Respect START/END for preview fallback

src/base/USong.pas

@@ -217,6 +217,7 @@   TSong = class
     destructor  Destroy; override;
     function    LoadSong(DuetChange: boolean): boolean;
     function    Analyse(const ReadCustomTags: Boolean = false; DuetChange: boolean = false; RapToFreestyle: boolean = false; OutOfBoundsToFreestyle: boolean = false; AudioLength: real = 0; ForceLoadNotes: boolean = false): boolean;
+    function    GetPreviewStart(AudioLength: real): real;
     procedure   SetMedleyMode();
     procedure   Clear();
     function    MD5SongFile(SongFileR: TTextFileStream): string;
@@ -1504,6 +1505,46 @@ function  TSong.GetErrorLineNo: integer;
     Result := -1;
 end;
 
+function TSong.GetPreviewStart(AudioLength: real): real;
+var
+  EffectiveAudioStart:  real;
+  EffectiveAudioEnd:    real;
+  EffectiveAudioLength: real;
+  PreviewOffset:        real;
+begin
+  if (AudioLength <= 0.0) then
+  begin
+    Result := 0.0;
+    Exit;
+  end;
+
+  // PreviewStart is an explicit absolute position in the audio file.
+  if ((PreviewStart > 0.0) or HasPreview) and InRange(PreviewStart, 0.0, AudioLength) then
+  begin
+    Result := PreviewStart;
+    Exit;
+  end;
+
+  EffectiveAudioStart := EnsureRange(Start, 0.0, AudioLength);
+  if (Finish > 0) then
+    EffectiveAudioEnd := EnsureRange(Finish / 1000.0, 0.0, AudioLength)
+  else
+    EffectiveAudioEnd := AudioLength;
+
+  if (EffectiveAudioEnd <= EffectiveAudioStart) then
+  begin
+    EffectiveAudioStart := 0.0;
+    EffectiveAudioEnd := AudioLength;
+  end;
+
+  EffectiveAudioLength := EffectiveAudioEnd - EffectiveAudioStart;
+  PreviewOffset := EffectiveAudioLength / 4.0;
+  if (PreviewOffset > 120.0) then
+    PreviewOffset := 60.0;
+
+  Result := EffectiveAudioStart + PreviewOffset;
+end;
+
 function TSong.GetMD5: string;
 var
   SongFile: TMemTextFileStream;

src/screens/UScreenScore.pas

@@ -1805,10 +1805,7 @@ procedure TScreenSCore.StartPreview;
 
       if AudioPlayback.Open(CatSongs.Song[select].Path.Append(CatSongs.Song[select].Audio),nil) then
       begin
-        if (CatSongs.Song[select].PreviewStart > 0) then
-          AudioPlayback.Position := CatSongs.Song[select].PreviewStart
-        else
-          AudioPlayback.Position := (AudioPlayback.Length / 4);
+        AudioPlayback.Position := CatSongs.Song[select].GetPreviewStart(AudioPlayback.Length);
 
         // set preview volume
         PreviewVolume := EnsureRange(Ini.PreviewVolume, 0, 100) / 100;

src/screens/UScreenSong.pas

@@ -3640,15 +3640,7 @@ procedure TScreenSong.StartMusicPreview();
   begin
     PreviewOpened := Interaction;
 
-    // preview start is either calculated (by finding the chorus) or pre-set, use it
-    if ((Song.PreviewStart > 0.0) or Song.HasPreview) and InRange(Song.PreviewStart, 0.0, AudioPlayback.Length) then
-      PreviewPos := Song.PreviewStart
-    else
-    begin // otherwise, fallback to simple preview calculation
-      PreviewPos := AudioPlayback.Length / 4;
-      // fix for invalid music file lengths
-      if (PreviewPos > 120.0) then PreviewPos := 60.0;
-    end;
+    PreviewPos := Song.GetPreviewStart(AudioPlayback.Length);
 
     AudioPlayback.Position := PreviewPos;