prob003.go

//usr/bin/env go run $0 $@ ; exit

package main

import (
    "fmt"
    "math"
)

func factor(n int) (int, int) {
    sqrt_n := int(math.Sqrt(float64(n)) + 0.1)
    if n & 1 == 0 {
        return 2, n / 2
    } else {
        for p := 3; p <= sqrt_n; p += 2 {
            if n % p == 0 {
                return p, n / p
            }
        }
    }
    return 1, n
}

func main() {
    max_p := 0
    n := 600851475143

    for n > 1{
        var p int
        p, n = factor(n)
        if p == 1 {
            if n > max_p {
                max_p = n
            }
            break
        }
        if p > max_p {
            max_p = p
        }
    }
    fmt.Println(max_p)
}