```ruby
require 'strscan'

module HtmlBeautifier
  class Parser

    def self.debug_block(&blk)
      @debug_block = blk
    end

    def self.debug(match, method)
      if defined? @debug_block
        @debug_block.call(match, method)
      end
    end

    def initialize(&blk)
      @maps = []
      if block_given?
        self.instance_eval(&blk)
      end
    end

    def map(pattern, method)
      @maps << [pattern, method]
    end

    def scan(subject, receiver)
      @scanner = StringScanner.new(subject)
      until @scanner.eos?
        dispatch(receiver)
      end
    end

    def source_so_far
      @scanner.string[0..(@scanner.pos-1)]
    end

    def source_line_number
      [source_so_far.chomp.split(/\n/).count, 1].max
    end

    def dispatch(receiver)
      @maps.each do |pattern, method|
        if @scanner.scan(pattern)
          params = []
          i = 1
          while @scanner[i]
            params << @scanner[i]
            i += 1
          end
          params = [@scanner[0]] if params.empty?
          self.class.debug(@scanner[0], method)
          receiver.__send__(method, *params)
          return
        end
      end
      raise "Unmatched sequence #{match.inspect}"
    end

  end
end
```

Use exclusive range to eliminate subtraction
user
```ruby
require 'strscan'

module HtmlBeautifier
  class Parser

    def self.debug_block(&blk)
      @debug_block = blk
    end

    def self.debug(match, method)
      if defined? @debug_block
        @debug_block.call(match, method)
      end
    end

    def initialize(&blk)
      @maps = []
      if block_given?
        self.instance_eval(&blk)
      end
    end

    def map(pattern, method)
      @maps << [pattern, method]
    end

    def scan(subject, receiver)
      @scanner = StringScanner.new(subject)
      until @scanner.eos?
        dispatch(receiver)
      end
    end

    def source_so_far
      @scanner.string[0...@scanner.pos]
    end

    def source_line_number
      [source_so_far.chomp.split(/\n/).count, 1].max
    end

    def dispatch(receiver)
      @maps.each do |pattern, method|
        if @scanner.scan(pattern)
          params = []
          i = 1
          while @scanner[i]
            params << @scanner[i]
            i += 1
          end
          params = [@scanner[0]] if params.empty?
          self.class.debug(@scanner[0], method)
          receiver.__send__(method, *params)
          return
        end
      end
      raise "Unmatched sequence #{match.inspect}"
    end

  end
end
```
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