math/big: add SetUint64 and Uint64 functions to *Int

Implementation is mostly identical to passing a non-negative int64 to
SetInt64, and calling Int64 with a non-negative value in the *Int.
Fixes #4389.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6929048

src/pkg/math/big/int.go

@@ -51,6 +51,13 @@ func (z *Int) SetInt64(x int64) *Int {
 	return z
 }
 
+// SetUint64 sets z to x and returns z.
+func (z *Int) SetUint64(x uint64) *Int {
+	z.abs = z.abs.setUint64(uint64(x))
+	z.neg = false
+	return z
+}
+
 // NewInt allocates and returns a new Int set to x.
 func NewInt(x int64) *Int {
 	return new(Int).SetInt64(x)
@@ -519,6 +526,19 @@ func (x *Int) Int64() int64 {
 	return v
 }
 
+// Uint64 returns the int64 representation of x.
+// If x cannot be represented in an uint64, the result is undefined.
+func (x *Int) Uint64() uint64 {
+	if len(x.abs) == 0 {
+		return 0
+	}
+	v := uint64(x.abs[0])
+	if _W == 32 && len(x.abs) > 1 {
+		v |= uint64(x.abs[1]) << 32
+	}
+	return v
+}
+
 // SetString sets z to the value of s, interpreted in the given base,
 // and returns z and a boolean indicating success. If SetString fails,
 // the value of z is undefined but the returned value is nil.

src/pkg/math/big/int_test.go

@@ -1135,6 +1135,36 @@ func TestInt64(t *testing.T) {
 	}
 }
 
+var uint64Tests = []uint64{
+	0,
+	1,
+	4294967295,
+	4294967296,
+	8589934591,
+	8589934592,
+	9223372036854775807,
+	9223372036854775808,
+	18446744073709551615, // 1<<64 - 1
+}
+
+func TestUint64(t *testing.T) {
+	in := new(Int)
+	for i, testVal := range uint64Tests {
+		in.SetUint64(testVal)
+		out := in.Uint64()
+
+		if out != testVal {
+			t.Errorf("#%d got %d want %d", i, out, testVal)
+		}
+
+		str := fmt.Sprint(testVal)
+		strOut := in.String()
+		if strOut != str {
+			t.Errorf("#%d.String got %s want %s", i, strOut, str)
+		}
+	}
+}
+
 var bitwiseTests = []struct {
 	x, y                 string
 	and, or, xor, andNot string