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opusenc: Handle LUFS reference loudness in FLAC

Handle the FLAC comment REPLAYGAIN_REFERENCE_LOUDNESS expressed in
either dB SPL (positive) or LUFS (negative).  Although the FLAC
reference software writes dB SPL, some other software (such as
loudgain -s e) writes LUFS.

Fixes https://gitlab.xiph.org/xiph/opus-tools/-/issues/2316

--- a/src/flac.c

+++ b/src/flac.c

@@ -122,8 +122,9 @@

         comments=metadata->data.vorbis_comment.comments;

         saw_album_gain=saw_track_gain=0;

         album_gain=track_gain=0;

-        /*The default reference loudness for ReplayGain is 89.0 dB*/

-        reference_loudness=89;

+        /*The default reference loudness for ReplayGain is 89 dB SPL,

+          or -18 LUFS measured according to ITU-R BS.1770 / EBU R128.*/

+        reference_loudness=-18;

         /*The code below uses strtod for the gain tags, so make sure the locale is C*/

         saved_locale=setlocale(LC_NUMERIC,"C");

         for(i=0;i<num_comments;i++){

@@ -134,11 +135,15 @@

           /*Check for ReplayGain tags.

             Parse the ones we have R128 equivalents for, and skip the others.*/

           if(tagcompare(entry,"REPLAYGAIN_REFERENCE_LOUDNESS=",30)==0){

+            /*Reference loundness may be in dB SPL (positive) or

+              LUFS (negative).  The 89 dB SPL reference is considered

+              to be the same loudness as -18 LUFS.*/

             gain=strtod(entry+30,&end);

             if(end<=entry+30){

               fprintf(stderr,_("WARNING: Invalid ReplayGain tag: %s\n"),entry);

             }

-            else reference_loudness=gain;

+            else if (gain<0) reference_loudness=gain;

+            else reference_loudness=gain-89-18;

             continue;

           }

           if(tagcompare(entry,"REPLAYGAIN_ALBUM_GAIN=",22)==0){

@@ -177,9 +182,9 @@

         }

         setlocale(LC_NUMERIC,saved_locale);

         /*Set the header gain to the album gain after converting to the R128

-          reference level.*/

+          reference level (-23 LUFS).*/

         if(saw_album_gain){

-          gain=256*(album_gain+(84-reference_loudness))+0.5;

+          gain=256*(album_gain+(-23-reference_loudness))+0.5;

           inopt->gain=gain<-32768?-32768:gain<32767?(int)floor(gain):32767;

         }

         /*If there was a track gain, then add an equivalent R128 tag for that.*/