uint64_t Bitwalker_Peek(unsigned int Start,
unsigned int Length,
uint8_t Bitstream[],
unsigned int BitstreamSize)
{
if ((Start + Length) > 8 * BitstreamSize)
return 0; // error: invalid_bit_sequence
//@ assert UINT64_MAX == (1 << 64) -1;
uint64_t retval = 0;
/*@
loop invariant 0 <= i <= Length;
loop invariant 0 <= retval < (1 << i);
loop assigns i, retval;
loop variant Length - i;
*/
for (unsigned int i = 0; i < Length; i++)
{
unsigned int pos = Start + i;
unsigned int byte_index = pos / 8;
unsigned int bit_index = inverse_modulo(pos, 8);
// treat as unsigned int for Frama-C
unsigned int shift = Bitstream[byte_index] >> bit_index;
unsigned int bit_as_byte = shift & 1;
//@ assert bit_as_byte == 0 || bit_as_byte == 1;
retval = 2 * retval + bit_as_byte;
}
return retval;
}
lld combi(lld n, lld k,lld m)
{
if(n<k)
return 0;
if(n-k<k)
return combi(n,n-k,m);
lld i,p=1,t=1;
for(i=n-k+1;i<=n;i++)
p=(p*i)%m;
for(i=1;i<=k;i++)
t=(t*i)%m;
return (p*inverse_modulo(t,m))%m;
}
