4 of 5 tidy up explanation

# Background

Imagine, you have a big array A, which is mostly zeroes, but contains a short subarray B which has only strictly positive entries. For instance:

|              B              |
[0   0   0   0   1   2   3   0]
|     A     |


Now say, we split the array B into consecutive subarrays of length d (with a shorter final array, if the d doesn't divide the length of B). For d = 3:

[0   0   0   0   1   2   3   0]
|           |
[0   0   0] [0   1   2] [3   0]


And now sum up each of those arrays:

[0 3 3]


[3 3]


# The Challenge

You're to write a function which prints that final array of sums in any convenient array format, given the following 5 parameters (or any subset thereof, if you don't need all of them):

• The small array B.
• The size of the small array B (3 in the above example).
• The size of the big array A (8 in the above example).
• The index where the small array begins (4 in the above example).
• The length d of the subarrays (3 in the above example).

# The Rules

• You are not allowed to construct the big array A explicitly!
• You are not allowed to construct any of the subarrays of length d explicitly!
• That means you have to get the computation directly from the B and the other parameters at your disposal!

Fewest characters wins!