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package java.nio;

import java.io.FileDescriptor;
import java.lang.ref.Reference;
import jdk.internal.misc.Unsafe;


/**
 * A direct byte buffer whose content is a memory-mapped region of a file.
 *
 * <p> Mapped byte buffers are created via the {@link
 * java.nio.channels.FileChannel#map FileChannel.map} method. This class
 * extends the {@link ByteBuffer} class with operations that are specific to
 * memory-mapped file regions.
 *
 * <p> A mapped byte buffer and the file mapping that it represents remain
 * valid until the buffer itself is garbage-collected.
 *
 * <p> The content of a mapped byte buffer can change at any time, for example
 * if the content of the corresponding region of the mapped file is changed by
 * this program or another. Whether or not such changes occur, and when they
 * occur, is operating-system dependent and therefore unspecified.
 *
 * <a id="inaccess"></a><p> All or part of a mapped byte buffer may become
 * inaccessible at any time, for example if the mapped file is truncated. An
 * attempt to access an inaccessible region of a mapped byte buffer will not
 * change the buffer's content and will cause an unspecified exception to be
 * thrown either at the time of the access or at some later time. It is
 * therefore strongly recommended that appropriate precautions be taken to
 * avoid the manipulation of a mapped file by this program, or by a
 * concurrently running program, except to read or write the file's content.
 *
 * <p> Mapped byte buffers otherwise behave no differently than ordinary direct
 * byte buffers. </p>
 *
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @since 1.4
 */

public abstract class MappedByteBuffer
 extends ByteBuffer
{

 // This is a little bit backwards: By rights MappedByteBuffer should be a
 // subclass of DirectByteBuffer, but to keep the spec clear and simple, and
 // for optimization purposes, it's easier to do it the other way around.
 // This works because DirectByteBuffer is a package-private class.

 // For mapped buffers, a FileDescriptor that may be used for mapping
 // operations if valid; null if the buffer is not mapped.
 private final FileDescriptor fd;

 // This should only be invoked by the DirectByteBuffer constructors
 //
 MappedByteBuffer(int mark, int pos, int lim, int cap, // package-private
 FileDescriptor fd)
 {
 super(mark, pos, lim, cap);
 this.fd = fd;
 }

 MappedByteBuffer(int mark, int pos, int lim, int cap) { // package-private
 super(mark, pos, lim, cap);
 this.fd = null;
 }

 // Returns the distance (in bytes) of the buffer from the page aligned address
 // of the mapping. Computed each time to avoid storing in every direct buffer.
 private long mappingOffset() {
 int ps = Bits.pageSize();
 long offset = address % ps;
 return (offset >= 0) ? offset : (ps + offset);
 }

 private long mappingAddress(long mappingOffset) {
 return address - mappingOffset;
 }

 private long mappingLength(long mappingOffset) {
 return (long)capacity() + mappingOffset;
 }

 /**
 * Tells whether or not this buffer's content is resident in physical
 * memory.
 *
 * <p> A return value of {@code true} implies that it is highly likely
 * that all of the data in this buffer is resident in physical memory and
 * may therefore be accessed without incurring any virtual-memory page
 * faults or I/O operations. A return value of {@code false} does not
 * necessarily imply that the buffer's content is not resident in physical
 * memory.
 *
 * <p> The returned value is a hint, rather than a guarantee, because the
 * underlying operating system may have paged out some of the buffer's data
 * by the time that an invocation of this method returns. </p>
 *
 * @return {@code true} if it is likely that this buffer's content
 * is resident in physical memory
 */
 public final boolean isLoaded() {
 if (fd == null) {
 return true;
 }
 if ((address == 0) || (capacity() == 0))
 return true;
 long offset = mappingOffset();
 long length = mappingLength(offset);
 return isLoaded0(mappingAddress(offset), length, Bits.pageCount(length));
 }

 // not used, but a potential target for a store, see load() for details.
 private static byte unused;

 /**
 * Loads this buffer's content into physical memory.
 *
 * <p> This method makes a best effort to ensure that, when it returns,
 * this buffer's content is resident in physical memory. Invoking this
 * method may cause some number of page faults and I/O operations to
 * occur. </p>
 *
 * @return This buffer
 */
 public final MappedByteBuffer load() {
 if (fd == null) {
 return this;
 }
 if ((address == 0) || (capacity() == 0))
 return this;
 long offset = mappingOffset();
 long length = mappingLength(offset);
 load0(mappingAddress(offset), length);

 // Read a byte from each page to bring it into memory. A checksum
 // is computed as we go along to prevent the compiler from otherwise
 // considering the loop as dead code.
 Unsafe unsafe = Unsafe.getUnsafe();
 int ps = Bits.pageSize();
 int count = Bits.pageCount(length);
 long a = mappingAddress(offset);
 byte x = 0;
 try {
 for (int i=0; i<count; i++) {
 // TODO consider changing to getByteOpaque thus avoiding
 // dead code elimination and the need to calculate a checksum
 x ^= unsafe.getByte(a);
 a += ps;
 }
 } finally {
 Reference.reachabilityFence(this);
 }
 if (unused != 0)
 unused = x;

 return this;
 }

 /**
 * Forces any changes made to this buffer's content to be written to the
 * storage device containing the mapped file.
 *
 * <p> If the file mapped into this buffer resides on a local storage
 * device then when this method returns it is guaranteed that all changes
 * made to the buffer since it was created, or since this method was last
 * invoked, will have been written to that device.
 *
 * <p> If the file does not reside on a local device then no such guarantee
 * is made.
 *
 * <p> If this buffer was not mapped in read/write mode ({@link
 * java.nio.channels.FileChannel.MapMode#READ_WRITE}) then invoking this
 * method has no effect. </p>
 *
 * @return This buffer
 */
 public final MappedByteBuffer force() {
 if (fd == null) {
 return this;
 }
 if ((address != 0) && (capacity() != 0)) {
 long offset = mappingOffset();
 force0(fd, mappingAddress(offset), mappingLength(offset));
 }
 return this;
 }

 private native boolean isLoaded0(long address, long length, int pageCount);
 private native void load0(long address, long length);
 private native void force0(FileDescriptor fd, long address, long length);

 // -- Covariant return type overrides

 /**
 * {@inheritDoc}
 */
 @Override
 public final MappedByteBuffer position(int newPosition) {
 super.position(newPosition);
 return this;
 }

 /**
 * {@inheritDoc}
 */
 @Override
 public final MappedByteBuffer limit(int newLimit) {
 super.limit(newLimit);
 return this;
 }

 /**
 * {@inheritDoc}
 */
 @Override
 public final MappedByteBuffer mark() {
 super.mark();
 return this;
 }

 /**
 * {@inheritDoc}
 */
 @Override
 public final MappedByteBuffer reset() {
 super.reset();
 return this;
 }

 /**
 * {@inheritDoc}
 */
 @Override
 public final MappedByteBuffer clear() {
 super.clear();
 return this;
 }

 /**
 * {@inheritDoc}
 */
 @Override
 public final MappedByteBuffer flip() {
 super.flip();
 return this;
 }

 /**
 * {@inheritDoc}
 */
 @Override
 public final MappedByteBuffer rewind() {
 super.rewind();
 return this;
 }
}