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Floating point printing and parsing library based on Grisu2 and Krosh algorithms

License: Apache License 2.0

CMake 0.03% C++ 99.95% Shell 0.02%

floating-point grisu ieee-754 parsing printing

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notimplemented uplinkcoder temerkhanov chpdev

floaxie's Issues

Incorrect boundary computation

Hi,

I was recently testing your library and noticed that some numbers fail to round-trip when printed using floaxie::ftoa. As it turned out, the computation of shift_amount in diy_fp::boundaries is incorrect.

Actually, shift_amount currently always equals 1 and the resulting rounding interval is therefore too large if the input is a power-of-2.

The nth_bit test in

floaxie/include/floaxie/diy_fp.h

Line 445 in 1a4243d

 const unsigned char shift_amount(1 + nth_bit(mi.m_f, std::numeric_limits<FloatType>::digits)); 

should be replaced by something like

constexpr int Prec
    = std::numeric_limits<FloatType>::digits;
constexpr int MinExp
    = std::numeric_limits<FloatType>::min_exponent - 1 - (Prec - 1);
constexpr mantissa_storage_type HiddenBit
    = static_cast<mantissa_storage_type>(1) << (Prec - 1);

const bool lower_boundary_is_closer
    = (mi.m_f == HiddenBit && mi.m_e > MinExp);
const unsigned char shift_amount
    = 1 + lower_boundary_is_closer;

though it turns out that

const unsigned char shift_amount = 1 + (mi.m_f == HiddenBit);

is sufficient for the current implementation.

Here are some test cases which currently fail to round-trip.
(The hexadecimal numbers are the IEEE representations of the floating-point numbers.)

Some incorrect single-precision numbers:

9.8607613e-32f == 2^-103 == 0x0C000000
6.3108872e-30f == 2^-97  == 0x0F000000
8.4703295e-22f == 2^-70  == 0x1C800000
8.6736174e-19f == 2^-60  == 0x21800000
7.1054274e-15f == 2^-47  == 0x28000000
7.2057594e+16f == 2^56   == 0x5B800000
3.7778932e+22f == 2^75   == 0x65000000
7.5557864e+22f == 2^76   == 0x65800000
4.8357033e+24f == 2^82   == 0x68800000
7.7371252e+25f == 2^86   == 0x6A800000
1.5474251e+26f == 2^87   == 0x6B000000
3.0948501e+26f == 2^88   == 0x6B800000
6.1897002e+26f == 2^89   == 0x6C000000
1.2379401e+27f == 2^90   == 0x6C800000
2.4758801e+27f == 2^91   == 0x6D000000
4.9517602e+27f == 2^92   == 0x6D800000
9.9035203e+27f == 2^93   == 0x6E000000
1.9807041e+28f == 2^94   == 0x6E800000
3.9614081e+28f == 2^95   == 0x6F000000
7.9228163e+28f == 2^96   == 0x6F800000

Some incorrect double-precision numbers:

1.7800590868057611e-307 == 2^-1019 == 0x0040000000000000
2.0522684006491881e-289 == 2^-959  == 0x0400000000000000
3.9696644133184383e-264 == 2^-875  == 0x0940000000000000
2.9290953396399042e-244 == 2^-809  == 0x0D60000000000000
2.5160737381238802e-234 == 2^-776  == 0x0F70000000000000
5.5329046628180653e-222 == 2^-735  == 0x1200000000000000
4.5965573598916705e-187 == 2^-619  == 0x1940000000000000
2.8451311993408992e-160 == 2^-530  == 0x1ED0000000000000
5.0052077379577523e-147 == 2^-486  == 0x2190000000000000
4.9569176510071274e-119 == 2^-393  == 0x2760000000000000
4.6816763546921983e-97  == 2^-320  == 0x2BF0000000000000
5.0978941156238473e-57  == 2^-187  == 0x3440000000000000
3.2311742677852644e-27  == 2^-88   == 0x3A70000000000000
3.8685626227668134e+25  == 2^85    == 0x4540000000000000
4.9039857307708443e+55  == 2^185   == 0x4B80000000000000
2.6074060497081422e+92  == 2^307   == 0x5320000000000000
4.8098152095208105e+111 == 2^371   == 0x5720000000000000
4.7634102635436893e+139 == 2^464   == 0x5CF0000000000000
4.4989137945431964e+161 == 2^537   == 0x6180000000000000
4.8988833106573424e+201 == 2^670   == 0x69D0000000000000
8.139666055761541e+236  == 2^787   == 0x7120000000000000
1.3207363278391631e+269 == 2^894   == 0x77D0000000000000

doesnt work on windows / visual studio

I tried to use your library on MS-Windows in my visual studio project. I copied over the files into my project. I got some compiler warnings - not sure if they are serious or not.

However, when running a simple conversion I get an infinite loop here:

        while (!(m_f & hidden_bit<original_matissa_bit_width>()))
        {
            m_f <<= 1;
            m_e--;
        }

its line 126 in diy_fp.h

The code I am trying to run is:

    char buffer[128];
    double num = 55.6666;
    floaxie::ftoa(num, buffer);

Has your code ever been run on windows at all?

thanks Peter

`0` is not printed correctly.

floaxie/include/floaxie/ftoa.h

Line 124 in 05c6259

buffer[3] = '\0';

Null character is written at buf[3] for zero.
If we provide uninitialized buffer, buf[1] and buf[2] contains garbage and printf/cout does not work well when we don't use returned the number of string length(e.g. directly construct std::string from char[] buffer)

Possible fix would be

buffer[0] = '0';
buffer[1] = '\0';

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