src/pkg/strconv/atof.go - go - Git at Google

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 // decimal to binary floating point conversion.
 // Algorithm:
 //   1) Store input in multiprecision decimal.
 //   2) Multiply/divide decimal by powers of two until in range [0.5, 1)
 //   3) Multiply by 2^precision and round to get mantissa.

 // The strconv package implements conversions to and from
 // string representations of basic data types.
 package strconv

 import (
 	"math";
 	"os";
 	"strconv";
 )

 var optimize = true	// can change for testing

 // TODO(rsc): Better truncation handling.
 func stringToDecimal(s string) (neg bool, d *decimal, trunc bool, ok bool) {
 	i := 0;

 	// optional sign
 	if i >= len(s) {
 		return;
 	}
 	switch {
 	case s[i] == '+':
 		i++;
 	case s[i] == '-':
 		neg = true;
 		i++;
 	}

 	// digits
 	b := new(decimal);
 	sawdot := false;
 	sawdigits := false;
 	for ; i < len(s); i++ {
 		switch {
 		case s[i] == '.':
 			if sawdot {
 				return;
 			}
 			sawdot = true;
 			b.dp = b.nd;
 			continue;

 		case '0' <= s[i] && s[i] <= '9':
 			sawdigits = true;
 			if s[i] == '0' && b.nd == 0 {	// ignore leading zeros
 				b.dp--;
 				continue;
 			}
 			b.d[b.nd] = s[i];
 			b.nd++;
 			continue;
 		}
 		break;
 	}
 	if !sawdigits {
 		return;
 	}
 	if !sawdot {
 		b.dp = b.nd;
 	}

 	// optional exponent moves decimal point.
 	// if we read a very large, very long number,
 	// just be sure to move the decimal point by
 	// a lot (say, 100000).  it doesn't matter if it's
 	// not the exact number.
 	if i < len(s) && s[i] == 'e' {
 		i++;
 		if i >= len(s) {
 			return;
 		}
 		esign := 1;
 		if s[i] == '+' {
 			i++;
 		} else if s[i] == '-' {
 			i++;
 			esign = -1;
 		}
 		if i >= len(s) || s[i] < '0' || s[i] > '9' {
 			return;
 		}
 		e := 0;
 		for ; i < len(s) && '0' <= s[i] && s[i] <= '9'; i++ {
 			if e < 10000 {
 				e = e*10 + int(s[i]) - '0';
 			}
 		}
 		b.dp += e*esign;
 	}

 	if i != len(s) {
 		return;
 	}

 	d = b;
 	ok = true;
 	return;
 }

 // decimal power of ten to binary power of two.
 var powtab = []int{
 	1, 3, 6, 9, 13, 16, 19, 23, 26
 }

 func decimalToFloatBits(neg bool, d *decimal, trunc bool, flt *floatInfo) (b uint64, overflow bool) {
 	var exp int;
 	var mant uint64;

 	// Zero is always a special case.
 	if d.nd == 0 {
 		mant = 0;
 		exp = flt.bias;
 		goto out;
 	}

 	// Obvious overflow/underflow.
 	// These bounds are for 64-bit floats.
 	// Will have to change if we want to support 80-bit floats in the future.
 	if d.dp > 310 {
 		goto overflow;
 	}
 	if d.dp < -330 {
 		// zero
 		mant = 0;
 		exp = flt.bias;
 		goto out;
 	}

 	// Scale by powers of two until in range [0.5, 1.0)
 	exp = 0;
 	for d.dp > 0 {
 		var n int;
 		if d.dp >= len(powtab) {
 			n = 27;
 		} else {
 			n = powtab[d.dp];
 		}
 		d.Shift(-n);
 		exp += n;
 	}
 	for d.dp < 0 || d.dp == 0 && d.d[0] < '5' {
 		var n int;
 		if -d.dp >= len(powtab) {
 			n = 27;
 		} else {
 			n = powtab[-d.dp];
 		}
 		d.Shift(n);
 		exp -= n;
 	}

 	// Our range is [0.5,1) but floating point range is [1,2).
 	exp--;

 	// Minimum representable exponent is flt.bias+1.
 	// If the exponent is smaller, move it up and
 	// adjust d accordingly.
 	if exp < flt.bias+1 {
 		n := flt.bias+1 - exp;
 		d.Shift(-n);
 		exp += n;
 	}

 	if exp-flt.bias >= 1<<flt.expbits - 1 {
 		goto overflow;
 	}

 	// Extract 1+flt.mantbits bits.
 	mant = d.Shift(int(1+flt.mantbits)).RoundedInteger();

 	// Rounding might have added a bit; shift down.
 	if mant == 2<<flt.mantbits {
 		mant >>= 1;
 		exp++;
 		if exp-flt.bias >= 1<<flt.expbits - 1 {
 			goto overflow;
 		}
 	}

 	// Denormalized?
 	if mant&(1<<flt.mantbits) == 0 {
 		exp = flt.bias;
 	}
 	goto out;

 overflow:
 	// ±Inf
 	mant = 0;
 	exp = 1<<flt.expbits - 1 + flt.bias;
 	overflow = true;

 out:
 	// Assemble bits.
 	bits := mant & (uint64(1)<<flt.mantbits - 1);
 	bits |= uint64((exp-flt.bias)&(1<<flt.expbits - 1)) << flt.mantbits;
 	if neg {
 		bits |= 1<<flt.mantbits<<flt.expbits;
 	}
 	return bits, overflow;
 }

 // Compute exact floating-point integer from d's digits.
 // Caller is responsible for avoiding overflow.
 func decimalAtof64Int(neg bool, d *decimal) float64 {
 	f := float64(0);
 	for i := 0; i < d.nd; i++ {
 		f = f*10 + float64(d.d[i] - '0');
 	}
 	if neg {
 		f *= -1;	// BUG work around 6g f = -f.
 	}
 	return f;
 }

 func decimalAtof32Int(neg bool, d *decimal) float32 {
 	f := float32(0);
 	for i := 0; i < d.nd; i++ {
 		f = f*10 + float32(d.d[i] - '0');
 	}
 	if neg {
 		f *= -1;	// BUG work around 6g f = -f.
 	}
 	return f;
 }

 // Exact powers of 10.
 var float64pow10 = []float64 {
 	1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9,
 	1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19,
 	1e20, 1e21, 1e22
 }
 var float32pow10 = []float32 {
 	1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10
 }

 // If possible to convert decimal d to 64-bit float f exactly,
 // entirely in floating-point math, do so, avoiding the expense of decimalToFloatBits.
 // Three common cases:
 //	value is exact integer
 //	value is exact integer * exact power of ten
 //	value is exact integer / exact power of ten
 // These all produce potentially inexact but correctly rounded answers.
 func decimalAtof64(neg bool, d *decimal, trunc bool) (f float64, ok bool) {
 	// Exact integers are <= 10^15.
 	// Exact powers of ten are <= 10^22.
 	if d.nd > 15 {
 		return;
 	}
 	switch {
 	case d.dp == d.nd:	// int
 		f := decimalAtof64Int(neg, d);
 		return f, true;

 	case d.dp > d.nd && d.dp <= 15+22:	// int * 10^k
 		f := decimalAtof64Int(neg, d);
 		k := d.dp - d.nd;
 		// If exponent is big but number of digits is not,
 		// can move a few zeros into the integer part.
 		if k > 22 {
 			f *= float64pow10[k-22];
 			k = 22;
 		}
 		return f*float64pow10[k], true;

 	case d.dp < d.nd && d.nd - d.dp <= 22:	// int / 10^k
 		f := decimalAtof64Int(neg, d);
 		return f/float64pow10[d.nd - d.dp], true;
 	}
 	return;
 }

 // If possible to convert decimal d to 32-bit float f exactly,
 // entirely in floating-point math, do so, avoiding the machinery above.
 func decimalAtof32(neg bool, d *decimal, trunc bool) (f float32, ok bool) {
 	// Exact integers are <= 10^7.
 	// Exact powers of ten are <= 10^10.
 	if d.nd > 7 {
 		return;
 	}
 	switch {
 	case d.dp == d.nd:	// int
 		f := decimalAtof32Int(neg, d);
 		return f, true;

 	case d.dp > d.nd && d.dp <= 7+10:	// int * 10^k
 		f := decimalAtof32Int(neg, d);
 		k := d.dp - d.nd;
 		// If exponent is big but number of digits is not,
 		// can move a few zeros into the integer part.
 		if k > 10 {
 			f *= float32pow10[k-10];
 			k = 10;
 		}
 		return f*float32pow10[k], true;

 	case d.dp < d.nd && d.nd - d.dp <= 10:	// int / 10^k
 		f := decimalAtof32Int(neg, d);
 		return f/float32pow10[d.nd - d.dp], true;
 	}
 	return;
 }

 // Atof32 converts the string s to a 32-bit floating-point number.
 //
 // If s is well-formed and near a valid floating point number,
 // Atof32 returns the nearest floating point number rounded
 // using IEEE754 unbiased rounding.
 //
 // The errors that Atof32 returns have concrete type *NumError
 // and include err.Num = s.
 //
 // If s is not syntactically well-formed, Atof32 returns err.Error = os.EINVAL.
 //
 // If s is syntactically well-formed but is more than 1/2 ULP
 // away from the largest floating point number of the given size,
 // Atof32 returns f = ±Inf, err.Error = os.ERANGE.
 func Atof32(s string) (f float32, err os.Error) {
 	neg, d, trunc, ok := stringToDecimal(s);
 	if !ok {
 		return 0, &NumError{s, os.EINVAL};
 	}
 	if optimize {
 		if f, ok := decimalAtof32(neg, d, trunc); ok {
 			return f, nil;
 		}
 	}
 	b, ovf := decimalToFloatBits(neg, d, trunc, &float32info);
 	f = math.Float32frombits(uint32(b));
 	if ovf {
 		err = &NumError{s, os.ERANGE};
 	}
 	return f, err
 }

 // Atof64 converts the string s to a 64-bit floating-point number.
 // Except for the type of its result, its definition is the same as that
 // of Atof32.
 func Atof64(s string) (f float64, err os.Error) {
 	neg, d, trunc, ok := stringToDecimal(s);
 	if !ok {
 		return 0, &NumError{s, os.EINVAL};
 	}
 	if optimize {
 		if f, ok := decimalAtof64(neg, d, trunc); ok {
 			return f, nil;
 		}
 	}
 	b, ovf := decimalToFloatBits(neg, d, trunc, &float64info);
 	f = math.Float64frombits(b);
 	if ovf {
 		err = &NumError{s, os.ERANGE};
 	}
 	return f, err
 }

 // Atof is like Atof32 or Atof64, depending on the size of float.
 func Atof(s string) (f float, err os.Error) {
 	if FloatSize == 32 {
 		f1, err1 := Atof32(s);
 		return float(f1), err1;
 	}
 	f1, err1 := Atof64(s);
 	return float(f1), err1;
 }
	// Copyright 2009 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	// decimal to binary floating point conversion.
	// Algorithm:
	// 1) Store input in multiprecision decimal.
	// 2) Multiply/divide decimal by powers of two until in range [0.5, 1)
	// 3) Multiply by 2^precision and round to get mantissa.

	// The strconv package implements conversions to and from
	// string representations of basic data types.
	package strconv

	import (
	"math";
	"os";
	"strconv";
	)

	var optimize = true // can change for testing

	// TODO(rsc): Better truncation handling.
	func stringToDecimal(s string) (neg bool, d *decimal, trunc bool, ok bool) {
	i := 0;

	// optional sign
	if i >= len(s) {
	return;
	}
	switch {
	case s[i] == '+':
	i++;
	case s[i] == '-':
	neg = true;
	i++;
	}

	// digits
	b := new(decimal);
	sawdot := false;
	sawdigits := false;
	for ; i < len(s); i++ {
	switch {
	case s[i] == '.':
	if sawdot {
	return;
	}
	sawdot = true;
	b.dp = b.nd;
	continue;

	case '0' <= s[i] && s[i] <= '9':
	sawdigits = true;
	if s[i] == '0' && b.nd == 0 { // ignore leading zeros
	b.dp--;
	continue;
	}
	b.d[b.nd] = s[i];
	b.nd++;
	continue;
	}
	break;
	}
	if !sawdigits {
	return;
	}
	if !sawdot {
	b.dp = b.nd;
	}

	// optional exponent moves decimal point.
	// if we read a very large, very long number,
	// just be sure to move the decimal point by
	// a lot (say, 100000). it doesn't matter if it's
	// not the exact number.
	if i < len(s) && s[i] == 'e' {
	i++;
	if i >= len(s) {
	return;
	}
	esign := 1;
	if s[i] == '+' {
	i++;
	} else if s[i] == '-' {
	i++;
	esign = -1;
	}
	if i >= len(s) \|\| s[i] < '0' \|\| s[i] > '9' {
	return;
	}
	e := 0;
	for ; i < len(s) && '0' <= s[i] && s[i] <= '9'; i++ {
	if e < 10000 {
	e = e*10 + int(s[i]) - '0';
	}
	}
	b.dp += e*esign;
	}

	if i != len(s) {
	return;
	}

	d = b;
	ok = true;
	return;
	}

	// decimal power of ten to binary power of two.
	var powtab = []int{
	1, 3, 6, 9, 13, 16, 19, 23, 26
	}

	func decimalToFloatBits(neg bool, d decimal, trunc bool, flt floatInfo) (b uint64, overflow bool) {
	var exp int;
	var mant uint64;

	// Zero is always a special case.
	if d.nd == 0 {
	mant = 0;
	exp = flt.bias;
	goto out;
	}

	// Obvious overflow/underflow.
	// These bounds are for 64-bit floats.
	// Will have to change if we want to support 80-bit floats in the future.
	if d.dp > 310 {
	goto overflow;
	}
	if d.dp < -330 {
	// zero
	mant = 0;
	exp = flt.bias;
	goto out;
	}

	// Scale by powers of two until in range [0.5, 1.0)
	exp = 0;
	for d.dp > 0 {
	var n int;
	if d.dp >= len(powtab) {
	n = 27;
	} else {
	n = powtab[d.dp];
	}
	d.Shift(-n);
	exp += n;
	}
	for d.dp < 0 \|\| d.dp == 0 && d.d[0] < '5' {
	var n int;
	if -d.dp >= len(powtab) {
	n = 27;
	} else {
	n = powtab[-d.dp];
	}
	d.Shift(n);
	exp -= n;
	}

	// Our range is [0.5,1) but floating point range is [1,2).
	exp--;

	// Minimum representable exponent is flt.bias+1.
	// If the exponent is smaller, move it up and
	// adjust d accordingly.
	if exp < flt.bias+1 {
	n := flt.bias+1 - exp;
	d.Shift(-n);
	exp += n;
	}

	if exp-flt.bias >= 1<<flt.expbits - 1 {
	goto overflow;
	}

	// Extract 1+flt.mantbits bits.
	mant = d.Shift(int(1+flt.mantbits)).RoundedInteger();

	// Rounding might have added a bit; shift down.
	if mant == 2<<flt.mantbits {
	mant >>= 1;
	exp++;
	if exp-flt.bias >= 1<<flt.expbits - 1 {
	goto overflow;
	}
	}

	// Denormalized?
	if mant&(1<<flt.mantbits) == 0 {
	exp = flt.bias;
	}
	goto out;

	overflow:
	// ±Inf
	mant = 0;
	exp = 1<<flt.expbits - 1 + flt.bias;
	overflow = true;

	out:
	// Assemble bits.
	bits := mant & (uint64(1)<<flt.mantbits - 1);
	bits \|= uint64((exp-flt.bias)&(1<<flt.expbits - 1)) << flt.mantbits;
	if neg {
	bits \|= 1<<flt.mantbits<<flt.expbits;
	}
	return bits, overflow;
	}

	// Compute exact floating-point integer from d's digits.
	// Caller is responsible for avoiding overflow.
	func decimalAtof64Int(neg bool, d *decimal) float64 {
	f := float64(0);
	for i := 0; i < d.nd; i++ {
	f = f*10 + float64(d.d[i] - '0');
	}
	if neg {
	f *= -1; // BUG work around 6g f = -f.
	}
	return f;
	}

	func decimalAtof32Int(neg bool, d *decimal) float32 {
	f := float32(0);
	for i := 0; i < d.nd; i++ {
	f = f*10 + float32(d.d[i] - '0');
	}
	if neg {
	f *= -1; // BUG work around 6g f = -f.
	}
	return f;
	}

	// Exact powers of 10.
	var float64pow10 = []float64 {
	1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9,
	1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19,
	1e20, 1e21, 1e22
	}
	var float32pow10 = []float32 {
	1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10
	}

	// If possible to convert decimal d to 64-bit float f exactly,
	// entirely in floating-point math, do so, avoiding the expense of decimalToFloatBits.
	// Three common cases:
	// value is exact integer
	// value is exact integer * exact power of ten
	// value is exact integer / exact power of ten
	// These all produce potentially inexact but correctly rounded answers.
	func decimalAtof64(neg bool, d *decimal, trunc bool) (f float64, ok bool) {
	// Exact integers are <= 10^15.
	// Exact powers of ten are <= 10^22.
	if d.nd > 15 {
	return;
	}
	switch {
	case d.dp == d.nd: // int
	f := decimalAtof64Int(neg, d);
	return f, true;

	case d.dp > d.nd && d.dp <= 15+22: // int * 10^k
	f := decimalAtof64Int(neg, d);
	k := d.dp - d.nd;
	// If exponent is big but number of digits is not,
	// can move a few zeros into the integer part.
	if k > 22 {
	f *= float64pow10[k-22];
	k = 22;
	}
	return f*float64pow10[k], true;

	case d.dp < d.nd && d.nd - d.dp <= 22: // int / 10^k
	f := decimalAtof64Int(neg, d);
	return f/float64pow10[d.nd - d.dp], true;
	}
	return;
	}

	// If possible to convert decimal d to 32-bit float f exactly,
	// entirely in floating-point math, do so, avoiding the machinery above.
	func decimalAtof32(neg bool, d *decimal, trunc bool) (f float32, ok bool) {
	// Exact integers are <= 10^7.
	// Exact powers of ten are <= 10^10.
	if d.nd > 7 {
	return;
	}
	switch {
	case d.dp == d.nd: // int
	f := decimalAtof32Int(neg, d);
	return f, true;

	case d.dp > d.nd && d.dp <= 7+10: // int * 10^k
	f := decimalAtof32Int(neg, d);
	k := d.dp - d.nd;
	// If exponent is big but number of digits is not,
	// can move a few zeros into the integer part.
	if k > 10 {
	f *= float32pow10[k-10];
	k = 10;
	}
	return f*float32pow10[k], true;

	case d.dp < d.nd && d.nd - d.dp <= 10: // int / 10^k
	f := decimalAtof32Int(neg, d);
	return f/float32pow10[d.nd - d.dp], true;
	}
	return;
	}

	// Atof32 converts the string s to a 32-bit floating-point number.
	//
	// If s is well-formed and near a valid floating point number,
	// Atof32 returns the nearest floating point number rounded
	// using IEEE754 unbiased rounding.
	//
	// The errors that Atof32 returns have concrete type *NumError
	// and include err.Num = s.
	//
	// If s is not syntactically well-formed, Atof32 returns err.Error = os.EINVAL.
	//
	// If s is syntactically well-formed but is more than 1/2 ULP
	// away from the largest floating point number of the given size,
	// Atof32 returns f = ±Inf, err.Error = os.ERANGE.
	func Atof32(s string) (f float32, err os.Error) {
	neg, d, trunc, ok := stringToDecimal(s);
	if !ok {
	return 0, &NumError{s, os.EINVAL};
	}
	if optimize {
	if f, ok := decimalAtof32(neg, d, trunc); ok {
	return f, nil;
	}
	}
	b, ovf := decimalToFloatBits(neg, d, trunc, &float32info);
	f = math.Float32frombits(uint32(b));
	if ovf {
	err = &NumError{s, os.ERANGE};
	}
	return f, err
	}

	// Atof64 converts the string s to a 64-bit floating-point number.
	// Except for the type of its result, its definition is the same as that
	// of Atof32.
	func Atof64(s string) (f float64, err os.Error) {
	neg, d, trunc, ok := stringToDecimal(s);
	if !ok {
	return 0, &NumError{s, os.EINVAL};
	}
	if optimize {
	if f, ok := decimalAtof64(neg, d, trunc); ok {
	return f, nil;
	}
	}
	b, ovf := decimalToFloatBits(neg, d, trunc, &float64info);
	f = math.Float64frombits(b);
	if ovf {
	err = &NumError{s, os.ERANGE};
	}
	return f, err
	}

	// Atof is like Atof32 or Atof64, depending on the size of float.
	func Atof(s string) (f float, err os.Error) {
	if FloatSize == 32 {
	f1, err1 := Atof32(s);
	return float(f1), err1;
	}
	f1, err1 := Atof64(s);
	return float(f1), err1;
	}