find_valid_u8_pos.go

package main

import (
	"fmt"
	"unicode/utf8"
)

const (
	t1 = 0b00000000
	tx = 0b10000000
	t2 = 0b00000110
	t3 = 0b00001110
	t4 = 0b00011110
)

func main() {
	a := []byte("こん😅にちは")
	l := FindValidUTF8Position(a, 12)
	fmt.Println(string(a[0:l]), utf8.Valid(a[0:l]), l)
}

func FindValidUTF8Position(a []byte, l int) int {
	if len(a) < l {
		return -1
	}
	for i := 0; i < 4; i++ {
		if validUtf8(a[l+i-1]) {
			return l + i - 1
		}
	}
	return -1
}

func validUtf8(tmp byte) bool {
	if tmp&tx == t1 {
		return true
	}
	tmp >>= 3
	if tmp == t4 {
		return true
	}
	tmp >>= 1
	if tmp == t3 {
		return true
	}
	tmp >>= 1
	if tmp == t2 {
		return true
	}
	return false
}