math/big: minor cleanups

- comment fixes
- s/z/x/ in (*rat).Float64 to match convention for functions
  returning a non-*Rat
- minor test output tweaking

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8327044

src/pkg/math/big/int.go

@@ -53,7 +53,7 @@ func (z *Int) SetInt64(x int64) *Int {
 
 // SetUint64 sets z to x and returns z.
 func (z *Int) SetUint64(x uint64) *Int {
-	z.abs = z.abs.setUint64(uint64(x))
+	z.abs = z.abs.setUint64(x)
 	z.neg = false
 	return z
 }
@@ -513,13 +513,7 @@ func (z *Int) Scan(s fmt.ScanState, ch rune) error {
 // Int64 returns the int64 representation of x.
 // If x cannot be represented in an int64, the result is undefined.
 func (x *Int) Int64() int64 {
-	if len(x.abs) == 0 {
-		return 0
-	}
-	v := int64(x.abs[0])
-	if _W == 32 && len(x.abs) > 1 {
-		v |= int64(x.abs[1]) << 32
-	}
+	v := int64(x.Uint64())
 	if x.neg {
 		v = -v
 	}
@@ -527,7 +521,7 @@ func (x *Int) Int64() int64 {
 }
 
 // Uint64 returns the uint64 representation of x.
-// If x cannot be represented in an uint64, the result is undefined.
+// If x cannot be represented in a uint64, the result is undefined.
 func (x *Int) Uint64() uint64 {
 	if len(x.abs) == 0 {
 		return 0

src/pkg/math/big/rat.go

@@ -163,16 +163,16 @@ func quotToFloat(a, b nat) (f float64, exact bool) {
 	return
 }
 
-// Float64 returns the nearest float64 value to z.
-// If z is exactly representable as a float64, Float64 returns exact=true.
-// If z is negative, so too is f, even if f==0.
-func (z *Rat) Float64() (f float64, exact bool) {
-	b := z.b.abs
+// Float64 returns the nearest float64 value for x and a bool indicating
+// whether f represents x exactly. The sign of f always matches the sign
+// of x, even if f == 0.
+func (x *Rat) Float64() (f float64, exact bool) {
+	b := x.b.abs
 	if len(b) == 0 {
 		b = b.set(natOne) // materialize denominator
 	}
-	f, exact = quotToFloat(z.a.abs, b)
-	if z.a.neg {
+	f, exact = quotToFloat(x.a.abs, b)
+	if x.a.neg {
 		f = -f
 	}
 	return

src/pkg/math/big/rat_test.go

@@ -503,9 +503,7 @@ func TestIssue3521(t *testing.T) {
 // Test inputs to Rat.SetString.  The prefix "long:" causes the test
 // to be skipped in --test.short mode.  (The threshold is about 500us.)
 var float64inputs = []string{
-	//
 	// Constants plundered from strconv/testfp.txt.
-	//
 
 	// Table 1: Stress Inputs for Conversion to 53-bit Binary, < 1/2 ULP
 	"5e+125",
@@ -583,9 +581,7 @@ var float64inputs = []string{
 	"75224575729e-45",
 	"459926601011e+15",
 
-	//
 	// Constants plundered from strconv/atof_test.go.
-	//
 
 	"0",
 	"1",
@@ -734,7 +730,7 @@ func TestFloat64SpecialCases(t *testing.T) {
 			case f == 0 && r.Num().BitLen() == 0:
 				// Ok: Rat(0) is equivalent to both +/- float64(0).
 			default:
-				t.Errorf("strconv.ParseFloat(%q) = %g (%b), want %g (%b); delta=%g", input, e, e, f, f, f-e)
+				t.Errorf("strconv.ParseFloat(%q) = %g (%b), want %g (%b); delta = %g", input, e, e, f, f, f-e)
 			}
 		}
 
@@ -795,7 +791,7 @@ func TestFloat64Distribution(t *testing.T) {
 
 					if !checkIsBestApprox(t, f, r) {
 						// Append context information.
-						t.Errorf("(input was mantissa %#x, exp %d; f=%g (%b); f~%g; r=%v)",
+						t.Errorf("(input was mantissa %#x, exp %d; f = %g (%b); f ~ %g; r = %v)",
 							b, exp, f, f, math.Ldexp(float64(b), exp), r)
 					}
 
@@ -830,7 +826,7 @@ func checkNonLossyRoundtrip(t *testing.T, f float64) {
 	}
 	f2, exact := r.Float64()
 	if f != f2 || !exact {
-		t.Errorf("Rat.SetFloat64(%g).Float64() = %g (%b), %v, want %g (%b), %v; delta=%b",
+		t.Errorf("Rat.SetFloat64(%g).Float64() = %g (%b), %v, want %g (%b), %v; delta = %b",
 			f, f2, f2, exact, f, f, true, f2-f)
 	}
 }