//! Memory-mapped storage with safe access patterns
//! 
//! Design: Unsafe raw pointers are contained within RawStorage.
//! All public access goes through IndexStorage with safe methods.

use memmap2::{MmapMut, MmapOptions};
use std::fs::{File, OpenOptions};
use std::io::{self, Write};
use std::path::{Path, PathBuf};
use std::sync::Arc;

/// Header stored at the beginning of the mmap file
#[repr(C)]
#[derive(Debug, Clone, Copy)]
pub struct IndexHeader {
    pub version: u64,
    pub magic: [u8; 8],
    pub entry_count: u64,
    pub last_modified: i64,
    pub checksum: u64,
}

impl IndexHeader {
    pub const MAGIC: [u8; 8] = *b"FETCHIDX";
    pub const VERSION: u64 = 1;
    pub const SIZE: usize = std::mem::size_of::<IndexHeader>();
    
    pub fn new() -> Self {
        Self {
            version: Self::VERSION,
            magic: Self::MAGIC,
            entry_count: 0,
            last_modified: 0,
            checksum: 0,
        }
    }
    
    pub fn is_valid(&self) -> bool {
        self.magic == Self::MAGIC && self.version == Self::VERSION
    }
}

/// Internal unsafe state - never exposed directly
struct RawStorage {
    mmap: MmapMut,
    header_ptr: *mut IndexHeader,
    data_offset: usize,
}

impl RawStorage {
    fn new(mmap: MmapMut) -> io::Result<Self> {
        let ptr = mmap.as_ptr() as *mut u8;
        let header_ptr = ptr as *mut IndexHeader;
        
        Ok(Self {
            mmap,
            header_ptr,
            data_offset: IndexHeader::SIZE,
        })
    }
    
    fn header(&self) -> &IndexHeader {
        unsafe { &*self.header_ptr }
    }
    
    fn header_mut(&mut self) -> &mut IndexHeader {
        unsafe { &mut *self.header_ptr }
    }
}

/// Public safe interface to mmap storage
pub struct IndexStorage {
    raw: RawStorage,
    path: PathBuf,
}

impl IndexStorage {
    /// Open or create storage at the given path
    pub fn open<P: AsRef<Path>>(path: P) -> io::Result<Self> {
        let path = path.as_ref().to_path_buf();
        std::fs::create_dir_all(&path)?;
        
        let file_path = path.join("index.dat");
        let file = OpenOptions::new()
            .read(true)
            .write(true)
            .create(true)
            .open(&file_path)?;
        
        // Ensure file is at least header size
        let file_size = file.metadata()?.len() as usize;
        let min_size = IndexHeader::SIZE;
        
        if file_size < min_size {
            let header = IndexHeader::new();
            let header_bytes = unsafe {
                std::slice::from_raw_parts(
                    &header as *const IndexHeader as *const u8,
                    IndexHeader::SIZE,
                )
            };
            file.set_len(min_size as u64)?;
            file.write_all_at(header_bytes, 0)?;
        }
        
        let mmap = unsafe { MmapOptions::new().map_mut(&file)? };
        let raw = RawStorage::new(mmap)?;
        
        Ok(Self { raw, path })
    }
    
    /// Get a reference to the header (read-only)
    pub fn header(&self) -> &IndexHeader {
        self.raw.header()
    }
    
    /// Get a mutable reference to the header
    pub fn header_mut(&mut self) -> &mut IndexHeader {
        self.raw.header_mut()
    }
    
    /// Verify header magic and version
    pub fn verify(&self) -> io::Result<()> {
        let header = self.header();
        if !header.is_valid() {
            return Err(io::Error::new(
                io::ErrorKind::InvalidData,
                "Invalid index header (wrong magic or version)",
            ));
        }
        Ok(())
    }
    
    /// Get the path to the storage directory
    pub fn path(&self) -> &Path {
        &self.path
    }
    
    /// Flush changes to disk
    pub fn flush(&mut self) -> io::Result<()> {
        self.raw.mmap.flush()
    }
}

/// Thread-safe wrapper for concurrent access
pub struct SharedStorage {
    inner: Arc<Mutex<IndexStorage>>,
}

impl SharedStorage {
    pub fn open<P: AsRef<Path>>(path: P) -> io::Result<Self> {
        let storage = IndexStorage::open(path)?;
        Ok(Self {
            inner: Arc::new(Mutex::new(storage)),
        })
    }
    
    pub fn lock(&self) -> MutexGuard<IndexStorage> {
        self.inner.lock().unwrap()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use tempfile::TempDir;
    
    #[test]
    fn test_storage_creation() {
        let temp = TempDir::new().unwrap();
        let storage = IndexStorage::open(temp.path()).unwrap();
        assert!(storage.header().is_valid());
    }
    
    #[test]
    fn test_storage_verify() {
        let temp = TempDir::new().unwrap();
        let storage = IndexStorage::open(temp.path()).unwrap();
        assert!(storage.verify().is_ok());
    }
    
    #[test]
    fn test_shared_storage() {
        let temp = TempDir::new().unwrap();
        let shared = SharedStorage::open(temp.path()).unwrap();
        
        {
            let storage = shared.lock();
            assert!(storage.header().is_valid());
        }
        
        {
            let mut storage = shared.lock();
            storage.header_mut().entry_count = 42;
        }
        
        {
            let storage = shared.lock();
            assert_eq!(storage.header().entry_count, 42);
        }
    }
}