// Package lifecycle provides task lifecycle management with explicit state transitions.
package lifecycle

import (
	"fmt"
	"slices"
	"time"

	"github.com/jfraeys/fetch_ml/internal/audit"
	"github.com/jfraeys/fetch_ml/internal/domain"
)

// TaskState represents the current state of a task in its lifecycle.
// These states form a finite state machine with valid transitions defined below.
type TaskState string

const (
	// StateQueued indicates the task is waiting to be picked up by a worker.
	StateQueued TaskState = "queued"
	// StatePreparing indicates the task is being prepared (workspace setup, data staging).
	StatePreparing TaskState = "preparing"
	// StateRunning indicates the task is currently executing in a container.
	StateRunning TaskState = "running"
	// StateCollecting indicates the task has finished execution and results are being collected.
	StateCollecting TaskState = "collecting"
	// StateCompleted indicates the task finished successfully.
	StateCompleted TaskState = "completed"
	// StateFailed indicates the task failed during execution.
	StateFailed TaskState = "failed"
	// StateCancelled indicates the task was cancelled by user or system.
	StateCancelled TaskState = "cancelled"
	// StateOrphaned indicates the worker was lost - task will be silently re-queued.
	StateOrphaned TaskState = "orphaned"
	// StateServing indicates a service job (Jupyter/vLLM) is up and ready.
	StateServing TaskState = "serving"
	// StateStopping indicates a service job is in graceful shutdown.
	StateStopping TaskState = "stopping"
)

// ValidTransitions defines the allowed state transitions.
// The key is the "from" state, the value is a list of valid "to" states.
// This enforces that state transitions follow the expected lifecycle.
var ValidTransitions = map[TaskState][]TaskState{
	StateQueued:     {StatePreparing, StateFailed, StateCancelled},
	StatePreparing:  {StateRunning, StateFailed, StateCancelled},
	StateRunning:    {StateCollecting, StateFailed, StateOrphaned, StateServing, StateCancelled},
	StateCollecting: {StateCompleted, StateFailed, StateCancelled},
	StateCompleted:  {},
	StateFailed:     {},
	StateCancelled:  {},
	StateOrphaned:   {StateQueued},
	StateServing:    {StateStopping, StateFailed, StateOrphaned, StateCancelled},
	StateStopping:   {StateCompleted, StateFailed},
}

// StateTransitionError is returned when an invalid state transition is attempted.
type StateTransitionError struct {
	From TaskState
	To   TaskState
}

func (e StateTransitionError) Error() string {
	return fmt.Sprintf("invalid state transition: %s -> %s", e.From, e.To)
}

// StateManager manages task state transitions with audit logging.
type StateManager struct {
	auditor *audit.Logger
	enabled bool
}

// NewStateManager creates a new state manager with the given audit logger.
func NewStateManager(auditor *audit.Logger) *StateManager {
	return &StateManager{
		enabled: auditor != nil,
		auditor: auditor,
	}
}

// Transition attempts to transition a task from its current state to a new state.
// It validates the transition, updates the task status, and logs the event.
// Returns StateTransitionError if the transition is not valid.
func (sm *StateManager) Transition(task *domain.Task, to TaskState) error {
	from := TaskState(task.Status)

	// Validate the transition
	if err := sm.validateTransition(from, to); err != nil {
		return err
	}

	// Audit the transition before updating
	if sm.enabled && sm.auditor != nil {
		sm.auditor.Log(audit.Event{
			EventType: audit.EventJobStarted,
			Timestamp: time.Now(),
			Resource:  task.ID,
			Action:    "task_state_change",
			Success:   true,
			Metadata: map[string]any{
				"job_name":  task.JobName,
				"old_state": string(from),
				"new_state": string(to),
			},
		})
	}

	// Update task state
	task.Status = string(to)

	return nil
}

// validateTransition checks if a transition from one state to another is valid.
func (sm *StateManager) validateTransition(from, to TaskState) error {
	// Check if "from" state is valid
	allowed, ok := ValidTransitions[from]
	if !ok {
		return StateTransitionError{From: from, To: to}
	}

	// Check if "to" state is in the allowed list
	if slices.Contains(allowed, to) {
		return nil
	}

	return StateTransitionError{From: from, To: to}
}

// IsTerminalState returns true if the state is terminal (no further transitions allowed).
func IsTerminalState(state TaskState) bool {
	return state == StateCompleted || state == StateFailed || state == StateCancelled
}

// CanTransition returns true if a transition from -> to is valid.
func CanTransition(from, to TaskState) bool {
	allowed, ok := ValidTransitions[from]
	if !ok {
		return false
	}

	return slices.Contains(allowed, to)
}