From 92e6d71ddbe23676477b74f96a048396481c3349 Mon Sep 17 00:00:00 2001
From: David Howden <dhowden@gmail.com>
Date: Tue, 30 Jun 2015 23:07:26 +1000
Subject: [PATCH] Refactor common code to skip last 128 bytes from ReadSeeker.

---
 sum.go | 35 +++++++++++++++++------------------
 1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/sum.go b/sum.go
index 29d8023..cc25e05 100644
--- a/sum.go
+++ b/sum.go
@@ -96,13 +96,25 @@ func SumAtoms(r io.ReadSeeker) (string, error) {
 	}
 }
 
+func sizeToEndOffset(r io.ReadSeeker, offset int64) (int64, error) {
+	n, err := r.Seek(-128, os.SEEK_END)
+	if err != nil {
+		return 0, fmt.Errorf("error seeking end offset (%d bytes): %v", offset, err)
+	}
+
+	_, err = r.Seek(-n, os.SEEK_CUR)
+	if err != nil {
+		return 0, fmt.Errorf("error seeking back to original position: %v", err)
+	}
+	return n, nil
+}
+
 // SumID3v1 constructs a checksum of MP3 audio file data (assumed to have ID3v1 tags) provided
 // by the io.ReadSeeker which is metadata invariant.
 func SumID3v1(r io.ReadSeeker) (string, error) {
-	// Need to stop before we hit potential ID3v1 data.
-	n, err := r.Seek(-128, os.SEEK_END)
+	n, err := sizeToEndOffset(r, 128)
 	if err != nil {
-		return "", fmt.Errorf("error seeking to the end of the file (minus ID3v1 header): %v", err)
+		return "", fmt.Errorf("error determining read size to ID3v1 header: %v", err)
 	}
 
 	// TODO: improve this check???
@@ -110,12 +122,6 @@ func SumID3v1(r io.ReadSeeker) (string, error) {
 		return "", fmt.Errorf("file size must be greater than 128 bytes (ID3v1 header size) for MP3")
 	}
 
-	// Seek back to the original position now!
-	_, err = r.Seek(-1*n, os.SEEK_SET)
-	if err != nil {
-		return "", fmt.Errorf("error seeking back to the start of the data: %v", err)
-	}
-
 	h := sha1.New()
 	_, err = io.CopyN(h, r, n)
 	if err != nil {
@@ -137,10 +143,9 @@ func SumID3v2(r io.ReadSeeker) (string, error) {
 		return "", fmt.Errorf("error seeking to end of ID3V2 header: %v", err)
 	}
 
-	// Need to stop before we hit potential ID3v1 data.
-	n, err := r.Seek(-128, os.SEEK_END)
+	n, err := sizeToEndOffset(r, 128)
 	if err != nil {
-		return "", fmt.Errorf("error seeking to the end of the file (minus ID3v1 header): %v", err)
+		return "", fmt.Errorf("error determining read size to ID3v1 header: %v", err)
 	}
 
 	// TODO: remove this check?????
@@ -148,12 +153,6 @@ func SumID3v2(r io.ReadSeeker) (string, error) {
 		return "", fmt.Errorf("file size must be greater than 128 bytes for MP3: %v bytes", n)
 	}
 
-	// Seek back to the original position now!
-	_, err = r.Seek(-1*n, os.SEEK_SET)
-	if err != nil {
-		return "", fmt.Errorf("error seeking back to the start of the data: %v", err)
-	}
-
 	h := sha1.New()
 	_, err = io.CopyN(h, r, n)
 	if err != nil {