multimerge

Multimerge

Multimerge is a Python package that implements an algorithm for lazily combining several sorted iterables into one longer sorted iterator. It is a drop-in replacement for heapq.merge in the Python standard library.

The API (from heapq.merge())

def merge(*iterables, key=None, reverse=False):
    '''Merge multiple sorted inputs into a single sorted output.

    Similar to sorted(itertools.chain(*iterables)) but returns a generator,
    does not pull the data into memory all at once, and assumes that each of
    the input streams is already sorted (smallest to largest).

    >>> list(merge([1,3,5,7], [0,2,4,8], [5,10,15,20], [], [25]))
    [0, 1, 2, 3, 4, 5, 5, 7, 8, 10, 15, 20, 25]
    
    If *key* is not None, applies a key function to each element to determine
    its sort order.
    
    >>> list(merge(['dog', 'horse'], ['cat', 'fish', 'kangaroo'], key=len))
    ['dog', 'cat', 'fish', 'horse', 'kangaroo']
    
    '''
    ...

Comparing the Algorithms

heapq.merge()

The standard-library implementation of merge, as its location suggests, involves maintaining a heap (priority queue) data structure. In this algorithm, each node of the heap stores both a unique item from an iterator as well as the index of the iterator from which the item came. This is how sort stability is maintained. At each step of the merge, the root of the heap is yielded, then a new item from that root's source iterator is found, which then replaces the root with a call to heapreplace.

multimerge.merge()

Multimerge uses a different data structure: a linked binary tree known as a "tournament tree of winners". It works as follows:

• Each leaf node remembers a particular iterator, as well as the most recent item yielded from that iterator.

• Each non-leaf node stores the highest-priority value among all of its descendent leaves' values, as well as which leaf that highest-priority value came from.

• At a typical step of the process, the root of the tree stored the highest-priority value that was just yielded. To replace it, look at the root's stored reference to the leaf of the item most recently yielded. We will replace this leaf's value with a new value from its iterator. Finally, to restore the invariant about descendent values, we need to re-evaluate several "games of the tournament"; at each ancestor of the leaf, re-evaluate which of its two children should have the higher priority, then acquire that child's value and leaf references. When this is complete, the value at the root is the highest-priority, and can be yielded next.

• When sorting values not by direct comparison but by some key function, it is cheaper to only bring the keys up the tree, keeping the values stored only at the leaves.

• When a leaf's iterator is exhausted, it can be deleted so that its sibling can be promoted, shrinking the data structure as the problem reduces and maintaining the invariant that each non-leaf node has exactly two children.

Benefits of multimerge.merge()

• There are fewer comparisons required on average, especially in the case where the input data has long runs where one particular iterator should "win".

• In the heap model, during a heapreplace call, the (it_index, key, value) tuples are compared lexicographically. This involves identifying the first place where two tuples differ, which will require evaluating key1 == key2, followed then by evaluating key1 < key2. This is not necessary in multimerge.merge()'s tournament tree approach, since the order of the input iterators part of the tree structure, so sort stability comes naturally.

Both of the above are demonstrated below:

class Int(int):
    lt = eq = 0
    def __lt__(self, other):
        __class__.lt += 1
        return int.__lt__(self, other)

    def __eq__(self, other):
        __class__.eq += 1
        return int.__eq__(self, other)

def comparisons(mergefunc, iterables):
    Int.lt = Int.eq = 0
    for _ in mergefunc(*iterables):
        pass
    return Int.lt, Int.eq

no_overlap = [
    # (0..999), (1_000..1_999), (2_000..2_999), ...
    list(map(Int, range(x, x+1_000)))
    for x in range(0, 16_000, 1_000)
]

interleaved = [
    # (0,16,32,...), (1,17,33,...), (2,18,34,...), ...
    list(map(Int, range(x, 16_000, 16)))
    for x in range(16)
]

def test_merge_func(mergefunc):
    print("No overlap: {:,} lt; {:,} eq".format(
        *comparisons(mergefunc, no_overlap)))
    print("Interleaved: {:,} lt; {:,} eq".format(
        *comparisons(mergefunc, interleaved)))

if __name__ == "__main__":
    import heapq
    print("======= heapq.merge ======")
    test_merge_func(heapq.merge)
    print()

    import multimerge
    print("==== multimerge.merge ====")
    test_merge_func(multimerge.merge)
    print()

Result:

======= heapq.merge ======
No overlap: 65,004 lt; 65,004 eq
Interleaved: 64,004 lt; 64,004 eq

==== multimerge.merge ====
No overlap: 32,000 lt; 0 eq
Interleaved: 63,968 lt; 0 eq

• These theoretical improvements, coupled with a fast C implementation, make multimerge up to 5 times faster than heapq for basic benchmarks:

py -m pyperf timeit -s "from random import random; from collections import deque; from heapq import merge;  iters = [sorted(random() for j in range(10_000)) for i in range(20)]" "deque(merge(*iters), maxlen=0)"

    Mean +- std dev: 80.8 ms +- 5.6 ms

py -m pyperf timeit -s "from random import random; from collections import deque; from multimerge import merge;  iters = [sorted(random() for j in range(10_000)) for i in range(20)]" "deque(merge(*iters), maxlen=0)"

    Mean +- std dev: 17.1 ms +- 1.4 ms