Read track number & index as BCD-encoded bytes

The track number and track index from the CUEX chunk are BCD-encoded
(Binary-Coded Decimal). Reading them as binary led to wrong track
numbers starting with track 10.

There is doubt whereas the index number is BCD or not, but two-digit
indices are seldom seen.

src/metadata/cuex.rs

@@ -153,8 +153,8 @@ pub fn read_nrg_cuex(fd: &mut File) -> Result<NrgCuex, NrgError> {
 fn read_nrg_cuex_track(fd: &mut File) -> Result<NrgCuexTrack, NrgError> {
     let mut track = NrgCuexTrack::new();
     track.mode = try!(read_u8(fd));
-    track.track_number = try!(read_u8(fd));
-    track.index_number = try!(read_u8(fd));
+    track.track_number = try!(read_u8_bcd(fd));
+    track.index_number = try!(read_u8_bcd(fd));
     track.padding = try!(read_u8(fd));
     track.position_sectors = try!(read_u32(fd)) as i32;
     Ok(track)

src/metadata/readers.rs

@@ -97,3 +97,19 @@ pub fn read_u8(fd: &mut File) -> Result<u8, NrgError> {
     try!(fd.read_exact(&mut buf));
     Ok(buf[0])
 }
+
+
+/// Reads a BCD-encoded byte from `fd`.
+///
+/// If the decoded value is more than 99, which is not a valid binary-coded
+/// decimal value, the byte read is returned as is, without decoding.
+pub fn read_u8_bcd(fd: &mut File) -> Result<u8, NrgError> {
+    let byte = try!(read_u8(fd));
+    let tens = (byte >> 4) * 10;
+    let units = (byte << 4) >> 4;
+    let value = tens + units;
+    if value < 100 {
+        return Ok(value);
+    }
+    Ok(byte)
+}