use anyhow::{Context, Result};
use serde::{Deserialize, Serialize};
use std::collections::HashMap;
use std::path::{Path, PathBuf};
use std::sync::Arc;
use tokio::sync::Mutex;
use tracing::{debug, info};

use crate::config;
use crate::crypto;
use crate::crypto::wal;

#[derive(Debug, Clone, Serialize, Deserialize)]
struct MtimeEntry {
    db_mt: u64,
    wal_mt: u64,
    path: String,
}

#[derive(Debug, Clone)]
struct CacheEntry {
    db_mtime: u64,
    wal_mtime: u64,
    decrypted_path: PathBuf,
}

/// 解密后数据库的 mtime-aware 缓存
///
/// 当数据库文件（.db）或 WAL 文件（.db-wal）的 mtime 发生变化时，
/// 自动重新解密并更新缓存。跨进程重启可通过持久化 mtime 文件复用已解密的 DB。
pub struct DbCache {
    db_dir: PathBuf,
    cache_dir: PathBuf,
    all_keys: HashMap<String, String>, // rel_key -> enc_key(hex)
    inner: Arc<Mutex<HashMap<String, CacheEntry>>>,
}

impl DbCache {
    pub async fn new(
        db_dir: PathBuf,
        all_keys: HashMap<String, String>,
    ) -> Result<Self> {
        let cache_dir = config::cache_dir();
        tokio::fs::create_dir_all(&cache_dir).await?;

        let inner: HashMap<String, CacheEntry> = HashMap::new();
        let cache = DbCache {
            db_dir,
            cache_dir,
            all_keys,
            inner: Arc::new(Mutex::new(inner)),
        };

        cache.load_persistent().await;
        Ok(cache)
    }

    fn cache_file_path(&self, rel_key: &str) -> PathBuf {
        let hash = format!("{:x}", md5::compute(rel_key.as_bytes()));
        self.cache_dir.join(format!("{}.db", hash))
    }

    /// 从持久化文件加载 mtime 记录，复用未过期的解密文件
    async fn load_persistent(&self) {
        let mtime_file = config::mtime_file();
        let content = match tokio::fs::read_to_string(&mtime_file).await {
            Ok(c) => c,
            Err(_) => return,
        };
        let saved: HashMap<String, MtimeEntry> = match serde_json::from_str(&content) {
            Ok(v) => v,
            Err(_) => return,
        };

        let mut inner = self.inner.lock().await;
        let mut reused = 0usize;
        for (rel_key, entry) in &saved {
            let dec_path = PathBuf::from(&entry.path);
            if !dec_path.exists() {
                continue;
            }
            let db_path = self.db_dir.join(rel_key.replace('\\', std::path::MAIN_SEPARATOR_STR).replace('/', std::path::MAIN_SEPARATOR_STR));
            let wal_path = wal_path_for(&db_path);

            let db_mt = mtime_nanos(&db_path);
            let wal_mt = if wal_path.exists() { mtime_nanos(&wal_path) } else { 0 };

            if db_mt == entry.db_mt && wal_mt == entry.wal_mt {
                inner.insert(rel_key.clone(), CacheEntry {
                    db_mtime: db_mt,
                    wal_mtime: wal_mt,
                    decrypted_path: dec_path,
                });
                reused += 1;
            }
        }
        if reused > 0 {
            info!(reused, "复用已解密 DB");
        }
    }

    /// 持久化 mtime 记录
    async fn save_persistent(&self) {
        let mtime_file = config::mtime_file();
        let inner = self.inner.lock().await;
        let data: HashMap<String, MtimeEntry> = inner.iter().map(|(k, v)| {
            (k.clone(), MtimeEntry {
                db_mt: v.db_mtime,
                wal_mt: v.wal_mtime,
                path: v.decrypted_path.to_string_lossy().into_owned(),
            })
        }).collect();
        drop(inner);

        if let Ok(json) = serde_json::to_string_pretty(&data) {
            let _ = tokio::fs::write(&mtime_file, json).await;
        }
    }

    /// 获取解密后的数据库路径
    ///
    /// 如果 mtime 未变，直接返回缓存路径；否则重新解密
    pub async fn get(&self, rel_key: &str) -> Result<Option<PathBuf>> {
        let enc_key_hex = match self.all_keys.get(rel_key) {
            Some(k) => k.clone(),
            None => return Ok(None),
        };

        let db_path = self.db_dir.join(
            rel_key.replace('\\', std::path::MAIN_SEPARATOR_STR)
                   .replace('/', std::path::MAIN_SEPARATOR_STR)
        );
        if !db_path.exists() {
            return Ok(None);
        }

        let wal_path = wal_path_for(&db_path);

        let db_mt = mtime_nanos(&db_path);
        let wal_mt = if wal_path.exists() { mtime_nanos(&wal_path) } else { 0 };

        // 检查缓存
        {
            let inner = self.inner.lock().await;
            if let Some(entry) = inner.get(rel_key) {
                if entry.db_mtime == db_mt
                    && entry.wal_mtime == wal_mt
                    && entry.decrypted_path.exists()
                {
                    debug!(db = rel_key, "缓存命中");
                    return Ok(Some(entry.decrypted_path.clone()));
                }
            }
        }

        // 需要重新解密
        let out_path = self.cache_file_path(rel_key);
        let enc_key_bytes = hex_to_32bytes(&enc_key_hex)
            .with_context(|| format!("密钥格式错误: {}", rel_key))?;

        let t0 = std::time::Instant::now();
        let db_path2 = db_path.clone();
        let out_path2 = out_path.clone();
        let key_copy = enc_key_bytes;
        tokio::task::spawn_blocking(move || {
            crypto::full_decrypt(&db_path2, &out_path2, &key_copy)
        }).await??;

        // 应用 WAL
        if wal_path.exists() {
            let out_path3 = out_path.clone();
            let wal_path3 = wal_path.clone();
            let key_copy2 = enc_key_bytes;
            tokio::task::spawn_blocking(move || {
                wal::apply_wal(&wal_path3, &out_path3, &key_copy2)
            }).await??;
        }

        let elapsed_ms = t0.elapsed().as_millis();
        info!(db = rel_key, elapsed_ms, "解密完成");

        // 更新内存缓存
        {
            let mut inner = self.inner.lock().await;
            inner.insert(rel_key.to_string(), CacheEntry {
                db_mtime: db_mt,
                wal_mtime: wal_mt,
                decrypted_path: out_path.clone(),
            });
        }

        self.save_persistent().await;
        Ok(Some(out_path))
    }
}

pub(super) fn mtime_nanos(path: &Path) -> u64 {
    std::fs::metadata(path)
        .and_then(|m| m.modified())
        .map(|t| t.duration_since(std::time::UNIX_EPOCH).unwrap_or_default().as_nanos() as u64)
        .unwrap_or(0)
}

/// `foo/bar.db` → `foo/bar.db-wal`（用 OsString 拼接，避免 display() 的 UTF-8 问题）
fn wal_path_for(db_path: &Path) -> PathBuf {
    let mut name = db_path.file_name().unwrap_or_default().to_os_string();
    name.push("-wal");
    db_path.with_file_name(name)
}

fn hex_to_32bytes(s: &str) -> Result<[u8; 32]> {
    if s.len() != 64 {
        anyhow::bail!("密钥 hex 长度应为 64，实际为 {}", s.len());
    }
    let mut out = [0u8; 32];
    for i in 0..32 {
        out[i] = u8::from_str_radix(&s[i * 2..i * 2 + 2], 16)
            .with_context(|| format!("非法 hex 字符 at {}", i * 2))?;
    }
    Ok(out)
}