/*
 * Hunt - A refined core library for D programming language.
 *
 * Copyright (C) 2018-2019 HuntLabs
 *
 * Website: https://www.huntlabs.net/
 *
 * Licensed under the Apache-2.0 License.
 *
 */

module hunt.collection.MappedByteBuffer;

import hunt.io.ByteBuffer;

import hunt.Exceptions;
import hunt.stream.FileDescriptor;

/**
 * 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 name="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
 */

abstract class MappedByteBuffer : 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 FileDescriptor fd;

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

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

    private void checkMapped() {
        if (fd is null)
            // Can only happen if a luser explicitly casts a direct byte buffer
            throw new UnsupportedOperationException();
    }

    // // 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 cast(long)capacity() + mappingOffset;
    // }

    // /**
    //  * Tells whether or not this buffer's content is resident in physical
    //  * memory.
    //  *
    //  * <p> A return value of <tt>true</tt> 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 <tt>false</tt> 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  <tt>true</tt> if it is likely that this buffer's content
    //  *          is resident in physical memory
    //  */
    // final bool isLoaded() {
    //     checkMapped();
    //     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
    //  */
    // final MappedByteBuffer load() {
    //     checkMapped();
    //     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;
    //     for (int i=0; i<count; i++) {
    //         x ^= unsafe.getByte(a);
    //         a += ps;
    //     }
    //     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
    //  */
    // final MappedByteBuffer force() {
    //     checkMapped();
    //     if ((address != 0) && (capacity() != 0)) {
    //         long offset = mappingOffset();
    //         force0(fd, mappingAddress(offset), mappingLength(offset));
    //     }
    //     return this;
    // }

    private bool isLoaded0(long address, long length, int pageCount) {
        implementationMissing(false);
        return false;
    }
    private void load0(long address, long length) {
        implementationMissing(false);
    }
    private void force0(FileDescriptor fd, long address, long length){
        implementationMissing(false);
    }
}