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3 & 4 have been replaced by A & B making 5 now the oldest data in the buffer.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]
