Optimize hexadecimal decoding

[kkHAIKE]

simple optimize hexToString..
hexToRune is copy from encoding/json

benchmark

func Benchmark_hexToString(b *testing.B) {
	for i := 0; i < b.N; i++ {
		hexToString([]byte("0001F680"), 8)
	}
}

goos: darwin
goarch: arm64
pkg: tstgo
Benchmark_hexToString
Benchmark_hexToString-8    	30278191	        39.64 ns/op	      16 B/op	       2 allocs/op
Benchmark_hexToString2
Benchmark_hexToString2-8   	134665851	         8.828 ns/op	       0 B/op	       0 allocs/op

[pelletier]

Thank you for your help on go-toml!

Out of curiosity, do you have benchmarks for this function? The existing benchmarks show no time improvement, but I suspect parsing hex strings represents a minority of the time in those.

Before merging this I'd love to see a better-or-equal execution time and allocations benchmark (I'm fine with equal, just want to check for regressions), as well as handling non-hex characters correctly.

Thank you!

[kkHAIKE]

sorry, i forgot to name it clearly. hexToString2 test is new function, 8.828 ns/op, 4.5x faster than old one...

[pelletier]

Doh! I'm sorry, I somehow didn't see the benchmark in the PR description.

Pushed a commit to benchmark using parser.parseBasicString instead of hexToString, as it should be closer to what the user experiences. This made me realize that this function was only used in basic (multiline) string parsing, with its result always appended to a string buffer. As a result I changed the signature to return the rune directly, which removes some allocation. Then I inline hexToRune to return an error directly from the loop to scope the returned error to the specific invalid character, which should help the user see where the problem is.

---

Benchmarks:

Baseline vs new hex2string implementation

name                           old time/op    new time/op     delta
ParseBasicStringWithUnicode/4     413ns ± 0%      259ns ± 0%  -37.37%  (p=0.000 n=9+9)
ParseBasicStringWithUnicode/8     353ns ± 0%      246ns ± 0%  -30.35%  (p=0.000 n=10+10)

name                           old speed      new speed       delta
ParseBasicStringWithUnicode/4  91.9MB/s ± 0%  146.8MB/s ± 0%  +59.65%  (p=0.000 n=9+9)
ParseBasicStringWithUnicode/8   119MB/s ± 0%    171MB/s ± 0%  +43.57%  (p=0.000 n=10+10)

name                           old alloc/op   new alloc/op    delta
ParseBasicStringWithUnicode/4      112B ± 0%        88B ± 0%  -21.43%  (p=0.000 n=10+10)
ParseBasicStringWithUnicode/8     96.0B ± 0%      80.0B ± 0%  -16.67%  (p=0.000 n=10+10)

name                           old allocs/op  new allocs/op   delta
ParseBasicStringWithUnicode/4      13.0 ± 0%        7.0 ± 0%  -46.15%  (p=0.000 n=10+10)
ParseBasicStringWithUnicode/8      9.00 ± 0%       5.00 ± 0%  -44.44%  (p=0.000 n=10+10)

Baseline vs direct hex2rune call

name                           old time/op    new time/op     delta
ParseBasicStringWithUnicode/4     413ns ± 0%      146ns ± 0%   -64.62%  (p=0.000 n=9+10)
ParseBasicStringWithUnicode/8     353ns ± 0%      172ns ± 0%   -51.35%  (p=0.000 n=10+9)

name                           old speed      new speed       delta
ParseBasicStringWithUnicode/4  91.9MB/s ± 0%  259.8MB/s ± 0%  +182.67%  (p=0.000 n=9+10)
ParseBasicStringWithUnicode/8   119MB/s ± 0%    244MB/s ± 0%  +105.52%  (p=0.000 n=10+9)

name                           old alloc/op   new alloc/op    delta
ParseBasicStringWithUnicode/4      112B ± 0%        64B ± 0%   -42.86%  (p=0.000 n=10+10)
ParseBasicStringWithUnicode/8     96.0B ± 0%      64.0B ± 0%   -33.33%  (p=0.000 n=10+10)

name                           old allocs/op  new allocs/op   delta
ParseBasicStringWithUnicode/4      13.0 ± 0%        1.0 ± 0%   -92.31%  (p=0.000 n=10+10)
ParseBasicStringWithUnicode/8      9.00 ± 0%       1.00 ± 0%   -88.89%  (p=0.000 n=10+10)

Side by side

name \ time/op                 baseline.txt   branch.txt      branch-2.txt
ParseBasicStringWithUnicode/4     413ns ± 0%      259ns ± 0%      146ns ± 0%
ParseBasicStringWithUnicode/8     353ns ± 0%      246ns ± 0%      172ns ± 0%

name \ speed                   baseline.txt   branch.txt      branch-2.txt
ParseBasicStringWithUnicode/4  91.9MB/s ± 0%  146.8MB/s ± 0%  259.8MB/s ± 0%
ParseBasicStringWithUnicode/8   119MB/s ± 0%    171MB/s ± 0%    244MB/s ± 0%

name \ alloc/op                baseline.txt   branch.txt      branch-2.txt
ParseBasicStringWithUnicode/4      112B ± 0%        88B ± 0%        64B ± 0%
ParseBasicStringWithUnicode/8     96.0B ± 0%      80.0B ± 0%      64.0B ± 0%

name \ allocs/op               baseline.txt   branch.txt      branch-2.txt
ParseBasicStringWithUnicode/4      13.0 ± 0%        7.0 ± 0%        1.0 ± 0%
ParseBasicStringWithUnicode/8      9.00 ± 0%       5.00 ± 0%       1.00 ± 0%

[pelletier]

I'm going to merge this as is given the significant improvement. Thank you so much for your patch and your help in improving go-toml!