"""
 quicksort.py

  $ python quicksort.py 
  numbers : [25, 42, 7, 77, 96, 81, 2, 93, 61, 71, 29, 63, 37, 24, 30, 78]
  sorted  : [2, 7, 24, 25, 29, 30, 37, 42, 61, 63, 71, 77, 78, 81, 93, 96]

 TODO:
  * add in step counter
  * put in parition examples & tests
  * return list_size vs number_of_steps for some examples

 Jim M | Feb 2013
"""

def partition1(array, istart, iend, ipivot):
    """ Modify array[istart] ... array[iend]
        in place, such that elements
        smaller than array[ipivot] come
        before it, and ones bigger, after.
        Return index of final pivot position.
        """
    # This version tries to make the idea
    # clear, without trying to do it efficiently.
    smaller = []    # numbers below pivot
    bigger = []     # numbers above bivot
    pivot = array[ipivot]
    for i in range(istart, iend+1):
        if i != ipivot:
            if array[i] < pivot:
                smaller.append(array[i])
            else:
                bigger.append(array[i])
    array[istart : iend+1] = smaller + [pivot] + bigger
    return len(smaller)

def swap(array, i, j):
    """ swap two elements in a array, in place """
    temp = array[i]
    array[i] = array[j]
    array[j] = temp

def partition(array, istart, iend, ipivot):
    """ Same ... but in place, without python list methods """
    pivot = array[ipivot]       # remember pivot value
    swap(array, ipivot, iend)   # put pivot on right end for now
    ibound = istart             # index of smaller, bigger boundary
    for i in range(istart, iend):
        if array[i] < pivot:
            swap(array, i, ibound)
            ibound += 1
    swap(array, ibound, iend)   # put pivot at boundary
    return ibound

def quicksort(array, istart=None, iend=None):
    if (istart, iend) == (None, None):
        (istart, iend) = (0, len(array) - 1)
    if istart < iend:
        ipivot = (istart + iend)/2
        new_ipivot = partition(array, istart, iend, ipivot)
        quicksort(array, istart, new_ipivot - 1)
        quicksort(array, new_ipivot + 1, iend)
    return array

def main():
    import random
    numbers = random.sample(range(100), 16)
    random.shuffle(numbers)
    print " numbers : " + str(numbers)
    quicksort(numbers)
    print " sorted  : " + str(numbers)

main()