A circular buffer, cyclic buffer or ring buffer is a data structure that uses a single, fixed-size buffer as if it were connected end-to-end.
A circular buffer first starts empty and of some predefined length. For example, this is a 7-element buffer:
[ ][ ][ ][ ][ ][ ][ ]
Assume that a 1 is written into the middle of the buffer (exact starting location does not matter in a circular buffer):
[ ][ ][ ][1][ ][ ][ ]
Then assume that two more elements are added — 2 & 3 — which get appended after the 1:
[ ][ ][ ][1][2][3][ ]
If two elements are then removed from the buffer, the oldest values inside the buffer are removed. The two elements removed, in this case, are 1 & 2, leaving the buffer with just a 3:
[ ][ ][ ][ ][ ][3][ ]
If the buffer has 7 elements then it is completely full:
[5][6][7][8][9][3][4]
When the buffer is full an error will be raised, alerting the client that further writes are blocked until a slot becomes free.
When the buffer is full, the client can opt to overwrite the oldest data with a forced write. In this case, two more elements — A & B — are added and they overwrite the 3 & 4:
[5][6][7][8][9][A][B]
3 & 4 have been replaced by A & B making 5 now the oldest data in the buffer. Finally, if two elements are removed then what would be returned is 5 & 6 yielding the buffer:
[ ][ ][7][8][9][A][B]
Because there is space available, if the client again uses overwrite to store C & D then the space where 5 & 6 were stored previously will be used not the location of 7 & 8. 7 is still the oldest element and the buffer is once again full.
[C][D][7][8][9][A][B]
This exercise introduces a couple of concepts you might not have seen before:
Words in 8th all belong to a namespace, e.g. a:new refers to the new word in the a namespace.
One can also define custom namespaces, which is what we do at the top of the circular-buffer.8th file:
ns: cb
This means that all words within that file can be accessed via cb:<word>, which is a great way to group related functionality.
For more information, check out the namespaces documentation.
The read and write words are both expected to throw an exception when they're called on a circular buffer with an invalid state.
For more information, check the exceptions and error handling documentation.
While you're free to implement the circular buffer as you like, you could consider using 8th's object support.
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In this video we take a look at Circular Buffer, what they are, where they're used and different implementations - including queues, static and dynamic arrays, immutable data structures and a fun agent based implementation.