Added factorial / factorialBig

math/fact.go

@@ -0,0 +1,60 @@
+package math
+
+import "math/big"
+
+var cache = []uint64{
+	1,                   // 0
+	1,                   // 1
+	2,                   // 2
+	6,                   // 3
+	24,                  // 4
+	120,                 // 5
+	720,                 // 6
+	5040,                // 7
+	40320,               // 8
+	362880,              // 9
+	3628800,             // 10
+	39916800,            // 11
+	479001600,           // 12
+	6227020800,          // 13
+	87178291200,         // 14
+	1307674368000,       // 15
+	20922789888000,      // 16
+	355687428096000,     // 17
+	6402373705728000,    // 18
+	121645100408832000,  // 19
+	2432902008176640000, // 20
+}
+
+// Factorial returns the factorial n!, if n > 20 it returns 0, use FactorialBig instead
+func Factorial(n uint) uint64 {
+	if n < uint(len(cache)) {
+		return cache[n]
+	}
+	return 0
+}
+
+var (
+	one    = big.NewInt(1)
+	twenty = big.NewInt(20)
+)
+
+func FactorialBig(n uint64) (r *big.Int) {
+	//fmt.Println("n = ", n)
+	bn := new(big.Int).SetUint64(n)
+	r = big.NewInt(1)
+
+	if bn.Cmp(one) <= 0 {
+		return
+	}
+
+	if bn.Cmp(twenty) <= 0 {
+		return r.SetUint64(Factorial(uint(n)))
+	}
+
+	for i := big.NewInt(2); i.Cmp(bn) <= 0; i.Add(i, one) {
+		r.Mul(r, i)
+	}
+
+	return
+}

math/fact_test.go

@@ -0,0 +1,18 @@
+package math
+
+import "testing"
+
+const fact100 = "93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000"
+
+func TestFactorialBig20(t *testing.T) {
+	if FactorialBig(20).Uint64() != Factorial(20) {
+		t.Errorf("FactorialBig(20) != Factorial(20)")
+	}
+}
+
+func TestFactorialBig100(t *testing.T) {
+	f100 := FactorialBig(100)
+	if f100.String() != fact100 {
+		t.Errorf("Factorial(100) returned %q, expected %q.", f100, fact100)
+	}
+}