ref: 5c448c618a89ab8933838da1b5b97ed7c4a42e08
dir: /sys/src/ape/lib/ap/stdio/_dtoa.c/
#include "fconv.h" static int quorem(Bigint *, Bigint *); /* dtoa for IEEE arithmetic (dmg): convert double to ASCII string. * * Inspired by "How to Print Floating-Point Numbers Accurately" by * Guy L. Steele, Jr. and Jon L. White [Proc. ACM SIGPLAN '90, pp. 92-101]. * * Modifications: * 1. Rather than iterating, we use a simple numeric overestimate * to determine k = floor(log10(d)). We scale relevant * quantities using O(log2(k)) rather than O(k) multiplications. * 2. For some modes > 2 (corresponding to ecvt and fcvt), we don't * try to generate digits strictly left to right. Instead, we * compute with fewer bits and propagate the carry if necessary * when rounding the final digit up. This is often faster. * 3. Under the assumption that input will be rounded nearest, * mode 0 renders 1e23 as 1e23 rather than 9.999999999999999e22. * That is, we allow equality in stopping tests when the * round-nearest rule will give the same floating-point value * as would satisfaction of the stopping test with strict * inequality. * 4. We remove common factors of powers of 2 from relevant * quantities. * 5. When converting floating-point integers less than 1e16, * we use floating-point arithmetic rather than resorting * to multiple-precision integers. * 6. When asked to produce fewer than 15 digits, we first try * to get by with floating-point arithmetic; we resort to * multiple-precision integer arithmetic only if we cannot * guarantee that the floating-point calculation has given * the correctly rounded result. For k requested digits and * "uniformly" distributed input, the probability is * something like 10^(k-15) that we must resort to the long * calculation. */ char * _dtoa(double darg, int mode, int ndigits, int *decpt, int *sign, char **rve) { /* Arguments ndigits, decpt, sign are similar to those of ecvt and fcvt; trailing zeros are suppressed from the returned string. If not null, *rve is set to point to the end of the return value. If d is +-Infinity or NaN, then *decpt is set to 9999. mode: 0 ==> shortest string that yields d when read in and rounded to nearest. 1 ==> like 0, but with Steele & White stopping rule; e.g. with IEEE P754 arithmetic , mode 0 gives 1e23 whereas mode 1 gives 9.999999999999999e22. 2 ==> max(1,ndigits) significant digits. This gives a return value similar to that of ecvt, except that trailing zeros are suppressed. 3 ==> through ndigits past the decimal point. This gives a return value similar to that from fcvt, except that trailing zeros are suppressed, and ndigits can be negative. 4-9 should give the same return values as 2-3, i.e., 4 <= mode <= 9 ==> same return as mode 2 + (mode & 1). These modes are mainly for debugging; often they run slower but sometimes faster than modes 2-3. 4,5,8,9 ==> left-to-right digit generation. 6-9 ==> don't try fast floating-point estimate (if applicable). Values of mode other than 0-9 are treated as mode 0. Sufficient space is allocated to the return value to hold the suppressed trailing zeros. */ int bbits, b2, b5, be, dig, i, ieps, ilim, ilim0, ilim1, j, j1, k, k0, k_check, leftright, m2, m5, s2, s5, spec_case, try_quick; long L; #ifndef Sudden_Underflow int denorm; unsigned long x; #endif Bigint *b, *b1, *delta, *mlo, *mhi, *S; double ds; Dul d2, eps; char *s, *s0; static Bigint *result; static int result_k; Dul d; d.d = darg; if (result) { result->k = result_k; result->maxwds = 1 << result_k; Bfree(result); result = 0; } if (word0(d) & Sign_bit) { /* set sign for everything, including 0's and NaNs */ *sign = 1; word0(d) &= ~Sign_bit; /* clear sign bit */ } else *sign = 0; #if defined(IEEE_Arith) + defined(VAX) #ifdef IEEE_Arith if ((word0(d) & Exp_mask) == Exp_mask) #else if (word0(d) == 0x8000) #endif { /* Infinity or NaN */ *decpt = 9999; s = #ifdef IEEE_Arith !word1(d) && !(word0(d) & 0xfffff) ? "Infinity" : #endif "NaN"; if (rve) *rve = #ifdef IEEE_Arith s[3] ? s + 8 : #endif s + 3; return s; } #endif #ifdef IBM d.d += 0; /* normalize */ #endif if (!d.d) { *decpt = 1; s = "0"; if (rve) *rve = s + 1; return s; } b = d2b(d.d, &be, &bbits); #ifdef Sudden_Underflow i = (int)(word0(d) >> Exp_shift1 & (Exp_mask>>Exp_shift1)); #else if (i = (int)(word0(d) >> Exp_shift1 & (Exp_mask>>Exp_shift1))) { #endif d2.d = d.d; word0(d2) &= Frac_mask1; word0(d2) |= Exp_11; #ifdef IBM if (j = 11 - hi0bits(word0(d2) & Frac_mask)) d2.d /= 1 << j; #endif /* log(x) ~=~ log(1.5) + (x-1.5)/1.5 * log10(x) = log(x) / log(10) * ~=~ log(1.5)/log(10) + (x-1.5)/(1.5*log(10)) * log10(d) = (i-Bias)*log(2)/log(10) + log10(d2) * * This suggests computing an approximation k to log10(d) by * * k = (i - Bias)*0.301029995663981 * + ( (d2-1.5)*0.289529654602168 + 0.176091259055681 ); * * We want k to be too large rather than too small. * The error in the first-order Taylor series approximation * is in our favor, so we just round up the constant enough * to compensate for any error in the multiplication of * (i - Bias) by 0.301029995663981; since |i - Bias| <= 1077, * and 1077 * 0.30103 * 2^-52 ~=~ 7.2e-14, * adding 1e-13 to the constant term more than suffices. * Hence we adjust the constant term to 0.1760912590558. * (We could get a more accurate k by invoking log10, * but this is probably not worthwhile.) */ i -= Bias; #ifdef IBM i <<= 2; i += j; #endif #ifndef Sudden_Underflow denorm = 0; } else { /* d is denormalized */ i = bbits + be + (Bias + (P-1) - 1); x = i > 32 ? word0(d) << 64 - i | word1(d) >> i - 32 : word1(d) << 32 - i; d2.d = x; word0(d2) -= 31*Exp_msk1; /* adjust exponent */ i -= (Bias + (P-1) - 1) + 1; denorm = 1; } #endif ds = (d2.d-1.5)*0.289529654602168 + 0.1760912590558 + i*0.301029995663981; k = floor(ds); k_check = 1; if (k >= 0 && k <= Ten_pmax) { if (d.d < tens[k]) k--; k_check = 0; } j = bbits - i - 1; if (j >= 0) { b2 = 0; s2 = j; } else { b2 = -j; s2 = 0; } if (k >= 0) { b5 = 0; s5 = k; s2 += k; } else { b2 -= k; b5 = -k; s5 = 0; } if (mode < 0 || mode > 9) mode = 0; try_quick = 1; if (mode > 5) { mode -= 4; try_quick = 0; } leftright = 1; switch(mode) { case 0: case 1: ilim = ilim1 = -1; i = 18; ndigits = 0; break; case 2: leftright = 0; /* no break */ case 4: if (ndigits <= 0) ndigits = 1; ilim = ilim1 = i = ndigits; break; case 3: leftright = 0; /* no break */ case 5: i = ndigits + k + 1; ilim = i; ilim1 = i - 1; if (i <= 0) i = 1; } j = sizeof(unsigned long); for(result_k = 0; sizeof(Bigint) - sizeof(unsigned long) + j <= i; j <<= 1) result_k++; result = Balloc(result_k); s = s0 = (char *)result; if (ilim >= 0 && ilim <= Quick_max && try_quick) { /* Try to get by with floating-point arithmetic. */ i = 0; d2.d = d.d; k0 = k; ilim0 = ilim; ieps = 2; /* conservative */ if (k > 0) { ds = tens[k&0xf]; j = k >> 4; if (j & Bletch) { /* prevent overflows */ j &= Bletch - 1; d.d /= bigtens[n_bigtens-1]; ieps++; } for(; j; j >>= 1, i++) if (j & 1) { ieps++; ds *= bigtens[i]; } d.d /= ds; } else if (j1 = -k) { d.d *= tens[j1 & 0xf]; for(j = j1 >> 4; j; j >>= 1, i++) if (j & 1) { ieps++; d.d *= bigtens[i]; } } if (k_check && d.d < 1. && ilim > 0) { if (ilim1 <= 0) goto fast_failed; ilim = ilim1; k--; d.d *= 10.; ieps++; } eps.d = ieps*d.d + 7.; word0(eps) -= (P-1)*Exp_msk1; if (ilim == 0) { S = mhi = 0; d.d -= 5.; if (d.d > eps.d) goto one_digit; if (d.d < -eps.d) goto no_digits; goto fast_failed; } #ifndef No_leftright if (leftright) { /* Use Steele & White method of only * generating digits needed. */ eps.d = 0.5/tens[ilim-1] - eps.d; for(i = 0;;) { L = floor(d.d); d.d -= L; *s++ = '0' + (int)L; if (d.d < eps.d) goto ret1; if (1. - d.d < eps.d) goto bump_up; if (++i >= ilim) break; eps.d *= 10.; d.d *= 10.; } } else { #endif /* Generate ilim digits, then fix them up. */ eps.d *= tens[ilim-1]; for(i = 1;; i++, d.d *= 10.) { L = floor(d.d); d.d -= L; *s++ = '0' + (int)L; if (i == ilim) { if (d.d > 0.5 + eps.d) goto bump_up; else if (d.d < 0.5 - eps.d) { while(*--s == '0'); s++; goto ret1; } break; } } #ifndef No_leftright } #endif fast_failed: s = s0; d.d = d2.d; k = k0; ilim = ilim0; } /* Do we have a "small" integer? */ if (be >= 0 && k <= Int_max) { /* Yes. */ ds = tens[k]; if (ndigits < 0 && ilim <= 0) { S = mhi = 0; if (ilim < 0 || d.d <= 5*ds) goto no_digits; goto one_digit; } for(i = 1;; i++) { L = floor(d.d / ds); d.d -= L*ds; #ifdef Check_FLT_ROUNDS /* If FLT_ROUNDS == 2, L will usually be high by 1 */ if (d.d < 0) { L--; d.d += ds; } #endif *s++ = '0' + (int)L; if (i == ilim) { d.d += d.d; if (d.d > ds || d.d == ds && L & 1) { bump_up: while(*--s == '9') if (s == s0) { k++; *s = '0'; break; } ++*s++; } break; } d.d *= 10.; if (d.d == 0.) break; } goto ret1; } m2 = b2; m5 = b5; mhi = mlo = 0; if (leftright) { if (mode < 2) { i = #ifndef Sudden_Underflow denorm ? be + (Bias + (P-1) - 1 + 1) : #endif #ifdef IBM 1 + 4*P - 3 - bbits + ((bbits + be - 1) & 3); #else 1 + P - bbits; #endif } else { j = ilim - 1; if (m5 >= j) m5 -= j; else { s5 += j -= m5; b5 += j; m5 = 0; } if ((i = ilim) < 0) { m2 -= i; i = 0; } } b2 += i; s2 += i; mhi = i2b(1); } if (m2 > 0 && s2 > 0) { i = m2 < s2 ? m2 : s2; b2 -= i; m2 -= i; s2 -= i; } if (b5 > 0) { if (leftright) { if (m5 > 0) { mhi = pow5mult(mhi, m5); b1 = mult(mhi, b); Bfree(b); b = b1; } if (j = b5 - m5) b = pow5mult(b, j); } else b = pow5mult(b, b5); } S = i2b(1); if (s5 > 0) S = pow5mult(S, s5); /* Check for special case that d is a normalized power of 2. */ if (mode < 2) { if (!word1(d) && !(word0(d) & Bndry_mask) #ifndef Sudden_Underflow && word0(d) & Exp_mask #endif ) { /* The special case */ b2 += Log2P; s2 += Log2P; spec_case = 1; } else spec_case = 0; } /* Arrange for convenient computation of quotients: * shift left if necessary so divisor has 4 leading 0 bits. * * Perhaps we should just compute leading 28 bits of S once * and for all and pass them and a shift to quorem, so it * can do shifts and ors to compute the numerator for q. */ #ifdef Pack_32 if (i = ((s5 ? 32 - hi0bits(S->x[S->wds-1]) : 1) + s2) & 0x1f) i = 32 - i; #else if (i = ((s5 ? 32 - hi0bits(S->x[S->wds-1]) : 1) + s2) & 0xf) i = 16 - i; #endif if (i > 4) { i -= 4; b2 += i; m2 += i; s2 += i; } else if (i < 4) { i += 28; b2 += i; m2 += i; s2 += i; } if (b2 > 0) b = lshift(b, b2); if (s2 > 0) S = lshift(S, s2); if (k_check) { if (cmp(b,S) < 0) { k--; b = multadd(b, 10, 0); /* we botched the k estimate */ if (leftright) mhi = multadd(mhi, 10, 0); ilim = ilim1; } } if (ilim <= 0 && mode > 2) { if (ilim < 0 || cmp(b,S = multadd(S,5,0)) <= 0) { /* no digits, fcvt style */ no_digits: k = -1 - ndigits; goto ret; } one_digit: *s++ = '1'; k++; goto ret; } if (leftright) { if (m2 > 0) mhi = lshift(mhi, m2); /* Compute mlo -- check for special case * that d is a normalized power of 2. */ mlo = mhi; if (spec_case) { mhi = Balloc(mhi->k); Bcopy(mhi, mlo); mhi = lshift(mhi, Log2P); } for(i = 1;;i++) { dig = quorem(b,S) + '0'; /* Do we yet have the shortest decimal string * that will round to d? */ j = cmp(b, mlo); delta = diff(S, mhi); j1 = delta->sign ? 1 : cmp(b, delta); Bfree(delta); #ifndef ROUND_BIASED if (j1 == 0 && !mode && !(word1(d) & 1)) { if (dig == '9') goto round_9_up; if (j > 0) dig++; *s++ = dig; goto ret; } #endif if (j < 0 || j == 0 && !mode #ifndef ROUND_BIASED && !(word1(d) & 1) #endif ) { if (j1 > 0) { b = lshift(b, 1); j1 = cmp(b, S); if ((j1 > 0 || j1 == 0 && dig & 1) && dig++ == '9') goto round_9_up; } *s++ = dig; goto ret; } if (j1 > 0) { if (dig == '9') { /* possible if i == 1 */ round_9_up: *s++ = '9'; goto roundoff; } *s++ = dig + 1; goto ret; } *s++ = dig; if (i == ilim) break; b = multadd(b, 10, 0); if (mlo == mhi) mlo = mhi = multadd(mhi, 10, 0); else { mlo = multadd(mlo, 10, 0); mhi = multadd(mhi, 10, 0); } } } else for(i = 1;; i++) { *s++ = dig = quorem(b,S) + '0'; if (i >= ilim) break; b = multadd(b, 10, 0); } /* Round off last digit */ b = lshift(b, 1); j = cmp(b, S); if (j > 0 || j == 0 && dig & 1) { roundoff: while(*--s == '9') if (s == s0) { k++; *s++ = '1'; goto ret; } ++*s++; } else { while(*--s == '0'); s++; } ret: Bfree(S); if (mhi) { if (mlo && mlo != mhi) Bfree(mlo); Bfree(mhi); } ret1: Bfree(b); *s = 0; *decpt = k + 1; if (rve) *rve = s; return s0; } static int quorem(Bigint *b, Bigint *S) { int n; long borrow, y; unsigned long carry, q, ys; unsigned long *bx, *bxe, *sx, *sxe; #ifdef Pack_32 long z; unsigned long si, zs; #endif n = S->wds; #ifdef DEBUG /*debug*/ if (b->wds > n) /*debug*/ Bug("oversize b in quorem"); #endif if (b->wds < n) return 0; sx = S->x; sxe = sx + --n; bx = b->x; bxe = bx + n; q = *bxe / (*sxe + 1); /* ensure q <= true quotient */ #ifdef DEBUG /*debug*/ if (q > 9) /*debug*/ Bug("oversized quotient in quorem"); #endif if (q) { borrow = 0; carry = 0; do { #ifdef Pack_32 si = *sx++; ys = (si & 0xffff) * q + carry; zs = (si >> 16) * q + (ys >> 16); carry = zs >> 16; y = (*bx & 0xffff) - (ys & 0xffff) + borrow; borrow = y >> 16; Sign_Extend(borrow, y); z = (*bx >> 16) - (zs & 0xffff) + borrow; borrow = z >> 16; Sign_Extend(borrow, z); Storeinc(bx, z, y); #else ys = *sx++ * q + carry; carry = ys >> 16; y = *bx - (ys & 0xffff) + borrow; borrow = y >> 16; Sign_Extend(borrow, y); *bx++ = y & 0xffff; #endif } while(sx <= sxe); if (!*bxe) { bx = b->x; while(--bxe > bx && !*bxe) --n; b->wds = n; } } if (cmp(b, S) >= 0) { q++; borrow = 0; carry = 0; bx = b->x; sx = S->x; do { #ifdef Pack_32 si = *sx++; ys = (si & 0xffff) + carry; zs = (si >> 16) + (ys >> 16); carry = zs >> 16; y = (*bx & 0xffff) - (ys & 0xffff) + borrow; borrow = y >> 16; Sign_Extend(borrow, y); z = (*bx >> 16) - (zs & 0xffff) + borrow; borrow = z >> 16; Sign_Extend(borrow, z); Storeinc(bx, z, y); #else ys = *sx++ + carry; carry = ys >> 16; y = *bx - (ys & 0xffff) + borrow; borrow = y >> 16; Sign_Extend(borrow, y); *bx++ = y & 0xffff; #endif } while(sx <= sxe); bx = b->x; bxe = bx + n; if (!*bxe) { while(--bxe > bx && !*bxe) --n; b->wds = n; } } return q; }