/**
 * Circuit breaker states
 */
export enum CircuitState {
  /** Circuit is closed, requests flow normally */
  CLOSED = 'closed',

  /** Circuit is open, requests are rejected immediately */
  OPEN = 'open',

  /** Circuit is half-open, testing if service recovered */
  HALF_OPEN = 'half_open',
}

/**
 * Circuit breaker configuration
 */
export interface CircuitBreakerConfig {
  /** Number of failures before opening circuit */
  failureThreshold: number;

  /** Number of successes in half-open state before closing */
  successThreshold: number;

  /** Time in milliseconds circuit stays open before attempting half-open */
  timeout: number;

  /** Time window in milliseconds for counting failures */
  rollingWindow: number;

  /** Whether circuit breaker is enabled */
  enabled: boolean;
}

/**
 * Default circuit breaker configuration
 */
export const DEFAULT_CIRCUIT_BREAKER_CONFIG: CircuitBreakerConfig = {
  failureThreshold: 5,
  successThreshold: 2,
  timeout: 60000, // 1 minute
  rollingWindow: 10000, // 10 seconds
  enabled: true,
};

/**
 * Circuit breaker error thrown when circuit is open
 */
export class CircuitBreakerOpenError extends Error {
  constructor(
    public readonly circuitName: string,
    public readonly nextAttemptTime: Date
  ) {
    super(
      `Circuit breaker "${circuitName}" is OPEN. Next attempt at ${nextAttemptTime.toISOString()}`
    );
    this.name = 'CircuitBreakerOpenError';
  }
}

/**
 * Failure record for tracking
 */
interface FailureRecord {
  timestamp: number;
  error: Error;
}

/**
 * Circuit breaker for preventing cascading failures
 *
 * @example
 * ```typescript
 * const breaker = new CircuitBreaker('elasticsearch', {
 *   failureThreshold: 5,
 *   timeout: 60000,
 * });
 *
 * try {
 *   const result = await breaker.execute(async () => {
 *     return await someElasticsearchOperation();
 *   });
 * } catch (error) {
 *   if (error instanceof CircuitBreakerOpenError) {
 *     // Circuit is open, handle gracefully
 *   }
 * }
 * ```
 */
export class CircuitBreaker {
  private config: CircuitBreakerConfig;
  private state: CircuitState = CircuitState.CLOSED;
  private failures: FailureRecord[] = [];
  private successCount = 0;
  private openedAt?: number;
  private nextAttemptTime?: number;

  constructor(
    private name: string,
    config: Partial<CircuitBreakerConfig> = {}
  ) {
    this.config = { ...DEFAULT_CIRCUIT_BREAKER_CONFIG, ...config };
  }

  /**
   * Execute an operation through the circuit breaker
   */
  async execute<T>(operation: () => Promise<T>): Promise<T> {
    if (!this.config.enabled) {
      return operation();
    }

    // Check circuit state
    this.updateState();

    if (this.state === CircuitState.OPEN) {
      const nextAttempt = this.nextAttemptTime
        ? new Date(this.nextAttemptTime)
        : new Date(Date.now() + this.config.timeout);
      throw new CircuitBreakerOpenError(this.name, nextAttempt);
    }

    try {
      const result = await operation();
      this.onSuccess();
      return result;
    } catch (error) {
      this.onFailure(error as Error);
      throw error;
    }
  }

  /**
   * Handle successful operation
   */
  private onSuccess(): void {
    this.removeOldFailures();

    if (this.state === CircuitState.HALF_OPEN) {
      this.successCount++;

      if (this.successCount >= this.config.successThreshold) {
        this.transitionTo(CircuitState.CLOSED);
      }
    }
  }

  /**
   * Handle failed operation
   */
  private onFailure(error: Error): void {
    this.failures.push({
      timestamp: Date.now(),
      error,
    });

    this.removeOldFailures();

    if (this.state === CircuitState.HALF_OPEN) {
      // Any failure in half-open state opens the circuit immediately
      this.transitionTo(CircuitState.OPEN);
    } else if (this.state === CircuitState.CLOSED) {
      // Check if we've exceeded failure threshold
      if (this.failures.length >= this.config.failureThreshold) {
        this.transitionTo(CircuitState.OPEN);
      }
    }
  }

  /**
   * Update circuit state based on time
   */
  private updateState(): void {
    if (this.state === CircuitState.OPEN && this.nextAttemptTime) {
      if (Date.now() >= this.nextAttemptTime) {
        this.transitionTo(CircuitState.HALF_OPEN);
      }
    }
  }

  /**
   * Transition to a new state
   */
  private transitionTo(newState: CircuitState): void {
    const previousState = this.state;
    this.state = newState;

    switch (newState) {
      case CircuitState.OPEN:
        this.openedAt = Date.now();
        this.nextAttemptTime = Date.now() + this.config.timeout;
        this.successCount = 0;
        break;

      case CircuitState.HALF_OPEN:
        this.successCount = 0;
        break;

      case CircuitState.CLOSED:
        this.failures = [];
        this.successCount = 0;
        this.openedAt = undefined;
        this.nextAttemptTime = undefined;
        break;
    }

    if (previousState !== newState) {
      this.onStateChange(previousState, newState);
    }
  }

  /**
   * Remove failures outside the rolling window
   */
  private removeOldFailures(): void {
    const cutoff = Date.now() - this.config.rollingWindow;
    this.failures = this.failures.filter((f) => f.timestamp >= cutoff);
  }

  /**
   * Callback when state changes (can be overridden)
   */
  protected onStateChange(from: CircuitState, to: CircuitState): void {
    // Override in subclass or use getState() to monitor
    console.log(`Circuit breaker "${this.name}" transitioned from ${from} to ${to}`);
  }

  /**
   * Get current circuit state
   */
  getState(): CircuitState {
    this.updateState();
    return this.state;
  }

  /**
   * Get circuit statistics
   */
  getStats(): {
    state: CircuitState;
    failureCount: number;
    successCount: number;
    openedAt?: Date;
    nextAttemptTime?: Date;
  } {
    this.removeOldFailures();
    this.updateState();

    return {
      state: this.state,
      failureCount: this.failures.length,
      successCount: this.successCount,
      ...(this.openedAt && { openedAt: new Date(this.openedAt) }),
      ...(this.nextAttemptTime && { nextAttemptTime: new Date(this.nextAttemptTime) }),
    };
  }

  /**
   * Manually open the circuit
   */
  open(): void {
    this.transitionTo(CircuitState.OPEN);
  }

  /**
   * Manually close the circuit
   */
  close(): void {
    this.transitionTo(CircuitState.CLOSED);
  }

  /**
   * Reset the circuit breaker
   */
  reset(): void {
    this.failures = [];
    this.successCount = 0;
    this.openedAt = undefined;
    this.nextAttemptTime = undefined;
    this.state = CircuitState.CLOSED;
  }

  /**
   * Check if circuit is open
   */
  isOpen(): boolean {
    this.updateState();
    return this.state === CircuitState.OPEN;
  }

  /**
   * Check if circuit is closed
   */
  isClosed(): boolean {
    this.updateState();
    return this.state === CircuitState.CLOSED;
  }

  /**
   * Check if circuit is half-open
   */
  isHalfOpen(): boolean {
    this.updateState();
    return this.state === CircuitState.HALF_OPEN;
  }
}