math: faster factorial function

vlib/math/math.v

@@ -76,7 +76,7 @@ pub fn cbrt(a f64) f64 {
 }
 
 // ceil returns the nearest integer greater or equal to the provided value.
-pub fn ceil(a f64) f64 {
+pub fn ceil(a f64) int {
 	return C.ceil(a)
 }
 
@@ -131,15 +131,45 @@ pub fn exp2(a f64) f64 {
 }
 
 // factorial calculates the factorial of the provided value.
-pub fn factorial(a int) i64 {
-	if a < 0 {
+fn recursive_product( n int, current_number_ptr &int) int{
+    mut m := n / 2
+    if (m == 0){
+        return *current_number_ptr += 2
+    }
+    if (n == 2){
+        return (*current_number_ptr += 2) * (*current_number_ptr += 2)
+    }
+    return recursive_product((n - m), *current_number_ptr) * recursive_product(m, *current_number_ptr)
+}
+
+pub fn factorial(n int) i64 {
+    if n < 0 {
         panic('factorial: Cannot find factorial of negative number')
     }
-	mut prod := 1
-	for i:= 0; i < a; i++ {
-		prod *= (i+1)
+    if n < 2 {
+        return i64(1)
     }
-	return prod
+    mut r := 1
+    mut p := 1
+    mut current_number := 1
+    mut h := 0
+    mut shift := 0
+    mut high := 1
+    mut len := high
+    mut log2n := int(floor(log2(n)))
+    for ;h != n; {
+        shift += h
+        h = n >> log2n
+        log2n -= 1
+        len = high
+        high = (h - 1) | 1
+        len = (high - len)/2
+        if (len > 0){
+            p *= recursive_product(len, &current_number)
+            r *= p
+        }
+    }
+    return i64((r << shift))
 }
 
 // floor returns the nearest integer lower or equal of the provided value.

vlib/math/math_test.v

@@ -28,6 +28,7 @@ fn test_digits() {
 }
 
 fn test_factorial() {
+	assert math.factorial(12) == 479001600
 	assert math.factorial(5) == 120
 	assert math.factorial(0) == 1
 }