require 'units/units_system'
require 'methodic_hash'

# A UnitsMeasure groups sets of units that measure the same quality.
class UnitsMeasure < MethodicHash
  
  # A Hash of UnitsMeasures to powers that represent the bases for this
  # UnitsMeasure if it is derived.
  attr_reader :derived
  attr_writer :derived # :nodoc:

  # A Methodic Hash mapping names of formats to procs that implement them.
  attr_reader :formats
  
  def initialize # :nodoc:
    @formats = MethodicHash.new
  end
  
  # Defines or extends a UnitsSystem. During definition, if forward
  # references between entities may occur and when encountered, are stored.
  # Upon completion of the definition, if any existing unresolved still
  # exist, an attempt is made to resolve them.
  def system(name,&block)
    Units.defining self
    system = (self[name] ||= UnitsSystem.new(self,name))
    block.call system if block_given?
    Units.resolve_forward_references
    Units.defining nil
    system
  end

  # Returns the names by which this UnitsMeasure is known.
  def names
    Units.names_of self
  end

  # Returns a new UnitsMeasure equal to the instance raised to the given
  # power. This is typically used to derive UnitsMeasures - for example,
  # if length is a UnitsMeasure, then length**3 could be used as the target
  # to derive volume.
  def **(exponent)
    UnitsMeasure.new.merge_derivation(self => exponent)
  end

  # Returns a new UnitsMeasure equal to the instance divided by another
  # UnitsMeasure. This is typically used to derive UnitsMeasures - for
  # example, if mass and volume are UnitsMeasures, then mass/volume could be
  # used as the target to derive density.
  def /(divisor)
    UnitsMeasure.new.merge_derivation(self).merge_derivation(divisor,-1)
  end

  # Returns a new UnitsMeasure equal to the instance multiplied by another
  # UnitsMeasure. This is typically used to derive UnitsMeasures - for
  # example, if length is a UnitsMeasure, then length*length could be
  # used as the target to derive area.
  def *(factor)
    UnitsMeasure.new.merge_derivation(self).merge_derivation(factor)
  end

  def merge_derivation derivation, multiplier=1 # :nodoc:
    current = (self.derived ||= {})
    if derivation.kind_of? UnitsMeasure
      if derivation.derived
        derivation.derived.each {|key,value|
          current[key] = (current[key] || 0) + multiplier*value } 
      else
        current[derivation] = (current[derivation] || 0) + multiplier
      end
    elsif derivation.kind_of? Hash
      derivation.each {|key,value|
        current[key] = ((current[key] || 0 ) + multiplier*value) }
    else
      raise UnitsException.new(
         "Cannot add #{derivation.self_name} to derivation")
    end
    self
  end
  
  # Returns a String containing the names of this UnitsMeasure and the
  # namse of the UnitsSystems defined within it. 
  def to_s
    "#{self_name} #{names} #{keys}"
  end

  # Associates the name of a format with an output formatter.
  def format(options)
    @formats[options[:name]] = options[:format]
  end
  
end