Kadane Variant: Maximum Product Subarray

Write a function `max_product_subarray(nums)` that takes a list of integers `nums` (may contain positive, negative, and zero values) and returns the largest product that can be obtained by multiplying a contiguous subarray (i.e., a non-empty slice `nums[i:j]` with `0 <= i < j <= len(nums)`). The array length is at least 1. The product may be large, so return it as a Python integer. Your solution should run in O(n) time and O(1) extra space.

>>> max_product_subarray([2, 3, -2, 4])
6
>>> max_product_subarray([-2, 0, -1])
0
>>> max_product_subarray([-2, -3, 4, -1])
24
>>> max_product_subarray([7])
7