// Package routines provides utilities for managing long-running goroutines.
package routines

import "io"
import "fmt"
import "log"
import "time"
import "sync"
import "errors"
import "context"

// Func is a routine created from a run function.
type Func func (context.Context) error

// Run runs the routine.
func (fun Func) Run (ctx context.Context) error {
	return fun(ctx)
}

// Routine is an object that can be run and stopped.
type Routine interface {
	// Run is a long-running function that does not return until it is
	// finished, or its context is cancelled. If the context is cancelled,
	// the function must perform necessary cleanup/shutdown operations and
	// exit. An error is returned if the routine exited due to an error.
	Run (context.Context) error
}

// InitRoutine is a routine that has to be initialized before it can be run.
type InitRoutine interface {
	Routine

	// Init is an initialization function that is called before any routines
	// are run, including this one. Routines must not depend on eachother in
	// this function.
	Init (context.Context) error
}

// Manager is a system capable of managing multiple routines, and restarting
// them if they fail.
type Manager struct {
	// Routines specifies a list of routines to manage. These are started
	// when Run() is called.
	Routines []Routine

	// RestartDeadline specifies the amount of time a routine has to be
	// running before failing to be restarted. This is to prevent routines
	// that immediately fail from just being restarted over and over again.
	RestartDeadline time.Duration

	// Logger, if non-nil, is where log messages will be written to. If it
	// is nil, messages will be written to the standard logger. To disable
	// logging altogether, this can be set to io.Discard.
	Logger     io.Writer
	loggerLock sync.Mutex

	ctx context.Context

	failed     map[Routine] struct { }
	failedLock sync.Mutex
}

// Run spawns all routines in the Routines slice. If a routine exits with an
// error and it was running for longer than RestartDeadline, it is restarted.
// Run returns only when all routines have exited.
func (this *Manager) Run (ctx context.Context) error {
	ctx, done := context.WithCancel(ctx)
	this.ctx = ctx
	this.failed = make(map[Routine] struct { })

	var waitGroup sync.WaitGroup
	for _, routine := range this.Routines {
		if routine != nil {
			waitGroup.Add(1)
			go this.initRoutine(routine, &waitGroup)
		}
	}
	waitGroup.Wait()
	
	for _, routine := range this.Routines {
		if _, failed := this.failed[routine]; !failed {
			waitGroup.Add(1)
			go this.runRoutine(routine, &waitGroup)
		}
	}
	this.failed = nil
	waitGroup.Wait()
	
	done()
	return nil
}

// Append adds one or more routines to the Routines slice. This has no effect if
// the manager is already running.
func (this *Manager) Append (routines ...Routine) {
	this.Routines = append(this.Routines, routines...)
}

func (this *Manager) log (message ...any) {
	this.loggerLock.Lock()
	defer this.loggerLock.Unlock()
	if this.Logger == nil {
		log.Println(message...)
	} else {
		fmt.Fprintln(this.Logger, message...)
	}
}

func (this *Manager) initRoutine (routine Routine, group *sync.WaitGroup) {
	defer group.Done()

	if initRoutine, ok := routine.(InitRoutine); ok {
		err := initRoutine.Init(this.ctx)
		if err != nil {
			this.log("XXX routine failed to initialize:", err)
			this.failedLock.Lock()
			defer this.failedLock.Unlock()
			this.failed[routine] = struct { } { }
		}
	}
}

func (this *Manager) runRoutine (routine Routine, group *sync.WaitGroup) {
	defer group.Done()

	for {
		lastStart := time.Now()
		err := panicWrap(routine.Run, this.ctx)
		
		if ctxErr := this.ctx.Err(); ctxErr != nil {
			if err == nil {
				this.log("(i) stopped routine")
			} else {
				this.log("!!! stopped routine, with error:", err)
			}
			break
		}
		
		if err == nil {
			this.log("(i) routine exited")
			break
		} else {
			this.log("XXX routine failed:", err)
		}
		
		if time.Since(lastStart) < this.RestartDeadline {
			this.log("!!! not restarting routine, failed too soon")
			break
		} else {
			this.log("(i) routine is being restarted")
		}
	}
}

func panicWrap (f func (context.Context) error, ctx context.Context) (err error) {
	defer func () {
		if pan := recover(); pan != nil {
			err = errors.New(fmt.Sprint(pan))
		}
	} ()
	
	err = f(ctx)
	return
}