Implement --factor option

[jasonrhansen]

Implements #19 with a difference from the Perl implementation in how sample counts get rounded, as explained in my comment on that issue. We need to decide if this is acceptable or not.

I've also combined all of the flamegraph tests into one file since they all duplicated the main testing code.

[codecov-io]

Codecov Report

Merging #65 into master will increase coverage by 0.04%.
The diff coverage is 93.47%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   77.54%   77.59%   +0.04%     
==========================================
  Files          15       13       -2     
  Lines        1287     1263      -24     
==========================================
- Hits          998      980      -18     
+ Misses        289      283       -6

Impacted Files	Coverage Δ
src/bin/flamegraph.rs	0% <0%> (ø)	⬆️
src/flamegraph/mod.rs	84.34% <100%> (ø)	⬆️
tests/flamegraph.rs	95.23% <100%> (ø)
src/flamegraph/merge.rs	92.13% <84.61%> (+0.46%)	⬆️
src/collapse/perf.rs	82.41% <0%> (+0.34%)	⬆️

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[jonhoo]

Looks good beyond my comment in #19 (comment). Specifically, I think it's not actually okay for us to round and truncate the result to an integer -- it may be fractional after multiplying by the factor.

[jonhoo]

Thinking some more, I think we specifically want f64::trunc, since f64::floor would give the next lower negative number. Not that we expect any of these to be negative, but 🤷‍♂️ See also rust-lang/rust#58812.

[jasonrhansen]

Using f64::trunc doesn't match the Perl version either. It's more complicated than that because of the weirdness of formatting floating point numbers. round actually works most of the time, but it's in the middle at .5 where we have issues.

printf "%.0f", 1.5 outputs 2
printf "%.0f", 2.5 outputs 2
printf "%.0f", 3.5 outputs 4

If we round we get 2, 3, 4.
If we truncate we get 1, 2, 3.

If we didn't care about thousands separators we could just format it the same way as the Perl version.

[jonhoo]

Wow, that's bizarre. Is that the case in Rust too? That makes no sense to me. But then yes, I think round() is fine!

[jasonrhansen]

Rust formats exactly the same way when using "{:.0}". I'll keep using round, but add a comment about how this slightly differs from the Perl version.

[jonhoo]

Sounds good! I wonder where that behavior comes from...

[jonhoo]

🎉 ❤️