"""Chaper 3.3, exercise 3.

Write a function called bisection that takes a sorted list and a target value
and returns the index of the value in the list if it's there, or None if it's
not.

"""

def bisection(a, item):
    """Returns the index of item in the sorted list a if it exists or None if
    not.

    Finds the "rightmost" item if it exists.

    """
    lo = 0
    hi = len(a)

    while lo < hi:
        mid = (lo + hi) // 2
        if item < a[mid]:
            hi = mid
        else:
            lo = mid + 1

    n = lo - 1
    return a[n] if n >= 0 and a[n] == item else None