PrettyPrint

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:

newline object and space generation block for ::new
optional width argument for #text
#breakable

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>

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}) click to toggle source

This is a convenience method which is same as follows:

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

 
               # File prettyprint.rb, line 41
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) click to toggle source

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 #text, the first argument sep of #breakable, the first argument newline of ::new, and the result of a given block for ::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.

 
               # File prettyprint.rb, line 78
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) click to toggle source

This is similar to ::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.

 
               # File prettyprint.rb, line 55
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() click to toggle source

Breaks the buffer into lines that are shorter than maxwidth

 
               # 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) click to toggle source

This says “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.

 
               # File prettyprint.rb, line 188
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() click to toggle source

Returns the group most recently added to the stack.

 
               # File prettyprint.rb, line 97
def current_group
  @group_stack.last
end

fill_breakable(sep=' ', width=sep.length) click to toggle source

This is similar to breakable except the decision to break or not is determined individually.

Two fill_breakable under a group may cause 4 results: (break,break), (break,non-break), (non-break,break), (non-break,non-break). This is different to breakable because two breakable under a group may cause 2 results: (break,break), (non-break,non-break).

The text sep+ is inserted if a line is not broken at this 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.

 
               # File prettyprint.rb, line 176
def fill_breakable(sep=' ', width=sep.length)
  group { breakable sep, width }
end

first?() click to toggle source

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.

 
               # File prettyprint.rb, line 118
def first?
  warn "PrettyPrint#first? is obsoleted at 1.8.2."
  current_group.first?
end

flush() click to toggle source

outputs buffered data.

 
               # File prettyprint.rb, line 251
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) click to toggle source

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.

 
               # File prettyprint.rb, line 213
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() click to toggle source

 
               # File prettyprint.rb, line 223
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) click to toggle source

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

 
               # File prettyprint.rb, line 240
def nest(indent)
  @indent += indent
  begin
    yield
  ensure
    @indent -= indent
  end
end

text(obj, width=obj.length) click to toggle source

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

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

 
               # 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

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