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/*
* Copyright (c) 2022-2025 See AUTHORS file.
*
* 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 com.github.tommyettinger.ds;
import org.checkerframework.checker.nullness.qual.NonNull;
import org.checkerframework.checker.nullness.qual.Nullable;
import java.util.Collection;
import java.util.Map;
/**
* A variant on {@link ObjectObjectMap} that compares keys by identity (using {@code ==}) instead of equality (using {@code equals()}).
* It also hashes with {@link System#identityHashCode(Object)} instead of calling the {@code hashCode()} of a key. This can be useful in
* some cases where keys may have invalid {@link Object#equals(Object)} and/or {@link Object#hashCode()} implementations, or if keys could
* be very large (making a hashCode() that uses all the items in the key slow). Oddly, {@link System#identityHashCode(Object)} tends to
* be slower than the hashCode() for most small keys, because an explicitly-written hashCode() typically doesn't need to do anything
* concurrently, but identityHashCode() needs to (concurrently) modify an internal JVM variable that ensures the results are unique, and
* that requires the JVM to do lots of extra work whenever identityHashCode() is called. Despite that, identityHashCode() doesn't depend
* on the quantity of variables in the key, so identityHashCode() gets relatively faster for larger keys. The equals() method used by
* ObjectObjectMap also tends to slow down for large keys, relative to the constant-time {@code ==} this uses.
*
* Note that the {@link #entrySet()}, {@link #keySet()} and individual Entry items this produces are those of an {@link ObjectObjectMap},
* and so do not compare by identity.
*/
public class IdentityObjectMap extends ObjectObjectMap {
/**
* Creates a new map with an initial capacity of 51 and a load factor of {@link Utilities#getDefaultLoadFactor()}.
*/
public IdentityObjectMap () {
super();
}
/**
* Creates a new map with the given starting capacity and a load factor of {@link Utilities#getDefaultLoadFactor()}.
*
* @param initialCapacity If not a power of two, it is increased to the next nearest power of two.
*/
public IdentityObjectMap (int initialCapacity) {
super(initialCapacity);
}
/**
* Creates a new map with the specified initial capacity and load factor. This map will hold initialCapacity items before
* growing the backing table.
*
* @param initialCapacity If not a power of two, it is increased to the next nearest power of two.
* @param loadFactor what fraction of the capacity can be filled before this has to resize; 0 < loadFactor <= 1
*/
public IdentityObjectMap (int initialCapacity, float loadFactor) {
super(initialCapacity, loadFactor);
}
/**
* Creates a new map identical to the specified map.
*
* @param map an ObjectObjectMap to copy, or a subclass such as this one
*/
public IdentityObjectMap (ObjectObjectMap map) {
super(map);
}
/**
* Creates a new map identical to the specified map.
*
* @param map a Map to copy
*/
public IdentityObjectMap (Map map) {
super(map);
}
/**
* Given two side-by-side arrays, one of keys, one of values, this constructs a map and inserts each pair of key and value into it.
* If keys and values have different lengths, this only uses the length of the smaller array.
*
* @param keys an array of keys
* @param values an array of values
*/
public IdentityObjectMap (K[] keys, V[] values) {
super(keys, values);
}
/**
* Given two side-by-side collections, one of keys, one of values, this constructs a map and inserts each pair of key and value into it.
* If keys and values have different lengths, this only uses the length of the smaller collection.
*
* @param keys a Collection of keys
* @param values a Collection of values
*/
public IdentityObjectMap (Collection keys, Collection values) {
super(keys, values);
}
/**
* Returns an index >= 0 and <= {@link #mask} for the specified {@code item}.
*
* This particular overload relies on the naturally-random nature of {@link System#identityHashCode(Object)}, and just
* masks the identity hash code so only the lower bits are used. This should be fine because the identity hash code
* defaults to a decent random number generator for its output, so it should collide over all bits very rarely, and
* collide only over the masked bits somewhat rarely.
*
* @param item a non-null Object; its identityHashCode is used here
*/
@Override
protected int place (@NonNull Object item) {
return (System.identityHashCode(item) & mask);
}
@Override
protected boolean equate (Object left, @Nullable Object right) {
return left == right;
}
@Override
public int hashCode () {
int h = size;
K[] keyTable = this.keyTable;
V[] valueTable = this.valueTable;
for (int i = 0, n = keyTable.length; i < n; i++) {
K key = keyTable[i];
if (key != null) {
h ^= System.identityHashCode(key);
V value = valueTable[i];
if (value != null) {h ^= value.hashCode();}
}
}
return h;
}
/**
* Effectively does nothing here because the hashMultiplier is no longer stored or used.
* Subclasses can use this as some kind of identifier or user data, though.
*
* @return any int; the value isn't used internally, but may be used by subclasses to identify something
*/
public int getHashMultiplier() {
return 0;
}
/**
* Effectively does nothing here because the hashMultiplier is no longer stored or used.
* Subclasses can use this to set some kind of identifier or user data, though.
*
* @param unused any int; will not be used as-is
*/
public void setHashMultiplier(int unused) {
}
/**
* Constructs an empty map given the types as generic type arguments.
* This is usually less useful than just using the constructor, but can be handy
* in some code-generation scenarios when you don't know how many arguments you will have.
*
* @param the type of keys
* @param the type of values
* @return a new map containing nothing
*/
public static IdentityObjectMap with () {
return new IdentityObjectMap<>(0);
}
/**
* Constructs a single-entry map given one key and one value.
* This is mostly useful as an optimization for {@link #with(Object, Object, Object...)}
* when there's no "rest" of the keys or values.
*
* @param key0 the first and only key
* @param value0 the first and only value
* @param the type of key0
* @param the type of value0
* @return a new map containing just the entry mapping key0 to value0
*/
public static IdentityObjectMap with (K key0, V value0) {
IdentityObjectMap map = new IdentityObjectMap<>(1);
map.put(key0, value0);
return map;
}
/**
* Constructs a map given alternating keys and values.
* This can be useful in some code-generation scenarios, or when you want to make a
* map conveniently by-hand and have it populated at the start. You can also use
* {@link #IdentityObjectMap(Object[], Object[])}, which takes all keys and then all values.
* This needs all keys to have the same type and all values to have the same type, because
* it gets those types from the first key parameter and first value parameter. Any keys that don't
* have K as their type or values that don't have V as their type have that entry skipped.
*
* @param key0 the first key; will be used to determine the type of all keys
* @param value0 the first value; will be used to determine the type of all values
* @param rest an array or varargs of alternating K, V, K, V... elements
* @param the type of keys, inferred from key0
* @param the type of values, inferred from value0
* @return a new map containing the given keys and values
*/
@SuppressWarnings("unchecked")
public static IdentityObjectMap with (K key0, V value0, Object... rest) {
IdentityObjectMap map = new IdentityObjectMap<>(1 + (rest.length >>> 1));
map.put(key0, value0);
for (int i = 1; i < rest.length; i += 2) {
try {
map.put((K)rest[i - 1], (V)rest[i]);
} catch (ClassCastException ignored) {
}
}
return map;
}
}