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# Source file src/math/big/float.go

## Documentation: math/big

```  // Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style

// This file implements multi-precision floating-point numbers.
// Like in the GNU MPFR library (http://www.mpfr.org/), operands
// can be of mixed precision. Unlike MPFR, the rounding mode is
// not specified with each operation, but with each operand. The
// rounding mode of the result operand determines the rounding
// mode of an operation. This is a from-scratch implementation.

package big

import (
"fmt"
"math"
"math/bits"
)

const debugFloat = false // enable for debugging

// A nonzero finite Float represents a multi-precision floating point number
//
//   sign × mantissa × 2**exponent
//
// with 0.5 <= mantissa < 1.0, and MinExp <= exponent <= MaxExp.
// A Float may also be zero (+0, -0) or infinite (+Inf, -Inf).
// All Floats are ordered, and the ordering of two Floats x and y
// is defined by x.Cmp(y).
//
// Each Float value also has a precision, rounding mode, and accuracy.
// The precision is the maximum number of mantissa bits available to
// represent the value. The rounding mode specifies how a result should
// be rounded to fit into the mantissa bits, and accuracy describes the
// rounding error with respect to the exact result.
//
// Unless specified otherwise, all operations (including setters) that
// specify a *Float variable for the result (usually via the receiver
// with the exception of MantExp), round the numeric result according
// to the precision and rounding mode of the result variable.
//
// If the provided result precision is 0 (see below), it is set to the
// precision of the argument with the largest precision value before any
// rounding takes place, and the rounding mode remains unchanged. Thus,
// uninitialized Floats provided as result arguments will have their
// precision set to a reasonable value determined by the operands and
// their mode is the zero value for RoundingMode (ToNearestEven).
//
// By setting the desired precision to 24 or 53 and using matching rounding
// mode (typically ToNearestEven), Float operations produce the same results
// as the corresponding float32 or float64 IEEE-754 arithmetic for operands
// that correspond to normal (i.e., not denormal) float32 or float64 numbers.
// Exponent underflow and overflow lead to a 0 or an Infinity for different
// values than IEEE-754 because Float exponents have a much larger range.
//
// The zero (uninitialized) value for a Float is ready to use and represents
// the number +0.0 exactly, with precision 0 and rounding mode ToNearestEven.
//
type Float struct {
prec uint32
mode RoundingMode
acc  Accuracy
form form
neg  bool
mant nat
exp  int32
}

// An ErrNaN panic is raised by a Float operation that would lead to
// a NaN under IEEE-754 rules. An ErrNaN implements the error interface.
type ErrNaN struct {
msg string
}

func (err ErrNaN) Error() string {
return err.msg
}

// NewFloat allocates and returns a new Float set to x,
// with precision 53 and rounding mode ToNearestEven.
// NewFloat panics with ErrNaN if x is a NaN.
func NewFloat(x float64) *Float {
if math.IsNaN(x) {
panic(ErrNaN{"NewFloat(NaN)"})
}
return new(Float).SetFloat64(x)
}

// Exponent and precision limits.
const (
MaxExp  = math.MaxInt32  // largest supported exponent
MinExp  = math.MinInt32  // smallest supported exponent
MaxPrec = math.MaxUint32 // largest (theoretically) supported precision; likely memory-limited
)

// Internal representation: The mantissa bits x.mant of a nonzero finite
// Float x are stored in a nat slice long enough to hold up to x.prec bits;
// the slice may (but doesn't have to) be shorter if the mantissa contains
// trailing 0 bits. x.mant is normalized if the msb of x.mant == 1 (i.e.,
// the msb is shifted all the way "to the left"). Thus, if the mantissa has
// trailing 0 bits or x.prec is not a multiple of the Word size _W,
// x.mant[0] has trailing zero bits. The msb of the mantissa corresponds
// to the value 0.5; the exponent x.exp shifts the binary point as needed.
//
// A zero or non-finite Float x ignores x.mant and x.exp.
//
// x                 form      neg      mant         exp
// ----------------------------------------------------------
// ±0                zero      sign     -            -
// 0 < |x| < +Inf    finite    sign     mantissa     exponent
// ±Inf              inf       sign     -            -

// A form value describes the internal representation.
type form byte

// The form value order is relevant - do not change!
const (
zero form = iota
finite
inf
)

// RoundingMode determines how a Float value is rounded to the
// desired precision. Rounding may change the Float value; the
// rounding error is described by the Float's Accuracy.
type RoundingMode byte

// These constants define supported rounding modes.
const (
ToNearestEven RoundingMode = iota // == IEEE 754-2008 roundTiesToEven
ToNearestAway                     // == IEEE 754-2008 roundTiesToAway
ToZero                            // == IEEE 754-2008 roundTowardZero
AwayFromZero                      // no IEEE 754-2008 equivalent
ToNegativeInf                     // == IEEE 754-2008 roundTowardNegative
ToPositiveInf                     // == IEEE 754-2008 roundTowardPositive
)

//go:generate stringer -type=RoundingMode

// Accuracy describes the rounding error produced by the most recent
// operation that generated a Float value, relative to the exact value.
type Accuracy int8

// Constants describing the Accuracy of a Float.
const (
Below Accuracy = -1
Exact Accuracy = 0
Above Accuracy = +1
)

//go:generate stringer -type=Accuracy

// SetPrec sets z's precision to prec and returns the (possibly) rounded
// value of z. Rounding occurs according to z's rounding mode if the mantissa
// cannot be represented in prec bits without loss of precision.
// SetPrec(0) maps all finite values to ±0; infinite values remain unchanged.
// If prec > MaxPrec, it is set to MaxPrec.
func (z *Float) SetPrec(prec uint) *Float {
z.acc = Exact // optimistically assume no rounding is needed

// special case
if prec == 0 {
z.prec = 0
if z.form == finite {
// truncate z to 0
z.acc = makeAcc(z.neg)
z.form = zero
}
return z
}

// general case
if prec > MaxPrec {
prec = MaxPrec
}
old := z.prec
z.prec = uint32(prec)
if z.prec < old {
z.round(0)
}
return z
}

func makeAcc(above bool) Accuracy {
if above {
return Above
}
return Below
}

// SetMode sets z's rounding mode to mode and returns an exact z.
// z remains unchanged otherwise.
// z.SetMode(z.Mode()) is a cheap way to set z's accuracy to Exact.
func (z *Float) SetMode(mode RoundingMode) *Float {
z.mode = mode
z.acc = Exact
return z
}

// Prec returns the mantissa precision of x in bits.
// The result may be 0 for |x| == 0 and |x| == Inf.
func (x *Float) Prec() uint {
return uint(x.prec)
}

// MinPrec returns the minimum precision required to represent x exactly
// (i.e., the smallest prec before x.SetPrec(prec) would start rounding x).
// The result is 0 for |x| == 0 and |x| == Inf.
func (x *Float) MinPrec() uint {
if x.form != finite {
return 0
}
return uint(len(x.mant))*_W - x.mant.trailingZeroBits()
}

// Mode returns the rounding mode of x.
func (x *Float) Mode() RoundingMode {
return x.mode
}

// Acc returns the accuracy of x produced by the most recent operation.
func (x *Float) Acc() Accuracy {
return x.acc
}

// Sign returns:
//
//	-1 if x <   0
//	 0 if x is ±0
//	+1 if x >   0
//
func (x *Float) Sign() int {
if debugFloat {
x.validate()
}
if x.form == zero {
return 0
}
if x.neg {
return -1
}
return 1
}

// MantExp breaks x into its mantissa and exponent components
// and returns the exponent. If a non-nil mant argument is
// provided its value is set to the mantissa of x, with the
// same precision and rounding mode as x. The components
// satisfy x == mant × 2**exp, with 0.5 <= |mant| < 1.0.
// Calling MantExp with a nil argument is an efficient way to
// get the exponent of the receiver.
//
// Special cases are:
//
//	(  ±0).MantExp(mant) = 0, with mant set to   ±0
//	(±Inf).MantExp(mant) = 0, with mant set to ±Inf
//
// x and mant may be the same in which case x is set to its
// mantissa value.
func (x *Float) MantExp(mant *Float) (exp int) {
if debugFloat {
x.validate()
}
if x.form == finite {
exp = int(x.exp)
}
if mant != nil {
mant.Copy(x)
if mant.form == finite {
mant.exp = 0
}
}
return
}

func (z *Float) setExpAndRound(exp int64, sbit uint) {
if exp < MinExp {
// underflow
z.acc = makeAcc(z.neg)
z.form = zero
return
}

if exp > MaxExp {
// overflow
z.acc = makeAcc(!z.neg)
z.form = inf
return
}

z.form = finite
z.exp = int32(exp)
z.round(sbit)
}

// SetMantExp sets z to mant × 2**exp and and returns z.
// The result z has the same precision and rounding mode
// as mant. SetMantExp is an inverse of MantExp but does
// not require 0.5 <= |mant| < 1.0. Specifically:
//
//	mant := new(Float)
//	new(Float).SetMantExp(mant, x.MantExp(mant)).Cmp(x) == 0
//
// Special cases are:
//
//	z.SetMantExp(  ±0, exp) =   ±0
//	z.SetMantExp(±Inf, exp) = ±Inf
//
// z and mant may be the same in which case z's exponent
// is set to exp.
func (z *Float) SetMantExp(mant *Float, exp int) *Float {
if debugFloat {
z.validate()
mant.validate()
}
z.Copy(mant)
if z.form != finite {
return z
}
z.setExpAndRound(int64(z.exp)+int64(exp), 0)
return z
}

// Signbit returns true if x is negative or negative zero.
func (x *Float) Signbit() bool {
return x.neg
}

// IsInf reports whether x is +Inf or -Inf.
func (x *Float) IsInf() bool {
return x.form == inf
}

// IsInt reports whether x is an integer.
// ±Inf values are not integers.
func (x *Float) IsInt() bool {
if debugFloat {
x.validate()
}
// special cases
if x.form != finite {
return x.form == zero
}
// x.form == finite
if x.exp <= 0 {
return false
}
// x.exp > 0
return x.prec <= uint32(x.exp) || x.MinPrec() <= uint(x.exp) // not enough bits for fractional mantissa
}

// debugging support
func (x *Float) validate() {
if !debugFloat {
// avoid performance bugs
panic("validate called but debugFloat is not set")
}
if x.form != finite {
return
}
m := len(x.mant)
if m == 0 {
panic("nonzero finite number with empty mantissa")
}
const msb = 1 << (_W - 1)
if x.mant[m-1]&msb == 0 {
panic(fmt.Sprintf("msb not set in last word %#x of %s", x.mant[m-1], x.Text('p', 0)))
}
if x.prec == 0 {
panic("zero precision finite number")
}
}

// round rounds z according to z.mode to z.prec bits and sets z.acc accordingly.
// sbit must be 0 or 1 and summarizes any "sticky bit" information one might
// have before calling round. z's mantissa must be normalized (with the msb set)
// or empty.
//
// CAUTION: The rounding modes ToNegativeInf, ToPositiveInf are affected by the
// sign of z. For correct rounding, the sign of z must be set correctly before
// calling round.
func (z *Float) round(sbit uint) {
if debugFloat {
z.validate()
}

z.acc = Exact
if z.form != finite {
// ±0 or ±Inf => nothing left to do
return
}
// z.form == finite && len(z.mant) > 0
// m > 0 implies z.prec > 0 (checked by validate)

m := uint32(len(z.mant)) // present mantissa length in words
bits := m * _W           // present mantissa bits; bits > 0
if bits <= z.prec {
// mantissa fits => nothing to do
return
}
// bits > z.prec

// Rounding is based on two bits: the rounding bit (rbit) and the
// sticky bit (sbit). The rbit is the bit immediately before the
// z.prec leading mantissa bits (the "0.5"). The sbit is set if any
// of the bits before the rbit are set (the "0.25", "0.125", etc.):
//
//   rbit  sbit  => "fractional part"
//
//   0     0        == 0
//   0     1        >  0  , < 0.5
//   1     0        == 0.5
//   1     1        >  0.5, < 1.0

// bits > z.prec: mantissa too large => round
r := uint(bits - z.prec - 1) // rounding bit position; r >= 0
rbit := z.mant.bit(r) & 1    // rounding bit; be safe and ensure it's a single bit
// The sticky bit is only needed for rounding ToNearestEven
// or when the rounding bit is zero. Avoid computation otherwise.
if sbit == 0 && (rbit == 0 || z.mode == ToNearestEven) {
sbit = z.mant.sticky(r)
}
sbit &= 1 // be safe and ensure it's a single bit

// cut off extra words
n := (z.prec + (_W - 1)) / _W // mantissa length in words for desired precision
if m > n {
copy(z.mant, z.mant[m-n:]) // move n last words to front
z.mant = z.mant[:n]
}

// determine number of trailing zero bits (ntz) and compute lsb mask of mantissa's least-significant word
ntz := n*_W - z.prec // 0 <= ntz < _W
lsb := Word(1) << ntz

// round if result is inexact
if rbit|sbit != 0 {
// Make rounding decision: The result mantissa is truncated ("rounded down")
// by default. Decide if we need to increment, or "round up", the (unsigned)
// mantissa.
inc := false
switch z.mode {
case ToNegativeInf:
inc = z.neg
case ToZero:
// nothing to do
case ToNearestEven:
inc = rbit != 0 && (sbit != 0 || z.mant[0]&lsb != 0)
case ToNearestAway:
inc = rbit != 0
case AwayFromZero:
inc = true
case ToPositiveInf:
inc = !z.neg
default:
panic("unreachable")
}

// A positive result (!z.neg) is Above the exact result if we increment,
// and it's Below if we truncate (Exact results require no rounding).
// For a negative result (z.neg) it is exactly the opposite.
z.acc = makeAcc(inc != z.neg)

if inc {
if addVW(z.mant, z.mant, lsb) != 0 {
// mantissa overflow => adjust exponent
if z.exp >= MaxExp {
// exponent overflow
z.form = inf
return
}
z.exp++
shrVU(z.mant, z.mant, 1)
// set msb == carry == 1 from the mantissa overflow above
const msb = 1 << (_W - 1)
z.mant[n-1] |= msb
}
}
}

// zero out trailing bits in least-significant word
z.mant[0] &^= lsb - 1

if debugFloat {
z.validate()
}
}

func (z *Float) setBits64(neg bool, x uint64) *Float {
if z.prec == 0 {
z.prec = 64
}
z.acc = Exact
z.neg = neg
if x == 0 {
z.form = zero
return z
}
// x != 0
z.form = finite
z.mant = z.mant.setUint64(x << uint(s))
z.exp = int32(64 - s) // always fits
if z.prec < 64 {
z.round(0)
}
return z
}

// SetUint64 sets z to the (possibly rounded) value of x and returns z.
// If z's precision is 0, it is changed to 64 (and rounding will have
// no effect).
func (z *Float) SetUint64(x uint64) *Float {
return z.setBits64(false, x)
}

// SetInt64 sets z to the (possibly rounded) value of x and returns z.
// If z's precision is 0, it is changed to 64 (and rounding will have
// no effect).
func (z *Float) SetInt64(x int64) *Float {
u := x
if u < 0 {
u = -u
}
// We cannot simply call z.SetUint64(uint64(u)) and change
// the sign afterwards because the sign affects rounding.
return z.setBits64(x < 0, uint64(u))
}

// SetFloat64 sets z to the (possibly rounded) value of x and returns z.
// If z's precision is 0, it is changed to 53 (and rounding will have
// no effect). SetFloat64 panics with ErrNaN if x is a NaN.
func (z *Float) SetFloat64(x float64) *Float {
if z.prec == 0 {
z.prec = 53
}
if math.IsNaN(x) {
panic(ErrNaN{"Float.SetFloat64(NaN)"})
}
z.acc = Exact
z.neg = math.Signbit(x) // handle -0, -Inf correctly
if x == 0 {
z.form = zero
return z
}
if math.IsInf(x, 0) {
z.form = inf
return z
}
// normalized x != 0
z.form = finite
fmant, exp := math.Frexp(x) // get normalized mantissa
z.mant = z.mant.setUint64(1<<63 | math.Float64bits(fmant)<<11)
z.exp = int32(exp) // always fits
if z.prec < 53 {
z.round(0)
}
return z
}

// fnorm normalizes mantissa m by shifting it to the left
// such that the msb of the most-significant word (msw) is 1.
// It returns the shift amount. It assumes that len(m) != 0.
func fnorm(m nat) int64 {
if debugFloat && (len(m) == 0 || m[len(m)-1] == 0) {
panic("msw of mantissa is 0")
}
s := nlz(m[len(m)-1])
if s > 0 {
c := shlVU(m, m, s)
if debugFloat && c != 0 {
panic("nlz or shlVU incorrect")
}
}
return int64(s)
}

// SetInt sets z to the (possibly rounded) value of x and returns z.
// If z's precision is 0, it is changed to the larger of x.BitLen()
// or 64 (and rounding will have no effect).
func (z *Float) SetInt(x *Int) *Float {
// TODO(gri) can be more efficient if z.prec > 0
// but small compared to the size of x, or if there
// are many trailing 0's.
bits := uint32(x.BitLen())
if z.prec == 0 {
z.prec = umax32(bits, 64)
}
z.acc = Exact
z.neg = x.neg
if len(x.abs) == 0 {
z.form = zero
return z
}
// x != 0
z.mant = z.mant.set(x.abs)
fnorm(z.mant)
z.setExpAndRound(int64(bits), 0)
return z
}

// SetRat sets z to the (possibly rounded) value of x and returns z.
// If z's precision is 0, it is changed to the largest of a.BitLen(),
// b.BitLen(), or 64; with x = a/b.
func (z *Float) SetRat(x *Rat) *Float {
if x.IsInt() {
return z.SetInt(x.Num())
}
var a, b Float
a.SetInt(x.Num())
b.SetInt(x.Denom())
if z.prec == 0 {
z.prec = umax32(a.prec, b.prec)
}
return z.Quo(&a, &b)
}

// SetInf sets z to the infinite Float -Inf if signbit is
// set, or +Inf if signbit is not set, and returns z. The
// precision of z is unchanged and the result is always
// Exact.
func (z *Float) SetInf(signbit bool) *Float {
z.acc = Exact
z.form = inf
z.neg = signbit
return z
}

// Set sets z to the (possibly rounded) value of x and returns z.
// If z's precision is 0, it is changed to the precision of x
// before setting z (and rounding will have no effect).
// Rounding is performed according to z's precision and rounding
// mode; and z's accuracy reports the result error relative to the
// exact (not rounded) result.
func (z *Float) Set(x *Float) *Float {
if debugFloat {
x.validate()
}
z.acc = Exact
if z != x {
z.form = x.form
z.neg = x.neg
if x.form == finite {
z.exp = x.exp
z.mant = z.mant.set(x.mant)
}
if z.prec == 0 {
z.prec = x.prec
} else if z.prec < x.prec {
z.round(0)
}
}
return z
}

// Copy sets z to x, with the same precision, rounding mode, and
// accuracy as x, and returns z. x is not changed even if z and
// x are the same.
func (z *Float) Copy(x *Float) *Float {
if debugFloat {
x.validate()
}
if z != x {
z.prec = x.prec
z.mode = x.mode
z.acc = x.acc
z.form = x.form
z.neg = x.neg
if z.form == finite {
z.mant = z.mant.set(x.mant)
z.exp = x.exp
}
}
return z
}

// msb32 returns the 32 most significant bits of x.
func msb32(x nat) uint32 {
i := len(x) - 1
if i < 0 {
return 0
}
if debugFloat && x[i]&(1<<(_W-1)) == 0 {
panic("x not normalized")
}
switch _W {
case 32:
return uint32(x[i])
case 64:
return uint32(x[i] >> 32)
}
panic("unreachable")
}

// msb64 returns the 64 most significant bits of x.
func msb64(x nat) uint64 {
i := len(x) - 1
if i < 0 {
return 0
}
if debugFloat && x[i]&(1<<(_W-1)) == 0 {
panic("x not normalized")
}
switch _W {
case 32:
v := uint64(x[i]) << 32
if i > 0 {
v |= uint64(x[i-1])
}
return v
case 64:
return uint64(x[i])
}
panic("unreachable")
}

// Uint64 returns the unsigned integer resulting from truncating x
// towards zero. If 0 <= x <= math.MaxUint64, the result is Exact
// if x is an integer and Below otherwise.
// The result is (0, Above) for x < 0, and (math.MaxUint64, Below)
// for x > math.MaxUint64.
func (x *Float) Uint64() (uint64, Accuracy) {
if debugFloat {
x.validate()
}

switch x.form {
case finite:
if x.neg {
return 0, Above
}
// 0 < x < +Inf
if x.exp <= 0 {
// 0 < x < 1
return 0, Below
}
// 1 <= x < Inf
if x.exp <= 64 {
// u = trunc(x) fits into a uint64
u := msb64(x.mant) >> (64 - uint32(x.exp))
if x.MinPrec() <= 64 {
return u, Exact
}
return u, Below // x truncated
}
// x too large
return math.MaxUint64, Below

case zero:
return 0, Exact

case inf:
if x.neg {
return 0, Above
}
return math.MaxUint64, Below
}

panic("unreachable")
}

// Int64 returns the integer resulting from truncating x towards zero.
// If math.MinInt64 <= x <= math.MaxInt64, the result is Exact if x is
// an integer, and Above (x < 0) or Below (x > 0) otherwise.
// The result is (math.MinInt64, Above) for x < math.MinInt64,
// and (math.MaxInt64, Below) for x > math.MaxInt64.
func (x *Float) Int64() (int64, Accuracy) {
if debugFloat {
x.validate()
}

switch x.form {
case finite:
// 0 < |x| < +Inf
acc := makeAcc(x.neg)
if x.exp <= 0 {
// 0 < |x| < 1
return 0, acc
}
// x.exp > 0

// 1 <= |x| < +Inf
if x.exp <= 63 {
// i = trunc(x) fits into an int64 (excluding math.MinInt64)
i := int64(msb64(x.mant) >> (64 - uint32(x.exp)))
if x.neg {
i = -i
}
if x.MinPrec() <= uint(x.exp) {
return i, Exact
}
return i, acc // x truncated
}
if x.neg {
// check for special case x == math.MinInt64 (i.e., x == -(0.5 << 64))
if x.exp == 64 && x.MinPrec() == 1 {
acc = Exact
}
return math.MinInt64, acc
}
// x too large
return math.MaxInt64, Below

case zero:
return 0, Exact

case inf:
if x.neg {
return math.MinInt64, Above
}
return math.MaxInt64, Below
}

panic("unreachable")
}

// Float32 returns the float32 value nearest to x. If x is too small to be
// represented by a float32 (|x| < math.SmallestNonzeroFloat32), the result
// is (0, Below) or (-0, Above), respectively, depending on the sign of x.
// If x is too large to be represented by a float32 (|x| > math.MaxFloat32),
// the result is (+Inf, Above) or (-Inf, Below), depending on the sign of x.
func (x *Float) Float32() (float32, Accuracy) {
if debugFloat {
x.validate()
}

switch x.form {
case finite:
// 0 < |x| < +Inf

const (
fbits = 32                //        float size
mbits = 23                //        mantissa size (excluding implicit msb)
ebits = fbits - mbits - 1 //     8  exponent size
bias  = 1<<(ebits-1) - 1  //   127  exponent bias
dmin  = 1 - bias - mbits  //  -149  smallest unbiased exponent (denormal)
emin  = 1 - bias          //  -126  smallest unbiased exponent (normal)
emax  = bias              //   127  largest unbiased exponent (normal)
)

// Float mantissa m is 0.5 <= m < 1.0; compute exponent e for float32 mantissa.
e := x.exp - 1 // exponent for normal mantissa m with 1.0 <= m < 2.0

// Compute precision p for float32 mantissa.
// If the exponent is too small, we have a denormal number before
// rounding and fewer than p mantissa bits of precision available
// (the exponent remains fixed but the mantissa gets shifted right).
p := mbits + 1 // precision of normal float
if e < emin {
// recompute precision
p = mbits + 1 - emin + int(e)
// If p == 0, the mantissa of x is shifted so much to the right
// that its msb falls immediately to the right of the float32
// mantissa space. In other words, if the smallest denormal is
// considered "1.0", for p == 0, the mantissa value m is >= 0.5.
// If m > 0.5, it is rounded up to 1.0; i.e., the smallest denormal.
// If m == 0.5, it is rounded down to even, i.e., 0.0.
// If p < 0, the mantissa value m is <= "0.25" which is never rounded up.
if p < 0 /* m <= 0.25 */ || p == 0 && x.mant.sticky(uint(len(x.mant))*_W-1) == 0 /* m == 0.5 */ {
// underflow to ±0
if x.neg {
var z float32
return -z, Above
}
return 0.0, Below
}
// otherwise, round up
// We handle p == 0 explicitly because it's easy and because
// Float.round doesn't support rounding to 0 bits of precision.
if p == 0 {
if x.neg {
return -math.SmallestNonzeroFloat32, Below
}
return math.SmallestNonzeroFloat32, Above
}
}
// p > 0

// round
var r Float
r.prec = uint32(p)
r.Set(x)
e = r.exp - 1

// Rounding may have caused r to overflow to ±Inf
// (rounding never causes underflows to 0).
// If the exponent is too large, also overflow to ±Inf.
if r.form == inf || e > emax {
// overflow
if x.neg {
return float32(math.Inf(-1)), Below
}
return float32(math.Inf(+1)), Above
}
// e <= emax

// Determine sign, biased exponent, and mantissa.
var sign, bexp, mant uint32
if x.neg {
sign = 1 << (fbits - 1)
}

// Rounding may have caused a denormal number to
// become normal. Check again.
if e < emin {
// denormal number: recompute precision
// Since rounding may have at best increased precision
// and we have eliminated p <= 0 early, we know p > 0.
// bexp == 0 for denormals
p = mbits + 1 - emin + int(e)
mant = msb32(r.mant) >> uint(fbits-p)
} else {
// normal number: emin <= e <= emax
bexp = uint32(e+bias) << mbits
mant = msb32(r.mant) >> ebits & (1<<mbits - 1) // cut off msb (implicit 1 bit)
}

return math.Float32frombits(sign | bexp | mant), r.acc

case zero:
if x.neg {
var z float32
return -z, Exact
}
return 0.0, Exact

case inf:
if x.neg {
return float32(math.Inf(-1)), Exact
}
return float32(math.Inf(+1)), Exact
}

panic("unreachable")
}

// Float64 returns the float64 value nearest to x. If x is too small to be
// represented by a float64 (|x| < math.SmallestNonzeroFloat64), the result
// is (0, Below) or (-0, Above), respectively, depending on the sign of x.
// If x is too large to be represented by a float64 (|x| > math.MaxFloat64),
// the result is (+Inf, Above) or (-Inf, Below), depending on the sign of x.
func (x *Float) Float64() (float64, Accuracy) {
if debugFloat {
x.validate()
}

switch x.form {
case finite:
// 0 < |x| < +Inf

const (
fbits = 64                //        float size
mbits = 52                //        mantissa size (excluding implicit msb)
ebits = fbits - mbits - 1 //    11  exponent size
bias  = 1<<(ebits-1) - 1  //  1023  exponent bias
dmin  = 1 - bias - mbits  // -1074  smallest unbiased exponent (denormal)
emin  = 1 - bias          // -1022  smallest unbiased exponent (normal)
emax  = bias              //  1023  largest unbiased exponent (normal)
)

// Float mantissa m is 0.5 <= m < 1.0; compute exponent e for float64 mantissa.
e := x.exp - 1 // exponent for normal mantissa m with 1.0 <= m < 2.0

// Compute precision p for float64 mantissa.
// If the exponent is too small, we have a denormal number before
// rounding and fewer than p mantissa bits of precision available
// (the exponent remains fixed but the mantissa gets shifted right).
p := mbits + 1 // precision of normal float
if e < emin {
// recompute precision
p = mbits + 1 - emin + int(e)
// If p == 0, the mantissa of x is shifted so much to the right
// that its msb falls immediately to the right of the float64
// mantissa space. In other words, if the smallest denormal is
// considered "1.0", for p == 0, the mantissa value m is >= 0.5.
// If m > 0.5, it is rounded up to 1.0; i.e., the smallest denormal.
// If m == 0.5, it is rounded down to even, i.e., 0.0.
// If p < 0, the mantissa value m is <= "0.25" which is never rounded up.
if p < 0 /* m <= 0.25 */ || p == 0 && x.mant.sticky(uint(len(x.mant))*_W-1) == 0 /* m == 0.5 */ {
// underflow to ±0
if x.neg {
var z float64
return -z, Above
}
return 0.0, Below
}
// otherwise, round up
// We handle p == 0 explicitly because it's easy and because
// Float.round doesn't support rounding to 0 bits of precision.
if p == 0 {
if x.neg {
return -math.SmallestNonzeroFloat64, Below
}
return math.SmallestNonzeroFloat64, Above
}
}
// p > 0

// round
var r Float
r.prec = uint32(p)
r.Set(x)
e = r.exp - 1

// Rounding may have caused r to overflow to ±Inf
// (rounding never causes underflows to 0).
// If the exponent is too large, also overflow to ±Inf.
if r.form == inf || e > emax {
// overflow
if x.neg {
return math.Inf(-1), Below
}
return math.Inf(+1), Above
}
// e <= emax

// Determine sign, biased exponent, and mantissa.
var sign, bexp, mant uint64
if x.neg {
sign = 1 << (fbits - 1)
}

// Rounding may have caused a denormal number to
// become normal. Check again.
if e < emin {
// denormal number: recompute precision
// Since rounding may have at best increased precision
// and we have eliminated p <= 0 early, we know p > 0.
// bexp == 0 for denormals
p = mbits + 1 - emin + int(e)
mant = msb64(r.mant) >> uint(fbits-p)
} else {
// normal number: emin <= e <= emax
bexp = uint64(e+bias) << mbits
mant = msb64(r.mant) >> ebits & (1<<mbits - 1) // cut off msb (implicit 1 bit)
}

return math.Float64frombits(sign | bexp | mant), r.acc

case zero:
if x.neg {
var z float64
return -z, Exact
}
return 0.0, Exact

case inf:
if x.neg {
return math.Inf(-1), Exact
}
return math.Inf(+1), Exact
}

panic("unreachable")
}

// Int returns the result of truncating x towards zero;
// or nil if x is an infinity.
// The result is Exact if x.IsInt(); otherwise it is Below
// for x > 0, and Above for x < 0.
// If a non-nil *Int argument z is provided, Int stores
// the result in z instead of allocating a new Int.
func (x *Float) Int(z *Int) (*Int, Accuracy) {
if debugFloat {
x.validate()
}

if z == nil && x.form <= finite {
z = new(Int)
}

switch x.form {
case finite:
// 0 < |x| < +Inf
acc := makeAcc(x.neg)
if x.exp <= 0 {
// 0 < |x| < 1
return z.SetInt64(0), acc
}
// x.exp > 0

// 1 <= |x| < +Inf
// determine minimum required precision for x
allBits := uint(len(x.mant)) * _W
exp := uint(x.exp)
if x.MinPrec() <= exp {
acc = Exact
}
// shift mantissa as needed
if z == nil {
z = new(Int)
}
z.neg = x.neg
switch {
case exp > allBits:
z.abs = z.abs.shl(x.mant, exp-allBits)
default:
z.abs = z.abs.set(x.mant)
case exp < allBits:
z.abs = z.abs.shr(x.mant, allBits-exp)
}
return z, acc

case zero:
return z.SetInt64(0), Exact

case inf:
return nil, makeAcc(x.neg)
}

panic("unreachable")
}

// Rat returns the rational number corresponding to x;
// or nil if x is an infinity.
// The result is Exact if x is not an Inf.
// If a non-nil *Rat argument z is provided, Rat stores
// the result in z instead of allocating a new Rat.
func (x *Float) Rat(z *Rat) (*Rat, Accuracy) {
if debugFloat {
x.validate()
}

if z == nil && x.form <= finite {
z = new(Rat)
}

switch x.form {
case finite:
// 0 < |x| < +Inf
allBits := int32(len(x.mant)) * _W
// build up numerator and denominator
z.a.neg = x.neg
switch {
case x.exp > allBits:
z.a.abs = z.a.abs.shl(x.mant, uint(x.exp-allBits))
z.b.abs = z.b.abs[:0] // == 1 (see Rat)
// z already in normal form
default:
z.a.abs = z.a.abs.set(x.mant)
z.b.abs = z.b.abs[:0] // == 1 (see Rat)
// z already in normal form
case x.exp < allBits:
z.a.abs = z.a.abs.set(x.mant)
t := z.b.abs.setUint64(1)
z.b.abs = t.shl(t, uint(allBits-x.exp))
z.norm()
}
return z, Exact

case zero:
return z.SetInt64(0), Exact

case inf:
return nil, makeAcc(x.neg)
}

panic("unreachable")
}

// Abs sets z to the (possibly rounded) value |x| (the absolute value of x)
// and returns z.
func (z *Float) Abs(x *Float) *Float {
z.Set(x)
z.neg = false
return z
}

// Neg sets z to the (possibly rounded) value of x with its sign negated,
// and returns z.
func (z *Float) Neg(x *Float) *Float {
z.Set(x)
z.neg = !z.neg
return z
}

func validateBinaryOperands(x, y *Float) {
if !debugFloat {
// avoid performance bugs
panic("validateBinaryOperands called but debugFloat is not set")
}
if len(x.mant) == 0 {
panic("empty mantissa for x")
}
if len(y.mant) == 0 {
panic("empty mantissa for y")
}
}

// z = x + y, ignoring signs of x and y for the addition
// but using the sign of z for rounding the result.
// x and y must have a non-empty mantissa and valid exponent.
func (z *Float) uadd(x, y *Float) {
// Note: This implementation requires 2 shifts most of the
// time. It is also inefficient if exponents or precisions
// differ by wide margins. The following article describes
// an efficient (but much more complicated) implementation
// compatible with the internal representation used here:
//
// Vincent Lefèvre: "The Generic Multiple-Precision Floating-
// Point Addition With Exact Rounding (as in the MPFR Library)"
// http://www.vinc17.net/research/papers/rnc6.pdf

if debugFloat {
validateBinaryOperands(x, y)
}

// compute exponents ex, ey for mantissa with "binary point"
// on the right (mantissa.0) - use int64 to avoid overflow
ex := int64(x.exp) - int64(len(x.mant))*_W
ey := int64(y.exp) - int64(len(y.mant))*_W

al := alias(z.mant, x.mant) || alias(z.mant, y.mant)

// TODO(gri) having a combined add-and-shift primitive
//           could make this code significantly faster
switch {
case ex < ey:
if al {
t := nat(nil).shl(y.mant, uint(ey-ex))
} else {
z.mant = z.mant.shl(y.mant, uint(ey-ex))
}
default:
// ex == ey, no shift needed
case ex > ey:
if al {
t := nat(nil).shl(x.mant, uint(ex-ey))
} else {
z.mant = z.mant.shl(x.mant, uint(ex-ey))
}
ex = ey
}
// len(z.mant) > 0

z.setExpAndRound(ex+int64(len(z.mant))*_W-fnorm(z.mant), 0)
}

// z = x - y for |x| > |y|, ignoring signs of x and y for the subtraction
// but using the sign of z for rounding the result.
// x and y must have a non-empty mantissa and valid exponent.
func (z *Float) usub(x, y *Float) {
// This code is symmetric to uadd.
// We have not factored the common code out because
// eventually uadd (and usub) should be optimized
// by special-casing, and the code will diverge.

if debugFloat {
validateBinaryOperands(x, y)
}

ex := int64(x.exp) - int64(len(x.mant))*_W
ey := int64(y.exp) - int64(len(y.mant))*_W

al := alias(z.mant, x.mant) || alias(z.mant, y.mant)

switch {
case ex < ey:
if al {
t := nat(nil).shl(y.mant, uint(ey-ex))
z.mant = t.sub(x.mant, t)
} else {
z.mant = z.mant.shl(y.mant, uint(ey-ex))
z.mant = z.mant.sub(x.mant, z.mant)
}
default:
// ex == ey, no shift needed
z.mant = z.mant.sub(x.mant, y.mant)
case ex > ey:
if al {
t := nat(nil).shl(x.mant, uint(ex-ey))
z.mant = t.sub(t, y.mant)
} else {
z.mant = z.mant.shl(x.mant, uint(ex-ey))
z.mant = z.mant.sub(z.mant, y.mant)
}
ex = ey
}

// operands may have canceled each other out
if len(z.mant) == 0 {
z.acc = Exact
z.form = zero
z.neg = false
return
}
// len(z.mant) > 0

z.setExpAndRound(ex+int64(len(z.mant))*_W-fnorm(z.mant), 0)
}

// z = x * y, ignoring signs of x and y for the multiplication
// but using the sign of z for rounding the result.
// x and y must have a non-empty mantissa and valid exponent.
func (z *Float) umul(x, y *Float) {
if debugFloat {
validateBinaryOperands(x, y)
}

// Note: This is doing too much work if the precision
// of z is less than the sum of the precisions of x
// and y which is often the case (e.g., if all floats
// have the same precision).
// TODO(gri) Optimize this for the common case.

e := int64(x.exp) + int64(y.exp)
if x == y {
z.mant = z.mant.sqr(x.mant)
} else {
z.mant = z.mant.mul(x.mant, y.mant)
}
z.setExpAndRound(e-fnorm(z.mant), 0)
}

// z = x / y, ignoring signs of x and y for the division
// but using the sign of z for rounding the result.
// x and y must have a non-empty mantissa and valid exponent.
func (z *Float) uquo(x, y *Float) {
if debugFloat {
validateBinaryOperands(x, y)
}

// mantissa length in words for desired result precision + 1
// (at least one extra bit so we get the rounding bit after
// the division)
n := int(z.prec/_W) + 1

// compute adjusted x.mant such that we get enough result precision
if d := n - len(x.mant) + len(y.mant); d > 0 {
// d extra words needed => add d "0 digits" to x
}
// TODO(gri): If we have too many digits (d < 0), we should be able
// to shorten x for faster division. But we must be extra careful
// with rounding in that case.

// Compute d before division since there may be aliasing of x.mant
// (via xadj) or y.mant with z.mant.

// divide
var r nat
z.mant, r = z.mant.div(nil, xadj, y.mant)
e := int64(x.exp) - int64(y.exp) - int64(d-len(z.mant))*_W

// The result is long enough to include (at least) the rounding bit.
// If there's a non-zero remainder, the corresponding fractional part
// (if it were computed), would have a non-zero sticky bit (if it were
// zero, it couldn't have a non-zero remainder).
var sbit uint
if len(r) > 0 {
sbit = 1
}

z.setExpAndRound(e-fnorm(z.mant), sbit)
}

// ucmp returns -1, 0, or +1, depending on whether
// |x| < |y|, |x| == |y|, or |x| > |y|.
// x and y must have a non-empty mantissa and valid exponent.
func (x *Float) ucmp(y *Float) int {
if debugFloat {
validateBinaryOperands(x, y)
}

switch {
case x.exp < y.exp:
return -1
case x.exp > y.exp:
return +1
}
// x.exp == y.exp

// compare mantissas
i := len(x.mant)
j := len(y.mant)
for i > 0 || j > 0 {
var xm, ym Word
if i > 0 {
i--
xm = x.mant[i]
}
if j > 0 {
j--
ym = y.mant[j]
}
switch {
case xm < ym:
return -1
case xm > ym:
return +1
}
}

return 0
}

// Handling of sign bit as defined by IEEE 754-2008, section 6.3:
//
// When neither the inputs nor result are NaN, the sign of a product or
// quotient is the exclusive OR of the operands’ signs; the sign of a sum,
// or of a difference x−y regarded as a sum x+(−y), differs from at most
// one of the addends’ signs; and the sign of the result of conversions,
// the quantize operation, the roundToIntegral operations, and the
// roundToIntegralExact (see 5.3.1) is the sign of the first or only operand.
// These rules shall apply even when operands or results are zero or infinite.
//
// When the sum of two operands with opposite signs (or the difference of
// two operands with like signs) is exactly zero, the sign of that sum (or
// difference) shall be +0 in all rounding-direction attributes except
// roundTowardNegative; under that attribute, the sign of an exact zero
// sum (or difference) shall be −0. However, x+x = x−(−x) retains the same
// sign as x even when x is zero.
//

// Add sets z to the rounded sum x+y and returns z. If z's precision is 0,
// it is changed to the larger of x's or y's precision before the operation.
// Rounding is performed according to z's precision and rounding mode; and
// z's accuracy reports the result error relative to the exact (not rounded)
// result. Add panics with ErrNaN if x and y are infinities with opposite
// signs. The value of z is undefined in that case.
//
// BUG(gri) When rounding ToNegativeInf, the sign of Float values rounded to 0 is incorrect.
func (z *Float) Add(x, y *Float) *Float {
if debugFloat {
x.validate()
y.validate()
}

if z.prec == 0 {
z.prec = umax32(x.prec, y.prec)
}

if x.form == finite && y.form == finite {
// x + y (common case)

// Below we set z.neg = x.neg, and when z aliases y this will
// change the y operand's sign. This is fine, because if an
// operand aliases the receiver it'll be overwritten, but we still
// want the original x.neg and y.neg values when we evaluate
// x.neg != y.neg, so we need to save y.neg before setting z.neg.
yneg := y.neg

z.neg = x.neg
if x.neg == yneg {
// x + y == x + y
// (-x) + (-y) == -(x + y)
} else {
// x + (-y) == x - y == -(y - x)
// (-x) + y == y - x == -(x - y)
if x.ucmp(y) > 0 {
z.usub(x, y)
} else {
z.neg = !z.neg
z.usub(y, x)
}
}
return z
}

if x.form == inf && y.form == inf && x.neg != y.neg {
// +Inf + -Inf
// -Inf + +Inf
// value of z is undefined but make sure it's valid
z.acc = Exact
z.form = zero
z.neg = false
panic(ErrNaN{"addition of infinities with opposite signs"})
}

if x.form == zero && y.form == zero {
// ±0 + ±0
z.acc = Exact
z.form = zero
z.neg = x.neg && y.neg // -0 + -0 == -0
return z
}

if x.form == inf || y.form == zero {
// ±Inf + y
// x + ±0
return z.Set(x)
}

// ±0 + y
// x + ±Inf
return z.Set(y)
}

// Sub sets z to the rounded difference x-y and returns z.
// Precision, rounding, and accuracy reporting are as for Add.
// Sub panics with ErrNaN if x and y are infinities with equal
// signs. The value of z is undefined in that case.
func (z *Float) Sub(x, y *Float) *Float {
if debugFloat {
x.validate()
y.validate()
}

if z.prec == 0 {
z.prec = umax32(x.prec, y.prec)
}

if x.form == finite && y.form == finite {
// x - y (common case)
yneg := y.neg
z.neg = x.neg
if x.neg != yneg {
// x - (-y) == x + y
// (-x) - y == -(x + y)
} else {
// x - y == x - y == -(y - x)
// (-x) - (-y) == y - x == -(x - y)
if x.ucmp(y) > 0 {
z.usub(x, y)
} else {
z.neg = !z.neg
z.usub(y, x)
}
}
return z
}

if x.form == inf && y.form == inf && x.neg == y.neg {
// +Inf - +Inf
// -Inf - -Inf
// value of z is undefined but make sure it's valid
z.acc = Exact
z.form = zero
z.neg = false
panic(ErrNaN{"subtraction of infinities with equal signs"})
}

if x.form == zero && y.form == zero {
// ±0 - ±0
z.acc = Exact
z.form = zero
z.neg = x.neg && !y.neg // -0 - +0 == -0
return z
}

if x.form == inf || y.form == zero {
// ±Inf - y
// x - ±0
return z.Set(x)
}

// ±0 - y
// x - ±Inf
return z.Neg(y)
}

// Mul sets z to the rounded product x*y and returns z.
// Precision, rounding, and accuracy reporting are as for Add.
// Mul panics with ErrNaN if one operand is zero and the other
// operand an infinity. The value of z is undefined in that case.
func (z *Float) Mul(x, y *Float) *Float {
if debugFloat {
x.validate()
y.validate()
}

if z.prec == 0 {
z.prec = umax32(x.prec, y.prec)
}

z.neg = x.neg != y.neg

if x.form == finite && y.form == finite {
// x * y (common case)
z.umul(x, y)
return z
}

z.acc = Exact
if x.form == zero && y.form == inf || x.form == inf && y.form == zero {
// ±0 * ±Inf
// ±Inf * ±0
// value of z is undefined but make sure it's valid
z.form = zero
z.neg = false
panic(ErrNaN{"multiplication of zero with infinity"})
}

if x.form == inf || y.form == inf {
// ±Inf * y
// x * ±Inf
z.form = inf
return z
}

// ±0 * y
// x * ±0
z.form = zero
return z
}

// Quo sets z to the rounded quotient x/y and returns z.
// Precision, rounding, and accuracy reporting are as for Add.
// Quo panics with ErrNaN if both operands are zero or infinities.
// The value of z is undefined in that case.
func (z *Float) Quo(x, y *Float) *Float {
if debugFloat {
x.validate()
y.validate()
}

if z.prec == 0 {
z.prec = umax32(x.prec, y.prec)
}

z.neg = x.neg != y.neg

if x.form == finite && y.form == finite {
// x / y (common case)
z.uquo(x, y)
return z
}

z.acc = Exact
if x.form == zero && y.form == zero || x.form == inf && y.form == inf {
// ±0 / ±0
// ±Inf / ±Inf
// value of z is undefined but make sure it's valid
z.form = zero
z.neg = false
panic(ErrNaN{"division of zero by zero or infinity by infinity"})
}

if x.form == zero || y.form == inf {
// ±0 / y
// x / ±Inf
z.form = zero
return z
}

// x / ±0
// ±Inf / y
z.form = inf
return z
}

// Cmp compares x and y and returns:
//
//   -1 if x <  y
//    0 if x == y (incl. -0 == 0, -Inf == -Inf, and +Inf == +Inf)
//   +1 if x >  y
//
func (x *Float) Cmp(y *Float) int {
if debugFloat {
x.validate()
y.validate()
}

mx := x.ord()
my := y.ord()
switch {
case mx < my:
return -1
case mx > my:
return +1
}
// mx == my

// only if |mx| == 1 we have to compare the mantissae
switch mx {
case -1:
return y.ucmp(x)
case +1:
return x.ucmp(y)
}

return 0
}

// ord classifies x and returns:
//
//	-2 if -Inf == x
//	-1 if -Inf < x < 0
//	 0 if x == 0 (signed or unsigned)
//	+1 if 0 < x < +Inf
//	+2 if x == +Inf
//
func (x *Float) ord() int {
var m int
switch x.form {
case finite:
m = 1
case zero:
return 0
case inf:
m = 2
}
if x.neg {
m = -m
}
return m
}

func umax32(x, y uint32) uint32 {
if x > y {
return x
}
return y
}

```

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