// Package updater performs the constrained on-disk activation of a prepared
// SoftBox self-update. It deliberately does not download, verify, or select an
// update package.
package updater

import (
	"context"
	"errors"
	"fmt"
	"time"
)

const (
	// ProductExecutableName is the only executable the updater may start.
	ProductExecutableName = "SoftBox.exe"
	// InternalHealthFlag is accepted only by SoftBox itself after an update.
	InternalHealthFlag = "--softbox-update-health"
)

var (
	ErrInvalidRequest     = errors.New("invalid self-update request")
	ErrUnsafeLayout       = errors.New("unsafe self-update layout")
	ErrStagingMissing     = errors.New("self-update staging directory is missing")
	ErrBackupExists       = errors.New("self-update backup directory already exists")
	ErrTransactionCorrupt = errors.New("self-update transaction is corrupt")
	ErrRecoveryRequired   = errors.New("self-update recovery is required")
	ErrParentWait         = errors.New("wait for SoftBox parent process")
	ErrLaunch             = errors.New("launch updated SoftBox")
	ErrHealthTimeout      = errors.New("updated SoftBox health confirmation timed out")
	ErrHealthInvalid      = errors.New("updated SoftBox health confirmation is invalid")
)

// Request identifies one prepared self-update. StagingDir and TargetDir are
// both checked against the fixed layout; callers cannot choose arbitrary move
// endpoints.
type Request struct {
	ParentPID  int
	StagingDir string
	TargetDir  string
	RequestID  string
}

// StartCommand contains the only process launch allowed by this package.
// Platform adapters must pass precisely the fixed internal health flag.
type StartCommand struct {
	Entrypoint       string
	WorkingDirectory string
	HealthRequestID  string
}

// ProcessWaiter waits for the old main-process PID to end naturally.
type ProcessWaiter interface {
	WaitForProcessExit(context.Context, int, time.Duration) error
}

// Launcher starts the verified new main executable without a shell.
type Launcher interface {
	StartSelfUpdate(StartCommand) (int, error)
}

// DirectorySyncer is implemented by the platform boundary. Directory flushing
// requires a Windows handle implementation and must not leak into core.
type DirectorySyncer interface {
	SyncDirectory(path string) error
}

// HealthWaiter observes the minimal acknowledgement written by the new main
// process. It must only accept the exact request ID.
type HealthWaiter interface {
	WaitForHealth(context.Context, string, string, time.Duration) error
}

// Timeouts controls externally-blocking update operations.
type Timeouts struct {
	ParentExit time.Duration
	Health     time.Duration
}

// Service owns one constrained self-update orchestration.
type Service struct {
	waiter   ProcessWaiter
	launcher Launcher
	syncer   DirectorySyncer
	health   HealthWaiter
	timeouts Timeouts
}

// NewService constructs an updater. Missing dependencies are reported by
// Update, keeping command composition straightforward.
func NewService(
	waiter ProcessWaiter,
	launcher Launcher,
	syncer DirectorySyncer,
	health HealthWaiter,
	timeouts Timeouts,
) *Service {
	if timeouts.ParentExit <= 0 {
		timeouts.ParentExit = 2 * time.Minute
	}
	if timeouts.Health <= 0 {
		timeouts.Health = 45 * time.Second
	}
	return &Service{waiter: waiter, launcher: launcher, syncer: syncer, health: health, timeouts: timeouts}
}

// Update waits for the old process, recovers a previous interrupted switch if
// needed, and activates the prepared directory. It never kills a process.
func (service *Service) Update(ctx context.Context, request Request) error {
	if service == nil || service.waiter == nil || service.launcher == nil || service.syncer == nil || service.health == nil {
		return fmt.Errorf("%w: updater dependencies are incomplete", ErrInvalidRequest)
	}
	layout, err := inspectRequest(request)
	if err != nil {
		return err
	}
	if err := service.waiter.WaitForProcessExit(ctx, request.ParentPID, service.timeouts.ParentExit); err != nil {
		return fmt.Errorf("%w: %w", ErrParentWait, err)
	}
	if err := recoverLayout(layout, service.syncer); err != nil {
		return err
	}
	if err := layout.validateReady(service.syncer); err != nil {
		return err
	}
	if err := removeHealth(layout, service.syncer); err != nil {
		return err
	}

	if err := writeTransaction(layout, phasePrepared, service.syncer); err != nil {
		return err
	}
	if err := renameDirectory(layout.target, layout.backup, layout, service.syncer, "back up current app"); err != nil {
		return service.failBeforeActivation(layout, err)
	}
	if err := writeTransaction(layout, phaseTargetBackedUp, service.syncer); err != nil {
		return service.rollback(layout, err)
	}
	if err := renameDirectory(layout.staging, layout.target, layout, service.syncer, "activate staged app"); err != nil {
		return service.rollback(layout, err)
	}
	if err := writeTransaction(layout, phaseStagingActivated, service.syncer); err != nil {
		return service.rollback(layout, err)
	}

	entrypoint := layout.entrypoint()
	if err := validateRegularFile(entrypoint); err != nil {
		return service.rollback(layout, fmt.Errorf("%w: %v", ErrUnsafeLayout, err))
	}
	if _, err := service.launcher.StartSelfUpdate(StartCommand{
		Entrypoint: entrypoint, WorkingDirectory: layout.target, HealthRequestID: layout.requestID,
	}); err != nil {
		return service.rollback(layout, fmt.Errorf("%w: %w", ErrLaunch, err))
	}
	if err := writeTransaction(layout, phaseLaunched, service.syncer); err != nil {
		return service.rollback(layout, err)
	}
	if err := service.health.WaitForHealth(ctx, layout.target, layout.requestID, service.timeouts.Health); err != nil {
		return service.rollback(layout, err)
	}
	if err := writeTransaction(layout, phaseCommitted, service.syncer); err != nil {
		return service.rollback(layout, err)
	}
	if err := removeManagedTree(layout.backup, layout, service.syncer); err != nil {
		return fmt.Errorf("commit self-update: %w", err)
	}
	if err := removeTransaction(layout, service.syncer); err != nil {
		return err
	}
	if err := removeHealth(layout, service.syncer); err != nil {
		return err
	}
	return nil
}

func (service *Service) failBeforeActivation(layout updateLayout, cause error) error {
	prepareErr := fmt.Errorf("prepare self-update: %w", cause)
	if recoveryErr := recoverLayout(layout, service.syncer); recoveryErr != nil {
		return errors.Join(prepareErr, recoveryErr)
	}
	return prepareErr
}

func (service *Service) rollback(layout updateLayout, cause error) error {
	rollbackErr := rollbackLayout(layout, service.syncer)
	if rollbackErr != nil {
		return errors.Join(cause, rollbackErr)
	}
	return cause
}

// Recover restores or finalizes a previously interrupted transaction for the
// fixed target directory. Callers must wait for any old parent process first.
func Recover(targetDir string, syncer DirectorySyncer) error {
	if syncer == nil {
		return fmt.Errorf("%w: directory syncer is required", ErrInvalidRequest)
	}
	layout, err := inspectTarget(targetDir)
	if err != nil {
		return err
	}
	return recoverLayout(layout, syncer)
}