math/big: change Float.SetMantExp to always multiply mant by 2**exp

Change-Id: If840e647376a2141f8c17729f7ef251bfff13f5f
Reviewed-on: https://go-review.googlesource.com/5810
Reviewed-by: Rob Pike <r@golang.org>

diff --git a/src/math/big/float.go b/src/math/big/float.go
index a89ef1021a9ba4d2a66e7f06439001bc479c7caa..015f1645b61b1d153c7a3dab38e6cb28a0ed4af6 100644 (file)
--- a/src/math/big/float.go
+++ b/src/math/big/float.go
@@ -219,22 +219,24 @@ func (x *Float) MantExp() (mant *Float, exp int) {
        return
 }
 
-// SetMantExp is the inverse of MantExp. It sets z to mant × 2**exp and
-// and returns z. The result z has the same precision and rounding mode
-// as mant.
+// 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:
+//
+//     new(Float).SetMantExp(x.MantExp()).Cmp(x) == 0
 //
 // Special cases are:
 //
 //     z.SetMantExp(  ±0, exp) =   ±0
 //     z.SetMantExp(±Inf, exp) = ±Inf
 //
-// The result is ±Inf if the magnitude of exp is > MaxExp.
 func (z *Float) SetMantExp(mant *Float, exp int) *Float {
        z.Copy(mant)
        if len(z.mant) == 0 || z.exp == infExp {
                return z
        }
-       z.setExp(int64(exp))
+       z.setExp(int64(z.exp) + int64(exp))
        return z
 }
 

diff --git a/src/math/big/float_test.go b/src/math/big/float_test.go
index 2789cfb9f3e0959ae4d4204e0ce2e1916dc6c273..6714a92bf00eb4bde82192f95add63092df8278c 100644 (file)
--- a/src/math/big/float_test.go
+++ b/src/math/big/float_test.go
@@ -167,11 +167,17 @@ func TestFloatSetMantExp(t *testing.T) {
                {"+Inf", -1234, "+Inf"},
                {"-Inf", -1234, "-Inf"},
                {"0", -MaxExp - 1, "0"},
-               {"1", -MaxExp - 1, "+Inf"},  // exponent magnitude too large
-               {"-1", -MaxExp - 1, "-Inf"}, // exponent magnitude too large
+               {"0.5", -MaxExp - 1, "+Inf"},  // exponent overflow
+               {"-0.5", -MaxExp - 1, "-Inf"}, // exponent overflow
+               {"0.5", MaxExp + 1, "+Inf"},   // exponent overflow
+               {"-0.5", MaxExp + 1, "-Inf"},  // exponent overflow
+               {"1", MaxExp, "+Inf"},         // exponent overflow
+               {"2", MaxExp - 1, "+Inf"},     // exponent overflow
                {"0.75", 1, "1.5"},
                {"0.5", 11, "1024"},
                {"-0.5", -2, "-0.125"},
+               {"32", 5, "1024"},
+               {"1024", -10, "1"},
        } {
                frac := makeFloat(test.frac)
                want := makeFloat(test.z)
@@ -180,6 +186,10 @@ func TestFloatSetMantExp(t *testing.T) {
                if !feq(&z, want) {
                        t.Errorf("SetMantExp(%s, %d) = %s; want %s", test.frac, test.exp, z.Format('g', 10), test.z)
                }
+               // test inverse property
+               if z.SetMantExp(want.MantExp()).Cmp(want) != 0 {
+                       t.Errorf("Inverse property not satisfied: got %s; want %s", z.Format('g', 10), test.z)
+               }
        }
 }