const (
	// Maximum size in bits for big.Ints before signaling
	// overflow and also mantissa precision for big.Floats.
	ConstPrec = 512
)

func BigFloat(v constant.Value) *big.Float {
	f := new(big.Float)
	f.SetPrec(ConstPrec)
	switch u := constant.Val(v).(type) {
	case int64:
		f.SetInt64(u)
	case *big.Int:
		f.SetInt(u)
	case *big.Float:
		f.Set(u)
	case *big.Rat:
		f.SetRat(u)
	default:

	if mode == ToNearestAway {
		panic("not yet implemented")
	}
	if mode == ToNearestEven && rbit == 1 && (sbit == 1 || sbit == 0 && bit0 != 0) || mode == AwayFromZero {
		// round away from zero
		f.SetMode(ToZero).SetPrec(prec)
		f.Add(f, Bits{int(r) + 1}.Float())
	}
	return f
}

// Float returns the *Float z of the smallest possible precision such that
// z = sum(2**bits[i]), with i = range bits. If multiple bits[i] are equal,

//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 {

	}

	// round mantissa to n bits
	var n uint
	if prec < 0 {
		n = 1 + (x.MinPrec()-1+3)/4*4 // round MinPrec up to 1 mod 4
	} else {
		n = 1 + 4*uint(prec)
	}
	// n%4 == 1
	x = new(Float).SetPrec(n).SetMode(x.mode).Set(x)

	// adjust mantissa to use exactly n bits
	m := x.mant
	switch w := uint(len(x.mant)) * _W; {
	case w < n:
		m = nat(nil).shl(m, n-w)
	case w > n:
		m = nat(nil).shr(m, w-n)

func (r *Decoder) bigInt() *big.Int {
	v := new(big.Int).SetBytes([]byte(r.String()))
	if r.Bool() {
		v.Neg(v)
	}
	return v
}

func (r *Decoder) bigFloat() *big.Float {
	v := new(big.Float).SetPrec(512)
	assert(v.UnmarshalText([]byte(r.String())) == nil)
	return v
}

// @@@ Helpers

// TODO(mdempsky): These should probably be removed. I think they're a

func (complexVal) implementsValue() {}

func newInt() *big.Int     { return new(big.Int) }
func newRat() *big.Rat     { return new(big.Rat) }
func newFloat() *big.Float { return new(big.Float).SetPrec(prec) }

func i64toi(x int64Val) intVal   { return intVal{newInt().SetInt64(int64(x))} }
func i64tor(x int64Val) ratVal   { return ratVal{newRat().SetInt64(int64(x))} }

pkg math/big, method (*Float) SetInt64(int64) *Float
pkg math/big, method (*Float) SetMantExp(*Float, int) *Float
pkg math/big, method (*Float) SetMode(RoundingMode) *Float
pkg math/big, method (*Float) SetPrec(uint) *Float
pkg math/big, method (*Float) SetRat(*Rat) *Float
pkg math/big, method (*Float) SetString(string) (*Float, bool)
pkg math/big, method (*Float) SetUint64(uint64) *Float
pkg math/big, method (*Float) Sign() int

		{"(*Float).SetInf", Method, 5},
		{"(*Float).SetInt", Method, 5},
		{"(*Float).SetInt64", Method, 5},
		{"(*Float).SetMantExp", Method, 5},
		{"(*Float).SetMode", Method, 5},
		{"(*Float).SetPrec", Method, 5},
		{"(*Float).SetRat", Method, 5},
		{"(*Float).SetString", Method, 5},
		{"(*Float).SetUint64", Method, 5},
		{"(*Float).Sign", Method, 5},
		{"(*Float).Signbit", Method, 5},