'''lecture_37_recursion_binary_search.py
Performs the binary search algorithm using recursion
Oliver W. Layton
CS 151: Computational Thinking: Visual Media
Fall 2020
'''


def binarySearch(theList, elem, leftIdx, rightIdx):
    '''Binary search algorithm, done recursively

    Parameters:
    -----------
    theList: the original list of numbers to search
    elem: the number we're looking for
    leftIdx: index of the left side of the "sublist range" we're searching in
    rightIdx: index of the right side the "sublist range" we're searching in

    Returns:
    -----------
    int. Index of `elem` in `theList`. If `elem` is not in `theList`, return -1.
    '''
    if leftIdx > rightIdx:
        # No way elem is in here!
        return -1

    mid = (leftIdx + rightIdx) // 2
    midItem = theList[mid]

    if midItem == elem:
        # We're good! Here it is! Return its index in the list
        return mid
    elif elem < midItem:
        # Repeat search in left half of list
        return binarySearch(theList, elem, leftIdx, mid-1)
    else:
        # Repeat search in right half of list
        return binarySearch(theList, elem, mid+1, rightIdx)


if __name__ == '__main__':
    myList = [1, 2, 8, 9, 11, 20]
    left = 0
    right = len(myList) - 1
    print(binarySearch(myList, 11, left, right))  # 4
    # print(binarySearch(myList, 2, left, right))  # 1
    # print(binarySearch(myList, 7, left, right))  # -1