"""
Redis-backed response caching utilities.

Design goals:
- Async Redis client with graceful degradation when Redis is unavailable
- Stable cache key builder
- JSON storage with ETag computation
- Simple helpers for endpoints to get/set cached responses
"""

from __future__ import annotations

import functools
import inspect
import logging
from typing import Any, Dict, Optional, Tuple
import hashlib
import asyncio

import orjson

logger = logging.getLogger(__name__)

try:
    from redis import asyncio as aioredis
except Exception:  # pragma: no cover - library import safety
    aioredis = None  # type: ignore

from app.core.config import settings


_redis_client: Optional["aioredis.Redis"] = None
_redis_lock = asyncio.Lock()


def _serialize(obj: Any) -> bytes:
    """Serialize Python object to JSON bytes using orjson."""
    return orjson.dumps(obj)


def _deserialize(data: Optional[bytes]) -> Optional[Dict[str, Any]]:
    if not data:
        return None
    try:
        return orjson.loads(data)
    except Exception as e:
        logger.debug("Cache deserialization failed: %s", e)
        return None


def compute_etag(payload_bytes: bytes) -> str:
    """Compute strong ETag for given payload bytes."""
    return hashlib.sha256(payload_bytes).hexdigest()


async def get_redis() -> Optional["aioredis.Redis"]:
    """Get a shared async Redis client. Returns None if Redis unavailable."""
    global _redis_client
    if aioredis is None:
        return None
    if _redis_client is not None:
        return _redis_client
    async with _redis_lock:
        if _redis_client is not None:
            return _redis_client
        try:
            _redis_client = aioredis.from_url(settings.REDIS_URL, encoding="utf-8", decode_responses=False)
            # Light-touch ping to verify connectivity (do not raise)
            try:
                await _redis_client.ping()
            except Exception as e:
                logger.debug("Redis ping failed: %s", e)
                pass
            return _redis_client
        except Exception as e:
            logger.debug("Redis connection failed: %s", e)
            return None


def build_cache_key(namespace: str, *parts: Any) -> str:
    """Build a stable cache key using namespace and parts.

    Each part is converted to string and stripped. Empty parts are skipped.
    """
    key_parts = [namespace]
    for p in parts:
        if p is None:
            continue
        s = str(p).strip()
        if not s:
            continue
        key_parts.append(s)
    return ":".join(key_parts)


async def get_cached_response(key: str) -> Optional[Tuple[Dict[str, Any], str]]:
    """Get cached response body and its ETag. Returns None if missing or on error.

    The cached value is stored as JSON with shape: {"etag": str, "body": {...}}.
    """
    client = await get_redis()
    if client is None:
        return None
    try:
        raw = await client.get(key)
        data = _deserialize(raw)
        if not data or "body" not in data:
            return None
        etag = data.get("etag")
        # If etag missing, compute it from body
        if not etag:
            etag = compute_etag(_serialize(data["body"]))
        return data["body"], etag
    except Exception as e:
        logger.debug("Cache get failed for key: %s", e)
        return None


async def set_cached_response(key: str, body: Dict[str, Any], ttl_seconds: Optional[int] = None) -> str:
    """Cache response body with ETag. Returns the computed ETag.

    If Redis is unavailable, this function is a no-op and returns the ETag anyway.
    """
    payload_bytes = _serialize(body)
    etag = compute_etag(payload_bytes)
    record = {"etag": etag, "body": body}
    client = await get_redis()
    if client is None:
        return etag
    try:
        if ttl_seconds is None:
            ttl_seconds = max(60, int(getattr(settings, "CACHE_TTL", 3600)))
        await client.set(key, _serialize(record), ex=ttl_seconds)
    except Exception as e:
        logger.debug("Cache set failed: %s", e)
    return etag


def with_cache(namespace: str, ttl: int = None, key_params: list[str] = None):
    """
    FastAPI endpoint caching decorator.
    - Detects force_refresh parameter automatically
    - Sets X-Cache/Cache-Control/ETag/X-Data-Source headers on Response
    - key_params: list of function argument names to include in cache key
    """
    def decorator(func):
        @functools.wraps(func)
        async def wrapper(*args, **kwargs):
            # Extract bound arguments
            sig = inspect.signature(func)
            bound = sig.bind(*args, **kwargs)
            bound.apply_defaults()
            arguments = bound.arguments

            # Build cache key from key_params
            parts = []
            for p in (key_params or []):
                val = arguments.get(p)
                if val is not None:
                    parts.append(str(val))
            cache_key = build_cache_key(namespace, *parts)

            # Check force_refresh
            force_refresh = arguments.get("force_refresh", False)

            # Get Response object for headers
            response = arguments.get("response")

            effective_ttl = ttl if ttl is not None else max(60, int(getattr(settings, "CACHE_TTL", 3600)))

            # Try cache
            if not force_refresh:
                cached = await get_cached_response(cache_key)
                if cached:
                    cached_body, etag = cached
                    if response is not None:
                        response.headers["X-Cache"] = "HIT"
                        response.headers["Cache-Control"] = f"public, max-age={effective_ttl}"
                        response.headers["ETag"] = etag
                        response.headers["X-Data-Source"] = "redis-cache"
                    return cached_body

            # Cache miss — call original function
            result = await func(*args, **kwargs)

            # Cache the result
            if isinstance(result, dict):
                body_dict = result
            elif hasattr(result, "model_dump"):
                body_dict = result.model_dump()
            else:
                body_dict = result

            etag = await set_cached_response(cache_key, body_dict, ttl_seconds=effective_ttl)
            if response is not None:
                response.headers["X-Cache"] = "MISS"
                response.headers["Cache-Control"] = f"public, max-age={effective_ttl}"
                response.headers["ETag"] = etag

            return result

        return wrapper
    return decorator