/**
 * Deterministic State Machine Core
 * 
 * A state machine that controls agent flow WITHOUT LLM decision-making.
 * States, transitions, and events are defined declaratively.
 * 
 * Key principle: The LLM does creative work, the state machine handles the plumbing.
 */

import { randomUUID } from 'crypto';
import { EventEmitter } from 'events';

// ============================================================================
// Types
// ============================================================================

export type StateStatus = 'idle' | 'active' | 'waiting' | 'completed' | 'failed' | 'paused';

export interface State {
  id: string;
  name: string;
  type: 'start' | 'end' | 'action' | 'parallel' | 'choice' | 'wait' | 'loop';
  agent?: string;           // Agent to invoke in this state
  action?: string;          // Action to execute
  timeout?: number;         // Timeout in ms
  retry?: RetryConfig;      // Retry configuration
  onEnter?: Transition[];   // Transitions on entering state
  onExit?: Transition[];    // Transitions on exiting state
  metadata?: Record<string, unknown>;
}

export interface Transition {
  event: string;            // Event that triggers this transition
  target: string;           // Target state ID
  condition?: Condition;    // Optional condition
  guard?: string;           // Guard function name
}

export interface Condition {
  type: 'equals' | 'contains' | 'exists' | 'custom';
  field: string;
  value?: unknown;
  expression?: string;
}

export interface RetryConfig {
  maxAttempts: number;
  backoff: 'fixed' | 'exponential' | 'linear';
  initialDelay: number;
  maxDelay: number;
}

export interface StateMachineDefinition {
  id: string;
  name: string;
  version: string;
  description?: string;
  initial: string;
  states: Record<string, State>;
  events?: string[];        // Allowed events
  context?: Record<string, unknown>;  // Initial context
  onError?: ErrorHandling;
}

export interface ErrorHandling {
  strategy: 'fail' | 'retry' | 'transition';
  targetState?: string;
  maxRetries?: number;
}

export interface StateMachineInstance {
  id: string;
  definition: StateMachineDefinition;
  currentState: string;
  previousState?: string;
  status: StateStatus;
  context: Record<string, unknown>;
  history: StateTransition[];
  createdAt: Date;
  updatedAt: Date;
  startedAt?: Date;
  completedAt?: Date;
  error?: string;
}

export interface StateTransition {
  from: string;
  to: string;
  event: string;
  timestamp: Date;
  context?: Record<string, unknown>;
}

export interface Event {
  type: string;
  source: string;
  target?: string;
  payload: unknown;
  timestamp: Date;
  correlationId?: string;
}

// ============================================================================
// State Machine Engine
// ============================================================================

/**
 * DeterministicStateMachine - Core engine for deterministic flow control
 */
export class DeterministicStateMachine extends EventEmitter {
  private definition: StateMachineDefinition;
  private instance: StateMachineInstance;
  private eventQueue: Event[] = [];
  private processing = false;
  private timeoutId?: ReturnType<typeof setTimeout>;

  constructor(definition: StateMachineDefinition, instanceId?: string) {
    super();
    this.definition = definition;
    this.instance = this.createInstance(instanceId);
  }

  /**
   * Create a new state machine instance
   */
  private createInstance(instanceId?: string): StateMachineInstance {
    return {
      id: instanceId || randomUUID(),
      definition: this.definition,
      currentState: this.definition.initial,
      status: 'idle',
      context: { ...this.definition.context } || {},
      history: [],
      createdAt: new Date(),
      updatedAt: new Date()
    };
  }

  /**
   * Start the state machine
   */
  start(): void {
    if (this.instance.status !== 'idle') {
      throw new Error(`Cannot start state machine in ${this.instance.status} status`);
    }

    this.instance.status = 'active';
    this.instance.startedAt = new Date();
    this.emit('started', { instance: this.instance });

    // Enter initial state
    this.enterState(this.instance.currentState);
  }

  /**
   * Send an event to the state machine
   */
  sendEvent(event: Omit<Event, 'timestamp'>): void {
    const fullEvent: Event = {
      ...event,
      timestamp: new Date()
    };

    this.eventQueue.push(fullEvent);
    this.emit('eventQueued', { event: fullEvent });
    
    this.processQueue();
  }

  /**
   * Process the event queue
   */
  private async processQueue(): Promise<void> {
    if (this.processing || this.eventQueue.length === 0) return;
    
    this.processing = true;

    try {
      while (this.eventQueue.length > 0 && this.instance.status === 'active') {
        const event = this.eventQueue.shift()!;
        await this.handleEvent(event);
      }
    } finally {
      this.processing = false;
    }
  }

  /**
   * Handle a single event
   */
  private async handleEvent(event: Event): Promise<void> {
    const currentState = this.getCurrentState();
    
    this.emit('eventProcessed', { event, state: currentState });

    // Find matching transition
    const transition = this.findTransition(currentState, event);
    
    if (!transition) {
      this.emit('noTransition', { event, state: currentState });
      return;
    }

    // Check condition if present
    if (transition.condition && !this.evaluateCondition(transition.condition)) {
      this.emit('conditionFailed', { event, transition });
      return;
    }

    // Execute transition
    await this.executeTransition(transition, event);
  }

  /**
   * Find a matching transition for the event
   */
  private findTransition(state: State, event: Event): Transition | undefined {
    const transitions = state.onExit || [];
    return transitions.find(t => {
      // Check event type match
      if (t.event !== event.type) return false;
      
      // Check target filter if event has specific target
      if (event.target && event.target !== this.instance.id) return false;
      
      return true;
    });
  }

  /**
   * Evaluate a transition condition
   */
  private evaluateCondition(condition: Condition): boolean {
    const value = this.getDeepValue(this.instance.context, condition.field);
    
    switch (condition.type) {
      case 'equals':
        return value === condition.value;
      case 'contains':
        if (Array.isArray(value)) {
          return value.includes(condition.value);
        }
        return String(value).includes(String(condition.value));
      case 'exists':
        return value !== undefined && value !== null;
      case 'custom':
        // Custom conditions would be evaluated by a condition registry
        return true;
      default:
        return false;
    }
  }

  /**
   * Execute a state transition
   */
  private async executeTransition(transition: Transition, event: Event): Promise<void> {
    const fromState = this.instance.currentState;
    const toState = transition.target;

    // Record transition
    const transitionRecord: StateTransition = {
      from: fromState,
      to: toState,
      event: event.type,
      timestamp: new Date(),
      context: { ...this.instance.context }
    };
    this.instance.history.push(transitionRecord);

    // Exit current state
    await this.exitState(fromState);

    // Update instance
    this.instance.previousState = fromState;
    this.instance.currentState = toState;
    this.instance.updatedAt = new Date();

    // Merge event payload into context
    if (event.payload && typeof event.payload === 'object') {
      this.instance.context = {
        ...this.instance.context,
        ...event.payload as Record<string, unknown>
      };
    }

    this.emit('transition', { from: fromState, to: toState, event });

    // Enter new state
    await this.enterState(toState);
  }

  /**
   * Enter a state
   */
  private async enterState(stateId: string): Promise<void> {
    const state = this.definition.states[stateId];
    if (!state) {
      this.handleError(`State ${stateId} not found`);
      return;
    }

    this.emit('enteringState', { state });

    // Handle state types
    switch (state.type) {
      case 'end':
        this.instance.status = 'completed';
        this.instance.completedAt = new Date();
        this.emit('completed', { instance: this.instance });
        break;

      case 'action':
        // Emit event for external action handler
        this.emit('action', { 
          state, 
          context: this.instance.context,
          instanceId: this.instance.id
        });
        
        // Set timeout if specified
        if (state.timeout) {
          this.setTimeout(state.timeout, stateId);
        }
        break;

      case 'parallel':
        this.handleParallelState(state);
        break;

      case 'choice':
        this.handleChoiceState(state);
        break;

      case 'wait':
        // Wait for external event
        this.instance.status = 'waiting';
        break;

      case 'loop':
        this.handleLoopState(state);
        break;

      default:
        // Process onEnter transitions
        if (state.onEnter) {
          for (const transition of state.onEnter) {
            // Auto-transitions trigger immediately
            if (transition.event === '*') {
              await this.executeTransition(transition, {
                type: '*',
                source: stateId,
                payload: {},
                timestamp: new Date()
              });
              break;
            }
          }
        }
    }

    this.emit('enteredState', { state });
  }

  /**
   * Exit a state
   */
  private async exitState(stateId: string): Promise<void> {
    const state = this.definition.states[stateId];
    
    // Clear any pending timeout
    if (this.timeoutId) {
      clearTimeout(this.timeoutId);
      this.timeoutId = undefined;
    }

    this.emit('exitingState', { state });
  }

  /**
   * Handle parallel state (fork into concurrent branches)
   */
  private handleParallelState(state: State): void {
    this.emit('parallel', {
      state,
      branches: state.onEnter?.map(t => t.target) || [],
      context: this.instance.context
    });
  }

  /**
   * Handle choice state (conditional branching)
   */
  private handleChoiceState(state: State): void {
    const transitions = state.onExit || [];
    
    for (const transition of transitions) {
      if (transition.condition && this.evaluateCondition(transition.condition)) {
        this.sendEvent({
          type: transition.event,
          source: state.id,
          payload: {}
        });
        return;
      }
    }

    // No condition matched - use default transition
    const defaultTransition = transitions.find(t => !t.condition);
    if (defaultTransition) {
      this.sendEvent({
        type: defaultTransition.event,
        source: state.id,
        payload: {}
      });
    }
  }

  /**
   * Handle loop state
   */
  private handleLoopState(state: State): void {
    const loopCount = (this.instance.context._loopCount as Record<string, number>)?.[state.id] || 0;
    const maxIterations = (state.metadata?.maxIterations as number) || 3;

    if (loopCount < maxIterations) {
      // Continue loop
      this.instance.context._loopCount = {
        ...this.instance.context._loopCount as Record<string, number>,
        [state.id]: loopCount + 1
      };
      
      this.emit('loopIteration', { 
        state, 
        iteration: loopCount + 1, 
        maxIterations 
      });
      
      // Trigger loop body
      const loopTransition = state.onExit?.find(t => t.event === 'continue');
      if (loopTransition) {
        this.sendEvent({
          type: 'continue',
          source: state.id,
          payload: { iteration: loopCount + 1 }
        });
      }
    } else {
      // Exit loop
      const exitTransition = state.onExit?.find(t => t.event === 'exit');
      if (exitTransition) {
        this.sendEvent({
          type: 'exit',
          source: state.id,
          payload: { iterations: loopCount }
        });
      }
    }
  }

  /**
   * Set a timeout for the current state
   */
  private setTimeout(duration: number, stateId: string): void {
    this.timeoutId = setTimeout(() => {
      this.emit('timeout', { stateId });
      this.sendEvent({
        type: 'timeout',
        source: stateId,
        payload: { timedOut: true }
      });
    }, duration);
  }

  /**
   * Handle errors
   */
  private handleError(error: string): void {
    this.instance.error = error;
    this.instance.status = 'failed';
    this.instance.completedAt = new Date();
    this.emit('error', { error, instance: this.instance });
  }

  /**
   * Get current state definition
   */
  getCurrentState(): State {
    return this.definition.states[this.instance.currentState];
  }

  /**
   * Get instance info
   */
  getInstance(): StateMachineInstance {
    return { ...this.instance };
  }

  /**
   * Update context
   */
  updateContext(updates: Record<string, unknown>): void {
    this.instance.context = { ...this.instance.context, ...updates };
    this.instance.updatedAt = new Date();
  }

  /**
   * Pause the state machine
   */
  pause(): void {
    if (this.instance.status === 'active') {
      this.instance.status = 'paused';
      this.emit('paused', { instance: this.instance });
    }
  }

  /**
   * Resume the state machine
   */
  resume(): void {
    if (this.instance.status === 'paused') {
      this.instance.status = 'active';
      this.emit('resumed', { instance: this.instance });
      this.processQueue();
    }
  }

  /**
   * Cancel the state machine
   */
  cancel(): void {
    this.instance.status = 'failed';
    this.instance.error = 'Cancelled';
    this.instance.completedAt = new Date();
    
    if (this.timeoutId) {
      clearTimeout(this.timeoutId);
    }
    
    this.eventQueue = [];
    this.emit('cancelled', { instance: this.instance });
  }

  /**
   * Get deep value from object by dot-notation path
   */
  private getDeepValue(obj: Record<string, unknown>, path: string): unknown {
    return path.split('.').reduce<unknown>((acc, key) => {
      if (acc && typeof acc === 'object' && key in acc) {
        return (acc as Record<string, unknown>)[key];
      }
      return undefined;
    }, obj);
  }
}

// ============================================================================
// State Machine Registry
// ============================================================================

/**
 * StateMachineRegistry - Manages multiple state machine instances
 */
export class StateMachineRegistry {
  private definitions: Map<string, StateMachineDefinition> = new Map();
  private instances: Map<string, DeterministicStateMachine> = new Map();

  /**
   * Register a state machine definition
   */
  register(definition: StateMachineDefinition): void {
    this.definitions.set(definition.id, definition);
  }

  /**
   * Create a new instance of a state machine
   */
  createInstance(definitionId: string, instanceId?: string): DeterministicStateMachine {
    const definition = this.definitions.get(definitionId);
    if (!definition) {
      throw new Error(`State machine definition ${definitionId} not found`);
    }

    const sm = new DeterministicStateMachine(definition, instanceId);
    this.instances.set(sm.getInstance().id, sm);
    
    return sm;
  }

  /**
   * Get an instance by ID
   */
  getInstance(instanceId: string): DeterministicStateMachine | undefined {
    return this.instances.get(instanceId);
  }

  /**
   * Get all instances
   */
  getAllInstances(): DeterministicStateMachine[] {
    return Array.from(this.instances.values());
  }

  /**
   * Get instances by status
   */
  getInstancesByStatus(status: StateStatus): DeterministicStateMachine[] {
    return this.getAllInstances().filter(sm => sm.getInstance().status === status);
  }

  /**
   * Remove an instance
   */
  removeInstance(instanceId: string): boolean {
    const sm = this.instances.get(instanceId);
    if (sm) {
      sm.cancel();
      return this.instances.delete(instanceId);
    }
    return false;
  }

  /**
   * Get statistics
   */
  getStats(): {
    definitions: number;
    instances: number;
    byStatus: Record<StateStatus, number>;
  } {
    const byStatus: Record<StateStatus, number> = {
      idle: 0,
      active: 0,
      waiting: 0,
      completed: 0,
      failed: 0,
      paused: 0
    };

    for (const sm of this.instances.values()) {
      byStatus[sm.getInstance().status]++;
    }

    return {
      definitions: this.definitions.size,
      instances: this.instances.size,
      byStatus
    };
  }
}

// Singleton registry
export const stateMachineRegistry = new StateMachineRegistry();