require 'eymiha/units/units_system'
require 'eymiha/util/methodic_hash'

module Eymiha

  # 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,
             "Cannot add #{derivation.class_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
      "#{class_name} #{names} #{keys}"
    end
    
    # Associates the name of a format with an output formatter.
    def format(options)
      @formats[options[:name]] = options[:format]
    end
    
  end
  
end