Class PrettyPrint
In: prettyprint.rb
Parent: Object

This class implements a pretty printing algorithm. It finds line breaks and nice indentations for grouped structure.

By default, the class assumes that primitive elements are strings and each byte in the strings have single column in width. But it can be used for other situations by giving suitable arguments for some methods:

There are several candidate uses:

  • text formatting using proportional fonts
  • multibyte characters which has columns different to number of bytes
  • non-string formatting

Bugs

  • Box based formatting?
  • Other (better) model/algorithm?

References

Christian Lindig, Strictly Pretty, March 2000, www.st.cs.uni-sb.de/~lindig/papers/#pretty

Philip Wadler, A prettier printer, March 1998, homepages.inf.ed.ac.uk/wadler/topics/language-design.html#prettier

Author

Tanaka Akira <akr@m17n.org>

Methods

break_outmost_groups   breakable   current_group   fill_breakable   first?   flush   format   group   group_sub   nest   new   singleline_format   text  

Classes and Modules

Class PrettyPrint::Breakable
Class PrettyPrint::Group
Class PrettyPrint::GroupQueue
Class PrettyPrint::SingleLine
Class PrettyPrint::Text

Attributes

genspace  [R] 
group_queue  [R] 
indent  [R] 
maxwidth  [R] 
newline  [R] 
output  [R] 

Public Class methods

format(output='', maxwidth=79, newline="\n", genspace=lambda {|n| ' ' * n} {|q| ...}

This is a convenience method which is same as follows: 

  begin
    q = PrettyPrint.new(output, maxwidth, newline, &genspace)
    ...
    q.flush
    output
  end

[Source]

# File prettyprint.rb, line 43
  def PrettyPrint.format(output='', maxwidth=79, newline="\n", genspace=lambda {|n| ' ' * n})
    q = PrettyPrint.new(output, maxwidth, newline, &genspace)
    yield q
    q.flush
    output
  end
new(output='', maxwidth=79, newline="\n", &genspace)

Creates a buffer for pretty printing.

output is an output target. If it is not specified, ’’ is assumed. It should have a << method which accepts the first argument obj of PrettyPrint#text, the first argument sep of PrettyPrint#breakable, the first argument newline of PrettyPrint.new, and the result of a given block for PrettyPrint.new.

maxwidth specifies maximum line length. If it is not specified, 79 is assumed. However actual outputs may overflow maxwidth if long non-breakable texts are provided.

newline is used for line breaks. "\n" is used if it is not specified.

The block is used to generate spaces. {|width| ’ ’ * width} is used if it is not given.

[Source]

# File prettyprint.rb, line 80
  def initialize(output='', maxwidth=79, newline="\n", &genspace)
    @output = output
    @maxwidth = maxwidth
    @newline = newline
    @genspace = genspace || lambda {|n| ' ' * n}

    @output_width = 0
    @buffer_width = 0
    @buffer = []

    root_group = Group.new(0)
    @group_stack = [root_group]
    @group_queue = GroupQueue.new(root_group)
    @indent = 0
  end
singleline_format(output='', maxwidth=nil, newline=nil, genspace=nil) {|q| ...}

This is similar to PrettyPrint::format but the result has no breaks.

maxwidth, newline and genspace are ignored.

The invocation of breakable in the block doesn‘t break a line and is treated as just an invocation of text.

[Source]

# File prettyprint.rb, line 57
  def PrettyPrint.singleline_format(output='', maxwidth=nil, newline=nil, genspace=nil)
    q = SingleLine.new(output)
    yield q
    output
  end

Public Instance methods

break_outmost_groups()

[Source]

# File prettyprint.rb, line 124
  def break_outmost_groups
    while @maxwidth < @output_width + @buffer_width
      return unless group = @group_queue.deq
      until group.breakables.empty?
        data = @buffer.shift
        @output_width = data.output(@output, @output_width)
        @buffer_width -= data.width
      end
      while !@buffer.empty? && Text === @buffer.first
        text = @buffer.shift
        @output_width = text.output(@output, @output_width)
        @buffer_width -= text.width
      end
    end
  end
breakable(sep=' ', width=sep.length)

This tells "you can break a line here if necessary", and a width\-column text sep is inserted if a line is not broken at the point.

If sep is not specified, " " is used.

If width is not specified, +sep.length+ is used. You will have to specify this when sep is a multibyte character, for example.

[Source]

# File prettyprint.rb, line 172
  def breakable(sep=' ', width=sep.length)
    group = @group_stack.last
    if group.break?
      flush
      @output << @newline
      @output << @genspace.call(@indent)
      @output_width = @indent
      @buffer_width = 0
    else
      @buffer << Breakable.new(sep, width, self)
      @buffer_width += width
      break_outmost_groups
    end
  end
current_group()

[Source]

# File prettyprint.rb, line 98
  def current_group
    @group_stack.last
  end
fill_breakable(sep=' ', width=sep.length)

[Source]

# File prettyprint.rb, line 160
  def fill_breakable(sep=' ', width=sep.length)
    group { breakable sep, width }
  end
first?()

first? is a predicate to test the call is a first call to first? with current group.

It is useful to format comma separated values as: 

  q.group(1, '[', ']') {
    xxx.each {|yyy|
      unless q.first?
        q.text ','
        q.breakable
      end
      ... pretty printing yyy ...
    }
  }

first? is obsoleted in 1.8.2.

[Source]

# File prettyprint.rb, line 119
  def first?
    warn "PrettyPrint#first? is obsoleted at 1.8.2."
    current_group.first?
  end
flush()

outputs buffered data.

[Source]

# File prettyprint.rb, line 235
  def flush
    @buffer.each {|data|
      @output_width = data.output(@output, @output_width)
    }
    @buffer.clear
    @buffer_width = 0
  end
group(indent=0, open_obj='', close_obj='', open_width=open_obj.length, close_width=close_obj.length) {|| ...}

Groups line break hints added in the block. The line break hints are all to be used or not.

If indent is specified, the method call is regarded as nested by nest(indent) { … }.

If open_obj is specified, text open_obj, open_width is called before grouping. If close_obj is specified, text close_obj, close_width is called after grouping.

[Source]

# File prettyprint.rb, line 197
  def group(indent=0, open_obj='', close_obj='', open_width=open_obj.length, close_width=close_obj.length)
    text open_obj, open_width
    group_sub {
      nest(indent) {
        yield
      }
    }
    text close_obj, close_width
  end
group_sub() {|| ...}

[Source]

# File prettyprint.rb, line 207
  def group_sub
    group = Group.new(@group_stack.last.depth + 1)
    @group_stack.push group
    @group_queue.enq group
    begin
      yield
    ensure
      @group_stack.pop
      if group.breakables.empty?
        @group_queue.delete group
      end
    end
  end
nest(indent) {|| ...}

Increases left margin after newline with indent for line breaks added in the block.

[Source]

# File prettyprint.rb, line 224
  def nest(indent)
    @indent += indent
    begin
      yield
    ensure
      @indent -= indent
    end
  end
text(obj, width=obj.length)

This adds obj as a text of width columns in width.

If width is not specified, obj.length is used.

[Source]

# File prettyprint.rb, line 144
  def text(obj, width=obj.length)
    if @buffer.empty?
      @output << obj
      @output_width += width
    else
      text = @buffer.last
      unless Text === text
        text = Text.new
        @buffer << text
      end
      text.add(obj, width)
      @buffer_width += width
      break_outmost_groups
    end
  end

[Validate]

ruby-doc.org is hosted and maintained by James Britt and Happy Camper Studios, a Ruby application development company in Phoenix, Arizona. The site was created in 2002 as part of the Ruby Documentation Project to promote the Ruby language and to help other Ruby hackers.

Documentation content on ruby-doc.org is provided by remarkable members of the Ruby community.

For more information on the Ruby programming language, visit ruby-lang.org.

Want to help improve Ruby's API docs? See Ruby Documentation Guidelines.