Added support for preserving existing album gain via CLI argument

README.md

@@ -87,6 +87,10 @@ tags are already present in the files. If you have files which had loudness
 levels calculated using the old ReplayGain algorithm, you can use this option
 to recalculate using the EBU R128 algorithm.
 
+`-p`/`--preserve-album-gain` causes regainer to preserve existing album gain.
+This is useful if you for example have multi-disc albums that have discs mastered
+at different loudness levels. NOTE: Using `-f` overrides this argument.
+
 `-j N` specifies the number of parallel jobs to run. By default it will
 use all CPUs available on the system. You can use this to reduce the amount
 of CPU that regainer will use.

regainer.py

@@ -195,7 +195,7 @@ def format_opus_gain(self, value):
         if value < -32768 or value > 32767:
             raise ValueError('Opus gain value out of range')
         return "{:d}".format(value)
-        
+
 
     def read_gain_id3(self):
         need_update = False
@@ -730,13 +730,18 @@ async def scan_album_gain(self):
             gain_scanner = GainScanner()
             self.gain = await gain_scanner.scan_album(included)
 
-    async def scan_gain(self):
-        album_task = asyncio.ensure_future(self.scan_album_gain())
-        track_tasks = [track.scan_gain() for track in self.tracks]
+    async def scan_gain(self, skip_album_scan=False):
+        tasks = [track.scan_gain() for track in self.tracks]
+
+        if not skip_album_scan:
+            album_task = asyncio.ensure_future(self.scan_album_gain())
+            tasks.append(album_task)
 
-        await asyncio.gather(album_task, *track_tasks)
+        await asyncio.gather(*tasks)
+
+        if not skip_album_scan:
+            self.gain.album_peak = max([t.gain.peak for t in self.tracks])
 
-        self.gain.album_peak = max([t.gain.peak for t in self.tracks])
         for track in self.tracks:
             track.gain.album_loudness = self.gain.album_loudness
             track.gain.album_peak = self.gain.album_peak
@@ -745,30 +750,35 @@ async def write_tags(self):
         track_tasks = [track.write_tags() for track in self.tracks]
         await asyncio.gather(*track_tasks)
 
-    async def scan(self, force=False, skip_save=False):
+    async def scan(self, force=False, skip_save=False, preserve_album_gain=False):
         await self.read_tags()
 
         need_scan = False
+        skip_album_scan = False
         for track in self.tracks:
             if track.gain.loudness is None or track.gain.peak is None:
                 need_scan = True
             if self.gain.album_loudness is None:
                 self.gain.album_loudness = track.gain.album_loudness
             if self.gain.album_loudness != track.gain.album_loudness:
-                need_scan = True
+                need_scan = need_scan or not preserve_album_gain
+                skip_album_scan = preserve_album_gain
             if self.gain.album_peak is None:
                 self.gain.album_peak = track.gain.album_peak
             if self.gain.album_peak != track.gain.album_peak:
-                need_scan = True
+                need_scan = need_scan or not preserve_album_gain
+                skip_album_scan = preserve_album_gain
         if self.gain.album_loudness is None or self.gain.album_peak is None:
             need_scan = True
+            skip_album_scan = False
         if force:
             need_scan = True
+            skip_album_scan = False
 
         need_save = any([track.tagger.need_album_update for track in self.tracks])
 
         if need_scan:
-            await self.scan_gain()
+            await self.scan_gain(skip_album_scan)
             need_save = True
 
         if need_save:
@@ -781,6 +791,8 @@ async def scan(self, force=False, skip_save=False):
             print(track.gain)
         if need_scan:
             print("Rescanned loudness")
+        if skip_album_scan:
+            print("Preserving missmatching album gains")
         if need_save:
             if not skip_save:
                 print("Updated tags")
@@ -808,6 +820,12 @@ def main(argv=None):
             Recalculate the ReplayGain values even if valid tags are already
             present in the files.
             ''')
+    parser.add_argument('-p', '--preserve-album-gain', default=False, action='store_true',
+            help='''
+            Preserve existing album ReplayGain values even if they missmatch within files.
+            Using this option will preserve different album gain values for given files.
+            Using -f/--force overrides this arguments.
+            ''')
     parser.add_argument('-j', '--jobs', type=int,
             default=multiprocessing.cpu_count(),
             help='''
@@ -855,7 +873,7 @@ def main(argv=None):
 
     tasks = []
     albums = [Album(album, job_sem) for album in args.album]
-    tasks += [album.scan(force=args.force, skip_save=args.dry_run)
+    tasks += [album.scan(force=args.force, skip_save=args.dry_run, preserve_album_gain=args.preserve_album_gain)
                     for album in albums]
     tracks = [Track(track, job_sem) for track in args.track]
     tasks += [track.scan(force=args.force, skip_save=args.dry_run)