length_utf8.go

// Copyright 2021 Matrix Origin
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package lengthutf8

var (
	table [256]uint8
)

func init() {
	table = [256]uint8{
		// start byte of 1-byte utf8 char: 0b0000'0000 ~ 0b0111'1111
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1,
		// continuation byte: 0b1000'0000 ~ 0b1011'1111
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
		// start byte of 2-byte utf8 char: 0b1100'0000 ~ 0b1101'1111
		2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
		2, 2, 2, 2, 2, 2, 2,
		// start byte of 3-byte utf8 char: 0b1110'0000 ~ 0b1110'1111
		3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
		// start byte of 4-byte utf8 char: 0b1111'0000 ~ 0b1111'0111
		// invalid utf8 byte: 0b1111'1000~ 0b1111'1111
		4, 4, 4, 4, 4, 4, 4, 4, 1, 1, 1, 1, 1, 1, 1, 1}
}

// this countUTF8CodePoints implementation bases on https://github.com/satanson/cpp_etudes/blob/master/include/string_functions.hh
func CountUTF8CodePoints(s []byte) uint64 {
	var charSize uint8
	var count uint64 = 0
	length := len(s)

	for i := 0; i < length; {
		charSize = table[s[i]]
		count++
		i = i + int(charSize)
	}
	return count
}