From b60e5290918f024e03f412c00ab46a7a131c1e02 Mon Sep 17 00:00:00 2001
From: David Howden <dhowden@gmail.com>
Date: Thu, 2 Jul 2015 23:07:17 +1000
Subject: [PATCH] Add Identity function for identifying metadata/filetypes.

---
 id.go  | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 tag.go | 24 +++++++++++---------
 2 files changed, 83 insertions(+), 11 deletions(-)
 create mode 100644 id.go

diff --git a/id.go b/id.go
new file mode 100644
index 0000000..d967f17
--- /dev/null
+++ b/id.go
@@ -0,0 +1,70 @@
+package tag
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+// Identify reads from the given ReadSeaker and
+func Identify(r io.ReadSeeker) (format Format, fileType FileType, err error) {
+	b, err := readBytes(r, 11)
+	if err != nil {
+		return
+	}
+
+	_, err = r.Seek(-11, os.SEEK_CUR)
+	if err != nil {
+		err = fmt.Errorf("could not seek back to original position: %v", err)
+		return
+	}
+
+	switch {
+	case string(b[0:4]) == "fLaC":
+		return VORBIS, FLAC, nil
+
+	case string(b[0:4]) == "OggS":
+		return VORBIS, OGG, nil
+
+	case string(b[4:11]) == "ftypM4A":
+		return AAC, MP4, nil
+
+	case string(b[0:3]) == "ID3":
+		b := b[3:]
+		switch uint(b[0]) {
+		case 2:
+			format = ID3v2_2
+		case 3:
+			format = ID3v2_3
+		case 4:
+			format = ID3v2_4
+		case 0, 1:
+			fallthrough
+		default:
+			err = fmt.Errorf("ID3 version: %v, expected: 2, 3 or 4", uint(b[0]))
+			return
+		}
+		return format, MP3, nil
+	}
+
+	n, err := r.Seek(-128, os.SEEK_END)
+	if err != nil {
+		return
+	}
+
+	tag, err := readString(r, 3)
+	if err != nil {
+		return
+	}
+
+	_, err = r.Seek(-n, os.SEEK_CUR)
+	if err != nil {
+		return
+	}
+
+	if tag != "TAG" {
+		err = ErrNoTagsFound
+		return
+	}
+	return ID3v1, MP3, nil
+}
diff --git a/tag.go b/tag.go
index 1186725..dabfaee 100644
--- a/tag.go
+++ b/tag.go
@@ -68,12 +68,13 @@ type Format string
 
 // Supported tag formats.
 const (
-	ID3v1   Format = "ID3v1"   // ID3v1 tag format.
-	ID3v2_2        = "ID3v2.2" // ID3v2.2 tag format.
-	ID3v2_3        = "ID3v2.3" // ID3v2.3 tag format (most common).
-	ID3v2_4        = "ID3v2.4" // ID3v2.4 tag format.
-	MP4            = "MP4"     // MP4 tag (atom) format.
-	VORBIS         = "VORBIS"  // Vorbis Comment tag format.
+	UnknownFormat Format = ""        // Unknown Format.
+	ID3v1                = "ID3v1"   // ID3v1 tag format.
+	ID3v2_2              = "ID3v2.2" // ID3v2.2 tag format.
+	ID3v2_3              = "ID3v2.3" // ID3v2.3 tag format (most common).
+	ID3v2_4              = "ID3v2.4" // ID3v2.4 tag format.
+	MP4                  = "MP4"     // MP4 tag (atom) format.
+	VORBIS               = "VORBIS"  // Vorbis Comment tag format.
 )
 
 // FileType is an enumeration of the audio file types supported by this package, in particular
@@ -83,11 +84,12 @@ type FileType string
 
 // Supported file types.
 const (
-	MP3  FileType = "MP3"  // MP3 file
-	AAC           = "AAC"  // M4A file (MP4)
-	ALAC          = "ALAC" // Apple Lossless file FIXME: actually detect this
-	FLAC          = "FLAC" // FLAC file
-	OGG           = "OGG"  // OGG file
+	UnknownFileType FileType = ""     // Unknown FileType.
+	MP3                      = "MP3"  // MP3 file
+	AAC                      = "AAC"  // M4A file (MP4)
+	ALAC                     = "ALAC" // Apple Lossless file FIXME: actually detect this
+	FLAC                     = "FLAC" // FLAC file
+	OGG                      = "OGG"  // OGG file
 )
 
 // Metadata is an interface which is used to describe metadata retrieved by this package.