ABC044D - 桁和

問題

https://atcoder.jp/contests/abc044/tasks/arc060_b

回答

defmodule Main do
  def main do
    :stdio
    |> IO.read(:all)
    |> solve()
    |> IO.puts()
  end

  defp split_lines(lines) do
    lines
    |> String.trim()
    |> String.split("\n")
  end

  defp digsum(b, n) when n < b, do: n

  defp digsum(b, n), do: rem(n, b) + digsum(b, div(n, b))

  def solve(input) do
    [n, s] =
      input
      |> split_lines()
      |> Enum.map(&String.to_integer/1)

    cond do
      n == s ->
        n + 1

      n < s ->
        -1

      true ->
        {small, big} =
          for i <- 1..(:math.sqrt(n - s) |> ceil() |> Kernel.+(1)),
              rem(n - s, i) == 0 do
            {i, div(n - s, i)}
          end
          |> Enum.reduce({[], []}, fn {small_i, big_i}, {small_acc, big_acc} ->
            {[small_i | small_acc], [big_i | big_acc]}
          end)

        [Enum.reverse(small), big]
        |> List.flatten()
        |> Enum.find(-2, &(digsum(&1 + 1, n) == s))
        |> Kernel.+(1)
    end
  end
end

"""
87654
30
"""
|> Main.solve()
|> then(&(&1 == 10))

"""
87654
138
"""
|> Main.solve()
|> then(&(&1 == 100))

"""
87654
45678
"""
|> Main.solve()
|> then(&(&1 == -1))

"""
31415926535
1
"""
|> Main.solve()
|> then(&(&1 == 31_415_926_535))

"""
1
31415926535
"""
|> Main.solve()
|> then(&(&1 == -1))