Anyone got a routine to portably read/write 64 bit IEEE numbers in
litle-endian format?
(I mean so that it is compatible with fwrite(&x, sizeof(double), 1,
fp) ona Windows PC, and won't break should the program be run on a
machine with a diferent internal floating-point unit).
For integer quantities I use hton[s|l]()/ntoh[s|l](). Generally
speaking though, how about encoding into IEEE-754? Here's some code I
copied a long time ago from...some fellow which I don't recall.
long long pack754(long double f, unsigned bits, unsigned expbits)
{
long double fnorm;
int shift;
long long sign, exp, significand;
unsigned significandbits = bits - expbits - 1; // -1 for sign bit
if (f == 0.0) return 0; // get this special case out of the way
// check sign and begin normalization
if (f < 0) { sign = 1; fnorm = -f; }
else { sign = 0; fnorm = f; }
// get the normalized form of f and track the exponent
shift = 0;
while(fnorm >= 2.0) { fnorm /= 2.0; shift++; }
while(fnorm < 1.0) { fnorm *= 2.0; shift--; }
fnorm = fnorm - 1.0;
// calculate the binary form (non-float) of the significand data
significand = fnorm * ((1LL<<significandbits) + 0.5f);
// get the biased exponent
exp = shift + ((1<<(expbits-1)) - 1); // shift + bias
// return the final answer
return (sign<<(bits-1)) | (exp<<(bits-expbits-1)) | significand;
}
long double unpack754(long long i, unsigned bits, unsigned expbits)
{
long double result;
long long shift;
unsigned bias;
unsigned significandbits = bits - expbits - 1; // -1 for sign bit
if (i == 0) return 0.0;
// pull the significand
result = (i&((1LL<<significandbits)-1)); // mask
result /= (1LL<<significandbits); // convert back to float
result += 1.0f; // add the one back on
// deal with the exponent
bias = (1<<(expbits-1)) - 1;
shift = ((i>>significandbits)&((1LL<<expbits)-1)) - bias;
while(shift > 0) { result *= 2.0; shift--; }
while(shift < 0) { result /= 2.0; shift++; }
// sign it
result *= (i>>(bits-1))&1? -1.0: 1.0;
return result;
}
Those don't handle NaN or infinity, but you wouldn't need a terrible
amount of time to do so.
You could also sprintf() encode if your application isn't bandwidth-
intensive.
