defmodule Calculator do
  @moduledoc """
  Basic idea: convert the infix expression to the postfix expression.
  Reference: https://leetcode.com/problems/basic-calculator-iii/discuss/113598/
  """
  @priority %{"*" => 1, "/" => 1, "+" => 2, "-" => 2}

  @doc """
    build the postfix list
  """

  def build_postfix(exp_list, postfix, ops) do
    if length(exp_list) == 0 do
      push_ops(postfix, ops)
    else
      [head | tail] = exp_list
      if is_integer(head) do
        build_postfix(tail, postfix ++ [head], ops)
      else
        if is_binary(head) and !Stack.isEmpty?(ops) do
          build_postfix_op(exp_list, postfix, ops)
        else
          build_postfix(tail, postfix, Stack.push(ops, head))
        end
      end
    end
  end

  @doc """
    build the postfix list when the head of current expression list is an operation, and the ops stack is not empty
  """

  def build_postfix_op(exp_list, postfix, ops) do
    current_op = Stack.peek(ops)
    [current_val | tail] = exp_list
    if (current_val == "(") do
      build_postfix(tail, postfix, Stack.push(ops, current_val))
    else if current_val == ")" and current_op != "(" do
      build_postfix(exp_list, postfix ++ [current_op], Stack.pop(ops))
    else if current_val == ")" and current_op == "(" do
      build_postfix(tail, postfix, Stack.pop(ops))
    else if current_val != "(" and @priority[current_op] < @priority[current_val] do
      build_postfix(exp_list, postfix ++ [current_op], Stack.pop(ops))
    else
      build_postfix(tail, postfix, Stack.push(ops, current_val))
    end
    end
    end
    end
  end

  @doc """
    When the operation stack is not empty, keep pushing the peek to postfix
  """

  def push_ops(postfix, ops) do
    if Stack.isEmpty?(ops) do
      postfix
    else
      push_ops(postfix ++ [Stack.peek(ops)], Stack.pop(ops))
    end
  end

  @doc """
    Calculate using postfix
  """
  def compute(postfix, temp) do
    if length(postfix) == 0 do
      {top, _bottom} = List.pop_at(temp, 0)
      top
    else
      [current_val | tail] = postfix
      if is_integer(current_val) do
        compute(tail, [current_val] ++ temp)
      else
        compute_portion(postfix, temp)
      end
    end
  end

  @doc """
    When the postfix is not empty, calculate using the top two elements in temp, and push the result to temp
  """
  def compute_portion(postfix, temp) do
    [op | tail] = postfix
    {top1, bottom1} = List.pop_at(temp, 0)
    {top2, bottom2} = List.pop_at(bottom1, 0)
    case op do
      "*" -> compute(tail, [top2 * top1] ++ bottom2)
      "/" -> compute(tail, [top2 / top1] ++ bottom2)
      "+" -> compute(tail, [top2 + top1] ++ bottom2)
      "-" -> compute(tail, [top2 - top1] ++ bottom2)
    end
  end

  @doc """
    If the string is a integer string, transform it to a integer. Otherwise return current str(operation)
  """

  def str_parse(str) do
    case Integer.parse(str) do
      {num, _} -> num
      :error -> str
    end
  end

  @doc """
    Concat the digit that split apart
  """
  def concat_number_str(exp_list, new_list) do
    if length(exp_list) == 0 or length(exp_list) == 1 do
      if length(exp_list) == 0 do
        new_list
      else
        [head | _tail] = exp_list
        new_list ++ [head]
      end
    else
      [head1 | tail1] = exp_list
      [head2 | tail2] = tail1
      if is_integer(str_parse(head1)) and is_integer(str_parse(head2)) do
        concat_result = Enum.join([head1, head2], "")
        concat_number_str([concat_result] ++ tail2, new_list)
      else if is_binary(str_parse(head1)) and is_integer(str_parse(head2)) do
        concat_number_str(tail1, new_list ++ [head1])
      else if is_integer(str_parse(head1)) and is_binary(str_parse(head2)) do
        concat_number_str(tail2, new_list ++ [head1] ++ [head2])
      else if is_binary(str_parse(head1)) and is_binary(str_parse(head2)) do
        concat_number_str(tail2, new_list ++ [head1] ++ [head2])
      end
      end
      end
      end
    end
  end

  @doc """
    deal with the situation that first character is `a`
  """
  def deal_first_neg(exp_list) do
    [head1 | tail1] = exp_list
    if head1 == "-" do
      [head2 | tail2] = tail1
      if is_integer(str_parse(head2)) do
        concat_number = Enum.join([head1, head2], "")
        [concat_number] ++ tail2
      else
        ["0"] ++ exp_list
      end
    else
      exp_list
    end
  end

  @doc """
  Main Evaluation function.
  Given an expression\n
  1. remove all the space\n
  2. return all the characters in the string as a list\n
  3. as we will hit enter in the terminal and consider as a character, remove the enter.\n
  4. Concat the integer string that split up\n
  5. parse each character in the list\n
  6. build a postfix list using current expression list, an empty postfix list and an empty stack\n
  7. compute using postfix list
  """
  def eval(exp) do
    exp
    |> String.replace(" ", "")
    |> String.codepoints()
    |> List.delete("\n")
    |> concat_number_str([])
    |> Enum.map(&str_parse/1)
    |> build_postfix([], Stack.new)
    |> compute([])
  end

  @doc """
    Main function.
  """
  def main do
    exp = IO.gets("> ")
    exp
    |> eval()
    |> IO.puts
    main()
  end

end