import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
/**
* Utility methods for operating on memory-efficient maps. All maps of size 0 or
* 1 are assumed to be immutable. All maps of size greater than 1 are assumed to
* be mutable.
*/
public class Maps {
private static final Class<?> MULTI_MAP_CLASS = HashMap.class;
private static final Class<?> SINGLETON_MAP_CLASS = Collections.singletonMap(
null, null).getClass();
public static <K, V> Map<K, V> create() {
return Collections.emptyMap();
}
public static <K, V> Map<K, V> create(K key, V value) {
return Collections.singletonMap(key, value);
}
public static <K, V> Map<K, V> normalize(Map<K, V> map) {
switch (map.size()) {
case 0:
return create();
case 1: {
if (map.getClass() == SINGLETON_MAP_CLASS) {
return map;
}
K key = map.keySet().iterator().next();
return create(key, map.get(key));
}
default:
if (map.getClass() == MULTI_MAP_CLASS) {
return map;
}
return new HashMap<K, V>(map);
}
}
public static <K, V> Map<K, V> normalizeUnmodifiable(Map<K, V> map) {
if (map.size() < 2) {
return normalize(map);
} else {
// TODO: implement an UnmodifiableHashMap?
return Collections.unmodifiableMap(normalize(map));
}
}
public static <K, V> Map<K, V> put(Map<K, V> map, K key, V value) {
switch (map.size()) {
case 0:
// Empty -> Singleton
return Collections.singletonMap(key, value);
case 1: {
if (map.containsKey(key)) {
return create(key, value);
}
// Singleton -> HashMap
Map<K, V> result = new HashMap<K, V>();
result.put(map.keySet().iterator().next(),
map.values().iterator().next());
result.put(key, value);
return result;
}
default:
// HashMap
map.put(key, value);
return map;
}
}
public static <K, V> Map<K, V> putAll(Map<K, V> map, Map<K, V> toAdd) {
switch (toAdd.size()) {
case 0:
// No-op.
return map;
case 1: {
// Add one element.
K key = toAdd.keySet().iterator().next();
return put(map, key, toAdd.get(key));
}
default:
// True list merge, result >= 2.
switch (map.size()) {
case 0:
return new HashMap<K, V>(toAdd);
case 1: {
HashMap<K, V> result = new HashMap<K, V>();
K key = map.keySet().iterator().next();
result.put(key, map.get(key));
result.putAll(toAdd);
return normalize(result);
}
default:
map.putAll(toAdd);
return map;
}
}
}
public static <K, V> Map<K, V> remove(Map<K, V> map, K key) {
switch (map.size()) {
case 0:
// Empty
return map;
case 1:
// Singleton -> Empty
if (map.containsKey(key)) {
return create();
}
return map;
case 2:
// HashMap -> Singleton
if (map.containsKey(key)) {
map.remove(key);
key = map.keySet().iterator().next();
return create(key, map.get(key));
}
return map;
default:
// IdentityHashMap
map.remove(key);
return map;
}
}
}
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