changed the String() method for UUID's

[cwndrws]

I was profiling some code for work, that uses the UUID.String() method a lot and saw it was using fmt.Sprintf(), which is a bit overkill, if you ask me. I changed it to use a switch statement. It is now between 4-6 times faster. Here is a benchmark.

[0x6e6562]

Very interesting. After having optimized this, what effect did you see on the performance of the app as a whole?

[cwndrws]

On Wed, May 21, 2014 at 02:50:51PM -0700, Ben Hood wrote:

Very interesting. After having optimized this, what effect did you see on the performance of the app as a whole?
Well the application is a server that handles many different kinds of
loads. Under a few specific test cases, where we are returning to an end
user and are turning all UUID's into strings, it was about twice as
fast as before.

[0x6e6562]

I've just run your benchmark myself:

go test -test.bench Benchmark* -test.run XXX
PASS
BenchmarkOldWay  1000000          2039 ns/op
BenchmarkNewWay 10000000           160 ns/op
ok      github.com/relops/gocql 3.848s

It does look like it has a significant performance improvement without affecting the functionality.

So I would be in favor of merging this - but I would like one of the other maintainers opinion on this, since this the first performance related patch we have received.

[nemosupremo]

Looking over the pr I have to ask:

1. hexString should be a const as it should never be changed
2. b1 and b2 shouldn't be global. This will cause corruptions if two goroutines run this function at the same time

[cwndrws]

good look @nemothekid I just pushed another commit addressing these issues.

[tux21b]

There is no need to export HexString or make it global. What do you think of this, sightly modified version?

const hexString = "0123456789abcdef"
var offsets = [...]int{0, 2, 4, 6, 9, 11, 14, 16, 19, 21, 24, 26, 28, 30, 32, 34}
r := make([]byte, 36)
for i, b := range uuid {
    r[offsets[i]] = hexString[b>>4]
    r[offsets[i]+1] = hexString[b&0xF]
}
r[8] = '-'
r[13] = '-'
r[18] = '-'
r[23] = '-'
return string(r)

It seems to perform sightly faster here. Oh, and it would be nice if you could also add your benchmark as BenchmarkUUIDString or something like that to the uuid_test.go file, so that we can track performance regressions in the future.

[cwndrws]

I have changed it to use the offsets array.

[tux21b]

LGTM. Many thanks for your contribution! :)