The way out of this is called unit testing. Code like this is just over engineered crap without it. Most of the effort is writing good tests. Show me the test that tells me in a concise way what this is actually supposed to do.

Mostly purity in this context boils down to weird combinations of premature optimization or complete disregard for that. Mostly it doesn't matter of course since code like this runs on trivial amounts of data so giving the garbage collector a little more work with silly stream objects, boxing/unboxing, etc. does not matter. Code like this does not matter, at all. Unless it's wrong. Hence the need for tests. Without tests it's just more likely to end up in tears. With tests, it doesn't really matter what the code looks like as long as the tests pass. If it's convoluted without tests, it's a problem waiting to happen.