strings,bytes: ToLower(), ToUpper() by using bitwise operator

[ksw2000]

When the string consists of only ASCII characters, we can convert lower case to upper case using bitwise operators. Bitwise operators are more efficient than using addition or subtraction.

go/src/strings/strings.go

Lines 616 to 625 in 3e67f46

	for i := 0; i < len(s); i++ {
	c := s[i]
	if 'a' <= c && c <= 'z' {
	c -= 'a' - 'A'
	if pos < i {
	b.WriteString(s[pos:i])
	}
	b.WriteByte(c)
	pos = i + 1
	}

For example, in Rust, they used bitwise operator to implement the converter.

https://github.com/rust-lang/rust/blob/c387f012b14a3d64e0d580b7ebe65e5325bcf822/library/core/src/num/mod.rs#L576-L579

---

I propose that we replace the addition and subtraction operators with bitwise operators in the following four places.

go/src/strings/strings.go

Lines 616 to 625 in 3e67f46

	for i := 0; i < len(s); i++ {
	c := s[i]
	if 'a' <= c && c <= 'z' {
	c -= 'a' - 'A'
	if pos < i {
	b.WriteString(s[pos:i])
	}
	b.WriteByte(c)
	pos = i + 1
	}

go/src/strings/strings.go

Lines 656 to 666 in 3e67f46

	for i := 0; i < len(s); i++ {
	c := s[i]
	if 'A' <= c && c <= 'Z' {
	c += 'a' - 'A'
	if pos < i {
	b.WriteString(s[pos:i])
	}
	b.WriteByte(c)
	pos = i + 1
	}
	}

go/src/bytes/bytes.go

Lines 638 to 652 in 3e67f46

	if isASCII { // optimize for ASCII-only byte slices.
	if !hasLower {
	// Just return a copy.
	return append([]byte(""), s...)
	}
	b := bytealg.MakeNoZero(len(s))
	for i := 0; i < len(s); i++ {
	c := s[i]
	if 'a' <= c && c <= 'z' {
	c -= 'a' - 'A'
	}
	b[i] = c
	}
	return b
	}

go/src/bytes/bytes.go

Lines 669 to 682 in 3e67f46

	if isASCII { // optimize for ASCII-only byte slices.
	if !hasUpper {
	return append([]byte(""), s...)
	}
	b := bytealg.MakeNoZero(len(s))
	for i := 0; i < len(s); i++ {
	c := s[i]
	if 'A' <= c && c <= 'Z' {
	c += 'a' - 'A'
	}
	b[i] = c
	}
	return b
	}

[Jorropo]

@ksw2000 I don't see in what way this is change the api or behavior, if this is purely an internal change you don't need a proposal and can just submit a patch. 🙂

[ksw2000]

Sorry, this is my first time creating an issue for Golang, and I'm not sure what type of issue to use. Can you please tell me what type of issue I should use for an issue like this or could I just pull request directly?

I am so sorry for my mistake. I will make sure to learn how to use it correctly next time.

[panjf2000]

Taking this out of the proposal milestone as it doesn't involve API changes.

[panjf2000]

@ksw2000
You only need a proposal when you're trying to make API changes or language changes, you can just send a CL directly for this without making a proposal since what you're intended to do here doesn't involve API or language changes.

See golang proposal for details.

[ksw2000]

@panjf2000
Thank you very much for your detailed explanation. I will send a CL directly. Should I close this issue?

[Jorropo]

I think you should, when you submit your CL don't forget to post benchmark results using benchstat. 🙂

[panjf2000]

Leave this issue and add a line Fixes #64361 in the commit message of your CL.