tabletop-companion/app/services/dice_roller.rb

# frozen_string_literal: true

class DiceRoller
  attr_reader :dice
  attr_reader :result

  def initialize(roll_command, stat: nil)
    @roll_command = roll_command
    @stat = stat&.value
    @dice = []
  end

  def roll
    result = 0
    operator = nil
    roll_command_parts.each do |part|
      case part
      when /\A\d*d\d+\z/
        operator.nil? ? (result += roll_dice(part)) : (result = result.send(operator, roll_dice(part)))
      when /\A\d+\z/
        operator.nil? ? (result += part.to_i) : (result = result.send(operator, part.to_i))
      when "self"
        operator.nil? ? (result += @stat) : (result = result.send(operator, @stat))
      when /\A[+\-*\/]\z/
        operator = part
      end
    end
    result
  end

  def valid?
    return if @roll_command.blank?

    # No repeated math operators
    return false if @roll_command.match?(/[+\-*\/]{2,}/)

    # No leading or trailing math operators
    return false if @roll_command.match?(/\A[+\-*\/]/) || @roll_command.match?(/[+\-*\/]\z/)

    @roll_command.match?(
      /
      \A(
        (\d*d\d*) |
        ([+\-*\/]) |
        (\d+) |
        (self)
      )*\z/xi,
    )
  end

  private

    def roll_command_parts
      @roll_command.scan(/([+\-*\/])|(\d*d\d+)|(\d+)|(self)/xi)
                   .flatten
                   .compact_blank
    end

    def roll_dice(command)
      parts = command.downcase.split("d").compact_blank
      die_type = parts.last
      dice_number = parts.length > 1 ? parts.first.to_i : 1

      result = 0
      dice_number.times do
        roll = rand(1..die_type.to_i)
        result += roll
        dice << [ die_type, result ]
      end
      result
    end
end