Obtain last 2 elements of nested object and perform logic on them

[hv09]

Hello all,

I've been reading ES docs, SO and this forums' posts about this for the past 2 days, but I can't seem to figure it out.

Given this data structure, which is a part of a bigger data structure that represents a user:

"reviews": [
            {
              "userEmail": "user1@email.com",
              "time": "2019-07-31T00:28:21.988315333Z",
              "userName": "User 1",
              "user": "1",
              "fieldIWantToCompare": 1
            },
            {
              "userEmail": "user2@email.com",
              "time": "2019-07-31T00:28:21.988315333Z",
              "userName": "User 2",
              "user": "2",
              "fieldIWantToCompare": 0
            },
            {
              "userEmail": "user3@email.com",
              "time": "2019-07-31T00:28:21.988315333Z",
              "userName": "User 3",
              "user": "3",
              "fieldIWantToCompare": 1
            }
            ...
          ]

reviews is a nested object.

How can I (if even possible at all) select the last 2 elements of this nested object to then perform some logic on their fields? What I want to do is compare a field on the last object and the penultimate object, and if they're not the same AND match a parameter, I want the query to filter the whole bigger structure in.

Here's a snippet of (Go) code that outlines what I'd do to get what I want on a normal application.
This is essentially what I want to do in ES:

previous := reviews[len(reviews) - 2]
current := reviews[len(reviews) - 1]

if previous.fieldIWantToCompare == <some_parameter_here> && previous.fieldIWantToCompare != current.fieldIWantToCompare {
    return true // The whole user would be returned by the query.
}

I've tried using scripts to get the length of fields but I can't seem to make it work as this is a nested object and I don't think I fully understand how they work.
I know they're flattened by ES like so:

{
"reviews.userEmail" : ["user1@email.com", "user2@email.com", "user3@email.com"],
"reviews.userName": ["User 1", "User 2", "User 3"]
...
}

But I don't understand how to access the lengths of these arrays or how to access their data, essentially by array index.

Thanks!