Forwarder: clean up packet_vec filter

[apfitzge]

Problem

Saw an odd use of filter_map

Summary of Changes

Use chained methods to do equivalent operations in less code

Fixes #

[lijunwangs]

The new code is cleaner definitely. Isn't the left side more efficient though without multiple loops?

[apfitzge]

It won't do multiple loops, iterators are a zero-cost abstraction: https://doc.rust-lang.org/book/ch13-04-performance.html

Closures and iterators are Rust features inspired by functional programming language ideas. They contribute to Rust’s capability to clearly express high-level ideas at low-level performance. The implementations of closures and iterators are such that runtime performance is not affected. This is part of Rust’s goal to strive to provide zero-cost abstractions.

They get compiled directly, so expansion isn't really a thing, but we kind of think how expanding these two versions might work...
Roughly the code before was:

    let mut packet_vec = vec![];
    for p in forwardable_packets {
        if !p.meta().forwarded() && self.data_budget.take(p.meta().size) {
            if let Some(data) = p.data(..) {
                packet_vec.push(data.to_vec());
            }
        }
    }

And after the change, (again roughly):

    let mut packet_vec = vec![];
    for p in forwardable_packets {
        if p.meta().forwarded() { continue; }
        if !p.data_budget.take(p.meta().size) { continue; }
        if let Some(data) = p.data(..) {
            packet_vec.push(data.to_vec());
        }
    }

[lijunwangs]

Good to know! Thanks

[lijunwangs]

LGTM

[codecov]

Codecov Report

Merging #30921 (bbacf68) into master (b53656b) will decrease coverage by 0.1%.
The diff coverage is 0.0%.

@@            Coverage Diff            @@
##           master   #30921     +/-   ##
=========================================
- Coverage    81.5%    81.5%   -0.1%     
=========================================
  Files         727      727             
  Lines      205118   205116      -2     
=========================================
- Hits       167256   167246     -10     
- Misses      37862    37870      +8