"""
Find the Equilibrium Index of an Array.
Reference: https://www.geeksforgeeks.org/equilibrium-index-of-an-array/

Python doctest can be run with the following command:
python -m doctest -v equilibrium_index.py

Given a sequence arr[] of size n, this function returns
an equilibrium index (if any) or -1 if no equilibrium index exists.

The equilibrium index of an array is an index such that the sum of
elements at lower indexes is equal to the sum of elements at higher indexes.



Example Input:
arr = [-7, 1, 5, 2, -4, 3, 0]
Output: 3

"""


def equilibrium_index(arr: list[int]) -> int:
    """
    Find the equilibrium index of an array.

    Args:
        arr (list[int]): The input array of integers.

    Returns:
        int: The equilibrium index or -1 if no equilibrium index exists.

    Examples:
        >>> equilibrium_index([-7, 1, 5, 2, -4, 3, 0])
        3
        >>> equilibrium_index([1, 2, 3, 4, 5])
        -1
        >>> equilibrium_index([1, 1, 1, 1, 1])
        2
        >>> equilibrium_index([2, 4, 6, 8, 10, 3])
        -1
    """
    total_sum = sum(arr)
    left_sum = 0

    for i, value in enumerate(arr):
        total_sum -= value
        if left_sum == total_sum:
            return i
        left_sum += value

    return -1


if __name__ == "__main__":
    import doctest

    doctest.testmod()