Batcher odd–even mergesort

Visualization of the odd–even mergesort network with eight inputs

Batcher's odd–even mergesort is a generic construction devised by Ken Batcher for sorting networks of size O(n (log n)²) and depth O((log n)²), where n is the number items to be sorted. Although it is not asymptotically optimal, Knuth concluded in 1998, with respect to the AKS network that "Batcher's method is much better, unless n exceeds the total memory capacity of all computers on earth!"

It is popularized by the second GPU Gems book, as an easy way of doing reasonably efficient sorts on graphics-processing hardware.

Example code

The following is an implementation of odd–even mergesort algorithm in Python. The input is a list x of length a power of 2. The output is a list sorted in ascending order.

def compare_and_swap(x, a, b):
    if x[a] > x[b]:
        x[a], x[b] = x[b], x[a]
 
def oddeven_merge(x, lo, hi, r):
    step = r * 2
    if step < hi - lo:
        oddeven_merge(x, lo, hi, step)
        oddeven_merge(x, lo + r, hi, step)
        for i in range(lo + r, hi - r, step):
            compare_and_swap(x, i, i + r)
    else:
        compare_and_swap(x, lo, lo + r)
 
def oddeven_merge_sort_range(x, lo, hi):
    """ sort the part of x with indices between lo and hi.

    Note: endpoints (lo and hi) are included.
    """
    if (hi - lo 1:
        # if there is more than one element, split the input
        # down the middle and first sort the first and second
        # half, followed by merging them.
        mid = lo + ((hi - lo) / 2)
        oddeven_merge_sort_range(x, lo, mid)
        oddeven_merge_sort_range(x, mid + 1, hi)
        oddeven_merge(x, lo, hi, 1)
 
def oddeven_merge_sort(x):
    oddeven_merge_sort_range(x, 0, len(x)-1)

>>> data = [4, 3, 5, 6, 1, 7, 8]
>>> oddeven_merge_sort(data)
>>> data
[1, 2, 3, 4, 5, 6, 7, 8]

External links

• Odd–even mergesort at fh-flensburg.de