Day10b.rb

class Corrupted < Exception
  def initialize(got)
    @got = got
  end

  attr_reader :got
end

class Parser
  def initialize(text)
    @text = text
    @pos = 0
    @completion = ""
  end

  def eof?
    @pos >= @text.length
  end

  def peek
    @text[@pos] or raise EOFError
  end

  def pop
    char = peek
    @pos += 1
    char
  end

  def unpop
    @pos -= 1
  end

  def expect(char)
    nchar = pop
    raise Corrupted.new(nchar) if nchar != char
  end

  def parse_chunk
    case nchar = pop
    when '('
      parse_chunk_body ')'
    when '['
      parse_chunk_body ']'
    when '{'
      parse_chunk_body '}'
    when '<'
      parse_chunk_body '>'
    else
      unpop
      raise Corrupted.new(nchar)
    end
  end

  def parse_chunk_body(delim)
    loop do
        if peek == delim
          pop
          break
        end
      parse_chunk
    rescue EOFError
      @completion << delim
      raise
    end
  end

  Scoring = {
    ')' => 1,
    ']' => 2,
    '}' => 3,
    '>' => 4,
  }
  def score
    @completion.chars.reduce(0) {|n, c| n * 5 + (Scoring[c] or 0)}
  end
end

scores = []
$<.each do |line|
  p = Parser.new(line.chomp)
  p.parse_chunk until p.eof?
rescue EOFError
  scores << p.score
rescue Corrupted
end

print scores.sort[scores.length / 2]