unicode/utf16: add RuneLen

[bouk]

Sometimes it's useful to determine how many 16-bit words it'll take to encode a unicode rune with UTF16. We already have utf8.RuneLen, and one for utf16 could be useful.

It would look something like this (those are private constants inside of utf16):

// RuneLen returns the number of 16-bit words required to encode the rune.
// It returns -1 if the rune is not a valid value to encode in UTF-16.
func RuneLen(r rune) int {
	switch {
	case 0 <= r && r < surr1, surr3 <= r && r < surrSelf:
		return 1
	case surrSelf <= r && r <= maxRune:
		return 2
	default:
		return -1
	}
}

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[bouk]

Excellent! I will work on a patch

[qmuntal]

@bouk any update? I'm interested in this proposal landing soon, if you are focused on other projects I can take over this implementation.

[bouk]

I've created a PR: #51894

[gopherbot]

Change https://go.dev/cl/395114 mentions this issue: unicode/utf16: add RuneLen

[gopherbot]

Change https://go.dev/cl/569755 mentions this issue: unicode/utf16: add func RuneLen