exercism/go/prime-factors/prime.go

package prime

import "fmt"

const testVersion = 2

var sieve []int64
var iter int

func Factors(num int64) []int64 {
	fmt.Println("Factors Called on ", num)
	if num == 1 {
		return []int64{}
	}
	for i := range sieve {
		if sieve[i] == num {
			// It's a prime already in the sieve
			return []int64{num}
		}
		if num%sieve[i] == 0 {
			div := (num / sieve[i])
			return append([]int64{sieve[i]}, Factors(div)...)
		}
	}
	// If we're here, we don't have the prime factors for this number
	// Add 1 to the highest prime that we have and find the next prime
	i := int64(2)
	if len(sieve) > 0 {
		i = sieve[len(sieve)-1]
	}
	fmt.Println("  Looking for a new prime starting at ", i)
	for ; i <= num; i++ {
		isPrime := true
		for j := range sieve {
			if i%sieve[j] == 0 {
				isPrime = false
			}
			if !isPrime {
				break
			}
		}
		if isPrime {
			sieve = append(sieve, i)
			// New prime in the sieve, try again
			return Factors(num)
		}
	}
	return []int64{}
}