module Liquid
  # A Liquid file system is way to let your templates retrieve other templates for use with the include tag.
  #
  # You can implement subclasses that retrieve templates from the database, from the file system using a different 
  # path structure, you can provide them as hard-coded inline strings, or any manner that you see fit.
  #
  # You can add additional instance variables, arguments, or methods as needed.
  #
  # Example:
  #
  # Liquid::Template.file_system = Liquid::LocalFileSystem.new(template_path)
  # liquid = Liquid::Template.parse(template)
  #
  # This will parse the template with a LocalFileSystem implementation rooted at 'template_path'.
  class BlankFileSystem
    # Called by Liquid to retrieve a template file
    def read_template_file(template_path)
      raise FileSystemError, "This liquid context does not allow includes."
    end
  end
  
  # This implements an abstract file system which retrieves template files named in a manner similar to Rails partials,
  # ie. with the template name prefixed with an underscore. The extension ".liquid" is also added.
  #
  # For security reasons, template paths are only allowed to contain letters, numbers, and underscore.
  #
  # Example:
  #
  # file_system = Liquid::LocalFileSystem.new("/some/path")
  # 
  # file_system.full_path("mypartial")       # => "/some/path/_mypartial.liquid"
  # file_system.full_path("dir/mypartial")   # => "/some/path/dir/_mypartial.liquid"
  #
  class LocalFileSystem
    attr_accessor :root
    
    def initialize(root)
      @root = root
    end
    
    def read_template_file(template_path)
      full_path = full_path(template_path)
      raise FileSystemError, "No such template '#{template_path}'" unless File.exists?(full_path)
      
      File.read(full_path)
    end
    
    def full_path(template_path)
      raise FileSystemError, "Illegal template name '#{template_path}'" unless template_path =~ /^[^.\/][a-zA-Z0-9_\/]+$/
      
      full_path = if template_path.include?('/')
        File.join(root, File.dirname(template_path), "_#{File.basename(template_path)}.liquid")
      else
        File.join(root, "_#{template_path}.liquid")
      end
      
      raise FileSystemError, "Illegal template path '#{File.expand_path(full_path)}'" unless File.expand_path(full_path) =~ /^#{File.expand_path(root)}/
      
      full_path
    end
  end
end