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Implement double to fp conversion

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Victor Zverovich 2018-05-06 13:37:49 -07:00
parent a4c7d99f70
commit 7d3de49715
1 changed files with 27 additions and 0 deletions
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27
include/fmt/format.h
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@ -260,7 +260,34 @@ inline dummy_int _isnan(...) { return dummy_int(); }
struct fp {
uint64_t f;
int e;
fp(uint64_t f, int e): f(f), e(e) {}
// Constructs fp from an IEEE754 double. It is a template to prevent compile
// errors on platforms where double is not IEEE754.
template <typename Double>
explicit fp(Double d) {
// Assume double is in the format [sign][exponent][significand].
typedef std::numeric_limits<Double> limits;
const int double_size =
sizeof(Double) * std::numeric_limits<unsigned char>::digits;
// Subtract 1 to account for an implicit most significant bit in the
// normalized form.
const int significand_size = limits::digits - 1;
const int exponent_size = double_size - significand_size - 1; // -1 for sign
const uint64_t implicit_bit = 1ull << significand_size;
const uint64_t significand_mask = implicit_bit - 1;
const uint64_t exponent_mask = (~0ull >> 1) & ~significand_mask;
const int exponent_bias = (1 << exponent_size) - limits::max_exponent - 1;
auto u = bit_cast<uint64_t>(d);
auto biased_e = (u & exponent_mask) >> significand_size;
f = u & significand_mask;
if (biased_e != 0)
f += implicit_bit;
else
biased_e = 1; // Subnormals use biased exponent 1 (min exponent).
e = biased_e - exponent_bias - significand_size;
}
};
// Returns an fp number representing x - y. Result may not be normalized.