class RBS::VarianceCalculator

def env
  builder.env
end

def function(type, result:, context:)
  type.each_param do |param|
    type(param.type, result: result, context: negate(context))
  end
  type(type.return_type, result: result, context: context)
end

def in_inherit(name:, args:, variables:)
  type = if name.class?
           Types::ClassInstance.new(name: name, args: args, location: nil)
         else
           Types::Interface.new(name: name, args: args, location: nil)
         end
  Result.new(variables: variables).tap do |result|
    type(type, result: result, context: :covariant)
  end
end

def in_method_type(method_type:, variables:)
  result = Result.new(variables: variables)
  function(method_type.type, result: result, context: :covariant)
  if block = method_type.block
    function(block.type, result: result, context: :contravariant)
  end
  result
end

def in_type_alias(name:)
  decl = env.type_alias_decls[name].decl or raise
  variables = decl.type_params.each.map(&:name)
  Result.new(variables: variables).tap do |result|
    type(decl.type, result: result, context: :covariant)
  end
end

def initialize(builder:)
  @builder = builder
end

def negate(variance)
  case variance
  when :invariant
    :invariant
  when :covariant
    :contravariant
  when :contravariant
    :covariant
  else
    raise
  end
end

def type(type, result:, context:)
  case type
  when Types::Variable
    if result.include?(type.name)
      case context
      when :covariant
        result.covariant(type.name)
      when :contravariant
        result.contravariant(type.name)
      when :invariant
        result.invariant(type.name)
      end
    end
  when Types::ClassInstance, Types::Interface, Types::Alias
    NoTypeFoundError.check!(type.name,
                            env: env,
                            location: type.location)
    type_params = case type
                  when Types::ClassInstance
                    env.class_decls[type.name].type_params
                  when Types::Interface
                    env.interface_decls[type.name].decl.type_params
                  when Types::Alias
                    env.type_alias_decls[type.name].decl.type_params
                  end
    type.args.each.with_index do |ty, i|
      if var = type_params[i]
        case var.variance
        when :invariant
          type(ty, result: result, context: :invariant)
        when :covariant
          type(ty, result: result, context: context)
        when :contravariant
          type(ty, result: result, context: negate(context))
        end
      end
    end
  when Types::Proc
    function(type.type, result: result, context: context)
  else
    type.each_type do |ty|
      type(ty, result: result, context: context)
    end
  end
end