'use strict';

/**
 * Circuit Breaker v2 — 增强版 per-Provider 熔断器
 *
 * 改进点:
 * 1. 半衰期自动恢复 (失败计数随时间衰减, 避免永久 OPEN)
 * 2. 可配置阈值 (通过环境变量覆盖)
 * 3. 管理端点支持 (getStatus + reset + resetAll)
 * 4. 统计指标 (总请求数、成功率、平均恢复时间)
 * 5. 事件日志 (状态变迁记录, 供审计)
 *
 * @module src/circuit-breaker
 */

const STATES = { CLOSED: 'closed', OPEN: 'open', HALF_OPEN: 'half_open' };

// 可通过环境变量覆盖
const FAILURE_THRESHOLD = parseInt(process.env.CB_FAILURE_THRESHOLD) || 5;
const RECOVERY_TIMEOUT = parseInt(process.env.CB_RECOVERY_TIMEOUT) || 30000;
const SUCCESS_THRESHOLD = parseInt(process.env.CB_SUCCESS_THRESHOLD) || 2;
const HALF_LIFE_MS = parseInt(process.env.CB_HALF_LIFE_MS) || 120000; // 2 分钟半衰期

const breakers = {};

// 状态变迁日志 (保留最近 100 条)
const MAX_LOG_SIZE = 100;
const transitionLog = [];

function _log(provider, from, to, reason) {
  const entry = {
    ts: new Date().toISOString(),
    provider,
    from,
    to,
    reason,
  };
  transitionLog.push(entry);
  if (transitionLog.length > MAX_LOG_SIZE) transitionLog.shift();
}

function _getBreaker(provider) {
  if (!breakers[provider]) {
    breakers[provider] = {
      state: STATES.CLOSED,
      failures: 0,
      lastFailureAt: 0,
      successCount: 0,
      totalTrips: 0,
      // 统计
      totalRequests: 0,
      totalSuccesses: 0,
      totalFailures: 0,
      lastTripAt: 0,
      lastRecoveryAt: 0,
      tripDurations: [], // 最近 10 次恢复耗时(ms)
    };
  }
  return breakers[provider];
}

/**
 * 半衰期衰减: failures 随时间指数衰减
 * 避免偶尔的瞬态错误累积到阈值
 */
function _decayedFailures(b) {
  if (b.failures === 0 || b.lastFailureAt === 0) return 0;
  const elapsed = Date.now() - b.lastFailureAt;
  if (elapsed <= 0) return b.failures;
  // 每过一个半衰期, failures 减半
  const decayFactor = Math.pow(0.5, elapsed / HALF_LIFE_MS);
  return b.failures * decayFactor;
}

/**
 * 检查是否允许向 provider 发送请求
 */
function canRequest(provider) {
  const b = _getBreaker(provider);

  if (b.state === STATES.CLOSED) {
    b.totalRequests++;
    return true;
  }

  if (b.state === STATES.OPEN) {
    if (Date.now() - b.lastFailureAt > RECOVERY_TIMEOUT) {
      const prev = b.state;
      b.state = STATES.HALF_OPEN;
      b.successCount = 0;
      _log(provider, prev, STATES.HALF_OPEN, 'recovery_timeout_elapsed');
      b.totalRequests++;
      return true;
    }
    b.totalRejected = (b.totalRejected || 0) + 1;
    return false;
  }

  // HALF_OPEN: 允许有限探测
  b.totalRequests++;
  return true;
}

/**
 * 记录成功
 */
function recordSuccess(provider) {
  const b = _getBreaker(provider);
  b.totalSuccesses++;

  if (b.state === STATES.HALF_OPEN) {
    b.successCount++;
    if (b.successCount >= SUCCESS_THRESHOLD) {
      const prev = b.state;
      b.state = STATES.CLOSED;
      b.failures = 0;
      b.successCount = 0;
      b.lastRecoveryAt = Date.now();
      // 记录恢复耗时
      if (b.lastTripAt > 0) {
        b.tripDurations.push(Date.now() - b.lastTripAt);
        if (b.tripDurations.length > 10) b.tripDurations.shift();
      }
      _log(provider, prev, STATES.CLOSED, `${SUCCESS_THRESHOLD}_consecutive_successes`);
    }
  } else if (b.state === STATES.CLOSED) {
    // 使用衰减后的失败数, 而非直接清零
    // 这样偶尔的成功会让累积的失败自然消散
    b.failures = Math.max(0, _decayedFailures(b) - 0.5);
  }
}

/**
 * 记录失败
 */
function recordFailure(provider) {
  const b = _getBreaker(provider);
  b.totalFailures++;
  b.lastFailureAt = Date.now();

  if (b.state === STATES.HALF_OPEN) {
    const prev = b.state;
    b.state = STATES.OPEN;
    b.totalTrips++;
    b.lastTripAt = Date.now();
    _log(provider, prev, STATES.OPEN, 'half_open_failure');
  } else if (b.state === STATES.CLOSED) {
    // 使用衰减后的值 + 1
    b.failures = _decayedFailures(b) + 1;
    if (b.failures >= FAILURE_THRESHOLD) {
      const prev = b.state;
      b.state = STATES.OPEN;
      b.totalTrips++;
      b.lastTripAt = Date.now();
      _log(provider, prev, STATES.OPEN, `${FAILURE_THRESHOLD}_failures_reached`);
    }
  }
}

/**
 * 获取所有熔断器状态快照 (管理后台用)
 */
function getStatus() {
  const snapshot = {};
  for (const [name, b] of Object.entries(breakers)) {
    const successRate = b.totalRequests > 0
      ? Math.round(b.totalSuccesses / b.totalRequests * 10000) / 100
      : 100;
    const avgRecoveryMs = b.tripDurations.length > 0
      ? Math.round(b.tripDurations.reduce((a, c) => a + c, 0) / b.tripDurations.length)
      : 0;

    snapshot[name] = {
      state: b.state,
      failures: Math.round(_decayedFailures(b) * 100) / 100,
      rawFailures: b.failures,
      totalTrips: b.totalTrips,
      lastFailureAt: b.lastFailureAt ? new Date(b.lastFailureAt).toISOString() : null,
      lastRecoveryAt: b.lastRecoveryAt ? new Date(b.lastRecoveryAt).toISOString() : null,
      stats: {
        totalRequests: b.totalRequests,
        totalSuccesses: b.totalSuccesses,
        totalFailures: b.totalFailures,
        successRate: successRate + '%',
        avgRecoveryMs,
      },
    };
  }
  return snapshot;
}

/**
 * 获取状态变迁日志
 */
function getTransitionLog(limit) {
  const n = limit || 20;
  return transitionLog.slice(-n);
}

/**
 * 手动重置单个 provider
 */
function reset(provider) {
  if (breakers[provider]) {
    const prev = breakers[provider].state;
    breakers[provider].state = STATES.CLOSED;
    breakers[provider].failures = 0;
    breakers[provider].lastFailureAt = 0;
    breakers[provider].successCount = 0;
    _log(provider, prev, STATES.CLOSED, 'manual_reset');
  }
}

/**
 * 重置所有 provider
 */
function resetAll() {
  for (const name of Object.keys(breakers)) {
    reset(name);
  }
}

module.exports = {
  canRequest, recordSuccess, recordFailure,
  getStatus, getTransitionLog,
  reset, resetAll,
  STATES,
};