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-rw-r--r--src/strconv/ftoa.go585
1 files changed, 585 insertions, 0 deletions
diff --git a/src/strconv/ftoa.go b/src/strconv/ftoa.go
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+// 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.
+
+// Binary to decimal floating point conversion.
+// Algorithm:
+// 1) store mantissa in multiprecision decimal
+// 2) shift decimal by exponent
+// 3) read digits out & format
+
+package strconv
+
+import "math"
+
+// TODO: move elsewhere?
+type floatInfo struct {
+ mantbits uint
+ expbits uint
+ bias int
+}
+
+var float32info = floatInfo{23, 8, -127}
+var float64info = floatInfo{52, 11, -1023}
+
+// FormatFloat converts the floating-point number f to a string,
+// according to the format fmt and precision prec. It rounds the
+// result assuming that the original was obtained from a floating-point
+// value of bitSize bits (32 for float32, 64 for float64).
+//
+// The format fmt is one of
+// 'b' (-ddddp±ddd, a binary exponent),
+// 'e' (-d.dddde±dd, a decimal exponent),
+// 'E' (-d.ddddE±dd, a decimal exponent),
+// 'f' (-ddd.dddd, no exponent),
+// 'g' ('e' for large exponents, 'f' otherwise),
+// 'G' ('E' for large exponents, 'f' otherwise),
+// 'x' (-0xd.ddddp±ddd, a hexadecimal fraction and binary exponent), or
+// 'X' (-0Xd.ddddP±ddd, a hexadecimal fraction and binary exponent).
+//
+// The precision prec controls the number of digits (excluding the exponent)
+// printed by the 'e', 'E', 'f', 'g', 'G', 'x', and 'X' formats.
+// For 'e', 'E', 'f', 'x', and 'X', it is the number of digits after the decimal point.
+// For 'g' and 'G' it is the maximum number of significant digits (trailing
+// zeros are removed).
+// The special precision -1 uses the smallest number of digits
+// necessary such that ParseFloat will return f exactly.
+func FormatFloat(f float64, fmt byte, prec, bitSize int) string {
+ return string(genericFtoa(make([]byte, 0, max(prec+4, 24)), f, fmt, prec, bitSize))
+}
+
+// AppendFloat appends the string form of the floating-point number f,
+// as generated by FormatFloat, to dst and returns the extended buffer.
+func AppendFloat(dst []byte, f float64, fmt byte, prec, bitSize int) []byte {
+ return genericFtoa(dst, f, fmt, prec, bitSize)
+}
+
+func genericFtoa(dst []byte, val float64, fmt byte, prec, bitSize int) []byte {
+ var bits uint64
+ var flt *floatInfo
+ switch bitSize {
+ case 32:
+ bits = uint64(math.Float32bits(float32(val)))
+ flt = &float32info
+ case 64:
+ bits = math.Float64bits(val)
+ flt = &float64info
+ default:
+ panic("strconv: illegal AppendFloat/FormatFloat bitSize")
+ }
+
+ neg := bits>>(flt.expbits+flt.mantbits) != 0
+ exp := int(bits>>flt.mantbits) & (1<<flt.expbits - 1)
+ mant := bits & (uint64(1)<<flt.mantbits - 1)
+
+ switch exp {
+ case 1<<flt.expbits - 1:
+ // Inf, NaN
+ var s string
+ switch {
+ case mant != 0:
+ s = "NaN"
+ case neg:
+ s = "-Inf"
+ default:
+ s = "+Inf"
+ }
+ return append(dst, s...)
+
+ case 0:
+ // denormalized
+ exp++
+
+ default:
+ // add implicit top bit
+ mant |= uint64(1) << flt.mantbits
+ }
+ exp += flt.bias
+
+ // Pick off easy binary, hex formats.
+ if fmt == 'b' {
+ return fmtB(dst, neg, mant, exp, flt)
+ }
+ if fmt == 'x' || fmt == 'X' {
+ return fmtX(dst, prec, fmt, neg, mant, exp, flt)
+ }
+
+ if !optimize {
+ return bigFtoa(dst, prec, fmt, neg, mant, exp, flt)
+ }
+
+ var digs decimalSlice
+ ok := false
+ // Negative precision means "only as much as needed to be exact."
+ shortest := prec < 0
+ if shortest {
+ // Use Ryu algorithm.
+ var buf [32]byte
+ digs.d = buf[:]
+ ryuFtoaShortest(&digs, mant, exp-int(flt.mantbits), flt)
+ ok = true
+ // Precision for shortest representation mode.
+ switch fmt {
+ case 'e', 'E':
+ prec = max(digs.nd-1, 0)
+ case 'f':
+ prec = max(digs.nd-digs.dp, 0)
+ case 'g', 'G':
+ prec = digs.nd
+ }
+ } else if fmt != 'f' {
+ // Fixed number of digits.
+ digits := prec
+ switch fmt {
+ case 'e', 'E':
+ digits++
+ case 'g', 'G':
+ if prec == 0 {
+ prec = 1
+ }
+ digits = prec
+ default:
+ // Invalid mode.
+ digits = 1
+ }
+ var buf [24]byte
+ if bitSize == 32 && digits <= 9 {
+ digs.d = buf[:]
+ ryuFtoaFixed32(&digs, uint32(mant), exp-int(flt.mantbits), digits)
+ ok = true
+ } else if digits <= 18 {
+ digs.d = buf[:]
+ ryuFtoaFixed64(&digs, mant, exp-int(flt.mantbits), digits)
+ ok = true
+ }
+ }
+ if !ok {
+ return bigFtoa(dst, prec, fmt, neg, mant, exp, flt)
+ }
+ return formatDigits(dst, shortest, neg, digs, prec, fmt)
+}
+
+// bigFtoa uses multiprecision computations to format a float.
+func bigFtoa(dst []byte, prec int, fmt byte, neg bool, mant uint64, exp int, flt *floatInfo) []byte {
+ d := new(decimal)
+ d.Assign(mant)
+ d.Shift(exp - int(flt.mantbits))
+ var digs decimalSlice
+ shortest := prec < 0
+ if shortest {
+ roundShortest(d, mant, exp, flt)
+ digs = decimalSlice{d: d.d[:], nd: d.nd, dp: d.dp}
+ // Precision for shortest representation mode.
+ switch fmt {
+ case 'e', 'E':
+ prec = digs.nd - 1
+ case 'f':
+ prec = max(digs.nd-digs.dp, 0)
+ case 'g', 'G':
+ prec = digs.nd
+ }
+ } else {
+ // Round appropriately.
+ switch fmt {
+ case 'e', 'E':
+ d.Round(prec + 1)
+ case 'f':
+ d.Round(d.dp + prec)
+ case 'g', 'G':
+ if prec == 0 {
+ prec = 1
+ }
+ d.Round(prec)
+ }
+ digs = decimalSlice{d: d.d[:], nd: d.nd, dp: d.dp}
+ }
+ return formatDigits(dst, shortest, neg, digs, prec, fmt)
+}
+
+func formatDigits(dst []byte, shortest bool, neg bool, digs decimalSlice, prec int, fmt byte) []byte {
+ switch fmt {
+ case 'e', 'E':
+ return fmtE(dst, neg, digs, prec, fmt)
+ case 'f':
+ return fmtF(dst, neg, digs, prec)
+ case 'g', 'G':
+ // trailing fractional zeros in 'e' form will be trimmed.
+ eprec := prec
+ if eprec > digs.nd && digs.nd >= digs.dp {
+ eprec = digs.nd
+ }
+ // %e is used if the exponent from the conversion
+ // is less than -4 or greater than or equal to the precision.
+ // if precision was the shortest possible, use precision 6 for this decision.
+ if shortest {
+ eprec = 6
+ }
+ exp := digs.dp - 1
+ if exp < -4 || exp >= eprec {
+ if prec > digs.nd {
+ prec = digs.nd
+ }
+ return fmtE(dst, neg, digs, prec-1, fmt+'e'-'g')
+ }
+ if prec > digs.dp {
+ prec = digs.nd
+ }
+ return fmtF(dst, neg, digs, max(prec-digs.dp, 0))
+ }
+
+ // unknown format
+ return append(dst, '%', fmt)
+}
+
+// roundShortest rounds d (= mant * 2^exp) to the shortest number of digits
+// that will let the original floating point value be precisely reconstructed.
+func roundShortest(d *decimal, mant uint64, exp int, flt *floatInfo) {
+ // If mantissa is zero, the number is zero; stop now.
+ if mant == 0 {
+ d.nd = 0
+ return
+ }
+
+ // Compute upper and lower such that any decimal number
+ // between upper and lower (possibly inclusive)
+ // will round to the original floating point number.
+
+ // We may see at once that the number is already shortest.
+ //
+ // Suppose d is not denormal, so that 2^exp <= d < 10^dp.
+ // The closest shorter number is at least 10^(dp-nd) away.
+ // The lower/upper bounds computed below are at distance
+ // at most 2^(exp-mantbits).
+ //
+ // So the number is already shortest if 10^(dp-nd) > 2^(exp-mantbits),
+ // or equivalently log2(10)*(dp-nd) > exp-mantbits.
+ // It is true if 332/100*(dp-nd) >= exp-mantbits (log2(10) > 3.32).
+ minexp := flt.bias + 1 // minimum possible exponent
+ if exp > minexp && 332*(d.dp-d.nd) >= 100*(exp-int(flt.mantbits)) {
+ // The number is already shortest.
+ return
+ }
+
+ // d = mant << (exp - mantbits)
+ // Next highest floating point number is mant+1 << exp-mantbits.
+ // Our upper bound is halfway between, mant*2+1 << exp-mantbits-1.
+ upper := new(decimal)
+ upper.Assign(mant*2 + 1)
+ upper.Shift(exp - int(flt.mantbits) - 1)
+
+ // d = mant << (exp - mantbits)
+ // Next lowest floating point number is mant-1 << exp-mantbits,
+ // unless mant-1 drops the significant bit and exp is not the minimum exp,
+ // in which case the next lowest is mant*2-1 << exp-mantbits-1.
+ // Either way, call it mantlo << explo-mantbits.
+ // Our lower bound is halfway between, mantlo*2+1 << explo-mantbits-1.
+ var mantlo uint64
+ var explo int
+ if mant > 1<<flt.mantbits || exp == minexp {
+ mantlo = mant - 1
+ explo = exp
+ } else {
+ mantlo = mant*2 - 1
+ explo = exp - 1
+ }
+ lower := new(decimal)
+ lower.Assign(mantlo*2 + 1)
+ lower.Shift(explo - int(flt.mantbits) - 1)
+
+ // The upper and lower bounds are possible outputs only if
+ // the original mantissa is even, so that IEEE round-to-even
+ // would round to the original mantissa and not the neighbors.
+ inclusive := mant%2 == 0
+
+ // As we walk the digits we want to know whether rounding up would fall
+ // within the upper bound. This is tracked by upperdelta:
+ //
+ // If upperdelta == 0, the digits of d and upper are the same so far.
+ //
+ // If upperdelta == 1, we saw a difference of 1 between d and upper on a
+ // previous digit and subsequently only 9s for d and 0s for upper.
+ // (Thus rounding up may fall outside the bound, if it is exclusive.)
+ //
+ // If upperdelta == 2, then the difference is greater than 1
+ // and we know that rounding up falls within the bound.
+ var upperdelta uint8
+
+ // Now we can figure out the minimum number of digits required.
+ // Walk along until d has distinguished itself from upper and lower.
+ for ui := 0; ; ui++ {
+ // lower, d, and upper may have the decimal points at different
+ // places. In this case upper is the longest, so we iterate from
+ // ui==0 and start li and mi at (possibly) -1.
+ mi := ui - upper.dp + d.dp
+ if mi >= d.nd {
+ break
+ }
+ li := ui - upper.dp + lower.dp
+ l := byte('0') // lower digit
+ if li >= 0 && li < lower.nd {
+ l = lower.d[li]
+ }
+ m := byte('0') // middle digit
+ if mi >= 0 {
+ m = d.d[mi]
+ }
+ u := byte('0') // upper digit
+ if ui < upper.nd {
+ u = upper.d[ui]
+ }
+
+ // Okay to round down (truncate) if lower has a different digit
+ // or if lower is inclusive and is exactly the result of rounding
+ // down (i.e., and we have reached the final digit of lower).
+ okdown := l != m || inclusive && li+1 == lower.nd
+
+ switch {
+ case upperdelta == 0 && m+1 < u:
+ // Example:
+ // m = 12345xxx
+ // u = 12347xxx
+ upperdelta = 2
+ case upperdelta == 0 && m != u:
+ // Example:
+ // m = 12345xxx
+ // u = 12346xxx
+ upperdelta = 1
+ case upperdelta == 1 && (m != '9' || u != '0'):
+ // Example:
+ // m = 1234598x
+ // u = 1234600x
+ upperdelta = 2
+ }
+ // Okay to round up if upper has a different digit and either upper
+ // is inclusive or upper is bigger than the result of rounding up.
+ okup := upperdelta > 0 && (inclusive || upperdelta > 1 || ui+1 < upper.nd)
+
+ // If it's okay to do either, then round to the nearest one.
+ // If it's okay to do only one, do it.
+ switch {
+ case okdown && okup:
+ d.Round(mi + 1)
+ return
+ case okdown:
+ d.RoundDown(mi + 1)
+ return
+ case okup:
+ d.RoundUp(mi + 1)
+ return
+ }
+ }
+}
+
+type decimalSlice struct {
+ d []byte
+ nd, dp int
+ neg bool
+}
+
+// %e: -d.ddddde±dd
+func fmtE(dst []byte, neg bool, d decimalSlice, prec int, fmt byte) []byte {
+ // sign
+ if neg {
+ dst = append(dst, '-')
+ }
+
+ // first digit
+ ch := byte('0')
+ if d.nd != 0 {
+ ch = d.d[0]
+ }
+ dst = append(dst, ch)
+
+ // .moredigits
+ if prec > 0 {
+ dst = append(dst, '.')
+ i := 1
+ m := min(d.nd, prec+1)
+ if i < m {
+ dst = append(dst, d.d[i:m]...)
+ i = m
+ }
+ for ; i <= prec; i++ {
+ dst = append(dst, '0')
+ }
+ }
+
+ // e±
+ dst = append(dst, fmt)
+ exp := d.dp - 1
+ if d.nd == 0 { // special case: 0 has exponent 0
+ exp = 0
+ }
+ if exp < 0 {
+ ch = '-'
+ exp = -exp
+ } else {
+ ch = '+'
+ }
+ dst = append(dst, ch)
+
+ // dd or ddd
+ switch {
+ case exp < 10:
+ dst = append(dst, '0', byte(exp)+'0')
+ case exp < 100:
+ dst = append(dst, byte(exp/10)+'0', byte(exp%10)+'0')
+ default:
+ dst = append(dst, byte(exp/100)+'0', byte(exp/10)%10+'0', byte(exp%10)+'0')
+ }
+
+ return dst
+}
+
+// %f: -ddddddd.ddddd
+func fmtF(dst []byte, neg bool, d decimalSlice, prec int) []byte {
+ // sign
+ if neg {
+ dst = append(dst, '-')
+ }
+
+ // integer, padded with zeros as needed.
+ if d.dp > 0 {
+ m := min(d.nd, d.dp)
+ dst = append(dst, d.d[:m]...)
+ for ; m < d.dp; m++ {
+ dst = append(dst, '0')
+ }
+ } else {
+ dst = append(dst, '0')
+ }
+
+ // fraction
+ if prec > 0 {
+ dst = append(dst, '.')
+ for i := 0; i < prec; i++ {
+ ch := byte('0')
+ if j := d.dp + i; 0 <= j && j < d.nd {
+ ch = d.d[j]
+ }
+ dst = append(dst, ch)
+ }
+ }
+
+ return dst
+}
+
+// %b: -ddddddddp±ddd
+func fmtB(dst []byte, neg bool, mant uint64, exp int, flt *floatInfo) []byte {
+ // sign
+ if neg {
+ dst = append(dst, '-')
+ }
+
+ // mantissa
+ dst, _ = formatBits(dst, mant, 10, false, true)
+
+ // p
+ dst = append(dst, 'p')
+
+ // ±exponent
+ exp -= int(flt.mantbits)
+ if exp >= 0 {
+ dst = append(dst, '+')
+ }
+ dst, _ = formatBits(dst, uint64(exp), 10, exp < 0, true)
+
+ return dst
+}
+
+// %x: -0x1.yyyyyyyyp±ddd or -0x0p+0. (y is hex digit, d is decimal digit)
+func fmtX(dst []byte, prec int, fmt byte, neg bool, mant uint64, exp int, flt *floatInfo) []byte {
+ if mant == 0 {
+ exp = 0
+ }
+
+ // Shift digits so leading 1 (if any) is at bit 1<<60.
+ mant <<= 60 - flt.mantbits
+ for mant != 0 && mant&(1<<60) == 0 {
+ mant <<= 1
+ exp--
+ }
+
+ // Round if requested.
+ if prec >= 0 && prec < 15 {
+ shift := uint(prec * 4)
+ extra := (mant << shift) & (1<<60 - 1)
+ mant >>= 60 - shift
+ if extra|(mant&1) > 1<<59 {
+ mant++
+ }
+ mant <<= 60 - shift
+ if mant&(1<<61) != 0 {
+ // Wrapped around.
+ mant >>= 1
+ exp++
+ }
+ }
+
+ hex := lowerhex
+ if fmt == 'X' {
+ hex = upperhex
+ }
+
+ // sign, 0x, leading digit
+ if neg {
+ dst = append(dst, '-')
+ }
+ dst = append(dst, '0', fmt, '0'+byte((mant>>60)&1))
+
+ // .fraction
+ mant <<= 4 // remove leading 0 or 1
+ if prec < 0 && mant != 0 {
+ dst = append(dst, '.')
+ for mant != 0 {
+ dst = append(dst, hex[(mant>>60)&15])
+ mant <<= 4
+ }
+ } else if prec > 0 {
+ dst = append(dst, '.')
+ for i := 0; i < prec; i++ {
+ dst = append(dst, hex[(mant>>60)&15])
+ mant <<= 4
+ }
+ }
+
+ // p±
+ ch := byte('P')
+ if fmt == lower(fmt) {
+ ch = 'p'
+ }
+ dst = append(dst, ch)
+ if exp < 0 {
+ ch = '-'
+ exp = -exp
+ } else {
+ ch = '+'
+ }
+ dst = append(dst, ch)
+
+ // dd or ddd or dddd
+ switch {
+ case exp < 100:
+ dst = append(dst, byte(exp/10)+'0', byte(exp%10)+'0')
+ case exp < 1000:
+ dst = append(dst, byte(exp/100)+'0', byte((exp/10)%10)+'0', byte(exp%10)+'0')
+ default:
+ dst = append(dst, byte(exp/1000)+'0', byte(exp/100)%10+'0', byte((exp/10)%10)+'0', byte(exp%10)+'0')
+ }
+
+ return dst
+}
+
+func min(a, b int) int {
+ if a < b {
+ return a
+ }
+ return b
+}
+
+func max(a, b int) int {
+ if a > b {
+ return a
+ }
+ return b
+}
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