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%g should not print 0 as infe-308 (#185)

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Graham Sanderson 2021-02-25 10:00:05 -06:00 committed by graham sanderson
parent 4335268a93
commit e526e4f850
1 changed files with 22 additions and 16 deletions
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38
src/rp2_common/pico_printf/printf.c
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@ -490,21 +490,27 @@ static size_t _etoa(out_fct_type out, char *buffer, size_t idx, size_t maxlen, d
} 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;
int expval;
if (conv.U) {
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
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;
}
} else {
expval = 0;
conv.F = 1;
}
// the exponent format is "%+03d" and largest value is "307", so set aside 4-5 characters
@ -513,7 +519,7 @@ static size_t _etoa(out_fct_type out, char *buffer, size_t idx, size_t maxlen, d
// 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 ((value == 0.0) || ((value >= 1e-4) && (value < 1e6))) {
if ((int) prec > expval) {
prec = (unsigned) ((int) prec - expval - 1);
} else {