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Implements ec_enc_patch_initial_bits()

--- a/libcelt/entenc.c

+++ b/libcelt/entenc.c

@@ -215,6 +215,31 @@

   _this->nbits_total+=_bits;

 }

+int ec_enc_patch_initial_bits(ec_enc *_this,unsigned _val,int _nbits){

+   int      shift;

+   unsigned mask;

+   if(_nbits<0||_nbits>EC_SYM_BITS)return -1;

+   shift=EC_SYM_BITS-_nbits;

+   mask=(1<<_nbits)-1<<shift;

+   if(_this->offs>0){

+     /*The first byte has been finalized.*/

+     _this->buf[0]=(unsigned char)(_this->buf[0]&~mask|_val<<shift);

+   }

+   else if(_this->rem>=0){

+     /*The first byte is still awaiting carry propagation.*/

+     _this->rem=_this->rem&~mask|_val<<shift;

+   }

+   else if(_this->rng<=EC_CODE_TOP>>shift){

+     /*The renormalization loop has never been run.*/

+     _this->val=_this->val&~((ec_uint32)mask<<EC_CODE_SHIFT)|

+      (ec_uint32)_val<<EC_CODE_SHIFT+shift;

+   }

+   /*The encoder hasn't even encoded _nbits of data yet.*/

+   else return -1;

+   /*Success!*/

+   return 0;

+}

+

 void ec_enc_shrink(ec_enc *_this,ec_uint32 _size){

   celt_assert(_this->offs+_this->end_offs<=_size);

   CELT_MOVE(_this->buf+_size-_this->end_offs,