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Python Program for Bitonic Sort

Last Updated : 03 Aug, 2022
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Bitonic Sequence: A sequence is called Bitonic if it is first increasing, then decreasing. In other words, an array arr[0..n-i] is Bitonic if there exists an index i where 0<=i<=n-1 such that 

x0 <= x1 …..<= xi  and  xi >= xi+1….. >= xn-1 
  1. A sequence, sorted in increasing order is considered Bitonic with the decreasing part as empty. Similarly, decreasing order sequence is considered Bitonic with the increasing part as empty.
  2. A rotation of the Bitonic Sequence is also bitonic.

Bitonic Sorting: It mainly involves two steps.  

  1. Forming a bitonic sequence (discussed above in detail). After this step we reach the fourth stage in below diagram, i.e., the array becomes {3, 4, 7, 8, 6, 5, 2, 1}
  2. Creating one sorted sequence from the bitonic sequence: After the first step, the first half is sorted in increasing order and the second half in decreasing order. 
    We compare the first element of the first half with the first element of the second half, then the second element of the first half with the second element of second and so on. We exchange elements if an element of the first half is smaller. 
    After the above compare and exchange steps, we get two bitonic sequences in the array. See the fifth stage below the diagram. In the fifth stage, we have {3, 4, 2, 1, 6, 5, 7, 8}. If we take a closer look at the elements, we can notice that there are two bitonic sequences of length n/2 such that all elements in the first bitonic sequence {3, 4, 2, 1} are smaller than all elements of the second bitonic sequence {6, 5, 7, 8}. 
    We repeat the same process within two bitonic sequences and we get four bitonic sequences of length n/4 such that all elements of the leftmost bitonic sequence are smaller and all elements of the rightmost. See sixth stage in below diagram, arrays is {2, 1, 3, 4, 6, 5, 7, 8}. 
    If we repeat this process one more time we get 8 bitonic sequences of size n/8 which is 1. Since all these bitonic sequences are sorted and every bitonic sequence has one element, we get the sorted array.



# Python program for Bitonic Sort. Note that this program
# works only when size of input is a power of 2.
# The parameter dir indicates the sorting direction, ASCENDING
# or DESCENDING; if (a[i] > a[j]) agrees with the direction,
# then a[i] and a[j] are interchanged.*/
def compAndSwap(a, i, j, dire):
    if (dire == 1 and a[i] > a[j]) or (dire == 0 and a[i] > a[j]):
        a[i], a[j] = a[j], a[i]
# It recursively sorts a bitonic sequence in ascending order,
# if dir = 1, and in descending order otherwise (means dir=0).
# The sequence to be sorted starts at index position low,
# the parameter cnt is the number of elements to be sorted.
def bitonicMerge(a, low, cnt, dire):
    if cnt > 1:
        k = cnt//2
        for i in range(low, low+k):
            compAndSwap(a, i, i+k, dire)
        bitonicMerge(a, low, k, dire)
        bitonicMerge(a, low+k, k, dire)
# This function first produces a bitonic sequence by recursively
# sorting its two halves in opposite sorting orders, and then
# calls bitonicMerge to make them in the same order
def bitonicSort(a, low, cnt, dire):
    if cnt > 1:
        k = cnt//2
        bitonicSort(a, low, k, 1)
        bitonicSort(a, low+k, k, 0)
        bitonicMerge(a, low, cnt, dire)
# Caller of bitonicSort for sorting the entire array of length N
# in ASCENDING order
def sort(a, N, up):
    bitonicSort(a, 0, N, up)
# Driver code to test above
a = [3, 7, 4, 8, 6, 2, 1, 5]
n = len(a)
up = 1
sort(a, n, up)
print("\n\nSorted array is")
for i in range(n):
    print("%d" % a[i], end=" ")


Sorted array is
1 5 2 6 3 7 4 8

Time Complexity: O(n*log2(n))

Auxiliary Space: O(n*log2(n))

Please refer complete article on Bitonic Sort for more details!

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