module Ovto
  class State
    # Mandatory value is omitted in the argument of State.new
    class MissingValue < StandardError; end
    # Unknown key is given
    class UnknownKey < StandardError; end

    # (internal) initialize subclass
    def self.inherited(subclass)
      subclass.instance_variable_set('@item_specs', [])
    end

    # Declare state item
    def self.item(name, options={})
      unless options.is_a?(Hash)
        raise ArgumentError, "options must be a Hash: item :#{name}, #{options.inspect}"
      end
      @item_specs << [name, options]
      # Define accessor
      define_method(name){ @values[name] }
    end

    # Return list of item specs (Array of `[name, options]`)
    def self.item_specs
      @item_specs
    end

    def initialize(hash = {})
      unknown_keys = hash.keys - self.class.item_specs.map(&:first)
      if unknown_keys.any?
        raise UnknownKey, "unknown key(s): #{unknown_keys.inspect}"
      end

      @values = self.class.item_specs.map{|name, options|
        if !hash.key?(name) && !options.key?(:default)
          raise MissingValue, ":#{name} is mandatory for #{self.class.name}.new"
        end
        # Note that `hash[key]` may be false or nil
        value = hash.key?(name) ? hash[name] : options[:default]
        [name, value]
      }.to_h
    end
    attr_reader :values

    # Create new state object from `self` and `hash`
    def merge(hash)
      unknown_keys = hash.keys - self.class.item_specs.map(&:first)
      if unknown_keys.any?
        raise UnknownKey, "unknown key(s): #{unknown_keys.inspect}"
      end
      self.class.new(@values.merge(hash))
    end

    # Return the value corresponds to `key`
    def [](key)
      @values[key]
    end

    # Return true if a State object `other` has same key-value paris as `self`
    def ==(other)
      other.is_a?(State) && self.values == other.values
    end

    def to_h
      @values
    end

    def inspect
      "#<#{self.class.name}:#{object_id} #{@values.inspect}>"
    end
  end
end