No, the author can use all entries in the buffer. The space cost is hidden in that one of the pointers need to hold X+1 values, ie for 256 entries it is not enough with 8-bit indices.
An unfortunate effect of this implementation is that both indices are modified by the consumer. It's not safe to write/read dats from different contexts, something that would be very useful in a driver.
Yeah your intuition is correct here. On an MSP430 (which is were I use it) the producer is in interrupt context but as a savvy redditor pointed out, I will need to enable/disable interrupts in the consumer function as well.
If I understand correctly he's storing X items in a buffer of size X, but using an invalid value in the head pointer to indicate that the buffer is full.
It's easier to understand without the decrementing. Initialize head and tail to 0, keep incrementing and cycling through the end as usual. When removing an element check if head and tail are equal before removing. When adding an element check if head and tail are equal after adding, and set the head to a sentinel value, say MAX_INT. Now you can check for the sentinel before attempting to add an element.
Comments
You still have a buffer or size X that may store X-Y items. I fail to see the "zero memory waste" (not that it's a good tradeoff).
No, the author can use all entries in the buffer. The space cost is hidden in that one of the pointers need to hold X+1 values, ie for 256 entries it is not enough with 8-bit indices.
An unfortunate effect of this implementation is that both indices are modified by the consumer. It's not safe to write/read dats from different contexts, something that would be very useful in a driver.
Yeah your intuition is correct here. On an MSP430 (which is were I use it) the producer is in interrupt context but as a savvy redditor pointed out, I will need to enable/disable interrupts in the consumer function as well.
Ah, thanks for explaining.
Now it makes sense.
If I understand correctly he's storing X items in a buffer of size X, but using an invalid value in the head pointer to indicate that the buffer is full.
It's easier to understand without the decrementing. Initialize head and tail to 0, keep incrementing and cycling through the end as usual. When removing an element check if head and tail are equal before removing. When adding an element check if head and tail are equal after adding, and set the head to a sentinel value, say MAX_INT. Now you can check for the sentinel before attempting to add an element.
Neat trick!