math/big: implement Int.TrailingZeroBits

Implemented via the underlying nat.trailingZeroBits.

Fixes #29578

Change-Id: If9876c5a74b107cbabceb7547bef4e44501f6745
Reviewed-on: https://go-review.googlesource.com/c/go/+/160681
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/big/int.go

@@ -448,6 +448,12 @@ func (x *Int) BitLen() int {
 	return x.abs.bitLen()
 }
 
+// TrailingZeroBits returns the number of consecutive least significant zero
+// bits of |x|.
+func (x *Int) TrailingZeroBits() uint {
+	return x.abs.trailingZeroBits()
+}
+
 // Exp sets z = x**y mod |m| (i.e. the sign of m is ignored), and returns z.
 // If m == nil or m == 0, z = x**y unless y <= 0 then z = 1.
 //

src/math/big/int_test.go

@@ -1335,6 +1335,31 @@ func TestBitSet(t *testing.T) {
 	}
 }
 
+var tzbTests = []struct {
+	in  string
+	out uint
+}{
+	{"0", 0},
+	{"1", 0},
+	{"-1", 0},
+	{"4", 2},
+	{"-8", 3},
+	{"0x4000000000000000000", 74},
+	{"-0x8000000000000000000", 75},
+}
+
+func TestTrailingZeroBits(t *testing.T) {
+	for i, test := range tzbTests {
+		in, _ := new(Int).SetString(test.in, 0)
+		want := test.out
+		got := in.TrailingZeroBits()
+
+		if got != want {
+			t.Errorf("#%d: got %v want %v", i, got, want)
+		}
+	}
+}
+
 func BenchmarkBitset(b *testing.B) {
 	z := new(Int)
 	z.SetBit(z, 512, 1)