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JavaCL is an Object-Oriented API that makes the C OpenCL API available to Java in a very natural way. It hides away the complexity of cross-platform C bindings, has a clean OO design (with generics, Java enums, NIO buffers, fully typed exceptions...), provides high-level features (OpenGL-interop, array reductions) and comes with samples and demos. For more info, please visit http://code.google.com/p/nativelibs4java/wiki/OpenCL.
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package com.nativelibs4java.opencl.util;

import java.io.IOException;
import java.nio.IntBuffer;
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.logging.Level;
import java.util.logging.Logger;

import com.nativelibs4java.opencl.CLBuildException;
import com.nativelibs4java.opencl.CLContext;
import com.nativelibs4java.opencl.CLEvent;
import com.nativelibs4java.opencl.CLIntBuffer;
import com.nativelibs4java.opencl.JavaCL;
import com.nativelibs4java.opencl.CLQueue;
import com.nativelibs4java.opencl.CLMem.Usage;
import com.nativelibs4java.util.NIOUtils;

/**
 *
 * @author ochafik
 */
public class ParallelRandom {

    protected final XORShiftRandom randomProgram;
    //private final IntBuffer outputBuffer;
    //private IntBuffer mappedOutputBuffer;
    protected final CLQueue queue;
    protected final CLContext context;
    protected final int parallelSize;
    protected final int[] globalWorkSizes;
	
	protected int consumedInts = 0;

	boolean preload;
	CLEvent preloadEvent;
	protected CLIntBuffer seeds, output;
	IntBuffer lastData;
	boolean isDataFresh;
	
	public ParallelRandom() throws IOException {
		this(JavaCL.createBestContext().createDefaultQueue(), 32 * 1024, System.currentTimeMillis());
	}
    public ParallelRandom(CLQueue queue, int parallelSize, final long seed) throws IOException {
        try {
            this.queue = queue;
            this.context = queue.getContext();
            randomProgram = new XORShiftRandom(context);
            this.parallelSize = parallelSize;

            int seedsNeededByWorkItem = 4;
            //int generatedNumbersByWorkItemIteration = 1;
            int maxUnits = queue.getDevice().getMaxComputeUnits();
            int unitsFactor = maxUnits < 10 ? 1 : 16;
            int scheduledWorkItems = maxUnits * unitsFactor;
            //int countByWorkItem = parallelSize / scheduledWorkItems;
            if (scheduledWorkItems > parallelSize / seedsNeededByWorkItem) {
                scheduledWorkItems = parallelSize / seedsNeededByWorkItem;
                scheduledWorkItems += parallelSize % seedsNeededByWorkItem;
            }
            //int iterationsByWorkItem = parallelCount / (generatedNumbersByWorkItemIteration * scheduledWorkItems);
            globalWorkSizes = new int[] { scheduledWorkItems };

            //int lws = 1;//(int)queue.getDevice().getMaxWorkGroupSize();
            //if (lws > 32)
            //    lws = 32;
            //localWorkSizes = new int[] { lws };

            randomProgram.getProgram().defineMacro("NUMBERS_COUNT", parallelSize);
            randomProgram.getProgram().defineMacro("WORK_ITEMS_COUNT", scheduledWorkItems);

            final int nSeeds = seedsNeededByWorkItem * parallelSize;
            final IntBuffer seedsBuf = NIOUtils.directInts(nSeeds, context.getKernelsDefaultByteOrder());
            initSeeds(seedsBuf, seed);
            //println(seedsBuf);
            this.seeds = context.createIntBuffer(Usage.InputOutput, seedsBuf, true);
            //this.lastOutputData = NIOUtils.directInts(parallelSize, context.getKernelsDefaultByteOrder());
            this.output = context.createIntBuffer(Usage.Output, parallelSize);
        } catch (InterruptedException ex) {
            Logger.getLogger(ParallelRandom.class.getName()).log(Level.SEVERE, null, ex);
            throw new RuntimeException("Failed to initialized parallel random", ex);
        }
    }
	
	static final int floatMask = 0x00ffffff;
	static final double floatDivid = (1 << 24);
	//static final int mask = (1 << 30) - 1;
	//static final double divid = (1 << 30);

	public int nextInt() {
		waitForData(1);
		return lastData.get(consumedInts++);		
	}
	
	public synchronized void setPreload(boolean preload) throws CLBuildException {
		this.preload = preload;
		if (preload && preloadEvent == null) {
			if (lastData == null) {
				preloadEvent = randomProgram.gen_numbers(queue, seeds, output, globalWorkSizes, null);
			} else if (consumedInts > 0) {
				preload();
			}
		}
	}
	private synchronized CLEvent preload() throws CLBuildException {
		return preloadEvent = randomProgram.gen_numbers(queue, seeds, output, globalWorkSizes, null, preloadEvent);
	}
	private synchronized void waitForData(int n) {
		try {
			if (lastData == null) {
				//lastOutputData = NIOUtils.directInts(parallelSize, context.getKernelsDefaultByteOrder());
				if (preloadEvent == null)
					preloadEvent = randomProgram.gen_numbers(queue, seeds, output, globalWorkSizes, null);
					
				readLastOutputData();
			}
			if (consumedInts > parallelSize - n) {
				preload().waitFor();
				consumedInts = 0;
				readLastOutputData();
			}
			if (preload && preloadEvent == null)
				preload();
		} catch (CLBuildException ex) {
			throw new RuntimeException(ex);
		}
	}
	private synchronized void readLastOutputData() {
		if (lastData == null)
			lastData = output.read(queue, preloadEvent);
		else
			output.read(queue, lastData, true, preloadEvent);
		preloadEvent = null;
	}
	public long nextLong() {
        return (((long)nextInt()) << 32) | nextInt();
    }
	
	private static final int intSignMask = 1 << 31;
	public int nextInt(int n) {
        if (n <= 0)
            throw new IllegalArgumentException("n must be positive");

        if ((n & -n) == n)  // i.e., n is a power of 2
            return (int)((n * (long)(nextInt() & intSignMask)) >> 31);

        int bits, val;
        do {
            bits = nextInt() & intSignMask;
            val = bits % n;
        } while (bits - val + (n-1) < 0);
        return val;
    }

	public float nextFloat() {
		return (float)((nextInt() & floatMask) / floatDivid);
	}
	
	private static final int doubleMask = (1 << 27) - 1;
	private static final double doubleDivid = 1L << 53;
	
	public double nextDouble() {
		return (((long)(nextInt() & doubleMask) << 27) | (nextInt() & doubleMask)) / doubleDivid;
	}

	public CLIntBuffer getSeeds() {
		return seeds;
	}
	public CLQueue getQueue() {
		return queue;
	}
	
    /**
     * Number of random numbers generated at each call of {@link ParallelRandom#next() } or {@link ParallelRandom#next(IntBuffer) }

     * The numbers might not all be generated exactly in parallel, the level of parallelism is implementation-dependent.
     * @return size of each buffer returned by {@link ParallelRandom#next() }
     */
    public int getParallelSize() {
        return parallelSize;
    }
    
    public synchronized CLEvent doNext() {
        try {
            //if (mappedOutputBuffer != null) {
            //    //output.unmap(queue, mappedOutputBuffer);
            //    mappedOutputBuffer = null;
            //}
            return randomProgram.gen_numbers(queue, seeds, //parallelSize,
                    output, globalWorkSizes, null);
        } catch (CLBuildException ex) {
            Logger.getLogger(ParallelRandom.class.getName()).log(Level.SEVERE, null, ex);
            throw new RuntimeException("Failed to compile the random number generation routine", ex);
        }
    }

    /**
     * Copies the next {@link ParallelRandom#getParallelSize() } random integers in the provided output buffer
     * @param output
     */
    public synchronized void next(IntBuffer output) {
        CLEvent evt = doNext();
        this.output.read(queue, output, true, evt);
    }

    
    /**
     * Returns a direct NIO buffer containing the next {@link ParallelRandom#getParallelSize() } random integers.

     * This buffer is read only and will only be valid until any of the "next" method is called again.
     * @param output buffer of capacity ; see {@link ParallelRandom#getParallelSize() }
     */
    public synchronized IntBuffer next() {
        CLEvent evt = doNext();
        //queue.finish(); evt = null;
        //return outputBuffer;
        //return (mappedOutputBuffer = output.map(queue, MapFlags.Read, evt)).asReadOnlyBuffer();
        return output.read(queue, evt);
    }

    private void initSeeds(final IntBuffer seedsBuf, final long seed) throws InterruptedException {
        final int nSeeds = seedsBuf.capacity();

        long start = System.nanoTime();

        // TODO benchmark threshold :
        boolean parallelize = nSeeds > 10000;
        //parallelize = false;
        if (parallelize) {
            Random random = new Random(seed);
            for (int i = nSeeds; i-- != 0;)
                seedsBuf.put(i, random.nextInt());
        } else {
            // Parallelize seeds initialization
            final int nThreads = Runtime.getRuntime().availableProcessors();// * 2;
            ExecutorService service = Executors.newFixedThreadPool(nThreads);
            for (int i = 0; i < nThreads; i++) {
                final int iThread = i;
                service.execute(new Runnable() {

                    public void run() {
                        int n = nSeeds / nThreads;
                        int offset = n * iThread;
                        Random random = new Random(seed + iThread);// * System.currentTimeMillis());
                        if (iThread == nThreads - 1)
                            n += nSeeds - n * nThreads;
                        
                        for (int i = n; i-- != 0;)
                            seedsBuf.put(offset++, random.nextInt());
                    }
                });
            }
            service.shutdown();
            service.awaitTermination(Long.MAX_VALUE, TimeUnit.DAYS);
        }

        long time = System.nanoTime() - start;
        Logger.getLogger(ParallelRandom.class.getName()).log(Level.INFO, "Initialization of " + nSeeds + " seeds took " + (time/1000000) + " ms (" + (time / (double)nSeeds) + " ns per seed)");
    }
}