How to efficiently wrap the index of a fixed-size circular buffer

Question

I have a fixed size circular buffer (implemented as an array): upon initialization, the buffer gets filled with the specified maximum number of elements which allows the use

Answer 1

FWIW, you could always do a parallel array: i = next[i];

But, really, I've always just done this: i++; if (i >= n) i = 0; OR i = (i+1) % n;

Regardless, I'd be really surprised if this is ever a significant performance issue.

Answer 2

Ensure that the buffer is always a power of two long and mask out the top bits.

Answer 3

I tested all 3 versions:

// plain wrap
public static int WrapIndex(int index, int endIndex, int maxSize)
{
    return (endIndex + index) > maxSize ? (endIndex + index) - maxSize : endIndex + index;
}

// wrap using mod
public static int WrapIndexMod(int index, int endIndex, int maxSize)
{
    return (endIndex + index) % maxSize;
}

// wrap by masking out the top bits
public static int WrapIndexMask(int index, int endIndex, int maxSize)
{
    return (endIndex + index) & (maxSize - 1);
}

The performance results (ticks):

Plain: 25 Mod: 16 Mask: 16 (maxSize = 512)
Plain: 25 Mod: 17 Mask: 17 (maxSize = 1024)
Plain: 25 Mod: 17 Mask: 17 (maxSize = 4096)

So it seems that the modulus is the better choice, because it does not require any restriction on the size of the buffer.

Answer 4

Best I've come up with is:

public static int Wrap(int index, int n)
{
    return ((index % n) + n) % n;
}

(Assuming you need to work with negative numbers)

Answer 5

It'll depend somewhat on the processor, but it's probably at least worth trying something like return (end_index + index) % buffer_size;

Answer 6

int GetElement(int index)
{
    return buffer[(end_index + index) % buffer_size];
}

See modulo operation for the more information on the modulus operator (%).