildren, index, children.length - index - 1);
return newChildren;
}
/** Iterates sequentially over a tree. */
private static class Iter implements Iterator {
final Deque> queue = new ArrayDeque<>();
final Function, O> transformer;
Iter(HamtPMap map, Function, O> transformer) {
this.transformer = transformer;
if (!map.isEmpty()) {
queue.add(map);
}
}
@Override
public boolean hasNext() {
return !queue.isEmpty();
}
@Override
public O next() {
HamtPMap top = queue.removeFirst();
for (int i = top.children.length - 1; i >= 0; i--) {
queue.add(top.children[i]);
}
return transformer.apply(top);
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}
/** Throws an assertion error if the map invariant is violated. */
@VisibleForTesting
HamtPMap assertCorrectStructure() {
if (isEmpty()) {
return this;
}
int hash = hash(key);
for (int i = 0; i < children.length; i++) {
int childHash = hash(children[i].key);
if (compareUnsigned(childHash, hash) < 0) {
throw new AssertionError("Invalid map has decreasing hash " + children[i].key + "("
+ Integer.toHexString(childHash) + ") beneath " + key + "("
+ Integer.toHexString(hash) + ": " + this);
}
children[i].assertCorrectStructure();
}
return this;
}
}