org.jcsp.util
Class OverWriteOldestBuffer

java.lang.Object
  org.jcsp.util.OverWriteOldestBuffer

All Implemented Interfaces:

Serializable, Cloneable, ChannelDataStore

public class OverWriteOldestBuffer
extends Object
implements ChannelDataStore, Serializable

This is used to create a buffered object channel that always accepts input, overwriting its oldest data if full.

Description

OverWriteOldestBuffer is an implementation of ChannelDataStore that yields a FIFO buffered semantics for a channel. When empty, the channel blocks readers. When full, a writer will overwrite the oldest item left unread in the channel. See the static construction methods of Channel (Channel.one2one(org.jcsp.util.ChannelDataStore) etc.).

The getState method returns EMPTY or NONEMPTYFULL, but never FULL.

Author:

P.D. Austin

See Also:

ZeroBuffer, Buffer, OverWritingBuffer, OverFlowingBuffer, InfiniteBuffer, Channel, Serialized Form

Field Summary

Fields inherited from interface org.jcsp.util.ChannelDataStore

EMPTY, FULL, NONEMPTYFULL

Constructor Summary

OverWriteOldestBuffer(int size)
Construct a new OverWriteOldestBuffer with the specified size.

Method Summary

Object	clone() Returns a new (and EMPTY) OverWriteOldestBuffer with the same creation parameters as this one.
void	endGet() This ends an extended rendezvous by the reader.
Object	get() Returns the oldest Object from the OverWriteOldestBuffer and removes it.
int	getState() Returns the current state of the OverWriteOldestBuffer.
void	put(Object value) Puts a new Object into the OverWriteOldestBuffer.
void	removeAll() Deletes all items in the buffer, leaving it empty.
Object	startGet() Begins an extended rendezvous by the reader.

Methods inherited from class java.lang.Object

equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

OverWriteOldestBuffer

public OverWriteOldestBuffer(int size)

Construct a new OverWriteOldestBuffer with the specified size.

Parameters:

size - the number of Objects the OverWriteOldestBuffer can store.

Throws:

BufferSizeError - if size is zero or negative. Note: no action should be taken to try/catch this exception - application code generating it is in error and needs correcting.

Method Detail

get

public Object get()

Returns the oldest Object from the OverWriteOldestBuffer and removes it.

Pre-condition: getState must not currently return EMPTY.

Specified by:

get in interface ChannelDataStore

Returns:

the oldest Object from the OverWriteOldestBuffer

startGet

public Object startGet()

Begins an extended rendezvous by the reader. The semantics of an extended rendezvous on an overwrite-oldest buffer are slightly complicated, but hopefully intuitive.

When a reader begins an extended rendezvous, the oldest value is returned from the buffer (as it would be for a call to get()). While an extended rendezvous is ongoing, the writer may (repeatedly) write to the buffer, without ever blocking.

When the reader finishes an extended rendezvous, the following options are possible:

• The writer has not written to the channel during the rendezvous. In this case, the value that was read at the start of the rendezvous is removed from the buffer.
• The writer has written to the channel during the rendezvous, but has not over-written the value that was read at the start of the rendezvous. In this case, the value that was read at the start of the rendezvous is removed from the buffer.
• The writer has written to the channel during the rendezvous, and has over-written (perhaps repeatedly) the value that was read at the start of the rendezvous. In this case, the value that was read at the start of the rendezvous is no longer in the buffer, and hence nothing is removed.

Specified by:

startGet in interface ChannelDataStore

Returns:

The oldest value in the buffer at this time

See Also:

ChannelDataStore.endGet()

endGet

public void endGet()

This ends an extended rendezvous by the reader.

Specified by:

endGet in interface ChannelDataStore

See Also:

startGet()

put

public void put(Object value)

Puts a new Object into the OverWriteOldestBuffer.

If OverWriteOldestBuffer is full, the oldest item left unread in the buffer will be overwritten.

Specified by:

put in interface ChannelDataStore

Parameters:

value - the Object to put into the OverWriteOldestBuffer

getState

public int getState()

Returns the current state of the OverWriteOldestBuffer.

Specified by:

getState in interface ChannelDataStore

Returns:

the current state of the OverWriteOldestBuffer (EMPTY or NONEMPTYFULL)

clone

public Object clone()

Returns a new (and EMPTY) OverWriteOldestBuffer with the same creation parameters as this one.

Note: Only the size and structure of the OverWriteOldestBuffer is cloned, not any stored data.

Specified by:

clone in interface ChannelDataStore

Overrides:

clone in class Object

Returns:

the cloned instance of this OverWriteOldestBuffer.

removeAll

public void removeAll()

Description copied from interface: ChannelDataStore

Deletes all items in the buffer, leaving it empty.

Specified by:

removeAll in interface ChannelDataStore