"""Redis-based distributed rate limiting for AgentForms.

Uses sliding window counters in Redis for accurate rate limiting
across multiple worker instances.

Fallback: in-memory rate limiting when Redis is unavailable
(prevents fail-open security issue).

Usage:
    from app.services.ratelimit import RateLimiter

    limiter = RateLimiter()

    # Check API rate limit (per user, 100 req/min)
    if not limiter.allow("api:user:123"):
        abort(429, "Rate limit exceeded")

    # Check IP-based limit for public endpoints
    if not limiter.allow("ip:" + request.remote_addr, max_requests=20, window=60):
        abort(429, "Rate limit exceeded")
"""

import os
import time
import threading
import logging

logger = logging.getLogger("agentforms.ratelimit")
from app.services.logging import log as struct_log


class InMemoryRateLimiter:
    """Simple in-memory rate limiter using sliding window counters.
    
    Used as fallback when Redis is unavailable. Per-process only
    (not shared across workers), but prevents fail-open.
    """
    
    def __init__(self):
        self._counts = {}  # key -> [(timestamp, count)]
        self._lock = threading.Lock()
    
    def check(self, key: str, max_requests: int = 100, window: int = 60) -> tuple:
        """Check rate limit using in-memory sliding window.
        
        Returns:
            (allowed, remaining, reset_at) — bool, int, float.
        """
        now = time.time()
        cutoff = now - window
        
        with self._lock:
            # Get or create window entry
            if key not in self._counts:
                self._counts[key] = []
            
            # Remove expired entries
            entries = self._counts[key]
            self._counts[key] = [t for t in entries if t > cutoff]
            entries = self._counts[key]
            
            count = len(entries)
            reset_at = (entries[0] + window) if entries else now + window
            
            if count >= max_requests:
                remaining = 0
                allowed = False
            else:
                # Add this request
                self._counts[key].append(now)
                remaining = max(0, max_requests - count - 1)
                allowed = True
            
            # Periodic cleanup of old keys (every 1000 checks)
            if len(self._counts) > 1000:
                self._cleanup(cutoff)
        
        return (allowed, remaining, reset_at)
    
    def _cleanup(self, cutoff: float):
        """Remove expired entries (called while holding lock)."""
        expired = [k for k, v in self._counts.items() if not v or max(v) < cutoff]
        for k in expired:
            del self._counts[k]


class RateLimiter:
    """Sliding window rate limiter backed by Redis.

    Uses atomic Redis operations (INCR + EXPIRE) for correctness
    across multiple worker instances.
    """

    def __init__(self, redis_client=None):
        self._redis = redis_client
        self._connected = bool(redis_client)
        self._local = InMemoryRateLimiter()  # Fallback when Redis is down

    def connect(self, redis_client):
        """Set the Redis client (called from app init)."""
        self._redis = redis_client
        self._connected = True

    def allow(self, key: str, max_requests: int = 100, window: int = 60) -> bool:
        """Check if a request is allowed under the rate limit.

        Args:
            key: Unique rate limit key (e.g., "api:user:123")
            max_requests: Maximum requests allowed in the window
            window: Time window in seconds

        Returns:
            True if the request is allowed, False if rate limited.
            Always returns True if Redis is unavailable (fail-open).
        """
        allowed, _, _ = self.check(key, max_requests=max_requests, window=window)
        return allowed

    def check(self, key: str, max_requests: int = 100, window: int = 60) -> tuple:
        """Check rate limit and return detailed info.

        Args:
            key: Unique rate limit key (e.g., "api:user:123")
            max_requests: Maximum requests allowed in the window
            window: Time window in seconds

        Returns:
            (allowed, remaining, reset_at) — bool, int, float.
            allowed: True if request is permitted.
            remaining: Requests left in the current window.
            reset_at: Unix timestamp when the window resets.
            Falls back to in-memory rate limiter if Redis is unavailable
            (prevents fail-open security issue).
        """
        if not self._connected:
            # Fallback to in-memory rate limiting — do NOT fail open
            return self._local.check(key, max_requests=max_requests, window=window)

        try:
            pipe = self._redis.pipeline()
            pipe.incr(key)
            pipe.expire(key, window)
            results = pipe.execute()
            count = results[0]

            remaining = max(0, max_requests - count)
            reset_at = time.time() + window

            allowed = count <= max_requests
            if not allowed:
                struct_log.warning(
                    "ratelimit",
                    "Rate limit exceeded",
                    key=key,
                    count=count,
                    max=max_requests,
                    window=window,
                )

            return (allowed, remaining, reset_at)

        except Exception as e:
            logger.error("Rate limiter Redis error: %s", e)
            # Fallback to in-memory on Redis error — do NOT fail open
            struct_log.warning("ratelimit", "Redis error, using in-memory fallback", error=str(e))
            self._connected = False  # Switch to fallback mode
            return self._local.check(key, max_requests=max_requests, window=window)

    def flush(self, pattern: str = "*") -> None:
        """Flush rate limit keys matching pattern. Use with care — intended for tests."""
        if not self._connected:
            return
        try:
            keys = self._redis.keys(pattern)
            if keys:
                self._redis.delete(*keys)
        except Exception as e:
            logger.error("Rate limiter flush error: %s", e)

    def get_remaining(self, key: str, max_requests: int = 100, window: int = 60) -> tuple:
        """Get remaining requests and reset time for a key.

        Returns:
            (remaining, reset_at) — remaining requests and window reset timestamp.
            Falls back to in-memory limiter if Redis unavailable.
        """
        if not self._connected:
            # In-memory fallback — estimate from last check result
            allowed, remaining, reset_at = self._local.check(key, max_requests=max_requests, window=window)
            return (remaining, reset_at)

        try:
            count = self._redis.get(key)
            ttl = self._redis.ttl(key)

            if count is None:
                return (max_requests, time.time() + window)

            remaining = max(0, max_requests - int(count))
            reset_at = time.time() + (ttl if ttl > 0 else window)

            return (remaining, reset_at)

        except Exception as e:
            logger.error("Rate limiter Redis error: %s", e)
            return (max_requests, 0)


# Module-level singleton
limiter = RateLimiter()
