// Mgmt
// Copyright (C) James Shubin and the project contributors
// Written by James Shubin <james@shubin.ca> and the project contributors
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program.  If not, see <https://www.gnu.org/licenses/>.
//
// Additional permission under GNU GPL version 3 section 7
//
// If you modify this program, or any covered work, by linking or combining it
// with embedded mcl code and modules (and that the embedded mcl code and
// modules which link with this program, contain a copy of their source code in
// the authoritative form) containing parts covered by the terms of any other
// license, the licensors of this program grant you additional permission to
// convey the resulting work. Furthermore, the licensors of this program grant
// the original author, James Shubin, additional permission to update this
// additional permission if he deems it necessary to achieve the goals of this
// additional permission.

package engine

import (
	"fmt"
)

// SendableRes is the interface a resource must implement to support sending
// named parameters. You must specify to the engine what kind of values (and
// with their types) you will be sending. This is used for static type checking.
// Formerly, you had to make sure not to overwrite omitted parameters, otherwise
// it will be as if you've now declared a fixed state for that param. For that
// example, if a parameter `Foo string` had the zero value to mean that it was
// undefined, and you learned that the value is actually `up`, then sending on
// that param would cause that state to be managed, when it was previously not.
// This new interface actually provides a different namespace for sending keys.
type SendableRes interface {
	Res // implement everything in Res but add the additional requirements

	// Sends returns a struct containing the defaults of the type we send.
	Sends() interface{}

	// Send is used in CheckApply to send the desired data. It returns an
	// error if the data is malformed or doesn't type check. You should use
	// the GenerateSendFunc helper function to build this function for use
	// in the resource internal state handle.
	//
	// The struct passed to Send is published as an immutable snapshot. The
	// engine may read it concurrently from other resource workers (for
	// example, while a receiver processes a send/recv edge) even as this
	// resource runs CheckApply again. The caller MUST NOT mutate the
	// struct, or anything it transitively points to, after Send returns. To
	// send updated data, construct and Send a new value instead.
	Send(st interface{}) error

	// Sent returns the most recently published snapshot. This is used by
	// the engine and may be called concurrently with Send. Treat the result
	// as read-only. It should not be called if an upstream value has not
	// already been produced since that would violate the DAG properties. If
	// no payload has been sent yet, a nil may be returned, but we must not
	// rely on this behaviour as it may change.
	Sent() interface{}

	// SendActive let's the resource know if it must send a value. This is
	// usually called during CheckApply, but it's probably safe to check it
	// during Init as well.
	// XXX: Not doing this for now. If a send/recv edge wasn't initially on,
	// and the sender ran CheckApply, but didn't cache the value to send,
	// and then the edge flipped on, we'd have to error. Better to always
	// generate the cache, and only consider adding this if we have a more
	// important privacy or performance situation that requires it.
	//SendActive() bool

	// SendSetActive is used by the compiler to store the "SendActive" bool
	// so that it will later know if it will need to send or not. Only the
	// engine should call this function.
	// TODO: We could instead pass in the various edges we will be sending,
	// and store a map of those for the resource to know more precisely.
	// XXX: Not doing this for now. If a send/recv edge wasn't initially on,
	// and the sender ran CheckApply, but didn't cache the value to send,
	// and then the edge flipped on, we'd have to error. Better to always
	// generate the cache, and only consider adding this if we have a more
	// important privacy or performance situation that requires it.
	//SendSetActive(bool)
}

// RecvableRes is the interface a resource must implement to support receiving
// on public parameters. The resource only has to include the correct trait for
// this interface to be fulfilled, as no additional methods need to be added. To
// get information about received changes, you can use the Recv method from the
// input API that comes in via Init.
type RecvableRes interface {
	Res

	// SetRecv stores the map of sendable data which should arrive here. It
	// is called by the GAPI when building the resource. Unlike the Send and
	// Sent payload (which is published and read across resource workers and
	// is therefore synchronized) this map is set once during graph build,
	// before any Worker starts, and only read afterwards, so it does not
	// need its own synchronization.
	SetRecv(recv map[string]*Send)

	// Recv is used by the resource to get information on changes. This data
	// can be used to invalidate caches, restart watches, or it can be
	// ignored entirely. You should use the GenerateRecvFunc helper function
	// to build this function for use in the resource internal state handle.
	Recv() map[string]*Send
}

// Send points to a value that a resource will send.
type Send struct {
	Res SendableRes // a handle to the resource which is sending a value
	Key string      // the key in the resource that we're sending

	// Changed is set to true when a new value was transferred into this
	// key, and it stays true until the engine has run a CheckApply which
	// had the opportunity to consume it. It is read only for resources.
	//
	// It must be sticky like this because Send/Recv can run more than once
	// for each CheckApply, and because a CheckApply might get skipped or
	// might error and get retried. Once a value has been transferred into
	// the receiver field, a subsequent Send/Recv run finds the field it
	// already wrote there and won't see a difference, so if this flag was
	// recomputed on each run, then the notification would be silently lost.
	Changed bool
}

// GenerateSendFunc generates the Send function using the resource of our choice
// for use in the resource internal state handle.
func GenerateSendFunc(res Res) func(interface{}) error {
	return func(st interface{}) error {
		//fmt.Printf("from: %+v\n", res)
		//fmt.Printf("send: %+v\n", st)
		r, ok := res.(SendableRes)
		if !ok {
			panic(fmt.Sprintf("res of kind `%s` does not support the Sendable trait", res.Kind()))
		}
		// XXX: type check this
		//expected := r.Sends()
		//if err := XXX_TYPE_CHECK(expected, st); err != nil {
		//	return err
		//}

		return r.Send(st) // send the struct
	}
}

// GenerateRecvFunc generates the Recv function using the resource of our choice
// for use in the resource internal state handle.
func GenerateRecvFunc(res Res) func() map[string]*Send {
	return func() map[string]*Send { // TODO: change this API?
		r, ok := res.(RecvableRes)
		if !ok {
			panic(fmt.Sprintf("res of kind `%s` does not support the Recvable trait", res.Kind()))
		}
		return r.Recv()
	}
}
