static size_t _etoa()

in RISC-V_RV32_THEAD_SMART_CDS/libs/libc/printf.c [561:665]

• 71 lines of code
• 24 McCabe index (conditional complexity)

static size_t _etoa(out_fct_type out, char* buffer, size_t idx, size_t maxlen, double value, unsigned int prec, unsigned int width, unsigned int flags)
{
  // check for NaN and special values
  if ((value != value) || (value > DBL_MAX) || (value < -DBL_MAX)) {
    return _ftoa(out, buffer, idx, maxlen, value, prec, width, flags);
  }

  // determine the sign
  const bool negative = value < 0;
  if (negative) {
    value = -value;
  }

  // default precision
  if (!(flags & FLAGS_PRECISION)) {
    prec = PRINTF_DEFAULT_FLOAT_PRECISION;
  }

  // determine the decimal exponent
  // based on the algorithm by David Gay (https://www.ampl.com/netlib/fp/dtoa.c)
  union {
    uint64_t U;
    double   F;
  } conv;

  conv.F = value;
  int exp2 = (int)((conv.U >> 52U) & 0x07FFU) - 1023;           // effectively log2
  conv.U = (conv.U & ((1ULL << 52U) - 1U)) | (1023ULL << 52U);  // drop the exponent so conv.F is now in [1,2)
  // now approximate log10 from the log2 integer part and an expansion of ln around 1.5
  int expval = (int)(0.1760912590558 + exp2 * 0.301029995663981 + (conv.F - 1.5) * 0.289529654602168);
  // now we want to compute 10^expval but we want to be sure it won't overflow
  exp2 = (int)(expval * 3.321928094887362 + 0.5);
  const double z  = expval * 2.302585092994046 - exp2 * 0.6931471805599453;
  const double z2 = z * z;
  conv.U = (uint64_t)(exp2 + 1023) << 52U;
  // compute exp(z) using continued fractions, see https://en.wikipedia.org/wiki/Exponential_function#Continued_fractions_for_ex
  conv.F *= 1 + 2 * z / (2 - z + (z2 / (6 + (z2 / (10 + z2 / 14)))));
  // correct for rounding errors
  if (value < conv.F) {
    expval--;
    conv.F /= 10;
  }

  // the exponent format is "%+03d" and largest value is "307", so set aside 4-5 characters
  unsigned int minwidth = ((expval < 100) && (expval > -100)) ? 4U : 5U;

  // in "%g" mode, "prec" is the number of *significant figures* not decimals
  if (flags & FLAGS_ADAPT_EXP) {
    // do we want to fall-back to "%f" mode?
    if ((value >= 1e-4) && (value < 1e6)) {
      if ((int)prec > expval) {
        prec = (unsigned)((int)prec - expval - 1);
      }
      else {
        prec = 0;
      }
      flags |= FLAGS_PRECISION;   // make sure _ftoa respects precision
      // no characters in exponent
      minwidth = 0U;
      expval   = 0;
    }
    else {
      // we use one sigfig for the whole part
      if ((prec > 0) && (flags & FLAGS_PRECISION)) {
        --prec;
      }
    }
  }

  // will everything fit?
  unsigned int fwidth = width;
  if (width > minwidth) {
    // we didn't fall-back so subtract the characters required for the exponent
    fwidth -= minwidth;
  } else {
    // not enough characters, so go back to default sizing
    fwidth = 0U;
  }
  if ((flags & FLAGS_LEFT) && minwidth) {
    // if we're padding on the right, DON'T pad the floating part
    fwidth = 0U;
  }

  // rescale the float value
  if (expval) {
    value /= conv.F;
  }

  // output the floating part
  const size_t start_idx = idx;
  idx = _ftoa(out, buffer, idx, maxlen, negative ? -value : value, prec, fwidth, flags & ~FLAGS_ADAPT_EXP);

  // output the exponent part
  if (minwidth) {
    // output the exponential symbol
    out((flags & FLAGS_UPPERCASE) ? 'E' : 'e', buffer, idx++, maxlen);
    // output the exponent value
    idx = _ntoa_long(out, buffer, idx, maxlen, (expval < 0) ? -expval : expval, expval < 0, 10, 0, minwidth-1, FLAGS_ZEROPAD | FLAGS_PLUS);
    // might need to right-pad spaces
    if (flags & FLAGS_LEFT) {
      while (idx - start_idx < width) out(' ', buffer, idx++, maxlen);
    }
  }
  return idx;
}