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math/big: When result prec == 0, use at least prec == 64 for SetInt, SetRat.

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This avoids surprises.

Change-Id: Iaae67da2d12e29c4e797ad6313e0895f7ce80cb1
Reviewed-on: https://go-review.googlesource.com/4480
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
This commit is contained in:
Robert Griesemer 2015-02-10 11:49:54 -08:00
parent a15818fed3
commit 950aa9f1bc
2 changed files with 35 additions and 11 deletions
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13
src/math/big/float.go
View File

@ -567,15 +567,15 @@ func fnorm(m nat) uint {
}
// SetInt sets z to the (possibly rounded) value of x and returns z.
// If z's precision is 0, it is changed to x.BitLen() (and rounding will have
// no effect).
// 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 := uint(x.BitLen())
if z.prec == 0 {
z.prec = bits
z.prec = umax(bits, 64)
}
z.acc = Exact
z.neg = x.neg
@ -595,9 +595,8 @@ func (z *Float) SetInt(x *Int) *Float {
}
// SetRat 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 a.BitLen()
// and b.BitLen(), where a and b are the numerator and denominator
// of x, respectively (x = a/b).
// 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 {
// TODO(gri) can be more efficient if x is an integer
var a, b Float
@ -1110,6 +1109,7 @@ func (z *Float) Rsh(x *Float, s uint, mode RoundingMode) *Float {
// 0 if x == y (incl. -0 == 0)
// +1 if x > y
//
// Infinities with matching sign are equal.
func (x *Float) Cmp(y *Float) int {
if debugFloat {
x.validate()
@ -1118,7 +1118,6 @@ func (x *Float) Cmp(y *Float) int {
mx := x.mag()
my := y.mag()
switch {
case mx < my:
return -1

33
src/math/big/float_test.go
View File

@ -490,8 +490,20 @@ func TestFloatSetInt(t *testing.T) {
t.Errorf("invalid integer %s", want)
continue
}
n := x.BitLen()
var f Float
f.SetInt(&x)
// check precision
if n < 64 {
n = 64
}
if prec := f.Precision(); prec != uint(n) {
t.Errorf("got prec = %d; want %d", prec, n)
}
// check value
got := f.Format('g', 100)
if got != want {
t.Errorf("got %s (%s); want %s", got, f.Format('p', 0), want)
@ -519,11 +531,24 @@ func TestFloatSetRat(t *testing.T) {
t.Errorf("invalid fraction %s", want)
continue
}
f := NewFloat(0, 1000, 0) // set a high precision - TODO(gri) find a cleaner way
f.SetRat(&x)
got := f.Format('g', 100)
n := max(x.Num().BitLen(), x.Denom().BitLen())
var f1 Float
var f2 = NewFloat(0, 1000, 0) // set a high precision - TODO(gri) find a cleaner way
f1.SetRat(&x)
f2.SetRat(&x)
// check precision when set automatically
if n < 64 {
n = 64
}
if prec := f1.Precision(); prec != uint(n) {
t.Errorf("got prec = %d; want %d", prec, n)
}
got := f2.Format('g', 100)
if got != want {
t.Errorf("got %s (%s); want %s", got, f.Format('p', 0), want)
t.Errorf("got %s (%s); want %s", got, f2.Format('p', 0), want)
}
}
}