Faster prime test

Rafal Gajdulewicz · Rafal Gajdulewicz · commit 9245e64c0dd7 · 2013-09-12T22:04:24.000-07:00
diff --git a/utils/utils.go b/utils/utils.go
@@ -4,8 +4,11 @@ import "math"
 
 func IsPrime(num int) bool {
 	floatNum := float64(num)
-	for i := 2; i <= int(math.Floor(math.Sqrt(floatNum))); i++ {
-		if num%i == 0 {
+	if (num%2 == 0) || (num%3 == 0) {
+		return false
+	}
+	for i := 1; (6*i - 1) <= int(math.Floor(math.Sqrt(floatNum))); i++ {
+		if num%(6*i-1) == 0 || num%(6*i+1) == 0 {
 			return false
 		}
 	}

Original file line number	Diff line number	Diff line change
`@@ -4,8 +4,11 @@ import "math"`
`4`	`4`
`5`	`5`	`func IsPrime(num int) bool {`
`6`	`6`	`floatNum := float64(num)`
`7`		`- for i := 2; i <= int(math.Floor(math.Sqrt(floatNum))); i++ {`
`8`		`- if num%i == 0 {`
	`7`	`+ if (num%2 == 0) \|\| (num%3 == 0) {`
	`8`	`+ return false`
	`9`	`+ }`
	`10`	`+ for i := 1; (6*i - 1) <= int(math.Floor(math.Sqrt(floatNum))); i++ {`
	`11`	`+ if num%(6i-1) == 0 \|\| num%(6i+1) == 0 {`
`9`	`12`	`return false`
`10`	`13`	`}`
`11`	`14`	`}`