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radixsort.py
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radixsort.py
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# -*- coding: utf-8 -*-
import quicksort
def msd(iterable, depth=0):
"""Iterate over the sequences in ``iterable`` in lexicographical order,
determined using MSD radix sort.
The optional ``depth`` parameter determines the position from which to
start comparing sequences. Default is 0.
Note that this function returns a generator, not a list. If a sorted
list is desired, invoke it with ``list(msd(iterable))``.
"""
# A naive recursive implementation would blow Python's recursion
# limit on larger inputs, so we emulate boundless recursion with
# a simple stack.
stack = [(iterable, depth)]
while stack:
bucket, depth = stack.pop()
if depth < 0:
# Negative depth is used to mark pre-sorted buckets; we can
# output these as-is.
for string in bucket:
yield string
else:
# A Python dict would be more intuitive here, but as it does not
# preserve key ordering, we allocate a list of buckets and use
# each character's Unicode code point number as its address in
# the table.
buckets = [list() for x in range(256)]
for string in bucket:
# Strings shorter than the sorting depth come first in the
# in-bucket ordering, so we can output them as-is. The
# rest are appended to the appropriate buckets.
if len(string) <= depth:
yield string
else:
buckets[ord(string[depth])].append(string)
# The newly filled buckets are pushed to the top of the stack in
# reverse order. This guarantees that the lexicographically first
# bucket is always at the top of the stack, so when its contents
# are eventually pushed to output, they come first as well.
for bucket in reversed(buckets):
# Buckets smaller than the alphabet are better sorted with
# string quicksort, so we use Arturs's implementation and mark
# the bucket as pre-sorted. The rest are pushed to the stack
# as-is, with the sorting depth incremented by one.
if len(bucket) < 256:
stack.append((quicksort.QuickSort(bucket, depth+1), -1))
else:
stack.append((bucket, depth+1))