# Filter let you specify common filtering options, in order to generate
# a filtered condition, useful for filtering from request params.
#
# Example usage:
#   @filter = Filter.new
#   @filter.multiple :types, "type_id"
#
#   @people = People.find(:all, :conditions => @filter.conditions)
#   
#   # Generates: SELECT * FROM people WHERE (type_id IN (1,2))
class Filter

  # Instantiate a new filter using the data hash as base.
  def initialize
    @filters = Array.new
  end

  # Defines a filtering that accepts multiple values.
  # This will match against the filtered column looking for ANY of the values in the key.
  #
  # Example:
  #   # This will match all the objects that have type_id = 1 OR type_id = 2
  #   @filter = Filter.new 
  #   @filter.multiple :types, 'type_id'
  def multiple(key, filtered)
    key = key.to_s 
    @filters << ["#{filtered} IN (?)", key] unless key.nil? || key.empty?
  end

  # Defines a filtering that checks for equality.
  #
  # Example:
  #   @filter = Filter.new
  #   @filter.equal params[:id], 'an_id'
  def equal(key, filtered)
    key = key.to_s
    @filters << ["#{filtered} = ?", key] unless key.nil? || key.empty?
  end

  # Defines a filtering that checks for great.
  #
  # Example:
  #   @filter = Filter.new 
  #   @filter.great :other_id, 'an_id'
  def great(key, filtered)
    key = key.to_s
    @filters << ["#{filtered} >= ?", key] unless key.nil? || key.empty?
  end

  # Defines a filtering that checks for less.
  #
  # Example:
  #   @filter = Filter.new 
  #   @filter.less :other_id, 'an_id'
  def less(key, filtered)
    key = key.to_s  
    @filters << ["#{filtered} <= ?", key] unless key.nil? || key.empty?
  end

  # Generate an ActiveRecord prepared condition using the specified filters.
  # Used to scope the query.
  #
  # Example:
  #   People.find(:all, :conditions => @filter.conditions)
  #
  # If the filter is empty, this method will return nil, which will be ignored by AR.
  def conditions
    unless self.empty?
      @filters.inject([""]) do |condition,filter|
        filter = filter.dup
        condition.first << " AND " unless condition.first.empty?
        condition.first << filter.shift
        condition += filter
      end
    end
  end

  # Returns true or false for whether this filter is empty or not.
  #
  # Example:
  #   @filter = Filter.new nil
  #   @filter.empty?  # => true
  #
  #   @filter = Filter.new {}
  #   @filter.empty?  # => true
  #
  #   @filter = Filter.new :foo => 'bar'
  #   @filter.empty?  # => false
  def empty?
    @filters.nil? or @filters.empty?
  end
end