prawn_column_layout.rb

module Madriska
  module Prawn
    module ColumnLayout

      # doc.columns %w[70 50% * *] do
      #   doc.column { doc.text "70pt wide" }
      #   doc.column { doc.text "50% of container" }
      #   doc.column { doc.text "Split the remainder" }
      #   doc.column { doc.text "Split the remainder" }
      # end
      def columns(column_spec)
        @columns = distribute_columns(column_spec, bounds.width)
        @column_x = 0
        @column_lowest_y = @y
        yield
        @columns = nil
        @y = @column_lowest_y
      end

      def column(&block)
        width = @columns.shift
        mask(:y) do
          bounding_box([@column_x, cursor], :width => width) do
            block.call
            @column_lowest_y = [@column_lowest_y, bounds.absolute_bottom].min
          end
        end
        @column_x += width
      end

      protected

      # Distributes the total +width+ given among the array of +columns+.
      # Syntax is similar to HTML frame specifications; each column can be:
      #
      # * Fixed width, such as "200"
      # * Percentage width, such as "30%"
      # * Elastic width, indicated by "*". These will stretch to fill the 
      #   available space, with all elastic columns having equal width.
      #
      # Examples:
      #
      #   distribute_columns %w[* * * *], 100  # => [25.0, 25.0, 25.0, 25.0]
      #   distribute_columns %w[32% * 19], 133 # => [42.56, 71.44, 19.0]
      #   distribute_columns %w[* 30% *], 800  # => [280.0, 240.0, 280.0]
      #   
      def distribute_columns(columns, width)
        raise ArgumentError, "Array expected" unless columns.is_a?(Array)
        # Calculate floating-point widths of fixed columns
        fixed_columns = columns.map do |c|
          case c
          when /%$/: (c.to_f / 100) * width
          when "*": nil # elastic column; patched up below
          else c.to_f
          end
        end
        
        # Distribute remaining elastic width among elastic columns
        fixed_width = fixed_columns.compact.inject(0.0){|s,x| s+x}
        num_elastic_columns = fixed_columns.select{|c| c.nil?}.length
        elastic_column_width = (width - fixed_width).to_f / num_elastic_columns
        
        # Patch elastic width into each elastic column
        fixed_columns.map{|x| x || elastic_column_width}
      end

    end
  end
end

require 'prawn'
Prawn::Document.send :include, Madriska::Prawn::ColumnLayout