big: implemented Int.Binomial (to be used in test/hilbert.go with the

     forthcoming implementation of big.Rat)

R=rsc
CC=golang-dev
https://golang.org/cl/1229047

src/pkg/big/int.go

@@ -39,7 +39,7 @@ func NewInt(x int64) *Int {
 }
 
 
-// Set sets z to x.
+// Set sets z to x and returns z.
 func (z *Int) Set(x *Int) *Int {
 	z.abs = z.abs.set(x.abs)
 	z.neg = x.neg
@@ -47,6 +47,14 @@ func (z *Int) Set(x *Int) *Int {
 }
 
 
+// Neg sets z to -x and returns z.
+func (z *Int) Neg(x *Int) *Int {
+	z.abs = z.abs.set(x.abs)
+	z.neg = len(z.abs) > 0 && !x.neg // 0 has no sign
+	return z
+}
+
+
 // Add sets z to the sum x+y and returns z.
 func (z *Int) Add(x, y *Int) *Int {
 	neg := x.neg
@@ -127,6 +135,15 @@ func (z *Int) MulRange(a, b int64) *Int {
 }
 
 
+// Binomial sets z to the binomial coefficient of (n, k) and returns z.
+func (z *Int) Binomial(n, k int64) *Int {
+	var a, b Int
+	a.MulRange(n-k+1, n)
+	b.MulRange(1, k)
+	return z.Quo(&a, &b)
+}
+
+
 // Quo sets z to the quotient x/y for y != 0 and returns z.
 // If y == 0, a division-by-zero run-time panic occurs.
 // See QuoRem for more details.
@@ -237,14 +254,6 @@ func (z *Int) DivMod(x, y, m *Int) (*Int, *Int) {
 }
 
 
-// Neg computes the negation z = -x.
-func (z *Int) Neg(x *Int) *Int {
-	z.abs = z.abs.set(x.abs)
-	z.neg = len(z.abs) > 0 && !x.neg // 0 has no sign
-	return z
-}
-
-
 // Cmp compares x and y and returns:
 //
 //   -1 if x <  y