require 'active_support/callbacks'

module ActiveSupport
  class ExecutionWrapper
    include ActiveSupport::Callbacks

    Null = Object.new # :nodoc:
    def Null.complete! # :nodoc:
    end

    define_callbacks :run
    define_callbacks :complete

    def self.to_run(*args, &block)
      set_callback(:run, *args, &block)
    end

    def self.to_complete(*args, &block)
      set_callback(:complete, *args, &block)
    end

    class RunHook < Struct.new(:hook) # :nodoc:
      def before(target)
        hook_state = target.send(:hook_state)
        hook_state[hook] = hook.run
      end
    end

    class CompleteHook < Struct.new(:hook) # :nodoc:
      def before(target)
        hook_state = target.send(:hook_state)
        if hook_state.key?(hook)
          hook.complete hook_state[hook]
        end
      end
      alias after before
    end

    # Register an object to be invoked during both the +run+ and
    # +complete+ steps.
    #
    # +hook.complete+ will be passed the value returned from +hook.run+,
    # and will only be invoked if +run+ has previously been called.
    # (Mostly, this means it won't be invoked if an exception occurs in
    # a preceding +to_run+ block; all ordinary +to_complete+ blocks are
    # invoked in that situation.)
    def self.register_hook(hook, outer: false)
      if outer
        to_run RunHook.new(hook), prepend: true
        to_complete :after, CompleteHook.new(hook)
      else
        to_run RunHook.new(hook)
        to_complete CompleteHook.new(hook)
      end
    end

    # Run this execution.
    #
    # Returns an instance, whose +complete!+ method *must* be invoked
    # after the work has been performed.
    #
    # Where possible, prefer +wrap+.
    def self.run!
      if active?
        Null
      else
        new.tap do |instance|
          success = nil
          begin
            instance.run!
            success = true
          ensure
            instance.complete! unless success
          end
        end
      end
    end

    # Perform the work in the supplied block as an execution.
    def self.wrap
      return yield if active?

      instance = run!
      begin
        yield
      ensure
        instance.complete!
      end
    end

    class << self # :nodoc:
      attr_accessor :active
    end

    def self.inherited(other) # :nodoc:
      super
      other.active = Concurrent::Hash.new
    end

    self.active = Concurrent::Hash.new

    def self.active? # :nodoc:
      @active[Thread.current]
    end

    def run! # :nodoc:
      self.class.active[Thread.current] = true
      run_callbacks(:run)
    end

    # Complete this in-flight execution. This method *must* be called
    # exactly once on the result of any call to +run!+.
    #
    # Where possible, prefer +wrap+.
    def complete!
      run_callbacks(:complete)
    ensure
      self.class.active.delete Thread.current
    end

    private
      def hook_state
        @_hook_state ||= {}
      end
  end
end
