Add mp3 support + command line option to limit tracks + general redesign for more modularity

mp3/mp3.go

@@ -0,0 +1,113 @@
+package mp3
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"hash/crc32"
+	"html"
+	"io"
+	"net/http"
+	"strconv"
+	"strings"
+)
+
+var (
+	ErrNoMetaint         = errors.New("mp3: key 'icy-metaint' not found in HTTP header")
+	ErrCorruptedMetadata = errors.New("mp3: corrupted metadata")
+	ErrNoStreamTitle     = errors.New("mp3: no 'StreamTitle' tag in metadata")
+)
+
+type Extractor struct {
+	metaint        int64 // Distance between two metadata chunks
+	hasStreamTitle bool
+	streamTitle    string // Metadata tag determining the filename
+}
+
+func NewExtractor(respHdr http.Header) (*Extractor, error) {
+	mi := respHdr.Get("icy-metaint")
+	if mi == "" {
+		return nil, ErrNoMetaint
+	}
+	miNum, _ := strconv.ParseInt(mi, 10, 64)
+	return &Extractor{
+		metaint: miNum,
+	}, nil
+}
+
+func (d *Extractor) ReadBlock(r io.Reader, w io.Writer) (isFirst bool, err error) {
+	var musicData bytes.Buffer
+
+	// We want to write everything to the output, as well as musicData for
+	// calculating the checksum.
+	multi := io.MultiWriter(w, &musicData)
+
+	// Read until the metadata chunk. The part that is read here is also what
+	// contains the actual mp3 music data.
+	io.CopyN(multi, r, d.metaint)
+
+	// Read number of metadata blocks (blocks within this function are not what
+	// is meant with `ReadBlock()`).
+	var numBlocks uint8
+	err = binary.Read(r, binary.LittleEndian, &numBlocks)
+
+	// Whether this block is the beginning of a new track.
+	var isBOF bool
+
+	// Read metadata blocks.
+	if numBlocks > 0 {
+		// Metadata is only actually stored in the first metadata chunk
+		// of a given file. Therefore, every metadata chunk with more than 1
+		// block always marks the beginning of a file.
+		isBOF = true
+
+		// Each block is 16 bytes in size. Any excess bytes in the last block
+		// are set to '\0', which is great because the `string()` conversion
+		// function ignores null bytes. The whole string is escaped via HTML.
+		// Metadata format: k0='v0';k1='v1';
+		raw := make([]byte, numBlocks*16)
+		if _, err := r.Read(raw); err != nil {
+			return false, err
+		}
+		rawString := html.UnescapeString(string(raw))
+		for _, data := range strings.Split(rawString, ";") {
+			s := strings.Split(data, "=")
+			if len(s) == 2 {
+				if s[0] == "StreamTitle" {
+					d.hasStreamTitle = true
+					// Strip stream title's first and last character (single
+					// quotes).
+					t := s[1]
+					if len(t) < 2 {
+						return false, ErrCorruptedMetadata
+					}
+					t = t[1 : len(t)-1]
+					if t == "Unknown" {
+						// If there is no stream title, use format:
+						// Unknown_<crc32 checksum>
+						// Where the checksum is only that of the first block.
+						sumStr := strconv.FormatInt(int64(crc32.ChecksumIEEE(musicData.Bytes())), 10)
+						d.streamTitle = "Unknown_" + sumStr
+					} else {
+						d.streamTitle = t
+					}
+				}
+			} else if len(s) != 1 {
+				return false, ErrCorruptedMetadata
+			}
+		}
+		if !d.hasStreamTitle {
+			return false, ErrNoStreamTitle
+		}
+	}
+
+	return isBOF, nil
+}
+
+func (d *Extractor) TryGetFilename() (filename string, hasFilename bool) {
+	if !d.hasStreamTitle {
+		return "", false
+	}
+	base := strings.ReplaceAll(d.streamTitle, "/", "_") // Replace invalid characters.
+	return base + ".mp3", true
+}