import { Injectable } from '@nestjs/common';

type Bucket = {
  requests: number;
  errors: number;
  totalDurationMs: number;
};

@Injectable()
export class MetricsService {
  private readonly bucket: Bucket = { requests: 0, errors: 0, totalDurationMs: 0 };
  private startedAt = Date.now();

  record(statusCode: number, durationMs: number): void {
    this.bucket.requests += 1;
    if (statusCode >= 400) this.bucket.errors += 1;
    this.bucket.totalDurationMs += durationMs;
  }

  snapshot() {
    const uptimeSec = Math.max(1, Math.floor((Date.now() - this.startedAt) / 1000));
    const qps = this.bucket.requests / uptimeSec;
    const avgLatencyMs = this.bucket.requests > 0 ? this.bucket.totalDurationMs / this.bucket.requests : 0;
    const errorRate = this.bucket.requests > 0 ? this.bucket.errors / this.bucket.requests : 0;

    return {
      requests: this.bucket.requests,
      errors: this.bucket.errors,
      qps,
      avgLatencyMs,
      errorRate,
      uptimeSec,
    };
  }
}