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Extended collection, iterator, and builder interfaces, lightweight cache sets and maps, and other foundational utilities
// Copyright 2015-2019 SWIM.AI inc.
//
// 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 swim.util;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
import java.util.concurrent.atomic.AtomicReferenceArray;
/**
* A hashed generational set evicts the least recently used value with the
* worst hit rate per hash bucket. HashGenSet is a concurrent and lock-free
* LRFU cache, with O(1) access time, that strongly references its values.
*
* Maintaining four "generations" of cached values per hash bucket, the cache
* discards from the younger generations based on least recent usage, and
* promotes younger generations to older generations based on most frequent
* usage. Cache misses count as negative usage of the older generations,
* biasing the cache against least recently used values with poor hit rates.
*
* The evict(V) method is guaranteed to be called when a value is displaced
* from the cache.
*/
public class HashGenSet {
final AtomicReferenceArray> buckets;
volatile int gen4Hits;
volatile int gen3Hits;
volatile int gen2Hits;
volatile int gen1Hits;
volatile int misses;
volatile int evicts;
public HashGenSet(int size) {
buckets = new AtomicReferenceArray>(size);
}
protected void evict(V value) { }
public V put(V value) {
if (buckets.length() == 0) {
return value;
}
HashGenSetBucket bucket;
HashGenSetBucket newBucket;
V evictVal = null;
V cacheVal;
final int index = Math.abs(value.hashCode()) % buckets.length();
do {
bucket = buckets.get(index);
if (bucket == null) {
newBucket = new HashGenSetBucket(value);
cacheVal = value;
} else {
if (bucket.gen4Val != null && value.equals(bucket.gen4Val)) {
GEN4_HITS.incrementAndGet(this);
BUCKET_GEN4_WEIGHT.incrementAndGet(bucket);
newBucket = bucket;
cacheVal = bucket.gen4Val;
} else if (bucket.gen3Val != null && value.equals(bucket.gen3Val)) {
GEN3_HITS.incrementAndGet(this);
if (BUCKET_GEN3_WEIGHT.incrementAndGet(bucket) > bucket.gen4Weight) {
newBucket = new HashGenSetBucket(
bucket.gen3Val, bucket.gen3Weight,
bucket.gen4Val, bucket.gen4Weight,
bucket.gen2Val, bucket.gen2Weight,
bucket.gen1Val, bucket.gen1Weight);
} else {
newBucket = bucket;
}
cacheVal = bucket.gen3Val;
} else if (bucket.gen2Val != null && value.equals(bucket.gen2Val)) {
GEN2_HITS.incrementAndGet(this);
if (BUCKET_GEN2_WEIGHT.incrementAndGet(bucket) > bucket.gen3Weight) {
newBucket = new HashGenSetBucket(
bucket.gen4Val, bucket.gen4Weight,
bucket.gen2Val, bucket.gen2Weight,
bucket.gen3Val, bucket.gen3Weight,
bucket.gen1Val, bucket.gen1Weight);
} else {
newBucket = bucket;
}
cacheVal = bucket.gen2Val;
} else if (bucket.gen1Val != null && value.equals(bucket.gen1Val)) {
GEN1_HITS.incrementAndGet(this);
if (BUCKET_GEN1_WEIGHT.incrementAndGet(bucket) > bucket.gen2Weight) {
newBucket = new HashGenSetBucket(
bucket.gen4Val, bucket.gen4Weight,
bucket.gen3Val, bucket.gen3Weight,
bucket.gen1Val, bucket.gen1Weight,
bucket.gen2Val, bucket.gen2Weight);
} else {
newBucket = bucket;
}
cacheVal = bucket.gen1Val;
} else {
MISSES.incrementAndGet(this);
evictVal = bucket.gen2Val;
// Penalize older gens for thrash. Promote gen1 to prevent nacent gens
// from flip-flopping. If sacrificed gen2 was worth keeping, it likely
// would have already been promoted.
newBucket = new HashGenSetBucket(
bucket.gen4Val, bucket.gen4Weight - 1,
bucket.gen3Val, bucket.gen3Weight - 1,
bucket.gen1Val, bucket.gen1Weight,
value, 1);
cacheVal = value;
}
}
} while (bucket != newBucket && !buckets.compareAndSet(index, bucket, newBucket));
if (evictVal != null) {
EVICTS.incrementAndGet(this);
evict(evictVal);
}
return cacheVal;
}
public boolean remove(V value) {
if (buckets.length() == 0) {
return false;
}
HashGenSetBucket bucket;
HashGenSetBucket newBucket;
boolean removed;
final int index = Math.abs(value.hashCode()) % buckets.length();
do {
bucket = buckets.get(index);
if (bucket == null) {
newBucket = null;
removed = false;
} else {
if (bucket.gen4Val != null && value.equals(bucket.gen4Val)) {
newBucket = new HashGenSetBucket(
bucket.gen3Val, bucket.gen3Weight,
bucket.gen2Val, bucket.gen2Weight,
bucket.gen1Val, bucket.gen1Weight,
null, 0);
removed = true;
} else if (bucket.gen3Val != null && value.equals(bucket.gen3Val)) {
newBucket = new HashGenSetBucket(
bucket.gen4Val, bucket.gen4Weight,
bucket.gen2Val, bucket.gen2Weight,
bucket.gen1Val, bucket.gen1Weight,
null, 0);
removed = true;
} else if (bucket.gen2Val != null && value.equals(bucket.gen2Val)) {
newBucket = new HashGenSetBucket(
bucket.gen4Val, bucket.gen4Weight,
bucket.gen3Val, bucket.gen3Weight,
bucket.gen1Val, bucket.gen1Weight,
null, 0);
removed = true;
} else if (bucket.gen1Val != null && value.equals(bucket.gen1Val)) {
newBucket = new HashGenSetBucket(
bucket.gen4Val, bucket.gen4Weight,
bucket.gen3Val, bucket.gen3Weight,
bucket.gen2Val, bucket.gen2Weight,
null, 0);
removed = true;
} else {
newBucket = bucket;
removed = false;
}
}
} while (bucket != newBucket && !buckets.compareAndSet(index, bucket, newBucket));
return removed;
}
public void clear() {
for (int i = 0; i < buckets.length(); i += 1) {
buckets.set(i, null);
}
}
public double hitRatio() {
final double hits = (double) gen4Hits + (double) gen3Hits + (double) gen2Hits + (double) gen1Hits;
return hits / (hits + (double) misses);
}
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> BUCKET_GEN4_WEIGHT =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSetBucket.class, "gen4Weight");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> BUCKET_GEN3_WEIGHT =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSetBucket.class, "gen3Weight");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> BUCKET_GEN2_WEIGHT =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSetBucket.class, "gen2Weight");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> BUCKET_GEN1_WEIGHT =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSetBucket.class, "gen1Weight");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> GEN4_HITS =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSet.class, "gen4Hits");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> GEN3_HITS =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSet.class, "gen3Hits");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> GEN2_HITS =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSet.class, "gen2Hits");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> GEN1_HITS =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSet.class, "gen1Hits");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> MISSES =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSet.class, "misses");
@SuppressWarnings("unchecked")
static final AtomicIntegerFieldUpdater> EVICTS =
AtomicIntegerFieldUpdater.newUpdater((Class>) (Class) HashGenSet.class, "evicts");
}
final class HashGenSetBucket {
final V gen4Val;
final V gen3Val;
final V gen2Val;
final V gen1Val;
volatile int gen4Weight;
volatile int gen3Weight;
volatile int gen2Weight;
volatile int gen1Weight;
HashGenSetBucket(V gen4Val, int gen4Weight,
V gen3Val, int gen3Weight,
V gen2Val, int gen2Weight,
V gen1Val, int gen1Weight) {
this.gen4Val = gen4Val;
this.gen4Weight = gen4Weight;
this.gen3Val = gen3Val;
this.gen3Weight = gen3Weight;
this.gen2Val = gen2Val;
this.gen2Weight = gen2Weight;
this.gen1Val = gen1Val;
this.gen1Weight = gen1Weight;
}
HashGenSetBucket(V value) {
this(null, 0, null, 0, null, 0, value, 1);
}
HashGenSetBucket() {
this(null, 0, null, 0, null, 0, null, 0);
}
}