//
//  VirtualRingBuffer.cpp
//  FilePlayer
//
/*
 Copyright (c) 2002, Kurt Revis.  All rights reserved.

 Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

 * Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
 * Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
 * Neither the name of Snoize nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

 THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <mach/mach.h>
#include <mach/mach_error.h>
#include "VirtualRingBuffer.h"
    
VirtualRingBuffer::VirtualRingBuffer(unsigned long length)
{
    // We need to allocate entire VM pages, so round the specified length up to the next page if necessary.
    bufferLength = round_page(length);

    buffer = allocateVirtualBuffer(bufferLength, &memoryEntry);
    if (buffer) {
        bufferEnd = (void *)((unsigned long)buffer + (unsigned long)bufferLength);
    } else {
         this->~VirtualRingBuffer();      
    }
    readPointer = NULL;
    writePointer = NULL;
}

VirtualRingBuffer::~VirtualRingBuffer(void)
{
    if (buffer)
        deallocateVirtualBuffer(buffer, bufferLength);
        mach_port_deallocate(mach_task_self(), memoryEntry);
}

void VirtualRingBuffer::clear(void)
{
    // Dumps everything that was previously writen to the buffer
    readPointer = NULL;
}

void VirtualRingBuffer::empty(void)
{
    // Assumption:
    // No one is reading or writing from the buffer, in any thread, when this method is called.

        readPointer = NULL;
        writePointer = NULL;
}

unsigned char VirtualRingBuffer::isEmpty(void)
{
    return (readPointer != NULL && writePointer != NULL);
}


//
// Theory of operation:
//
// This class keeps a pointer to the next byte to be read (readPointer) and a pointer to the next byte to be written (writePointer).
// readPointer is only advanced in the reading thread (except for one case: when the buffer first has data written to it).
// writePointer is only advanced in the writing thread.
//
// Since loading and storing word length data is atomic, each pointer can safely be modified in one thread while the other thread
// uses it, IF each thread is careful to make a local copy of the "opposite" pointer when necessary.
// 

//
// Read operations
//

unsigned long VirtualRingBuffer::lengthAvailableToReadReturningPointer(void **returnedReadPointer)
{
    // Assumptions:
    // returnedReadPointer != NULL

    unsigned long length=0;
    // Read this pointer exactly once, so we're safe in case it is changed in another thread
    void *localWritePointer = writePointer;

    // Depending on out-of-order execution and memory storage, either one of these may be NULL when the buffer is empty. So we must check both.
    if (!readPointer || !localWritePointer) {
        // The buffer is empty
        length = 0;
    } else if (localWritePointer > readPointer) {
        // Write is ahead of read in the buffer
        length = (unsigned long)localWritePointer - (unsigned long)readPointer;
    } else {
        // Write has wrapped around past read, OR write == read (the buffer is full)
        length = (unsigned long)bufferLength - ((unsigned long)readPointer - (unsigned long)localWritePointer);
    }

    *returnedReadPointer = readPointer;
    return length;
}

void VirtualRingBuffer::didReadLength(unsigned long length)
{
    // Assumptions:
    // [self lengthAvailableToReadReturningPointer:] currently returns a value >= length
    // length > 0

    void *newReadPointer;

    newReadPointer = (void *)((unsigned long)readPointer + (unsigned long)length);
    if (newReadPointer >= bufferEnd)
        newReadPointer = (void *)((unsigned long)newReadPointer - (unsigned long)bufferLength);

    if (newReadPointer == writePointer) {
        // We just read the last data out of the buffer, so it is now empty.
        newReadPointer = NULL;
    }

    // Store the new read pointer. This is the only place this happens in the read thread.
    readPointer = newReadPointer;    
}


//
// Write operations
//

unsigned long VirtualRingBuffer::lengthAvailableToWriteReturningPointer(void **returnedWritePointer)
{
    // Assumptions:
    // returnedWritePointer != NULL
    
    unsigned long length=0;
    // Read this pointer exactly once, so we're safe in case it is changed in another thread
    void *localReadPointer = readPointer;
    
    // Either one of these may be NULL when the buffer is empty. So we must check both.
    if (!localReadPointer || !writePointer) {
        // The buffer is empty. Set it up to be written into.
        // This is one of the two places the write pointer can change; both are in the write thread.
        writePointer = buffer;
        length = bufferLength;
    } else if (writePointer <= localReadPointer) {
        // Write is before read in the buffer, OR write == read (meaning that the buffer is full).
        length = (unsigned long)localReadPointer - (unsigned long)writePointer;
    } else {
        // Write is behind read in the buffer. The available space wraps around.
        length = ((unsigned long)bufferEnd - (unsigned long)writePointer) + ((unsigned long)localReadPointer - (unsigned long)buffer);
    }

    *returnedWritePointer = writePointer;
    return length;
}

void VirtualRingBuffer::didWriteLength(unsigned long length)
{
    // Assumptions:
    // [self lengthAvailableToWriteReturningPointer:] currently returns a value >= length
    // length > 0

    void *oldWritePointer = writePointer;
    void *newWritePointer;

    // Advance the write pointer, wrapping around if necessary.
    newWritePointer = (void*)((unsigned long)writePointer + (unsigned long)length);
    if (newWritePointer >= bufferEnd)
        newWritePointer = (void*)((unsigned long)newWritePointer - (unsigned long)bufferLength);

    // This is one of the two places the write pointer can change; both are in the write thread.
    writePointer = newWritePointer;

    // Also, if the read pointer is NULL, then we just wrote into a previously empty buffer, so set the read pointer.
    // This is the only place the read pointer is changed in the write thread.
    // The read thread should never change the read pointer when it is NULL, so this is safe.
    if (!readPointer)
        readPointer = oldWritePointer;
}

void *VirtualRingBuffer::allocateVirtualBuffer(unsigned long bufferLength, mach_port_t *port)
{
    kern_return_t error;
    vm_address_t originalAddress = NULL;
    vm_address_t realAddress = NULL;
    vm_size_t memoryEntryLength;
    vm_address_t virtualAddress = NULL;

    // We want to find where we can get 2 * bufferLength bytes of contiguous address space.
    // So let's just allocate that space, remember its address, and deallocate it.
    // (This doesn't actually have to touch all of that memory so it's not terribly expensive.)
    error = vm_allocate(mach_task_self(), &originalAddress, 2 * bufferLength, TRUE);
    if (error) {
        return NULL;
    }

    error = vm_deallocate(mach_task_self(), originalAddress, 2 * bufferLength);
    if (error) {
        return NULL;
    }

    // Then allocate a "real" block of memory at the same address, but with the normal bufferLength.
    realAddress = originalAddress;
    error = vm_allocate(mach_task_self(), &realAddress, bufferLength, FALSE);
    if (error) {
        return NULL;
    }
    if (realAddress != originalAddress) {
       goto errorReturn;
    }
    // lock the buffer into wired memory
    // everything is already page aligned!
//    mlock((const void*)realAddress, bufferLength);
    
    // Then make a memory entry for the area we just allocated.
    memoryEntryLength = bufferLength;
    error = mach_make_memory_entry(mach_task_self(), &memoryEntryLength, realAddress, VM_PROT_READ | VM_PROT_WRITE, port, NULL);
    if (error) {
        goto errorReturn;
    }
    if (!(*port)) {
        goto errorReturn;
    }
    if (memoryEntryLength != bufferLength) {
        goto errorReturn;
    }

    // And map the area immediately after the first block, with length bufferLength, to that memory entry.
    virtualAddress = realAddress + bufferLength;
    error = vm_map(mach_task_self(), &virtualAddress, bufferLength, 0, FALSE, *port, 0, FALSE, VM_PROT_READ | VM_PROT_WRITE, VM_PROT_READ | VM_PROT_WRITE, VM_INHERIT_DEFAULT);
    if (error) {
        virtualAddress = NULL;
        goto errorReturn;
    }
    if (virtualAddress != realAddress + bufferLength) {
        goto errorReturn;
    }
    
    // Success!
    return (void *)realAddress;
    
errorReturn:
    if (realAddress)
        vm_deallocate(mach_task_self(), realAddress, bufferLength);
    if (virtualAddress)
        vm_deallocate(mach_task_self(), virtualAddress, bufferLength);

    return NULL;
}

void VirtualRingBuffer::deallocateVirtualBuffer(void *buffer, unsigned long bufferLength)
{
//    munlock(buffer, bufferLength);
    // deallocate real & shodawed buffer
    vm_deallocate(mach_task_self(), (vm_address_t)buffer, bufferLength * 2);
}