Circular queue size using mod

Question

Let's say I implement a circular queue using an array. How could I calculate the size of the queue? By size I mean the number of elements between the front and the rear. I want to use the modulo operation.

I have the capacity of the array, and the positions of the front and the rear of the queue. I don't known what to do now.

score 1 · Accepted Answer · answered Oct 09 '16 at 09:46

1

How could I calculate the size of the queue?

I would use

size = (start - end + mod) % mod;

This assumes the buffer is never completely at capacity. An alernative is to use a start and end which is without modding

size = lastWriteIndex - nextReadIndex;

You can mod these value when you look up the index.

answered Oct 09 '16 at 09:46

Peter Lawrey

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`size = (end - start + mod) % mod;`? – Oct 09 '16 at 10:12
@saka1029 for a circular buffer the start index could be before the end index. – Peter Lawrey Oct 09 '16 at 11:34
Do you mean `start` is `lastWriteIndex`? – Oct 09 '16 at 11:46
@saka1029 it would be the nextWriteIndex. – Peter Lawrey Oct 09 '16 at 11:57

Circular queue size using mod

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