libgsmfrp: new refined implementation The previous implementation exhibited the following defects, which are now fixed: 1) The last received valid SID was cached forever for the purpose of handling future invalid SIDs - we could have received some valid SID ages ago, then lots of speech or NO_DATA, and if we then get an invalid SID, we would resurrect the last valid SID from ancient history - a bad design. In our new design, we handle invalid SID based on the current state, much like BFI. 2) GSM 06.11 spec says clearly that after the second lost SID (received BFI=1 && TAF=1 in CN state) we need to gradually decrease the output level, rather than jump directly to emitting silence frames - we previously failed to implement such logic. 3) Per GSM 06.12 section 5.2, Xmaxc should be the same in all 4 subframes in a SID frame. What should we do if we receive an otherwise valid SID frame with different Xmaxc? Our previous approach would replicate this Xmaxc oddity in every subsequent generated CN frame, which is rather bad. In our new design, the very first CN frame (which can be seen as a transformation of the SID frame itself) retains the original 4 distinct Xmaxc, but all subsequent CN frames are based on the Xmaxc from the last subframe of the most recent SID.

/*
 * Unfortunately the code we got from ETSI makes heavy use of two global
 * Boolean flags named Carry and Overflow that function like equally named
 * processor state flags on many CPU architectures.  They are not part
 * of persistent codec session state for either the encoder or the decoder,
 * instead they are "short-term" globals much like UNIX errno.
 *
 * Given this unfortunate reality plus the natural desire to make our
 * EFR library thread-safe (a transcoding MGW handling a large volume of
 * simultaneous calls is exactly the kind of application that would benefit
 * from utilitizing all CPU cores), our current workaround is to use
 * thread-local storage.
 */

#include <stdint.h>
#include "typedef.h"

__thread Flag EFR__Carry, EFR__Overflow;