1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // decimal to binary floating point conversion.
7 // 1) Store input in multiprecision decimal.
8 // 2) Multiply/divide decimal by powers of two until in range [0.5, 1)
9 // 3) Multiply by 2^precision and round to get mantissa.
11 // The strconv package implements conversions to and from
12 // string representations of basic data types.
20 var optimize = true // can change for testing
22 func equalIgnoreCase(s1, s2 string) bool {
23 if len(s1) != len(s2) {
26 for i := 0; i < len(s1); i++ {
28 if 'A' <= c1 && c1 <= 'Z' {
32 if 'A' <= c2 && c2 <= 'Z' {
42 func special(s string) (f float64, ok bool) {
44 case equalIgnoreCase(s, "nan"):
45 return math.NaN(), true
46 case equalIgnoreCase(s, "-inf"):
47 return math.Inf(-1), true
48 case equalIgnoreCase(s, "+inf"):
49 return math.Inf(1), true
50 case equalIgnoreCase(s, "inf"):
51 return math.Inf(1), true
56 // TODO(rsc): Better truncation handling.
57 func stringToDecimal(s string) (neg bool, d *decimal, trunc bool, ok bool) {
76 for ; i < len(s); i++ {
86 case '0' <= s[i] && s[i] <= '9':
88 if s[i] == '0' && b.nd == 0 { // ignore leading zeros
105 // optional exponent moves decimal point.
106 // if we read a very large, very long number,
107 // just be sure to move the decimal point by
108 // a lot (say, 100000). it doesn't matter if it's
109 // not the exact number.
110 if i < len(s) && (s[i] == 'e' || s[i] == 'E') {
118 } else if s[i] == '-' {
122 if i >= len(s) || s[i] < '0' || s[i] > '9' {
126 for ; i < len(s) && '0' <= s[i] && s[i] <= '9'; i++ {
128 e = e*10 + int(s[i]) - '0'
143 // decimal power of ten to binary power of two.
144 var powtab = []int{1, 3, 6, 9, 13, 16, 19, 23, 26}
146 func decimalToFloatBits(neg bool, d *decimal, trunc bool, flt *floatInfo) (b uint64, overflow bool) {
150 // Zero is always a special case.
157 // Obvious overflow/underflow.
158 // These bounds are for 64-bit floats.
159 // Will have to change if we want to support 80-bit floats in the future.
170 // Scale by powers of two until in range [0.5, 1.0)
174 if d.dp >= len(powtab) {
182 for d.dp < 0 || d.dp == 0 && d.d[0] < '5' {
184 if -d.dp >= len(powtab) {
193 // Our range is [0.5,1) but floating point range is [1,2).
196 // Minimum representable exponent is flt.bias+1.
197 // If the exponent is smaller, move it up and
198 // adjust d accordingly.
199 if exp < flt.bias+1 {
200 n := flt.bias + 1 - exp
205 if exp-flt.bias >= 1<<flt.expbits-1 {
209 // Extract 1+flt.mantbits bits.
210 mant = d.Shift(int(1 + flt.mantbits)).RoundedInteger()
212 // Rounding might have added a bit; shift down.
213 if mant == 2<<flt.mantbits {
216 if exp-flt.bias >= 1<<flt.expbits-1 {
222 if mant&(1<<flt.mantbits) == 0 {
230 exp = 1<<flt.expbits - 1 + flt.bias
235 bits := mant & (uint64(1)<<flt.mantbits - 1)
236 bits |= uint64((exp-flt.bias)&(1<<flt.expbits-1)) << flt.mantbits
238 bits |= 1 << flt.mantbits << flt.expbits
240 return bits, overflow
243 // Compute exact floating-point integer from d's digits.
244 // Caller is responsible for avoiding overflow.
245 func decimalAtof64Int(neg bool, d *decimal) float64 {
247 for i := 0; i < d.nd; i++ {
248 f = f*10 + float64(d.d[i]-'0')
251 f *= -1 // BUG work around 6g f = -f.
256 func decimalAtof32Int(neg bool, d *decimal) float32 {
258 for i := 0; i < d.nd; i++ {
259 f = f*10 + float32(d.d[i]-'0')
262 f *= -1 // BUG work around 6g f = -f.
267 // Exact powers of 10.
268 var float64pow10 = []float64{
269 1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9,
270 1e10, 1e11, 1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19,
273 var float32pow10 = []float32{1e0, 1e1, 1e2, 1e3, 1e4, 1e5, 1e6, 1e7, 1e8, 1e9, 1e10}
275 // If possible to convert decimal d to 64-bit float f exactly,
276 // entirely in floating-point math, do so, avoiding the expense of decimalToFloatBits.
277 // Three common cases:
278 // value is exact integer
279 // value is exact integer * exact power of ten
280 // value is exact integer / exact power of ten
281 // These all produce potentially inexact but correctly rounded answers.
282 func decimalAtof64(neg bool, d *decimal, trunc bool) (f float64, ok bool) {
283 // Exact integers are <= 10^15.
284 // Exact powers of ten are <= 10^22.
289 case d.dp == d.nd: // int
290 f := decimalAtof64Int(neg, d)
293 case d.dp > d.nd && d.dp <= 15+22: // int * 10^k
294 f := decimalAtof64Int(neg, d)
296 // If exponent is big but number of digits is not,
297 // can move a few zeros into the integer part.
299 f *= float64pow10[k-22]
302 return f * float64pow10[k], true
304 case d.dp < d.nd && d.nd-d.dp <= 22: // int / 10^k
305 f := decimalAtof64Int(neg, d)
306 return f / float64pow10[d.nd-d.dp], true
311 // If possible to convert decimal d to 32-bit float f exactly,
312 // entirely in floating-point math, do so, avoiding the machinery above.
313 func decimalAtof32(neg bool, d *decimal, trunc bool) (f float32, ok bool) {
314 // Exact integers are <= 10^7.
315 // Exact powers of ten are <= 10^10.
320 case d.dp == d.nd: // int
321 f := decimalAtof32Int(neg, d)
324 case d.dp > d.nd && d.dp <= 7+10: // int * 10^k
325 f := decimalAtof32Int(neg, d)
327 // If exponent is big but number of digits is not,
328 // can move a few zeros into the integer part.
330 f *= float32pow10[k-10]
333 return f * float32pow10[k], true
335 case d.dp < d.nd && d.nd-d.dp <= 10: // int / 10^k
336 f := decimalAtof32Int(neg, d)
337 return f / float32pow10[d.nd-d.dp], true
342 // Atof32 converts the string s to a 32-bit floating-point number.
344 // If s is well-formed and near a valid floating point number,
345 // Atof32 returns the nearest floating point number rounded
346 // using IEEE754 unbiased rounding.
348 // The errors that Atof32 returns have concrete type *NumError
349 // and include err.Num = s.
351 // If s is not syntactically well-formed, Atof32 returns err.Error = os.EINVAL.
353 // If s is syntactically well-formed but is more than 1/2 ULP
354 // away from the largest floating point number of the given size,
355 // Atof32 returns f = ±Inf, err.Error = os.ERANGE.
356 func Atof32(s string) (f float32, err os.Error) {
357 if val, ok := special(s); ok {
358 return float32(val), nil
361 neg, d, trunc, ok := stringToDecimal(s)
363 return 0, &NumError{s, os.EINVAL}
366 if f, ok := decimalAtof32(neg, d, trunc); ok {
370 b, ovf := decimalToFloatBits(neg, d, trunc, &float32info)
371 f = math.Float32frombits(uint32(b))
373 err = &NumError{s, os.ERANGE}
378 // Atof64 converts the string s to a 64-bit floating-point number.
379 // Except for the type of its result, its definition is the same as that
381 func Atof64(s string) (f float64, err os.Error) {
382 if val, ok := special(s); ok {
386 neg, d, trunc, ok := stringToDecimal(s)
388 return 0, &NumError{s, os.EINVAL}
391 if f, ok := decimalAtof64(neg, d, trunc); ok {
395 b, ovf := decimalToFloatBits(neg, d, trunc, &float64info)
396 f = math.Float64frombits(b)
398 err = &NumError{s, os.ERANGE}
403 // AtofN converts the string s to a 64-bit floating-point number,
404 // but it rounds the result assuming that it will be stored in a value
405 // of n bits (32 or 64).
406 func AtofN(s string, n int) (f float64, err os.Error) {
408 f1, err1 := Atof32(s)
409 return float64(f1), err1
411 f1, err1 := Atof64(s)