This renames ec_dec_cdf() to ec_dec_icdf(), and changes the
functionality to use an "inverse" CDF table, where
icdf[i]=ft-cdf[i+1].
The first entry is omitted entirely.
It also adds a corresonding ec_enc_icdf() to the encoder, which uses
the same table.
One could use ec_encode_bin() by converting the values in the tables
back to normal CDF values, but the icdf[] table already has them in
the form ec_encode_bin() wants to use them, so there's no reason to
translate them and then translate them back.
This is done primarily to allow SILK to use the range coder with
8-bit probability tables containing cumulative frequencies that
span the full range 0...256.
With an 8-bit table, the final 256 of a normal CDF becomes 0 in the
"inverse" CDF.
It's the 0 at the start of a normal CDF which would become 256, but
this is the value we omit, as it already has to be special-cased in
the encoder, and is not used at all in the decoder.
For our current usage, this doesn't matter, but is more consistent
with the rest of the API.
We may want to reduce this to an unsigned char[], but I'd rather
coordinate that optimization with SILK's planned reduction to
8-bit CDFs, as we may be able to use the same code.
This ensures that the various alternative routines in the entropy
encoder and decoder (e.g., ec_{enc|dec}_bit_logp()) really are
just specialized optimizations of the same general ec_encode()
and ec_decode() routines.
This is done by randomly picking one to encode with for each symbol,
and randomly picking a different one to decode with.
This simplifies a good bit of the error handling, and should make it
impossible to overrun the buffer in the encoder or decoder, while
still allowing tell() to operate correctly after a bust.
The encoder now tries to keep the range coder data intact after a
bust instead of corrupting it with extra bits data, though this is
not a guarantee (too many extra bits may have already been flushed).
It also now correctly reports errors when the bust occurs merging the
last byte of range coder and extra bits.
A number of abstraction barrier violations were cleaned up, as well.
This patch also includes a number of minor performance improvements:
ec_{enc|dec}_bits() in particular should be much faster.
Finally, tf_select was changed to be coded with the range coder
rather than extra bits, so that it is at the front of the packet
(for unequal error protection robustness).
Making it so all the information encoded directly with ec_enc_bits() gets
stored at the end of the stream, without going through the range coder. This
should be both faster and reduce the effects of bit errors.
Conflicts:
tests/ectest.c
When I removed the special case for EC_ILOG(0) in commit
06390d082d, it broke ec_dec_uint() with _ft=1
(which should encode the value 0 using 0 bits).
This feature was tested by ectest.c, but not actually used by libcelt.
An assert has been added to ec_dec_uint() to ensure that we don't try to use
this feature by accident.
ec_enc_uint() was actually correct, but support for this feature has been
removed and the assert put in its place.
