_dtoa.c

这是一个同样来自贝尔实验室的和UNIX有着渊源的操作系统, 其简洁的设计和实现易于我们学习和理解

			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;
	}