refactor: use io.Reader instead of custom method

algo/kwm/kwm.go

@@ -2,8 +2,9 @@ package kwm
 
 import (
 	"bytes"
-	"encoding/binary"
 	"errors"
+	"fmt"
+	"io"
 	"strconv"
 	"strings"
 	"unicode"
@@ -11,95 +12,63 @@ import (
 	"github.com/unlock-music/cli/algo/common"
 )
 
-var (
-	magicHeader = []byte{
-		0x79, 0x65, 0x65, 0x6C, 0x69, 0x6F, 0x6E, 0x2D,
-		0x6B, 0x75, 0x77, 0x6F, 0x2D, 0x74, 0x6D, 0x65}
-	ErrKwFileSize    = errors.New("kwm invalid file size")
-	ErrKwMagicHeader = errors.New("kwm magic header not matched")
-)
-
+const magicHeader = "yeelion-kuwo-tme"
 const keyPreDefined = "MoOtOiTvINGwd2E6n0E1i7L5t2IoOoNk"
 
 type Decoder struct {
-	file []byte
+	cipher common.StreamDecoder
+
+	rd     io.ReadSeeker
+	offset int
 
-	key       []byte
 	outputExt string
 	bitrate   int
-	mask      []byte
-
-	audio []byte
-}
-
-func (d *Decoder) GetAudioData() []byte {
-	return d.audio
 }
 
 func (d *Decoder) GetAudioExt() string {
 	return "." + d.outputExt
 }
 
-func (d *Decoder) GetMeta() common.Meta {
-	return nil
-}
-
-func NewDecoder(data []byte) common.Decoder {
-	//todo: Notice the input data will be changed for now
-	return &Decoder{file: data}
+func NewDecoder(rd io.ReadSeeker) common.Decoder {
+	return &Decoder{rd: rd}
 }
 
 func (d *Decoder) Validate() error {
-	lenData := len(d.file)
-	if lenData < 1024 {
-		return ErrKwFileSize
-	}
-	if !bytes.Equal(magicHeader, d.file[:16]) {
-		return ErrKwMagicHeader
-	}
-
-	return nil
-}
-
-func generateMask(key []byte) []byte {
-	keyInt := binary.LittleEndian.Uint64(key)
-	keyStr := strconv.FormatUint(keyInt, 10)
-	keyStrTrim := padOrTruncate(keyStr, 32)
-	mask := make([]byte, 32)
-	for i := 0; i < 32; i++ {
-		mask[i] = keyPreDefined[i] ^ keyStrTrim[i]
-	}
-	return mask
-}
-
-func (d *Decoder) parseBitrateAndType() {
-	bitType := string(bytes.TrimRight(d.file[0x30:0x38], string(byte(0))))
-	charPos := 0
-	for charPos = range bitType {
-		if !unicode.IsNumber(rune(bitType[charPos])) {
-			break
-		}
-	}
-	var err error
-	d.bitrate, err = strconv.Atoi(bitType[:charPos])
+	header := make([]byte, 0x400) // kwm header is fixed to 1024 bytes
+	_, err := io.ReadFull(d.rd, header)
 	if err != nil {
-		d.bitrate = 0
+		return fmt.Errorf("kwm read header: %w", err)
 	}
-	d.outputExt = strings.ToLower(bitType[charPos:])
 
+	// check magic header, 0x00 - 0x0F
+	if !bytes.Equal([]byte(magicHeader), header[:len(magicHeader)]) {
+		return errors.New("kwm magic header not matched")
+	}
+
+	d.cipher = newKwmCipher(header[0x18:0x20])                      // Crypto Key, 0x18 - 0x1F
+	d.bitrate, d.outputExt = parseBitrateAndType(header[0x30:0x38]) // Bitrate & File Extension, 0x30 - 0x38
+
+	return nil
 }
 
-func (d *Decoder) Decode() error {
-	d.parseBitrateAndType()
+func parseBitrateAndType(header []byte) (int, string) {
+	tmp := strings.TrimRight(string(header), "\x00")
+	sep := strings.IndexFunc(tmp, func(r rune) bool {
+		return !unicode.IsDigit(r)
+	})
 
-	d.mask = generateMask(d.file[0x18:0x20])
+	bitrate, _ := strconv.Atoi(tmp[:sep]) // just ignore the error
+	outputExt := strings.ToLower(tmp[sep:])
+	return bitrate, outputExt
+}
 
-	d.audio = d.file[1024:]
-	dataLen := len(d.audio)
-	for i := 0; i < dataLen; i++ {
-		d.audio[i] ^= d.mask[i&0x1F] //equals: [i % 32]
+func (d *Decoder) Read(b []byte) (int, error) {
+	n, err := d.rd.Read(b)
+	if n > 0 {
+		d.cipher.Decrypt(b[:n], d.offset)
+		d.offset += n
 	}
-	return nil
+	return n, err
 }
 
 func padOrTruncate(raw string, length int) string {