Number of Sub-arrays With Odd Sum

lcpy gen -n 1524   # by problem number
lcpy gen -s number_of_subarrays_with_odd_sum   # by problem name

Input: arr = [1,3,5]
Output: 4
Explanation: All subarrays are [[1],[1,3],[1,3,5],[3],[3,5],[5]]
All sub-arrays sum are [1,4,9,3,8,5].
Odd sums are [1,9,3,5] so the answer is 4.

Input: arr = [2,4,6]
Output: 0
Explanation: All subarrays are [[2],[2,4],[2,4,6],[4],[4,6],[6]]
All sub-arrays sum are [2,6,12,4,10,6].
All sub-arrays have even sum and the answer is 0.

Input: arr = [1,2,3,4,5,6,7]
Output: 16

class Solution:
    # Time: O(n)
    # Space: O(1)
    def num_of_subarrays(self, arr: list[int]) -> int:
        mod = 1_000_000_007
        result = 0
        odd_prefixes = 0
        even_prefixes = 1  # empty prefix has even sum
        parity = 0
        for value in arr:
            parity ^= value & 1
            if parity:
                result += even_prefixes
                odd_prefixes += 1
            else:
                result += odd_prefixes
                even_prefixes += 1
        return result % mod