Optimize marshalTags()

[benbjohnson]

Overview

This pull request optimizes the marshaling of tag sets. Previously the strings.Join() function was used with added a lot of memory allocations. The new implementation manually allocates and appends to a byte slice.

The number of allocations has dropped to 3:

• keys := make([]string, 0, len(tags))
• sort.Strings(keys)
• b := make([]byte, sz)

The new implementation manually handles the append instead of using the built-in append() function because appending a string to a []byte requires a memory allocation whereas a copy() does not.

A table test suite was also added to verify correctness.

Benchmark

Previous implementation

$ go test -bench=BenchmarkMarshalTags -v
PASS
BenchmarkMarshalTags_KeyN1   2000000           824 ns/op         176 B/op          6 allocs/op
BenchmarkMarshalTags_KeyN3   1000000          1319 ns/op         416 B/op          6 allocs/op
BenchmarkMarshalTags_KeyN5   1000000          2173 ns/op         656 B/op          6 allocs/op
BenchmarkMarshalTags_KeyN10   500000          3929 ns/op        1280 B/op          6 allocs/op

New implementation

$ go test -bench=BenchmarkMarshalTags -v
PASS
BenchmarkMarshalTags_KeyN1   3000000           458 ns/op          80 B/op          3 allocs/op
BenchmarkMarshalTags_KeyN3   2000000           753 ns/op         160 B/op          3 allocs/op
BenchmarkMarshalTags_KeyN5   1000000          1193 ns/op         240 B/op          3 allocs/op
BenchmarkMarshalTags_KeyN10   500000          2477 ns/op         448 B/op          3 allocs/op

[toddboom]

Nice.