Given an array of increasing integers, find the length of the longest fibonacci-like subsequence of the array. If one does not exist, return 0. A sequence is “fibonacci-like” if X_i + X_{i+1} = X_{i+2}.

fibonacci_like.py

def fibonacci_like(arr: list):
    """Returns the length of the longest fibonacci-like subsequence of the
    arr.

    If one does not exist, returns 0.
    """
    seen_arr = set(arr)
    max_length = 0
    for i in range(len(arr)):
        for j in range(i + 1, len(arr)):
            current, expected = arr[j], arr[i] + arr[j]
            curr_length = 2

            while expected in seen_arr:
                current, expected = expected, current + expected
                curr_length += 1
                max_length = max(curr_length, max_length)
    return max_length if max_length >= 3 else 0


if __name__ == '__main__':
    CASES = (
        ([1, 3, 7, 11, 12, 14, 18], 3),
        ([1, 2, 3, 4, 5, 6, 7, 8], 5),
        ([1, 3, 5], 0),
    )

    for arr, expected in CASES:
        result = fibonacci_like(arr)
        assert result == expected, f'{result} != {expected}'