module Math
  {% if compare_versions(env("CRYSTAL_VERSION") || "0.0.0", "1.2.0") < 0 %}
    def isqrt(value : Int::Primitive)
      raise ArgumentError.new "Input must be non-negative integer" if value < 0
      return value if value < 2
      res = value.class.zero
      bit = res.succ << (res.leading_zeros_count - 2)
      bit >>= value.leading_zeros_count & ~0x3
      while (bit != 0)
        if value >= res + bit
          value -= res + bit
          res = (res >> 1) + bit
        else
          res >>= 1
        end
        bit >>= 2
      end
      res
    end
  {% end %}
end

class PrimeFactor
  def initialize(@n : Int32)
    s = (@n + 1) // 2
    sieve = Array.new(s, true)

    if @n < 2
      @primes = [] of Int32
      return
    end

    m = (Math.isqrt(n) - 1) // 2
    (1..m).each do |p|
      if sieve[p]
        (p*3+1...s).step(p*2+1) do |q|
          sieve[q] = false
        end
      end
    end

    @primes = [2]
    (1...s).each do |p|
      @primes << p*2+1 if sieve[p]
    end
  end

  def self.sqrt(n : Int)
    self.new(Math.isqrt(n).to_i32)
  end

  getter primes : Array(Int32)

  record Factor(T), prime : T, exp : Int32

  def div(x : T) forall T
    factors = [] of Factor(T)
    t = Math.isqrt(x)
    @primes.each do |p|
      break if p > t
      c = 0
      while x%p == 0
        c += 1
        x //= p
      end
      factors << Factor.new(T.new(p), c) if c > 0
      break if x == 1
    end
    factors << Factor.new(x, 1) if x > 1
    factors
  end

  def divisors(x : T) forall T
    factors = div(x)
    r = divisors_proc(factors, 0, T.multiplicative_identity)
    r.sort!
  end


  def divisors_proc(factors : Array(Factor(T)), i : Int32, c : T) forall T
    return [c] if i == factors.size
    r = [] of T
    (0..factors[i].exp).each do |j|
      r.concat(divisors_proc(factors, i+1, c * factors[i].prime**j))
    end
    r
  end
end