Fun read. I haven't used Java in a long time, so even the "idiomatic Java code that would pass muster with any seasoned Java developer" was a bit shocking.

I know it's a cliche that someone brings up Rust in any programming thread, but I can't help myself. ;-) Several of the techniques here are easy enough in Rust that I just do them routinely.

* Rayon makes it easy to process line-wise in reasonable-sized chunks without allocating a string per line.

* The standard integer parsing stuff takes a slice, not an owned string.

* The standard hash map doesn't have the same expectation "that we pass in an instance of the key class that is equal to an existing instance"; you can call HashMap<String>::get with a &str. (One limitation is that std::collections::HashMap::entry does expect a K, so there's some redundancy if you need to insert afterward, but you could drop to hashbrown::HashMap::raw_entry_mut to avoid that.)