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feat: add floating-point support to format functions in libc

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This commit is contained in:
Daniel Hammer
2026-07-09 11:54:59 +02:00
parent 08d3cbe218
commit 9b66d3f470
3 changed files with 236 additions and 7 deletions
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programs/lib/libc/libc.c
+235 -6
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@@ -1174,6 +1174,226 @@ static void _pf_putnum(struct _pf_state *st, unsigned long val, int base, int up
_pf_putc(st, tmp[--i]);
}
/* Union for double bit manipulation (used by float formatting and math). */
typedef union { double d; uint64_t u; } _dbl_bits;
/* Base-10 exponent of the leading digit of a positive finite value, i.e. the
E such that value = m * 10^E with m in [1, 10). No rounding is applied. */
static int _pf_exp10(double v) {
int E = (int)floor(log10(v));
double s = v / pow(10.0, (double)E);
if (s >= 10.0) E++;
else if (s < 1.0) E--;
return E;
}
/* Generate `nsig` significant decimal digits of a positive finite value into
`out` (ASCII, no terminator). Returns the base-10 exponent of the leading
digit, i.e. value ~= (out[0].out[1..]) * 10^E. Rounding is half-up and may
carry into a new leading digit, bumping the returned exponent. */
static int _pf_gendigits(double v, int nsig, char *out) {
int E = (int)floor(log10(v));
double scale = v / pow(10.0, (double)E);
/* Correct estimate errors so scale lands in [1, 10). */
if (scale >= 10.0) { scale /= 10.0; E++; }
else if (scale < 1.0) { scale *= 10.0; E--; }
for (int i = 0; i < nsig; i++) {
int d = (int)scale;
if (d < 0) d = 0; else if (d > 9) d = 9;
out[i] = (char)('0' + d);
scale = (scale - d) * 10.0;
}
if (scale >= 5.0) { /* round up based on the next digit */
int i = nsig - 1;
while (i >= 0) {
if (out[i] == '9') { out[i] = '0'; i--; }
else { out[i]++; break; }
}
if (i < 0) { out[0] = '1'; E++; } /* all nines carried out */
}
return E;
}
/* Append the two-or-more-digit exponent field of %e/%g ("e+NN"). */
static void _pf_put_exp(char *mbuf, int *mlen, int E, int upper) {
int m = *mlen;
mbuf[m++] = upper ? 'E' : 'e';
int ex = E; char es = '+';
if (ex < 0) { es = '-'; ex = -ex; }
mbuf[m++] = es;
char eb[8]; int el = 0;
if (ex == 0) eb[el++] = '0';
while (ex > 0) { eb[el++] = (char)('0' + ex % 10); ex /= 10; }
while (el < 2) eb[el++] = '0';
while (el > 0) mbuf[m++] = eb[--el];
*mlen = m;
}
/* Format a floating-point value for the %e/%f/%g conversions (and uppercase
variants), honoring precision, width, padding and sign flags. */
static void _pf_putfloat(struct _pf_state *st, double v, char conv,
int precision, int width, char pad,
int left_align, int plus, int space, int alt) {
int upper = (conv >= 'A' && conv <= 'Z');
char lc = upper ? (char)(conv - 'A' + 'a') : conv;
_dbl_bits bits; bits.d = v;
int neg = (int)(bits.u >> 63);
uint64_t expo = (bits.u >> 52) & 0x7FF;
uint64_t mant = bits.u & 0x000FFFFFFFFFFFFFULL;
char sign = 0;
if (neg) sign = '-';
else if (plus) sign = '+';
else if (space) sign = ' ';
/* Infinity / NaN: never zero-padded. */
if (expo == 0x7FF) {
const char *word = mant ? (upper ? "NAN" : "nan") : (upper ? "INF" : "inf");
if (mant) sign = 0;
char tmp[8]; int tl = 0;
if (sign) tmp[tl++] = sign;
for (const char *p = word; *p; p++) tmp[tl++] = *p;
if (!left_align) for (int w = tl; w < width; w++) _pf_putc(st, ' ');
for (int i = 0; i < tl; i++) _pf_putc(st, tmp[i]);
if (left_align) for (int w = tl; w < width; w++) _pf_putc(st, ' ');
return;
}
if (precision < 0) precision = 6;
if (precision > 340) precision = 340;
double av = neg ? -v : v;
char mbuf[800];
int mlen = 0;
if (lc == 'e') {
int nsig = precision + 1;
char dig[400];
int E;
if (av == 0.0) { for (int i = 0; i < nsig; i++) dig[i] = '0'; E = 0; }
else E = _pf_gendigits(av, nsig, dig);
mbuf[mlen++] = dig[0];
if (precision > 0 || alt) {
mbuf[mlen++] = '.';
for (int i = 1; i <= precision; i++) mbuf[mlen++] = dig[i];
}
_pf_put_exp(mbuf, &mlen, E, upper);
}
else if (lc == 'f') {
if (av == 0.0) {
mbuf[mlen++] = '0';
if (precision > 0 || alt) {
mbuf[mlen++] = '.';
for (int i = 0; i < precision; i++) mbuf[mlen++] = '0';
}
} else {
int Ef = _pf_exp10(av);
int keep = Ef + 1 + precision;
if (keep <= 0) {
char d1[4];
_pf_gendigits(av, 1, d1);
int roundUp = (keep == 0 && d1[0] >= '5');
mbuf[mlen++] = '0';
if (precision > 0 || alt) {
mbuf[mlen++] = '.';
for (int i = 0; i < precision; i++) mbuf[mlen++] = '0';
}
if (roundUp) mbuf[mlen - 1] = '1';
} else {
if (keep > 380) keep = 380;
char dig[400];
int E = _pf_gendigits(av, keep, dig);
int len = keep;
int intlen = (E >= 0) ? (E + 1) : 0;
if (intlen == 0) mbuf[mlen++] = '0';
else for (int i = 0; i < intlen; i++) mbuf[mlen++] = (i < len ? dig[i] : '0');
if (precision > 0 || alt) {
mbuf[mlen++] = '.';
if (E < 0) {
int lead = -E - 1;
for (int k = 0; k < precision; k++) {
if (k < lead) mbuf[mlen++] = '0';
else { int idx = k - lead; mbuf[mlen++] = (idx < len ? dig[idx] : '0'); }
}
} else {
for (int k = 0; k < precision; k++) {
int idx = intlen + k;
mbuf[mlen++] = (idx < len ? dig[idx] : '0');
}
}
}
}
}
}
else { /* 'g' / 'G' */
int P = precision; if (P == 0) P = 1;
char dig[400];
int E;
if (av == 0.0) { E = 0; for (int i = 0; i < P; i++) dig[i] = '0'; }
else E = _pf_gendigits(av, P, dig);
int len = P;
int useExp = (E < -4 || E >= P);
int hasDot = 0;
if (useExp) {
int fp = P - 1;
mbuf[mlen++] = dig[0];
if (fp > 0 || alt) {
mbuf[mlen++] = '.'; hasDot = 1;
for (int i = 1; i <= fp; i++) mbuf[mlen++] = dig[i];
}
if (!alt && hasDot) {
while (mbuf[mlen - 1] == '0') mlen--;
if (mbuf[mlen - 1] == '.') mlen--;
}
_pf_put_exp(mbuf, &mlen, E, upper);
} else {
int fp = P - 1 - E;
if (fp < 0) fp = 0;
int intlen = (E >= 0) ? (E + 1) : 0;
if (intlen == 0) mbuf[mlen++] = '0';
else for (int i = 0; i < intlen; i++) mbuf[mlen++] = (i < len ? dig[i] : '0');
if (fp > 0 || alt) {
mbuf[mlen++] = '.'; hasDot = 1;
if (E < 0) {
int lead = -E - 1;
for (int k = 0; k < fp; k++) {
if (k < lead) mbuf[mlen++] = '0';
else { int idx = k - lead; mbuf[mlen++] = (idx < len ? dig[idx] : '0'); }
}
} else {
for (int k = 0; k < fp; k++) {
int idx = intlen + k;
mbuf[mlen++] = (idx < len ? dig[idx] : '0');
}
}
}
if (!alt && hasDot) {
while (mbuf[mlen - 1] == '0') mlen--;
if (mbuf[mlen - 1] == '.') mlen--;
}
}
}
int total = (sign ? 1 : 0) + mlen;
if (!left_align && pad == '0') {
if (sign) _pf_putc(st, sign);
for (int w = total; w < width; w++) _pf_putc(st, '0');
for (int i = 0; i < mlen; i++) _pf_putc(st, mbuf[i]);
} else if (!left_align) {
for (int w = total; w < width; w++) _pf_putc(st, ' ');
if (sign) _pf_putc(st, sign);
for (int i = 0; i < mlen; i++) _pf_putc(st, mbuf[i]);
} else {
if (sign) _pf_putc(st, sign);
for (int i = 0; i < mlen; i++) _pf_putc(st, mbuf[i]);
for (int w = total; w < width; w++) _pf_putc(st, ' ');
}
}
int vsnprintf(char *buf, size_t size, const char *fmt, va_list ap) {
struct _pf_state st;
st.buf = buf;
@@ -1188,12 +1408,16 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list ap) {
fmt++;
char pad = ' ';
int left_align = 0;
while (*fmt == '0' || *fmt == '-' || *fmt == '+' || *fmt == ' ') {
int left_align = 0, plus = 0, space = 0, alt = 0;
while (*fmt == '0' || *fmt == '-' || *fmt == '+' || *fmt == ' ' || *fmt == '#') {
if (*fmt == '0') pad = '0';
if (*fmt == '-') { left_align = 1; pad = ' '; }
else if (*fmt == '-') left_align = 1;
else if (*fmt == '+') plus = 1;
else if (*fmt == ' ') space = 1;
else if (*fmt == '#') alt = 1;
fmt++;
}
if (left_align) pad = ' ';
int width = 0;
if (*fmt == '*') {
@@ -1299,6 +1523,14 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list ap) {
_pf_putc(&st, c);
break;
}
case 'f': case 'F':
case 'e': case 'E':
case 'g': case 'G': {
double dv = va_arg(ap, double);
_pf_putfloat(&st, dv, *fmt, precision, width, pad,
left_align, plus, space, alt);
break;
}
case '%':
_pf_putc(&st, '%');
break;
@@ -2386,9 +2618,6 @@ double cos(double x) {
/* ---- log via exponent extraction + polynomial on [1, 2) ---- */
/* Union for double bit manipulation */
typedef union { double d; uint64_t u; } _dbl_bits;
double log(double x) {
if (x <= 0.0) return -HUGE_VAL;
if (x == 1.0) return 0.0;