ulam.go

package main

import (
	"fmt"
	"math"
	"strings"
)

const (
	RIGHT = iota
	UP
	LEFT
	DOWN
)

func generate(size, max int) {
	matrix := make([][]string, size)

	for i := 0; i < size; i++ {
		matrix[i] = make([]string, size)
	}

	dir := RIGHT
	y := size / 2
	var x int

	if size%2 == 0 {
		x = y - 1
	} else {
		x = y
	}

	for j := 1; j <= size*size; j++ {
		if j > max {
			matrix[y][x] = " "
		} else if isPrime(j) {
			matrix[y][x] = "#"
		} else {
			matrix[y][x] = "."
		}

		switch dir {
		case RIGHT:
			if x <= size-1 && matrix[y-1][x] == "" && j > 1 {
				dir = UP
			}
		case UP:
			if matrix[y][x-1] == "" {
				dir = LEFT
			}
		case LEFT:
			if x == 0 || matrix[y+1][x] == "" {
				dir = DOWN
			}
		case DOWN:
			if matrix[y][x+1] == "" {
				dir = RIGHT
			}
		}

		switch dir {
		case RIGHT:
			x += 1
		case UP:
			y -= 1
		case LEFT:
			x -= 1
		case DOWN:
			y += 1
		}
	}

	for _, row := range matrix {
		out := strings.Join(row, "")
		out = strings.Replace(out, " ", "o", -1)
		if !strings.ContainsAny(out, "# .") {
			continue
		}
		out = strings.TrimRight(out, "o")
		out = strings.Replace(out, "o", " ", -1)
		fmt.Println(out)
	}
}

func isPrime(a int) bool {
	if a == 2 {
		return true
	}
	if a <= 1 || a%2 == 0 {
		return false
	}
	max := int(math.Sqrt(float64(a)))
	for n := 3; n <= max; n += 2 {
		if a%n == 0 {
			return false
		}
	}
	return true
}

func getSize(max int) float64 {
	for {
		sqrt := math.Sqrt(float64(max))
		if sqrt*sqrt == float64(int(sqrt)*int(sqrt)) {
			return math.Sqrt(float64(max))
		}
		max++
	}
}

func main() {
	var input int
	fmt.Scanf("%d", &input)
	if input != 1 {
		generate(int(getSize(input)), input)
	} else {
		fmt.Println(".")
	}
}