package utils

import (
	"bytes"
	"golang.org/x/text/encoding/simplifiedchinese"
	"golang.org/x/text/transform"
	"io/ioutil"
	"unicode/utf8"
)

func IsGBK(data []byte) bool {
	length := len(data)
	var i = 0
	for i < length {
		if data[i] <= 0x7f {
			//编码0~127,只有一个字节的编码，兼容ASCII码
			i++
			continue
		} else {
			//大于127的使用双字节编码，落在gbk编码范围内的字符
			if data[i] >= 0x81 &&
				data[i] <= 0xfe &&
				data[i+1] >= 0x40 &&
				data[i+1] <= 0xfe &&
				data[i+1] != 0xf7 {
				i += 2
				continue
			} else {
				return false
			}
		}
	}
	return true
}

const (
	GBK     string = "GBK"
	UTF8    string = "UTF8"
	UNKNOWN string = "UNKNOWN"
)

func GetStrCoding(data []byte) string {
	if utf8.Valid(data) {
		return UTF8
	} else if IsGBK(data) {
		return GBK
	} else {
		return UNKNOWN
	}
}

func GbkToUtf8(s []byte) ([]byte, error) {
	reader := transform.NewReader(bytes.NewReader(s), simplifiedchinese.GBK.NewDecoder())
	d, e := ioutil.ReadAll(reader)
	if e != nil {
		return nil, e
	}
	return d, nil
}