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/*
 * Copyright (c) 2018-Present Pivotal Software Inc, All Rights Reserved.
 *
 * 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
 *
 *       https://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 reactor.pool;

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;

/**
 * Various pre-made {@link AllocationStrategy} for internal use.
 *
 * @author Simon Baslé
 */
final class AllocationStrategies {

    //FIXME tests
    static final class UnboundedAllocationStrategy extends AtomicInteger implements AllocationStrategy {

        @Override
        public int getPermits(int desired) {
            if (desired <= 0) {
                return 0;
            }
            int overflowCheck = addAndGet(desired); //+desired currently live
            if (overflowCheck < 0) {
                compareAndSet(overflowCheck, Integer.MAX_VALUE);
            }
            return desired;
        }

        @Override
        public int estimatePermitCount() {
            return Integer.MAX_VALUE;
        }

        @Override
        public void returnPermits(int returned) {
            int updated = addAndGet(-returned);
            if (updated < 0) {
                compareAndSet(updated, 0);
            }
        }

        @Override
        public int permitMinimum() {
            return 0;
        }

        @Override
        public int permitMaximum() {
            return Integer.MAX_VALUE;
        }

        @Override
        public int permitGranted() {
            return get();
        }
    }

    static final class SizeBasedAllocationStrategy implements AllocationStrategy {

        final int min;
        final int max;

        volatile int permits;
        static final AtomicIntegerFieldUpdater PERMITS = AtomicIntegerFieldUpdater.newUpdater(SizeBasedAllocationStrategy.class, "permits");

        SizeBasedAllocationStrategy(int min, int max) {
            if (min < 0) throw new IllegalArgumentException("min must be positive or zero");
            if (max < 1) throw new IllegalArgumentException("max must be strictly positive");
            if (min > max) throw new IllegalArgumentException("min must be less than or equal to max");
            this.min = min;
            this.max = max;
            PERMITS.lazySet(this, this.max);
        }

        @Override
        public int getPermits(int desired) {
            if (desired < 0) return 0;

            //impl note: this should be more efficient compared to the previous approach for desired == 1
            // (incrementAndGet + decrementAndGet compensation both induce a CAS loop, vs single loop here)
            for (;;) {
                int target;
                int p = permits;
                int granted = max - p;

                if (desired >= p) {
                    target = p;
                }
                else if (granted < min) {
                    target = Math.max(desired, min - granted);
                }
                else {
                    target = desired;
                }

                if (PERMITS.compareAndSet(this, p, p - target)) {
                    return target;
                }
            }
        }

        @Override
        public int estimatePermitCount() {
            return PERMITS.get(this);
        }

        @Override
        public int permitMinimum() {
            return min;
        }

        @Override
        public int permitMaximum() {
            return max;
        }

        @Override
        public int permitGranted() {
            return max - PERMITS.get(this);
        }

        @Override
        public void returnPermits(int returned) {
            PERMITS.addAndGet(this, returned);
        }
    }
}