from __future__ import annotations

from dataclasses import dataclass
from hashlib import sha256
import logging
import pickle
from threading import RLock
import time
from typing import Callable, Protocol, TypeVar

from app.core.config import get_settings

try:
    import redis
except Exception:  # pragma: no cover - optional dependency in local dev
    redis = None

T = TypeVar("T")
logger = logging.getLogger(__name__)


class CacheBackend(Protocol):
    def get(self, key: str) -> object | None: ...
    def set(self, key: str, value: object, ttl_seconds: int = 30) -> object: ...
    def get_or_set(self, key: str, factory: Callable[[], T], ttl_seconds: int = 30) -> T: ...
    def bump_revision(self) -> int: ...
    def revision(self) -> int: ...


@dataclass
class _CacheEntry:
    value: object
    expires_at: float
    revision: int


class MemoryRuntimeCache:
    """Simple in-memory cache for local/single-instance environments."""

    def __init__(self) -> None:
        self._lock = RLock()
        self._revision = 0
        self._entries: dict[str, _CacheEntry] = {}

    def get(self, key: str) -> object | None:
        now = time.time()
        with self._lock:
            entry = self._entries.get(key)
            if not entry:
                return None
            if entry.expires_at <= now or entry.revision != self._revision:
                self._entries.pop(key, None)
                return None
            return entry.value

    def set(self, key: str, value: object, ttl_seconds: int = 30) -> object:
        now = time.time()
        with self._lock:
            self._entries[key] = _CacheEntry(
                value=value,
                expires_at=now + max(ttl_seconds, 1),
                revision=self._revision,
            )
            if len(self._entries) > 2000:
                self._entries.clear()
        return value

    def get_or_set(self, key: str, factory: Callable[[], T], ttl_seconds: int = 30) -> T:
        cached = self.get(key)
        if cached is not None:
            return cached  # type: ignore[return-value]
        return self.set(key, factory(), ttl_seconds=ttl_seconds)  # type: ignore[return-value]

    def bump_revision(self) -> int:
        with self._lock:
            self._revision += 1
            if self._revision > 1_000_000_000:
                self._revision = 1
            self._entries.clear()
            return self._revision

    def revision(self) -> int:
        with self._lock:
            return self._revision


class RedisRuntimeCache:
    """Redis-backed cache for multi-instance deployments."""

    def __init__(self, *, redis_url: str, prefix: str, default_ttl_seconds: int = 30) -> None:
        if redis is None:
            raise RuntimeError("redis_package_not_installed")
        self._redis = redis.Redis.from_url(redis_url, decode_responses=False)
        self._prefix = prefix.strip() or "memberapi"
        self._default_ttl_seconds = max(int(default_ttl_seconds), 1)
        self._revision_key = f"{self._prefix}:cache:revision"
        self._rev_cache_value = 0
        self._rev_cache_expires_at = 0.0

    def _cache_key(self, key: str, revision: int) -> str:
        key_hash = sha256(key.encode("utf-8")).hexdigest()
        return f"{self._prefix}:cache:{revision}:{key_hash}"

    def _get_revision_cached(self) -> int:
        now = time.time()
        if now < self._rev_cache_expires_at:
            return self._rev_cache_value
        try:
            raw = self._redis.get(self._revision_key)
            value = int(raw) if raw else 0
        except Exception:
            return 0
        self._rev_cache_value = value
        self._rev_cache_expires_at = now + 1.0
        return value

    def get(self, key: str) -> object | None:
        try:
            revision = self._get_revision_cached()
            raw = self._redis.get(self._cache_key(key, revision))
            if raw is None:
                return None
            return pickle.loads(raw)
        except Exception:
            return None

    def set(self, key: str, value: object, ttl_seconds: int = 30) -> object:
        ttl = max(int(ttl_seconds), 1) if ttl_seconds else self._default_ttl_seconds
        try:
            revision = self._get_revision_cached()
            self._redis.setex(self._cache_key(key, revision), ttl, pickle.dumps(value))
        except Exception:
            # Keep request path healthy even when Redis has issues.
            pass
        return value

    def get_or_set(self, key: str, factory: Callable[[], T], ttl_seconds: int = 30) -> T:
        cached = self.get(key)
        if cached is not None:
            return cached  # type: ignore[return-value]
        return self.set(key, factory(), ttl_seconds=ttl_seconds)  # type: ignore[return-value]

    def bump_revision(self) -> int:
        try:
            value = int(self._redis.incr(self._revision_key))
            self._rev_cache_value = value
            self._rev_cache_expires_at = time.time() + 1.0
            return value
        except Exception:
            # Fail-open: keep app usable; caller still succeeds.
            return self._get_revision_cached()

    def revision(self) -> int:
        return self._get_revision_cached()


def _build_runtime_cache() -> CacheBackend:
    settings = get_settings()
    backend = (settings.cache_backend or "memory").strip().lower()
    if backend == "redis":
        try:
            cache = RedisRuntimeCache(
                redis_url=settings.cache_redis_url,
                prefix=settings.cache_prefix,
                default_ttl_seconds=settings.cache_default_ttl_seconds,
            )
            logger.info("runtime cache backend: redis")
            return cache
        except Exception as exc:
            logger.warning("redis cache unavailable, fallback to memory: %s", exc)
    logger.info("runtime cache backend: memory")
    return MemoryRuntimeCache()


runtime_cache: CacheBackend = _build_runtime_cache()