// Mgmt
// Copyright (C) 2013-2018+ 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 <http://www.gnu.org/licenses/>.

package lang // TODO: move this into a sub package of lang/$name?

import (
	"fmt"
	"io"
	"log"
	"sync"

	"github.com/purpleidea/mgmt/lang/funcs"
	_ "github.com/purpleidea/mgmt/lang/funcs/core"       // import so the funcs register
	_ "github.com/purpleidea/mgmt/lang/funcs/facts/core" // import so the facts register
	"github.com/purpleidea/mgmt/lang/interfaces"
	"github.com/purpleidea/mgmt/lang/unification"
	"github.com/purpleidea/mgmt/pgraph"
	"github.com/purpleidea/mgmt/resources"

	errwrap "github.com/pkg/errors"
)

const (
	// FileNameExtension is the filename extension used for languages files.
	FileNameExtension = "mcl" // alternate suggestions welcome!

	// make these available internally without requiring the import
	operatorFuncName = funcs.OperatorFuncName
	historyFuncName  = funcs.HistoryFuncName
	containsFuncName = funcs.ContainsFuncName
)

// Lang is the main language lexer/parser object.
type Lang struct {
	Input    io.Reader // os.Stdin or anything that satisfies this interface
	Hostname string
	World    resources.World
	Debug    bool

	ast   interfaces.Stmt // store main prog AST here
	funcs *funcs.Engine   // function event engine

	loadedChan chan struct{} // loaded signal

	streamChan chan error // signals a new graph can be created or problem
	//streamBurst bool // should we try and be bursty with the stream events?

	closeChan chan struct{} // close signal
	wg        *sync.WaitGroup
}

// Init initializes the lang struct, and starts up the initial data sources.
// NOTE: The trick is that we need to get the list of funcs to watch AND start
// watching them, *before* we pull their values, that way we'll know if they
// changed from the values we wanted.
func (obj *Lang) Init() error {
	obj.loadedChan = make(chan struct{})
	obj.streamChan = make(chan error)
	obj.closeChan = make(chan struct{})
	obj.wg = &sync.WaitGroup{}

	once := &sync.Once{}
	loadedSignal := func() { close(obj.loadedChan) } // only run once!

	// run the lexer/parser and build an AST
	log.Printf("%s: Lexing/Parsing...", Name)
	ast, err := LexParse(obj.Input)
	if err != nil {
		return errwrap.Wrapf(err, "could not generate AST")
	}
	if obj.Debug {
		log.Printf("%s: behold, the AST: %+v", Name, ast)
	}

	// TODO: should we validate the structure of the AST?
	// TODO: should we do this *after* interpolate, or trust it to behave?
	//if err := ast.Validate(); err != nil {
	//	return errwrap.Wrapf(err, "could not validate AST")
	//}

	log.Printf("%s: Interpolating...", Name)
	// interpolate strings and other expansionable nodes in AST
	interpolated, err := ast.Interpolate()
	if err != nil {
		return errwrap.Wrapf(err, "could not interpolate AST")
	}
	obj.ast = interpolated

	// top-level, built-in, initial global scope
	scope := &interfaces.Scope{
		Variables: map[string]interfaces.Expr{
			"purpleidea": &ExprStr{V: "hello world!"}, // james says hi
			// TODO: change to a func when we can change hostname dynamically!
			"hostname": &ExprStr{V: obj.Hostname},
		},
	}

	log.Printf("%s: Building Scope...", Name)
	// propagate the scope down through the AST...
	if err := obj.ast.SetScope(scope); err != nil {
		return errwrap.Wrapf(err, "could not set scope")
	}

	// apply type unification
	logf := func(format string, v ...interface{}) {
		if obj.Debug { // unification only has debug messages...
			log.Printf(Name+": unification: "+format, v...)
		}
	}
	log.Printf("%s: Running Type Unification...", Name)
	if err := unification.Unify(obj.ast, unification.SimpleInvariantSolverLogger(logf)); err != nil {
		return errwrap.Wrapf(err, "could not unify types")
	}

	log.Printf("%s: Building Function Graph...", Name)
	// we assume that for some given code, the list of funcs doesn't change
	// iow, we don't support variable, variables or absurd things like that
	graph, err := obj.ast.Graph() // build the graph of functions
	if err != nil {
		return errwrap.Wrapf(err, "could not generate function graph")
	}

	if obj.Debug {
		log.Printf("%s: function graph: %+v", Name, graph)
		graph.Logf("%s: ", Name) // log graph with this printf prefix...
	}

	if graph.NumVertices() == 0 { // no funcs to load!
		// send only one signal since we won't ever send after this!
		log.Printf("%s: Static graph found", Name)
		obj.wg.Add(1)
		go func() {
			defer obj.wg.Done()
			defer close(obj.streamChan) // no more events are coming!
			select {
			case obj.streamChan <- nil: // send one signal
				// pass
			case <-obj.closeChan:
				return
			}
			close(obj.loadedChan) // signal
		}()
		return nil // exit early, no funcs to load!
	}

	obj.funcs = &funcs.Engine{
		Graph:    graph, // not the same as the output graph!
		Hostname: obj.Hostname,
		World:    obj.World,
		Debug:    obj.Debug,
		Logf: func(format string, v ...interface{}) {
			log.Printf(Name+": funcs: "+format, v...)
		},
		Glitch: false, // FIXME: verify this functionality is perfect!
	}

	log.Printf("%s: Function Engine Initializing...", Name)
	if err := obj.funcs.Init(); err != nil {
		return errwrap.Wrapf(err, "init error with func engine")
	}

	log.Printf("%s: Function Engine Validating...", Name)
	if err := obj.funcs.Validate(); err != nil {
		return errwrap.Wrapf(err, "validate error with func engine")
	}

	log.Printf("%s: Function Engine Starting...", Name)
	// On failure, we expect the caller to run Close() to shutdown all of
	// the currently initialized (and running) funcs... This is needed if
	// we successfully ran `Run` but isn't needed only for Init/Validate.
	if err := obj.funcs.Run(); err != nil {
		return errwrap.Wrapf(err, "run error with func engine")
	}

	// wait for some activity
	log.Printf("%s: Stream...", Name)
	stream := obj.funcs.Stream()
	obj.wg.Add(1)
	go func() {
		log.Printf("%s: Loop...", Name)
		defer obj.wg.Done()
		defer close(obj.streamChan) // no more events are coming!
		for {
			var err error
			var ok bool
			select {
			case err, ok = <-stream:
				if !ok {
					log.Printf("%s: Stream closed", Name)
					return
				}
				if err == nil {
					// only do this once, on the first event
					once.Do(loadedSignal) // signal
				}

			case <-obj.closeChan:
				return
			}

			select {
			case obj.streamChan <- err: // send
				if err != nil {
					log.Printf("%s: Stream error: %+v", Name, err)
					return
				}

			case <-obj.closeChan:
				return
			}
		}
	}()
	return nil
}

// Stream returns a channel of graph change requests or errors. These are
// usually sent when a func output changes.
func (obj *Lang) Stream() chan error {
	return obj.streamChan
}

// Interpret runs the interpreter and returns a graph and corresponding error.
func (obj *Lang) Interpret() (*pgraph.Graph, error) {
	select {
	case <-obj.loadedChan: // funcs are now loaded!
		// pass
	default:
		// if this is hit, someone probably called this too early!
		// it should only be called in response to a stream event!
		return nil, fmt.Errorf("funcs aren't loaded yet")
	}

	log.Printf("%s: Running interpret...", Name)
	// this call returns the graph
	graph, err := interpret(obj.ast)
	if err != nil {
		return nil, errwrap.Wrapf(err, "could not interpret")
	}

	return graph, nil // return a graph
}

// Close shuts down the lang struct and causes all the funcs to shutdown. It
// must be called when finished after any successful Init ran.
func (obj *Lang) Close() error {
	var err error
	if obj.funcs != nil {
		err = obj.funcs.Close()
	}
	close(obj.closeChan)
	obj.wg.Wait()
	return err
}