Describe an advanced data structure : Efficiently Counting Subarrays with a Given Sum

The video tutorial explains how to count subarrays with a given sum using prefix sums. It starts with an introduction to the problem, followed by a naive solution and an efficient solution using prefix sums. The algorithm is explained in detail, and the implementation is demonstrated with tests to verify the solution. The tutorial also references a similar modulus problem from a previous section.

How does the algorithm ensure that it counts all valid subarrays that sum to the given number?

What steps are taken to update the prefix sum counts dictionary during the algorithm?