⬥ ID3 encoding/decoding library in Go ⬥
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package util
import (
"strings"
euni "golang.org/x/text/encoding/unicode"
)
// got the logic from: https://github.com/bogem/id3v2 , thank you very much.
const first7BitsMask = uint32(254 << 24) // shifting 11111110 to the end of uint32
// Converts given bytes into integer
func BytesToInt(gBytes []byte) uint32 {
var integer uint32 = 0
for _, b := range gBytes {
integer = integer << 8
integer = integer | uint32(b)
}
return integer
}
// Decodes given integer bytes into integer, ignores the first bit
// of every given byte in binary form
func BytesToIntSynchsafe(gBytes []byte) uint32 {
var integer uint32 = 0
for _, b := range gBytes {
integer = integer << 7
integer = integer | uint32(b)
}
return integer
}
// The exact opposite of what `BytesToIntSynchsafe` does
// Finally understood with the help of: https://github.com/bogem/id3v2/blob/master/size.go ,
// thank you very much !
func IntToBytesSynchsafe(gInt uint32) []byte {
synchsafeIBytes := make([]byte, 4)
// skip 4 0`ed bits
gInt = gInt << 4
// int32 == 4 bytes
for i := 0; i < 32/8; i++ {
// get first 7 bits
first7Bits := gInt & first7BitsMask
// shift captured bits to the beginning
first7Bits = first7Bits >> (3*8 + 1)
b := byte(first7Bits)
synchsafeIBytes = append(synchsafeIBytes, b)
// prepare next 7 bits for the next iteration
gInt = gInt << 7
}
return synchsafeIBytes
}
// Converts given bytes into string, ignoring the first 31 non-printable ASCII characters.
// (LOSSY, if given bytes contain some nasty ones)
func ToStringLossy(gBytes []byte) string {
var runes []rune
for _, b := range gBytes {
if b <= 31 {
continue
}
runes = append(runes, rune(b))
}
return strings.ToValidUTF8(string(runes), "")
}
// Decodes the given frame`s contents
func DecodeText(fContents []byte) string {
textEncoding := fContents[0] // the first byte is the encoding
switch textEncoding {
case 0:
// ISO-8859-1
return ToStringLossy(fContents[1:])
case 1:
// UTF-16 with BOM
encoding := euni.UTF16(euni.BigEndian, euni.ExpectBOM)
decoder := encoding.NewDecoder()
decodedBytes := make([]byte, len(fContents)*2)
_, _, err := decoder.Transform(decodedBytes, fContents[1:], true)
if err != nil {
return ""
}
return string(decodedBytes)
case 2:
// UTF-16
encoding := euni.UTF16(euni.BigEndian, euni.IgnoreBOM)
decoder := encoding.NewDecoder()
decodedBytes := make([]byte, len(fContents)*2)
_, _, err := decoder.Transform(decodedBytes, fContents[1:], true)
if err != nil {
return ""
}
return string(decodedBytes)
case 3:
// UTF-8
return ToStringLossy(fContents[1:])
}
return ""
}