ihash.rb

# This class works as a proxy for an instance
# and presents it as Hash.
class InstanceHash
  include Enumerable

  def initialize(obj, default_val = nil, &default_proc)
    @obj = obj
    @default = default_val
    @default_proc = default_proc || lambda {|h,k| @default}
  end

  def size
    keys.size
  end

  def empty?
    keys.empty?
  end

  def keys
    @obj.instance_variables
  end

  def values
    keys.map {|iv| get_val(iv)}
  end

  def has_key? k
    check_key(normalize(k))
  end

  alias key? has_key?

  def has_value? v
    keys.any? {|iv| v.eql? get_val(iv)}
  end

  alias value? has_value?

  def each_pair
    keys.each do |iv|
      yield iv, get_val(iv)
    end

    self
  end

  alias each each_pair

  def each_key(&b)
    keys.each(&b)
    self
  end

  def each_value
    keys.each do |iv|
      yield get_val(iv)
    end

    self
  end

  def fetch(key, &fallback)
    iv = normalize(key)

    if check_key(iv)
      get_val(iv)
    elsif fallback
      fallback[iv]
    else
      raise KeyError, 'key not found'
    end
  end

  def [] key
    fetch key do |iv|
      @default_proc[self, iv]
    end
  end

  def []= key, val
    @obj.instance_variable_set(normalize(key), val)
  end

  def assoc key
    iv = normalize(key)
    check_key(iv) && [iv, get_val(iv)]
  end

  def rassoc val
    each_pair do |iv, v|
      return [iv, v] if val.eql? v
    end

    nil
  end

private

  # check a key
  # iv must be normalized
  def check_key(iv)
    @obj.instance_variable_defined? iv
  end

  # get a value
  # iv must be normalized
  def get_val(iv)
    @obj.instance_variable_get(iv)
  end

  # Convert a String or Symbol into a proper
  # ivar name
  def normalize(var_name)
    case var_name
    when /\A@/ # covers String and Symbol
      var_name
    when String,Symbol
      :"@#{var_name}"
    else
      raise ArgumentError, "Not a valid instance variable key: %p" % var_name
    end
  end
end