Print the ulam spiral

ulamspiral.go

package main

import (
    "os"
    "fmt"
    "flag"
    "strconv"
)

func isPrime(n int) (bool) {

    if n == 2 || n == 3 {
        return true
    }

    if n == 1 || n % 2 == 0 || n % 3 == 0 {
        return false
    }

    sqrt := 1
    for sqrt*sqrt < n {
        sqrt++
    }

    for j := 5; j <= sqrt; j += 6 {
        if n%j == 0 || n%(j+2) == 0 {
            return false
        }
    }

    return true
}

func main() {

    var size int

    flag.Parse()

    if flag.NArg() != 1 {
        fmt.Println("specify one integer argument")
        os.Exit(1)
    } else {
        size, _ = strconv.Atoi(flag.Arg(0))
    }

    grid := make([][]int, size)
    for k := 0; k < size; k++ {
        grid[k] = make([]int, size)
    }

    mid := (size - 1)/2
    grid[mid][mid] = 1
    x, y := mid, mid

    step, count := 1, 2
    dx, dy := 0, 1
    for ; count <= size*size; step++ {
            for k := 1; count <= size*size && k <= 2; k++ {
                for i := 1; count <= size*size && i <= step; i++ {
                    x+=dx
                    y+=dy
                    grid[x][y] = count
                    count++
                }
                dx, dy = dy, dx
            }
            dx, dy = -dx, -dy
    }

    for i := 0; i < size; i++ {
        for j := 0; j < size; j++ {
            if isPrime(grid[i][j]) {
                fmt.Printf("%d\t", grid[i][j])
            } else {
                fmt.Printf("\t")
            }
        }
        fmt.Printf("\n")
    }
}