HealthLog/src/lib/rate-limit.ts

/**
 * PostgreSQL-based sliding window rate limiter.
 * Uses atomic upsert via raw SQL for correctness across multiple instances.
 */

import { prisma } from "@/lib/db";

export interface RateLimitResult {
  allowed: boolean;
  remaining: number;
  resetAt: number;
}

/**
 * Check rate limit for a given key.
 * Uses a single atomic SQL upsert — safe for concurrent requests.
 * @param key Unique identifier (e.g., "auth:login:1.2.3.4")
 * @param limit Max requests per window
 * @param windowMs Window duration in milliseconds
 */
export async function checkRateLimit(
  key: string,
  limit: number,
  windowMs: number,
): Promise<RateLimitResult> {
  const windowInterval = `${windowMs} milliseconds`;

  const rows = await prisma.$queryRaw<{ count: number; reset_at: Date }[]>`
    INSERT INTO rate_limits (key, count, reset_at)
    VALUES (${key}, 1, NOW() + ${windowInterval}::interval)
    ON CONFLICT (key) DO UPDATE SET
      count = CASE
        WHEN rate_limits.reset_at < NOW() THEN 1
        ELSE rate_limits.count + 1
      END,
      reset_at = CASE
        WHEN rate_limits.reset_at < NOW() THEN NOW() + ${windowInterval}::interval
        ELSE rate_limits.reset_at
      END
    RETURNING count, reset_at
  `;

  const row = rows[0];
  const resetAt = row.reset_at.getTime();
  const count = Number(row.count);

  return {
    allowed: count <= limit,
    remaining: Math.max(0, limit - count),
    resetAt,
  };
}

/**
 * Rate limit response headers.
 */
export function rateLimitHeaders(
  result: RateLimitResult,
): Record<string, string> {
  return {
    "X-RateLimit-Remaining": String(result.remaining),
    "X-RateLimit-Reset": new Date(result.resetAt).toISOString(),
  };
}