math/big: implement Rat.SetUint64

Implemented via the underlying Int.SetUint64.
Added tests for Rat.SetInt64 and Rat.SetUint64.

Fixes #29579

Change-Id: I03faaffc93e36873b202b58ae72b139dea5c40f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/160682
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/math/big/rat.go

@@ -339,6 +339,13 @@ func (z *Rat) SetInt64(x int64) *Rat {
 	return z
 }
 
+// SetUint64 sets z to x and returns z.
+func (z *Rat) SetUint64(x uint64) *Rat {
+	z.a.SetUint64(x)
+	z.b.abs = z.b.abs[:0]
+	return z
+}
+
 // Set sets z to x (by making a copy of x) and returns z.
 func (z *Rat) Set(x *Rat) *Rat {
 	if z != x {

src/math/big/rat_test.go

@@ -620,3 +620,54 @@ func TestIsFinite(t *testing.T) {
 		}
 	}
 }
+
+func TestRatSetInt64(t *testing.T) {
+	var testCases = []int64{
+		0,
+		1,
+		-1,
+		12345,
+		-98765,
+		math.MaxInt64,
+		math.MinInt64,
+	}
+	var r = new(Rat)
+	for i, want := range testCases {
+		r.SetInt64(want)
+		if !r.IsInt() {
+			t.Errorf("#%d: Rat.SetInt64(%d) is not an integer", i, want)
+		}
+		num := r.Num()
+		if !num.IsInt64() {
+			t.Errorf("#%d: Rat.SetInt64(%d) numerator is not an int64", i, want)
+		}
+		got := num.Int64()
+		if got != want {
+			t.Errorf("#%d: Rat.SetInt64(%d) = %d, but expected %d", i, want, got, want)
+		}
+	}
+}
+
+func TestRatSetUint64(t *testing.T) {
+	var testCases = []uint64{
+		0,
+		1,
+		12345,
+		^uint64(0),
+	}
+	var r = new(Rat)
+	for i, want := range testCases {
+		r.SetUint64(want)
+		if !r.IsInt() {
+			t.Errorf("#%d: Rat.SetUint64(%d) is not an integer", i, want)
+		}
+		num := r.Num()
+		if !num.IsUint64() {
+			t.Errorf("#%d: Rat.SetUint64(%d) numerator is not a uint64", i, want)
+		}
+		got := num.Uint64()
+		if got != want {
+			t.Errorf("#%d: Rat.SetUint64(%d) = %d, but expected %d", i, want, got, want)
+		}
+	}
+}