Code to generate nCk combinations for a n-length array (Won't work with sequences having duplicate values)

combinations.py

def prepArray(idxs,array):
    return [array[x] for x in idxs] #This step is O(n) too

def combinations(array,k):
    """
    Code to generate nCk combinations for a n-length array (Won't work with sequences having duplicate values)
    Maybe not so effecient but my implementation, Don't know about algo implemented in standard library
    Standard lib - itertools.combinations
    Just Wrote code to pass the time. :)

    Here is the idea:
    I am keeping indexes in the k-length array - idxs, which follow the invariant that it is STRICTLY INCREASING.
    Keeping that invariant in mind, I am generating the sequences in lexicographical increasing order.
    Lets see if you can figure out the pattern from this pattern of idxs.
    
    For , n = 6, k = 4 , *Idxs* are generated as follows.

    [0, 1, 2, 4]
    [0, 1, 2, 5]
    [0, 1, 3, 4]
    [0, 1, 3, 5]
    [0, 1, 4, 5]
    [0, 2, 3, 4]
    [0, 2, 3, 5]
    [0, 2, 4, 5]
    [0, 3, 4, 5]
    [1, 2, 3, 4]
    [1, 2, 3, 5]
    [1, 2, 4, 5]
    [1, 3, 4, 5]
    [2, 3, 4, 5]
    """

    array = list(array)
    yield array[:k]
    idxs = range(k)
    max_k = len(array)

    while True:
        while idxs[k-1] < max_k - 1:
            idxs[k-1]+=1
            yield prepArray(idxs,array)
        a = k-2
        while a >= 0:
            if idxs[a] < a + (max_k - k):
                idxs[a]+=1
                idxs[a+1:] = range(idxs[a]+1, idxs[a]+1+ (k - a - 1)) #Maybe we can do better here, the step is O(n)
                yield prepArray(idxs, array)
                break
            else:
                a-=1
        if a < 0:
            break