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/**
 * Copyright (C) 2014 Dasasian ([email protected])
 *
 * 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.dasasian.chok.util;

//import com.dasasian.chok.util.PositionedReadable;
//import com.dasasian.chok.util.Seekable;

import java.io.IOException;
import java.io.InputStream;
import java.io.InterruptedIOException;

/**
 * An {@link InputStream} which throttles the amount of bytes which is read from
 * the underlying {@link InputStream} in a given time frame.
 * 

 * Usage Example: 

 * //creates an throttled input stream which reads 1024 bytes/sec from the
 * underlying input stream at the most  

 * 
 * ThrottledInputStream throttledInputStream = new ThrottledInputStream(otherIputStream, new ThrottleSemaphore(1024));
 * 

 * 

 * Usage over multiple {@link InputStream}s: 

 * //throttle the read of multiple input streams at the rate of 1024
 * bytes/sec  

 * 
 * ThrottleSemaphore semaphore = new ThrottleSemaphore(1024);

 * ThrottledInputStream throttledInputStream1 = new ThrottledInputStream(otherIputStream1, semaphore);

 * ThrottledInputStream throttledInputStream2 = new ThrottledInputStream(otherIputStream2, semaphore);

 * ...
 * 

 */
public class ThrottledInputStream extends InputStream {

    private final InputStream inputStream;
    private final ThrottleSemaphore semaphore;

    public ThrottledInputStream(InputStream inputStream, ThrottleSemaphore semaphore) {
        this.inputStream = inputStream;
        this.semaphore = semaphore;
    }

    @Override
    public int read() throws IOException {
        semaphore.aquireBytes(1);
        return inputStream.read();
    }

    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        len = semaphore.aquireBytes(len);
        return inputStream.read(b, off, len);
    }

    @Override
    public void close() throws IOException {
        inputStream.close();
    }

    /**
     * This semaphore maintains the permitted bytes in a given timeframe. Each
     * {@link #aquireBytes(int)} blocks if necessary until at least one byte can
     * be acquired.
     * 

     * The time unit is bytes/second whereas the window of one second is splitted
     * into smaller windows to allow more steadied operations.
     * 

     * This class is thread safe and one instance can be used by multiple threads/
     * {@link ThrottledInputStream}s. (But it might not be fair to the different
     * treads)
     */
    public static class ThrottleSemaphore {

        private static final int SECOND = 1000;
        private final int _maxBytesPerWindow;
        private final int _windowsPerSecond;
        private volatile int _remainingBytesInCurrentWindow;
        private volatile long _nextWindowStartTime;

        public ThrottleSemaphore(float bytesPerSecond) {
            this(bytesPerSecond, 10);
        }

        public ThrottleSemaphore(float bytesPerSecond, int windowsPerSecond) {
            checkForGreaterZero((int) bytesPerSecond);
            checkForGreaterZero(windowsPerSecond);
            _windowsPerSecond = windowsPerSecond;
            _maxBytesPerWindow = (int) (bytesPerSecond / windowsPerSecond);
        }

        private void checkForGreaterZero(int value) {
            if (value <= 0) {
                throw new IllegalArgumentException("argument must be greater the 0 but is " + value);
            }
        }

        public synchronized int aquireBytes(int desired) throws IOException {
            try {
                waitForAllowedBytes();
                int aquiredBytes = Math.min(desired, _remainingBytesInCurrentWindow);
                _remainingBytesInCurrentWindow -= aquiredBytes;
                return aquiredBytes;
            } catch (InterruptedException e) {
                throw new InterruptedIOException();
            }
        }

        private void waitForAllowedBytes() throws InterruptedException {
            updateWindow();
            while (_remainingBytesInCurrentWindow <= 0) {
                updateWindow();
                Thread.sleep(_nextWindowStartTime - System.currentTimeMillis());
            }
        }

        private void updateWindow() {
            long now = System.currentTimeMillis();
            while (now >= _nextWindowStartTime) {
                if (now >= _nextWindowStartTime + SECOND) {
                    _nextWindowStartTime = now + SECOND / _windowsPerSecond;
                    _remainingBytesInCurrentWindow = _maxBytesPerWindow;
                }
                _nextWindowStartTime += SECOND / _windowsPerSecond;
                _remainingBytesInCurrentWindow += _maxBytesPerWindow;
            }
        }
    }

}