advent-2020-18-2-ruby

class Advent18
  def perform
    ARGF.map { |line| eat_expr(line.delete!(" \t\n"), false) }.reduce(&:+)
  end

  def eat_expr(line, eagier)
    op1 = looking_at(line, '(') ?  eat_expr(eat_till_closing_paren(line), false) : eat_digit(line)
    continuation(line, op1, eagier)
  end

  def continuation(line, op1, eagier)
    (line.empty? || eagier) ? op1 : eat_operation(line, op1, eagier)
  end

  def eat_operation(line, op1, eagier)
    raise "Unknown operation: #{line[0]}" unless %w[+ *].include? line[0]
    op = eat_char(line).to_sym
    partial = op1.send(op, eat_expr(line, op == :+))
    continuation(line, partial, eagier)
  end

  def looking_at(line, ch)
    eat_char(line) if result = (line[0] == ch)
    result
  end

  def eat_char(line)
    ch = line[0]
    line[0] = ''
    ch
  end

  def eat_digit(line)
    raise "Digit expected" unless line[0] =~ /\d/
    eat_char(line).to_i
  end

  def eat_till_closing_paren(line)
    expr = ''
    level = 0
    while !line.empty?
      if line[0] == ')'
        if level == 0
          eat_char(line)
          return expr
        end
        expr << eat_char(line)
        next level-= 1
      end
      level += 1 if line[0] == '('
      expr << eat_char(line)
    end
    raise "Unbalanced parens"
  end
end

puts "Total sum is #{Advent18.new.perform}"