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"""

A Python implementation of the quick select algorithm, which is efficient for

calculating the value that would appear in the index of a list if it would be

sorted, even if it is not already sorted

https://en.wikipedia.org/wiki/Quickselect

"""

import random

def _partition(data: list, pivot) -> tuple:

"""

Three way partition the data into smaller, equal and greater lists,

in relationship to the pivot

:param data: The data to be sorted (a list)

:param pivot: The value to partition the data on

:return: Three list: smaller, equal and greater

"""

less, equal, greater = [], [], []

for element in data:

if element < pivot:

less.append(element)

elif element > pivot:

greater.append(element)

else:

equal.append(element)

return less, equal, greater

def quick_select(items: list, index: int):

"""

>>> quick_select([2, 4, 5, 7, 899, 54, 32], 5)

>>> quick_select([2, 4, 5, 7, 899, 54, 32], 1)

>>> quick_select([5, 4, 3, 2], 2)

>>> quick_select([3, 5, 7, 10, 2, 12], 3)

"""

# index = len(items) // 2 when trying to find the median

# (value of index when items is sorted)

# invalid input

if index >= len(items) or index < 0:

return None

pivot = items[random.randint(0, len(items) - 1)]

count = 0

smaller, equal, larger = _partition(items, pivot)

count = len(equal)

m = len(smaller)

# index is the pivot

if m <= index < m + count:

return pivot

# must be in smaller

elif m > index:

return quick_select(smaller, index)

# must be in larger

else:

return quick_select(larger, index - (m + count))

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