Binary search - find the left border

find_left_border.py

def find_left_border(arr, target):
    left = 0
    right = len(arr) - 1

    left_border = -1        # keep track of the latest smaller number
    while left <= right:
        mid = left + (right - left) // 2
        if arr[mid] < target:
            left_border = mid
            left = mid + 1
        else:   # in case arr[mid] >= target
            right = mid - 1

    return left_border