math/big: add big.Int methods Inc() and Dec()

src/math/big/int.go

@@ -133,6 +133,17 @@ func (z *Int) Add(x, y *Int) *Int {
 	return z
 }
 
+// Inc sets z to the sum z+1 and returns z.
+func (z *Int) Inc() *Int {
+	if z.neg {
+		z.abs = z.abs.sub(z.abs, intOne.abs)
+		z.neg = len(z.abs) > 0
+	} else {
+		z.abs = z.abs.add(z.abs, intOne.abs)
+	}
+	return z
+}
+
 // Sub sets z to the difference x-y and returns z.
 func (z *Int) Sub(x, y *Int) *Int {
 	neg := x.neg
@@ -154,6 +165,19 @@ func (z *Int) Sub(x, y *Int) *Int {
 	return z
 }
 
+// Dec sets z to the difference z-1 and returns z.
+func (z *Int) Dec() *Int {
+	if z.neg {
+		z.abs = z.abs.add(z.abs, intOne.abs)
+	} else if len(z.abs) > 0 {
+		z.abs = z.abs.sub(z.abs, intOne.abs)
+	} else {
+		z.abs = z.abs.add(z.abs, intOne.abs)
+		z.neg = true
+	}
+	return z
+}
+
 // Mul sets z to the product x*y and returns z.
 func (z *Int) Mul(x, y *Int) *Int {
 	// x * y == x * y

src/math/big/int_test.go

@@ -1766,6 +1766,56 @@ func TestIssue22830(t *testing.T) {
 	}
 }
 
+func TestInc(t *testing.T) {
+	var i int64
+	for i = -1; i <= 1; i++ {
+		inc := NewInt(i).Inc()
+		add := new(Int).Add(NewInt(i), intOne)
+		if inc.Cmp(add) != 0 {
+			t.Fatalf("%v != %v", inc, add)
+		}
+	}
+}
+
+func TestDec(t *testing.T) {
+	var i int64
+	for i = -1; i <= 1; i++ {
+		dec := NewInt(i).Dec()
+		sub := new(Int).Sub(NewInt(i), intOne)
+		if dec.Cmp(sub) != 0 {
+			t.Fatalf("(%v != %v", dec, sub)
+		}
+	}
+}
+
+func BenchmarkInc(b *testing.B) {
+	z := new(Int)
+	for i := 0; i < b.N; i++ {
+		z.Inc()
+	}
+}
+
+func BenchmarkIncByAdd(b *testing.B) {
+	z := new(Int)
+	for i := 0; i < b.N; i++ {
+		z.Add(z, intOne)
+	}
+}
+
+func BenchmarkDec(b *testing.B) {
+	z := new(Int)
+	for i := 0; i < b.N; i++ {
+		z.Dec()
+	}
+}
+
+func BenchmarkDecBySub(b *testing.B) {
+	z := new(Int)
+	for i := 0; i < b.N; i++ {
+		z.Sub(z, intOne)
+	}
+}
+
 func BenchmarkSqrt(b *testing.B) {
 	n, _ := new(Int).SetString("1"+strings.Repeat("0", 1001), 10)
 	b.ResetTimer()