## Python, sort, msd_radix_sort.py

``````"""
Python implementation of the MSD radix sort algorithm.
It used the binary representation of the integers to sort
them.
https://en.wikipedia.org/wiki/Radix_sort
"""
from __future__ import annotations

def msd_radix_sort(list_of_ints: list[int]) -> list[int]:
"""
Implementation of the MSD radix sort algorithm. Only works
with positive integers
:param list_of_ints: A list of integers
:return: Returns the sorted list
>>> msd_radix_sort([40, 12, 1, 100, 4])
[1, 4, 12, 40, 100]
>>> msd_radix_sort([])
[]
>>> msd_radix_sort([123, 345, 123, 80])
[80, 123, 123, 345]
>>> msd_radix_sort([1209, 834598, 1, 540402, 45])
[1, 45, 1209, 540402, 834598]
>>> msd_radix_sort([-1, 34, 45])
Traceback (most recent call last):
...
ValueError: All numbers must be positive
"""
if not list_of_ints:
return []

if min(list_of_ints) < 0:
raise ValueError("All numbers must be positive")

most_bits = max(len(bin(x)[2:]) for x in list_of_ints)
return _msd_radix_sort(list_of_ints, most_bits)

def _msd_radix_sort(list_of_ints: list[int], bit_position: int) -> list[int]:
"""
Sort the given list based on the bit at bit_position. Numbers with a
0 at that position will be at the start of the list, numbers with a
1 at the end.
:param list_of_ints: A list of integers
:param bit_position: the position of the bit that gets compared
:return: Returns a partially sorted list
>>> _msd_radix_sort([45, 2, 32], 1)
[2, 32, 45]
>>> _msd_radix_sort([10, 4, 12], 2)
[4, 12, 10]
"""
if bit_position == 0 or len(list_of_ints) in [0, 1]:
return list_of_ints

zeros = list()
ones = list()
# Split numbers based on bit at bit_position from the right
for number in list_of_ints:
if (number >> (bit_position - 1)) & 1:
# number has a one at bit bit_position
ones.append(number)
else:
# number has a zero at bit bit_position
zeros.append(number)

# recursively split both lists further
zeros = _msd_radix_sort(zeros, bit_position - 1)
ones = _msd_radix_sort(ones, bit_position - 1)

# recombine lists
res = zeros
res.extend(ones)

return res

def msd_radix_sort_inplace(list_of_ints: list[int]):
"""
Inplace implementation of the MSD radix sort algorithm.
Sorts based on the binary representation of the integers.
>>> lst = [1, 345, 23, 89, 0, 3]
>>> msd_radix_sort_inplace(lst)
>>> lst == sorted(lst)
True
>>> lst = [1, 43, 0, 0, 0, 24, 3, 3]
>>> msd_radix_sort_inplace(lst)
>>> lst == sorted(lst)
True
>>> lst = []
>>> msd_radix_sort_inplace(lst)
>>> lst == []
True
>>> lst = [-1, 34, 23, 4, -42]
>>> msd_radix_sort_inplace(lst)
Traceback (most recent call last):
...
ValueError: All numbers must be positive
"""

length = len(list_of_ints)
if not list_of_ints or length == 1:
return

if min(list_of_ints) < 0:
raise ValueError("All numbers must be positive")

most_bits = max(len(bin(x)[2:]) for x in list_of_ints)
_msd_radix_sort_inplace(list_of_ints, most_bits, 0, length)

def _msd_radix_sort_inplace(
list_of_ints: list[int], bit_position: int, begin_index: int, end_index: int
):
"""
Sort the given list based on the bit at bit_position. Numbers with a
0 at that position will be at the start of the list, numbers with a
1 at the end.
>>> lst = [45, 2, 32, 24, 534, 2932]
>>> _msd_radix_sort_inplace(lst, 1, 0, 3)
>>> lst == [32, 2, 45, 24, 534, 2932]
True
>>> lst = [0, 2, 1, 3, 12, 10, 4, 90, 54, 2323, 756]
>>> _msd_radix_sort_inplace(lst, 2, 4, 7)
>>> lst == [0, 2, 1, 3, 12, 4, 10, 90, 54, 2323, 756]
True
"""
if bit_position == 0 or end_index - begin_index <= 1:
return

bit_position -= 1

i = begin_index
j = end_index - 1
while i <= j:
changed = False
if not ((list_of_ints[i] >> bit_position) & 1):
# found zero at the beginning
i += 1
changed = True
if (list_of_ints[j] >> bit_position) & 1:
# found one at the end
j -= 1
changed = True

if changed:
continue

list_of_ints[i], list_of_ints[j] = list_of_ints[j], list_of_ints[i]
j -= 1
if not j == i:
i += 1

_msd_radix_sort_inplace(list_of_ints, bit_position, begin_index, i)
_msd_radix_sort_inplace(list_of_ints, bit_position, i, end_index)

if __name__ == "__main__":
import doctest

doctest.testmod()
``````