math/big: cleaner handling of exponent under/overflow

Fixed several corner-case bugs and added corresponding tests.

Change-Id: I23096b9caeeff0956f65ab59fa91e168d0e47bb8
Reviewed-on: https://go-review.googlesource.com/7001
Reviewed-by: Alan Donovan <adonovan@google.com>

1 files changed, 8 insertions, 4 deletions

diff --git a/src/math/big/floatconv.go b/src/math/big/floatconv.go
index a3d0ff97a8..f6a78b794c 100644
--- a/src/math/big/floatconv.go
+++ b/src/math/big/floatconv.go
@@ -101,8 +101,6 @@ func (z *Float) Scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
 	}
 	// len(z.mant) > 0
 
-	z.form = finite
-
 	// The mantissa may have a decimal point (fcount <= 0) and there
 	// may be a nonzero exponent exp. The decimal point amounts to a
 	// division by b**(-fcount). An exponent means multiplication by
@@ -142,7 +140,14 @@ func (z *Float) Scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
 	// we don't need exp anymore
 
 	// apply 2**exp2
-	z.setExp(exp2)
+	if MinExp <= exp2 && exp2 <= MaxExp {
+		z.form = finite
+		z.exp = int32(exp2)
+	} else {
+		f = nil
+		err = fmt.Errorf("exponent overflow")
+		return
+	}
 
 	if exp10 == 0 {
 		// no decimal exponent to consider
@@ -160,7 +165,6 @@ func (z *Float) Scan(r io.ByteScanner, base int) (f *Float, b int, err error) {
 	fpowTen := new(Float).SetInt(new(Int).SetBits(powTen))
 
 	// apply 10**exp10
-	// (uquo and umul do the rounding)
 	if exp10 < 0 {
 		z.uquo(z, fpowTen)
 	} else {