1495 banks carry no RIFF and take a separate path that hardcoded both the
offset and stereo. Across a random 48-bank sample there was NOT ONE where the
old stereo-at-1392 pair beat the best mono offset; median gain 184x, individual
banks going from 0-4816 decoded bytes to 180000-380000. Stereo shows the same
stop-after-one-frame signature already recorded for the leading segment.
With no RIFF the offset cannot be derived, so scan_data_offset picks among the
four disc offsets by XMA1 packet-header plausibility. Validated on the LABELLED
set -- all 7358 banks that do have a RIFF, where the answer is forced: 7330
correct (99.62%), and all 28 misses are ties on the top score, never a wrong
unique winner. Ties fall back to 1392.
The winning offsets also reproduce, by directory, the distribution measured
independently from the RIFF-bearing banks. jpn\etc splits 1468/1600, so path
alone is not sufficient -- which is why this is a scan and not a lookup table.
7 disc tests pass (build-reborn test -p sylpheed-formats --test
slb_leading_segment_disc, SYLPHEED_DISC wired up).
HEADERLESS_DATA_OFFSET is the value the offset takes in <lang>\etc\, not a
property of the format. The leading stream is a whole number of 2048-byte XMA1
packets ending at the first RIFF, so its start is first_riff % XMA1_PACKET.
Disc-wide that takes four values -- 1392, 1468, 1600, 1728 -- varying by
language and subdirectory.
Verified by decoding, not by arithmetic: on a random 140-bank sample with a
non-empty leading region, the derived offset yields more audio in 85, identical
in 54 (the eng\etc controls, where it must and does reproduce the old
behaviour) and less in 1. Median gain among the improved is 70x --
eng\Voice\VOICE_TCAF_592 goes 1506 -> 97152 bytes, jpn 2910 -> 127178.
This withdraws my own claim from earlier today that the Japanese banks were a
different undecoded layout. They are the same format with a different offset;
I had treated a constant derived from one subdirectory as a property of the
format. The same error was hiding the identical defect in 1873 eng\Voice banks.
