trim_preserve_decimal_digits()

static double trim_preserve_decimal_digits ( double  d, int32_t  decimal_digits  )

static

Definition at line 2637 of file lwgeom.c.

{
        uint64_t dint = 0;
        memcpy(&dint, &d, sizeof(double));
        /* Extract the exponent from the IEEE 754 integer representation, which */
        /* corresponds to floor(log2(fabs(d))) */
        const int exponent = (int)((dint >> 52) & 2047) - 1023;
        /* (x * 851 + 255) / 256 == 1 + (int)(x * log2(10)) for x in [0,30] */
        int bits_needed = 1 + exponent + (decimal_digits * 851 + 255) / 256;
        /* for negative values, (x * 851 + 255) / 256 == (int)(x * log2(10)), so */
        /* subtract one */
        if (decimal_digits < 0)
                bits_needed --;
 
        /* This will also handle NaN and Inf since exponent = 1023, and thus for */
        /* reasonable decimal_digits values bits_needed will be > 52 */
        if (bits_needed >= 52)
        {
                return d;
        }
        if (bits_needed < 1 )
                bits_needed = 1;
        const uint64_t mask = 0xffffffffffffffffULL << (52 - bits_needed);
        dint &= mask;
        memcpy(&d, &dint, sizeof(double));
        return d;
}

Referenced by lwgeom_trim_bits_in_place().

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