From eb371e4840b84e7d710629f5176c88d282de0083 Mon Sep 17 00:00:00 2001
From: Unbewohnte <unbewohnte@gmail.com>
Date: Sun, 27 Jun 2021 12:34:09 +0300
Subject: [PATCH] Moved constant values into their own file, started working on
 ID3v2

---
 constants.go | 11 +++++++++++
 id3v10.go    | 16 +++++++---------
 id3v11.go    |  8 ++++----
 id3v20.go    | 38 ++++++++++++++++++++++++++++++++++++++
 4 files changed, 60 insertions(+), 13 deletions(-)
 create mode 100644 constants.go
 create mode 100644 id3v20.go

diff --git a/constants.go b/constants.go
new file mode 100644
index 0000000..d464fa3
--- /dev/null
+++ b/constants.go
@@ -0,0 +1,11 @@
+package id3ed
+
+// ID3v1
+const ID3v1IDENTIFIER string = "TAG"
+const ID3v1SIZE int = 128 // bytes
+const ID3v1INVALIDGENRE int = 255
+
+//ID3v2
+const ID3v2IDENTIFIER string = "ID3"
+const ID3v2HEADERSIZE int = 10     // bytes
+const ID3v2MAXSIZE int = 268435456 // bytes (256 MB)
diff --git a/id3v10.go b/id3v10.go
index 9949e74..536d081 100644
--- a/id3v10.go
+++ b/id3v10.go
@@ -11,8 +11,6 @@ import (
 
 // https://id3.org/ID3v1 - documentation
 
-const ID3V1SIZE int = 128
-
 type ID3v1Tags struct {
 	SongName string
 	Artist   string
@@ -25,7 +23,7 @@ type ID3v1Tags struct {
 // Retrieves ID3v1 field values of provided io.ReadSeeker (usually a file)
 func GetID3v1Tags(rs io.ReadSeeker) (*ID3v1Tags, error) {
 	// set reader to the last 128 bytes
-	_, err := rs.Seek(-int64(ID3V1SIZE), io.SeekEnd)
+	_, err := rs.Seek(-int64(ID3v1SIZE), io.SeekEnd)
 	if err != nil {
 		return nil, fmt.Errorf("could not seek: %s", err)
 	}
@@ -35,7 +33,7 @@ func GetID3v1Tags(rs io.ReadSeeker) (*ID3v1Tags, error) {
 		return nil, err
 	}
 
-	if !bytes.Equal(tag, []byte("TAG")) {
+	if !bytes.Equal(tag, []byte(ID3v1IDENTIFIER)) {
 		// no TAG, given file does not use ID3v1
 		return nil, fmt.Errorf("does not use ID3v1: expected %s; got %s", "TAG", tag)
 	}
@@ -97,7 +95,7 @@ func (tags *ID3v1Tags) Write(dst io.WriteSeeker) error {
 	dst.Seek(0, io.SeekEnd)
 
 	// TAG
-	_, err := dst.Write([]byte("TAG"))
+	_, err := dst.Write([]byte(ID3v1IDENTIFIER))
 	if err != nil {
 		return err
 	}
@@ -136,7 +134,7 @@ func (tags *ID3v1Tags) Write(dst io.WriteSeeker) error {
 	genreCode := getKey(id3v1genres, tags.Genre)
 	if genreCode == -1 {
 		// if no genre found - encode genre code as 255
-		genreCode = 255
+		genreCode = ID3v1INVALIDGENRE
 	}
 	genrebyte := make([]byte, 1)
 	binary.PutVarint(genrebyte, int64(genreCode))
@@ -154,14 +152,14 @@ func (tags *ID3v1Tags) WriteToFile(f *os.File) error {
 	defer f.Close()
 
 	// check for existing ID3v1 tag
-	f.Seek(-int64(ID3V1SIZE), io.SeekEnd)
+	f.Seek(-int64(ID3v1SIZE), io.SeekEnd)
 
 	tag, err := read(f, 3)
 	if err != nil {
 		return err
 	}
 
-	if !bytes.Equal(tag, []byte("TAG")) {
+	if !bytes.Equal(tag, []byte(ID3v1IDENTIFIER)) {
 		// no existing tag, just write given tags
 		err = tags.Write(f)
 		if err != nil {
@@ -176,7 +174,7 @@ func (tags *ID3v1Tags) WriteToFile(f *os.File) error {
 		return err
 	}
 
-	err = f.Truncate(fStats.Size() - int64(ID3V1SIZE))
+	err = f.Truncate(fStats.Size() - int64(ID3v1SIZE))
 	if err != nil {
 		return nil
 	}
diff --git a/id3v11.go b/id3v11.go
index ce5cb8e..6cd2537 100644
--- a/id3v11.go
+++ b/id3v11.go
@@ -22,7 +22,7 @@ type ID3v11Tags struct {
 // Retrieves ID3v1.1 field values of provided io.ReadSeeker
 func GetID3v11Tags(rs io.ReadSeeker) (*ID3v11Tags, error) {
 	// set reader to the last 128 bytes
-	_, err := rs.Seek(-int64(ID3V1SIZE), io.SeekEnd)
+	_, err := rs.Seek(-int64(ID3v1SIZE), io.SeekEnd)
 	if err != nil {
 		return nil, fmt.Errorf("could not seek: %s", err)
 	}
@@ -164,7 +164,7 @@ func (tags *ID3v11Tags) Write(dst io.WriteSeeker) error {
 	genreCode := getKey(id3v1genres, tags.Genre)
 	if genreCode == -1 {
 		// if no genre found - encode genre code as 255
-		genreCode = 255
+		genreCode = ID3v1INVALIDGENRE
 	}
 	genrebyte := make([]byte, 1)
 	binary.PutVarint(genrebyte, int64(genreCode))
@@ -182,7 +182,7 @@ func (tags *ID3v11Tags) WriteToFile(f *os.File) error {
 	defer f.Close()
 
 	// check for existing ID3v1.1 tag
-	f.Seek(-int64(ID3V1SIZE), io.SeekEnd)
+	f.Seek(-int64(ID3v1SIZE), io.SeekEnd)
 
 	tag, err := read(f, 3)
 	if err != nil {
@@ -204,7 +204,7 @@ func (tags *ID3v11Tags) WriteToFile(f *os.File) error {
 		return err
 	}
 
-	err = f.Truncate(fStats.Size() - int64(ID3V1SIZE))
+	err = f.Truncate(fStats.Size() - int64(ID3v1SIZE))
 	if err != nil {
 		return nil
 	}
diff --git a/id3v20.go b/id3v20.go
new file mode 100644
index 0000000..fc80b97
--- /dev/null
+++ b/id3v20.go
@@ -0,0 +1,38 @@
+package id3ed
+
+//////////////////////////////////////
+//(ᗜˬᗜ)~⭐//Under construction//(ᗜ‸ᗜ)//
+//////////////////////////////////////
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+)
+
+type Header struct {
+	Identifier string
+	Version    int
+	Flags      int
+	Size       int64
+}
+
+func GetHeader(rs io.ReadSeeker) (*Header, error) {
+	var header Header
+
+	rs.Seek(0, io.SeekStart)
+
+	identifier, err := read(rs, 3)
+	if err != nil {
+		return nil, err
+	}
+	// check if ID3v2 is used
+	if !bytes.Equal([]byte(ID3v2IDENTIFIER), identifier) {
+		return nil, fmt.Errorf("does not use ID3v2")
+	}
+	////
+
+	header.Identifier = string(identifier)
+
+	return &header, nil
+}