import type { TCircuitState, IUpstreamResilienceConfig } from './interfaces.upstream.js';
import { DEFAULT_RESILIENCE_CONFIG } from './interfaces.upstream.js';

/**
 * Circuit breaker implementation for upstream resilience.
 *
 * States:
 * - CLOSED: Normal operation, requests pass through
 * - OPEN: Circuit is tripped, requests fail fast
 * - HALF_OPEN: Testing if upstream has recovered
 *
 * Transitions:
 * - CLOSED → OPEN: When failure count exceeds threshold
 * - OPEN → HALF_OPEN: After reset timeout expires
 * - HALF_OPEN → CLOSED: On successful request
 * - HALF_OPEN → OPEN: On failed request
 */
export class CircuitBreaker {
  /** Unique identifier for logging and metrics */
  public readonly id: string;

  /** Current circuit state */
  private state: TCircuitState = 'CLOSED';

  /** Count of consecutive failures */
  private failureCount: number = 0;

  /** Timestamp when circuit was opened */
  private openedAt: number = 0;

  /** Number of successful requests in half-open state */
  private halfOpenSuccesses: number = 0;

  /** Configuration */
  private readonly config: IUpstreamResilienceConfig;

  /** Number of successes required to close circuit from half-open */
  private readonly halfOpenThreshold: number = 2;

  constructor(id: string, config?: Partial<IUpstreamResilienceConfig>) {
    this.id = id;
    this.config = { ...DEFAULT_RESILIENCE_CONFIG, ...config };
  }

  /**
   * Get current circuit state.
   */
  public getState(): TCircuitState {
    // Check if we should transition from OPEN to HALF_OPEN
    if (this.state === 'OPEN') {
      const elapsed = Date.now() - this.openedAt;
      if (elapsed >= this.config.circuitBreakerResetMs) {
        this.transitionTo('HALF_OPEN');
      }
    }
    return this.state;
  }

  /**
   * Check if circuit allows requests.
   * Returns true if requests should be allowed.
   */
  public canRequest(): boolean {
    const currentState = this.getState();
    return currentState !== 'OPEN';
  }

  /**
   * Record a successful request.
   * May transition circuit from HALF_OPEN to CLOSED.
   */
  public recordSuccess(): void {
    if (this.state === 'HALF_OPEN') {
      this.halfOpenSuccesses++;
      if (this.halfOpenSuccesses >= this.halfOpenThreshold) {
        this.transitionTo('CLOSED');
      }
    } else if (this.state === 'CLOSED') {
      // Reset failure count on success
      this.failureCount = 0;
    }
  }

  /**
   * Record a failed request.
   * May transition circuit from CLOSED/HALF_OPEN to OPEN.
   */
  public recordFailure(): void {
    if (this.state === 'HALF_OPEN') {
      // Any failure in half-open immediately opens circuit
      this.transitionTo('OPEN');
    } else if (this.state === 'CLOSED') {
      this.failureCount++;
      if (this.failureCount >= this.config.circuitBreakerThreshold) {
        this.transitionTo('OPEN');
      }
    }
  }

  /**
   * Force circuit to open state.
   * Useful for manual intervention or external health checks.
   */
  public forceOpen(): void {
    this.transitionTo('OPEN');
  }

  /**
   * Force circuit to closed state.
   * Useful for manual intervention after fixing upstream issues.
   */
  public forceClose(): void {
    this.transitionTo('CLOSED');
  }

  /**
   * Reset circuit to initial state.
   */
  public reset(): void {
    this.state = 'CLOSED';
    this.failureCount = 0;
    this.openedAt = 0;
    this.halfOpenSuccesses = 0;
  }

  /**
   * Get circuit metrics for monitoring.
   */
  public getMetrics(): ICircuitBreakerMetrics {
    return {
      id: this.id,
      state: this.getState(),
      failureCount: this.failureCount,
      openedAt: this.openedAt > 0 ? new Date(this.openedAt) : null,
      timeUntilHalfOpen: this.state === 'OPEN'
        ? Math.max(0, this.config.circuitBreakerResetMs - (Date.now() - this.openedAt))
        : 0,
      halfOpenSuccesses: this.halfOpenSuccesses,
      threshold: this.config.circuitBreakerThreshold,
      resetMs: this.config.circuitBreakerResetMs,
    };
  }

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

    switch (newState) {
      case 'OPEN':
        this.openedAt = Date.now();
        this.halfOpenSuccesses = 0;
        break;
      case 'HALF_OPEN':
        this.halfOpenSuccesses = 0;
        break;
      case 'CLOSED':
        this.failureCount = 0;
        this.openedAt = 0;
        this.halfOpenSuccesses = 0;
        break;
    }

    // Log state transition (useful for debugging and monitoring)
    // In production, this would emit events or metrics
    if (previousState !== newState) {
      // State changed - could emit event here
    }
  }
}

/**
 * Metrics for circuit breaker monitoring.
 */
export interface ICircuitBreakerMetrics {
  /** Circuit breaker identifier */
  id: string;
  /** Current state */
  state: TCircuitState;
  /** Number of consecutive failures */
  failureCount: number;
  /** When circuit was opened (null if never opened) */
  openedAt: Date | null;
  /** Milliseconds until circuit transitions to half-open (0 if not open) */
  timeUntilHalfOpen: number;
  /** Number of successes in half-open state */
  halfOpenSuccesses: number;
  /** Failure threshold for opening circuit */
  threshold: number;
  /** Reset timeout in milliseconds */
  resetMs: number;
}

/**
 * Execute a function with circuit breaker protection.
 *
 * @param breaker The circuit breaker to use
 * @param fn The async function to execute
 * @param fallback Optional fallback function when circuit is open
 * @returns The result of fn or fallback
 * @throws CircuitOpenError if circuit is open and no fallback provided
 */
export async function withCircuitBreaker<T>(
  breaker: CircuitBreaker,
  fn: () => Promise<T>,
  fallback?: () => Promise<T>,
): Promise<T> {
  if (!breaker.canRequest()) {
    if (fallback) {
      return fallback();
    }
    throw new CircuitOpenError(breaker.id);
  }

  try {
    const result = await fn();
    breaker.recordSuccess();
    return result;
  } catch (error) {
    breaker.recordFailure();
    throw error;
  }
}

/**
 * Error thrown when circuit is open and no fallback is provided.
 */
export class CircuitOpenError extends Error {
  public readonly circuitId: string;

  constructor(circuitId: string) {
    super(`Circuit breaker '${circuitId}' is open`);
    this.name = 'CircuitOpenError';
    this.circuitId = circuitId;
  }
}