AbstractFifo Class Reference

Encapsulates the logic required to implement a lock-free FIFO. More...

Classes
class	ScopedReadWrite
	Class for a scoped reader/writer. More...

Public Types
using	ScopedRead = ScopedReadWrite<ReadOrWrite::read>
using	ScopedWrite = ScopedReadWrite<ReadOrWrite::write>

Public Member Functions
	AbstractFifo (int capacity) noexcept
	Creates a FIFO to manage a buffer with the specified capacity.
int	getTotalSize () const noexcept
	Returns the total size of the buffer being managed.
int	getFreeSpace () const noexcept
	Returns the number of items that can currently be added to the buffer without it overflowing.
int	getNumReady () const noexcept
	Returns the number of items that can currently be read from the buffer.
void	reset () noexcept
	Clears the buffer positions, so that it appears empty.
void	setTotalSize (int newSize) noexcept
	Changes the buffer's total size.
void	prepareToWrite (int numToWrite, int &startIndex1, int &blockSize1, int &startIndex2, int &blockSize2) const noexcept
	Returns the location within the buffer at which an incoming block of data should be written.
void	finishedWrite (int numWritten) noexcept
	Called after writing from the FIFO, to indicate that this many items have been added.
void	prepareToRead (int numWanted, int &startIndex1, int &blockSize1, int &startIndex2, int &blockSize2) const noexcept
	Returns the location within the buffer from which the next block of data should be read.
void	finishedRead (int numRead) noexcept
	Called after reading from the FIFO, to indicate that this many items have now been consumed.
ScopedRead	read (int numToRead) noexcept
	Replaces prepareToRead/finishedRead with a single function.
ScopedWrite	write (int numToWrite) noexcept
	Replaces prepareToWrite/finishedWrite with a single function.

Detailed Description

Encapsulates the logic required to implement a lock-free FIFO.

This class handles the logic needed when building a single-reader, single-writer FIFO.

It doesn't actually hold any data itself, but your FIFO class can use one of these to manage its position and status when reading or writing to it.

To use it, you can call prepareToWrite() to determine the position within your own buffer that an incoming block of data should be stored, and prepareToRead() to find out when the next outgoing block should be read from.

e.g.

struct MyFifo
{
    void addToFifo (const int* someData, int numItems)
    {
        const auto scope = abstractFifo.write (numItems);
 
        if (scope.blockSize1 > 0)
            copySomeData (myBuffer + scope.startIndex1, someData, scope.blockSize1);
 
        if (scope.blockSize2 > 0)
            copySomeData (myBuffer + scope.startIndex2, someData + scope.blockSize1, scope.blockSize2);
    }
 
    void readFromFifo (int* someData, int numItems)
    {
        const auto scope = abstractFifo.read (numItems);
 
        if (scope.blockSize1 > 0)
            copySomeData (someData, myBuffer + scope.startIndex1, scope.blockSize1);
 
        if (scope.blockSize2 > 0)
            copySomeData (someData + scope.blockSize1, myBuffer + scope.startIndex2, scope.blockSize2);
    }
 
    AbstractFifo abstractFifo { 1024 };
    int myBuffer[1024];
};

Member Typedef Documentation

ScopedRead

using AbstractFifo::ScopedRead = ScopedReadWrite<ReadOrWrite::read>

ScopedWrite

using AbstractFifo::ScopedWrite = ScopedReadWrite<ReadOrWrite::write>

Constructor & Destructor Documentation

AbstractFifo()

AbstractFifo::AbstractFifo ( int capacity )

noexcept

Creates a FIFO to manage a buffer with the specified capacity.

Referenced by AbstractFifo::ScopedReadWrite< ReadOrWrite::read >::ScopedReadWrite(), and write().

Member Function Documentation

getTotalSize()

int AbstractFifo::getTotalSize ( ) const

noexcept

Returns the total size of the buffer being managed.

getFreeSpace()

int AbstractFifo::getFreeSpace ( ) const

noexcept

Returns the number of items that can currently be added to the buffer without it overflowing.

References getFreeSpace().

Referenced by getFreeSpace().

getNumReady()

int AbstractFifo::getNumReady ( ) const

noexcept

Returns the number of items that can currently be read from the buffer.

References getNumReady().

Referenced by getNumReady().

reset()

void AbstractFifo::reset ( )

noexcept

Clears the buffer positions, so that it appears empty.

References reset().

Referenced by reset().

setTotalSize()

void AbstractFifo::setTotalSize ( int newSize )

noexcept

Changes the buffer's total size.

Note that this isn't thread-safe, so don't call it if there's any danger that it might overlap with a call to any other method in this class!

References setTotalSize().

Referenced by setTotalSize().

prepareToWrite()

void AbstractFifo::prepareToWrite ( int numToWrite, int & startIndex1, int & blockSize1, int & startIndex2, int & blockSize2 ) const

noexcept

Returns the location within the buffer at which an incoming block of data should be written.

Because the section of data that you want to add to the buffer may overlap the end and wrap around to the start, two blocks within your buffer are returned, and you should copy your data into the first one, with any remaining data spilling over into the second.

If the number of items you ask for is too large to fit within the buffer's free space, then blockSize1 + blockSize2 may add up to a lower value than numToWrite. If this happens, you may decide to keep waiting and re-trying the method until there's enough space available.

After calling this method, if you choose to write your data into the blocks returned, you must call finishedWrite() to tell the FIFO how much data you actually added.

e.g.

void addToFifo (const int* someData, int numItems)
{
    int start1, size1, start2, size2;
    prepareToWrite (numItems, start1, size1, start2, size2);
 
    if (size1 > 0)
        copySomeData (myBuffer + start1, someData, size1);
 
    if (size2 > 0)
        copySomeData (myBuffer + start2, someData + size1, size2);
 
    finishedWrite (size1 + size2);
}

Parameters

numToWrite	indicates how many items you'd like to add to the buffer
startIndex1	on exit, this will contain the start index in your buffer at which your data should be written
blockSize1	on exit, this indicates how many items can be written to the block starting at startIndex1
startIndex2	on exit, this will contain the start index in your buffer at which any data that didn't fit into the first block should be written
blockSize2	on exit, this indicates how many items can be written to the block starting at startIndex2

See also

finishedWrite

References prepareToWrite().

Referenced by prepareToWrite().

finishedWrite()

void AbstractFifo::finishedWrite ( int numWritten )

noexcept

Called after writing from the FIFO, to indicate that this many items have been added.

See also

prepareToWrite

References finishedWrite().

Referenced by finishedWrite().

prepareToRead()

void AbstractFifo::prepareToRead ( int numWanted, int & startIndex1, int & blockSize1, int & startIndex2, int & blockSize2 ) const

noexcept

Returns the location within the buffer from which the next block of data should be read.

Because the section of data that you want to read from the buffer may overlap the end and wrap around to the start, two blocks within your buffer are returned, and you should read from both of them.

If the number of items you ask for is greater than the amount of data available, then blockSize1 + blockSize2 may add up to a lower value than numWanted. If this happens, you may decide to keep waiting and re-trying the method until there's enough data available.

After calling this method, if you choose to read the data, you must call finishedRead() to tell the FIFO how much data you have consumed.

e.g.

void readFromFifo (int* someData, int numItems)
{
    int start1, size1, start2, size2;
    prepareToRead (numSamples, start1, size1, start2, size2);
 
    if (size1 > 0)
        copySomeData (someData, myBuffer + start1, size1);
 
    if (size2 > 0)
        copySomeData (someData + size1, myBuffer + start2, size2);
 
    finishedRead (size1 + size2);
}

Parameters

numWanted	indicates how many items you'd like to read from the buffer
startIndex1	on exit, this will contain the start index in your buffer at which your data should be written
blockSize1	on exit, this indicates how many items can be written to the block starting at startIndex1
startIndex2	on exit, this will contain the start index in your buffer at which any data that didn't fit into the first block should be written
blockSize2	on exit, this indicates how many items can be written to the block starting at startIndex2

See also

finishedRead

References prepareToRead().

Referenced by prepareToRead().

finishedRead()

void AbstractFifo::finishedRead ( int numRead )

noexcept

Called after reading from the FIFO, to indicate that this many items have now been consumed.

See also

prepareToRead

References finishedRead(), read(), and write().

Referenced by finishedRead().

read()

ScopedRead AbstractFifo::read ( int numToRead )

noexcept

Replaces prepareToRead/finishedRead with a single function.

This function returns an object which contains the start indices and block sizes, and also automatically finishes the read operation when it goes out of scope.

{
    auto readHandle = fifo.read (4);
 
    for (auto i = 0; i != readHandle.blockSize1; ++i)
    {
        // read the item at index readHandle.startIndex1 + i
    }
 
    for (auto i = 0; i != readHandle.blockSize2; ++i)
    {
        // read the item at index readHandle.startIndex2 + i
    }
} // readHandle goes out of scope here, finishing the read operation

Referenced by finishedRead().

write()

ScopedWrite AbstractFifo::write ( int numToWrite )

noexcept

Replaces prepareToWrite/finishedWrite with a single function.

This function returns an object which contains the start indices and block sizes, and also automatically finishes the write operation when it goes out of scope.

{
    auto writeHandle = fifo.write (5);
 
    for (auto i = 0; i != writeHandle.blockSize1; ++i)
    {
        // write the item at index writeHandle.startIndex1 + i
    }
 
    for (auto i = 0; i != writeHandle.blockSize2; ++i)
    {
        // write the item at index writeHandle.startIndex2 + i
    }
} // writeHandle goes out of scope here, finishing the write operation

References AbstractFifo().

Referenced by finishedRead().