//! Helpers for revlog file reading and writing.

use std::{
    cell::RefCell,
    fs::File,
    io::{Read, Seek, SeekFrom, Write},
    path::{Path, PathBuf},
    sync::{Arc, Mutex},
};

use crate::{
    errors::{HgError, IoResultExt},
    vfs::Vfs,
};

/// Wraps accessing arbitrary chunks of data within a file and reusing handles.
/// This is currently useful for accessing a revlog's data file, only reading
/// the ranges that are currently relevant, like a sort of basic and manual
/// file-based mmap.
///
/// XXX should this just be replaced with `mmap` + `madvise` ranges?
/// The upcoming `UncompressedChunkCache` will make up for most of the slowness
/// of re-reading the same chunks, so this might not be as useful. Aside from
/// the major benefit of having less code to take care of, using `mmap` will
/// allow multiple processes to share the same pages, especially for the
/// changelog and manifest, which would make a difference in server contexts.
pub struct RandomAccessFile {
    /// The current store VFS to pass it to [`FileHandle`]
    vfs: Box<dyn Vfs>,
    /// Filename of the open file, relative to the vfs root
    pub filename: PathBuf,
    /// The current read-only handle on the file, if any
    pub reading_handle: RefCell<Option<FileHandle>>,
    /// The current read-write handle on the file, if any
    pub writing_handle: RefCell<Option<FileHandle>>,
}

impl RandomAccessFile {
    /// Wrap a file for random access
    pub fn new(vfs: Box<dyn Vfs>, filename: PathBuf) -> Self {
        assert!(filename.is_relative());
        Self {
            vfs,
            filename,
            reading_handle: RefCell::new(None),
            writing_handle: RefCell::new(None),
        }
    }

    /// Read a chunk of bytes from the file.
    pub fn read_chunk(
        &self,
        offset: usize,
        length: usize,
    ) -> Result<Vec<u8>, HgError> {
        let mut handle = self.get_read_handle()?;
        handle
            .seek(SeekFrom::Start(offset as u64))
            .when_reading_file(&self.filename)?;
        handle.read_exact(length).when_reading_file(&self.filename)
    }

    /// `pub` only for hg-cpython
    #[doc(hidden)]
    pub fn get_read_handle(&self) -> Result<FileHandle, HgError> {
        if let Some(handle) = &*self.writing_handle.borrow() {
            // Use a file handle being actively used for writes, if available.
            // There is some danger to doing this because reads will seek the
            // file.
            // However, [`Revlog::write_entry`] performs a `SeekFrom::End(0)`
            // before all writes, so we should be safe.
            return Ok(handle.clone());
        }
        if let Some(handle) = &*self.reading_handle.borrow() {
            return Ok(handle.clone());
        }
        // early returns done to work around borrowck being overzealous
        // See https://github.com/rust-lang/rust/issues/103108
        let new_handle = FileHandle::new(
            dyn_clone::clone_box(&*self.vfs),
            &self.filename,
            false,
            false,
        )?;
        *self.reading_handle.borrow_mut() = Some(new_handle.clone());
        Ok(new_handle)
    }

    /// `pub` only for hg-cpython
    #[doc(hidden)]
    pub fn exit_reading_context(&self) {
        self.reading_handle.take();
    }

    // Returns whether this file currently open
    pub fn is_open(&self) -> bool {
        self.reading_handle.borrow().is_some()
            || self.writing_handle.borrow().is_some()
    }
}

/// A buffer that holds new changelog index data that needs to be written
/// after the manifest and filelogs so that the repo is updated atomically to
/// external processes.
#[derive(Clone, Debug, Default)]
pub struct DelayedBuffer {
    // The actual in-memory bytes storing the delayed writes
    pub(super) buffer: Vec<u8>,
    /// The current offset into the virtual file composed of file + buffer
    offset: u64,
    /// The size of the file at the time of opening
    file_size: u64,
}

impl DelayedBuffer {
    /// Returns the length of the full data (on-disk + buffer length).
    pub fn len(&self) -> u64 {
        self.buffer.len() as u64 + self.file_size
    }

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }
}

/// Holds an open [`File`] and the related data. This can be used for reading
/// and writing. Writes can be delayed to a buffer before touching the disk,
/// if relevant (in the changelog case), but reads are transparent.
pub struct FileHandle {
    /// The actual open file
    pub file: File,
    /// The VFS with which the file was opened
    vfs: Box<dyn Vfs>,
    /// Filename of the open file, relative to the repo root
    filename: PathBuf,
    /// Buffer of delayed entry writes to the changelog index. This points
    /// back to the buffer inside the revlog this handle refers to.
    delayed_buffer: Option<Arc<Mutex<DelayedBuffer>>>,
}

impl std::fmt::Debug for FileHandle {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("FileHandle")
            .field("filename", &self.filename)
            .field("delayed_buffer", &self.delayed_buffer)
            .field("file", &self.file)
            .finish()
    }
}

impl Clone for FileHandle {
    fn clone(&self) -> Self {
        Self {
            vfs: dyn_clone::clone_box(&*self.vfs),
            filename: self.filename.clone(),
            delayed_buffer: self.delayed_buffer.clone(),
            // This can only fail if the OS doesn't have the file handle
            // anymore, so we're not going to do anything useful anyway.
            file: self.file.try_clone().expect("couldn't clone file handle"),
        }
    }
}

impl FileHandle {
    /// Get a (read or write) file handle to `filename`. Only creates the file
    /// if `create` is `true`.
    pub fn new(
        vfs: Box<dyn Vfs>,
        filename: impl AsRef<Path>,
        create: bool,
        write: bool,
    ) -> Result<Self, HgError> {
        let file = if create {
            vfs.create(filename.as_ref())?
        } else if write {
            vfs.open(filename.as_ref())?
        } else {
            vfs.open_read(filename.as_ref())?
        };
        Ok(Self {
            vfs,
            filename: filename.as_ref().to_owned(),
            delayed_buffer: None,
            file,
        })
    }

    /// Get a file handle to `filename`, but writes go to a [`DelayedBuffer`].
    pub fn new_delayed(
        vfs: Box<dyn Vfs>,
        filename: impl AsRef<Path>,
        create: bool,
        delayed_buffer: Arc<Mutex<DelayedBuffer>>,
    ) -> Result<Self, HgError> {
        let mut file = if create {
            vfs.create(filename.as_ref())?
        } else {
            vfs.open(filename.as_ref())?
        };
        let size = vfs.file_size(&file)?;
        let offset = file
            .stream_position()
            .when_reading_file(filename.as_ref())?;

        {
            let mut buf = delayed_buffer.lock().unwrap();
            buf.file_size = size;
            buf.offset = offset;
        }

        Ok(Self {
            vfs,
            filename: filename.as_ref().to_owned(),
            delayed_buffer: Some(delayed_buffer),
            file,
        })
    }

    /// Wrap an existing [`File`]
    pub fn from_file(
        file: File,
        vfs: Box<dyn Vfs>,
        filename: impl AsRef<Path>,
    ) -> Self {
        Self {
            vfs,
            filename: filename.as_ref().to_owned(),
            delayed_buffer: None,
            file,
        }
    }

    /// Wrap an existing [`File`], but writes go to a [`DelayedBuffer`].
    pub fn from_file_delayed(
        mut file: File,
        vfs: Box<dyn Vfs>,
        filename: impl AsRef<Path>,
        delayed_buffer: Arc<Mutex<DelayedBuffer>>,
    ) -> Result<Self, HgError> {
        let size = vfs.file_size(&file)?;
        let offset = file
            .stream_position()
            .when_reading_file(filename.as_ref())?;

        {
            let mut buf = delayed_buffer.lock().unwrap();
            buf.file_size = size;
            buf.offset = offset;
        }

        Ok(Self {
            vfs,
            filename: filename.as_ref().to_owned(),
            delayed_buffer: Some(delayed_buffer),
            file,
        })
    }

    /// Move the position of the handle to `pos`,
    /// spanning the [`DelayedBuffer`] if defined. Will return an error if
    /// an invalid seek position is asked, or for any standard io error.
    pub fn seek(&mut self, pos: SeekFrom) -> Result<u64, std::io::Error> {
        if let Some(delay_buf) = &self.delayed_buffer {
            let mut delay_buf = delay_buf.lock().unwrap();
            // Virtual file offset spans real file and data
            match pos {
                SeekFrom::Start(offset) => delay_buf.offset = offset,
                SeekFrom::End(offset) => {
                    delay_buf.offset =
                        delay_buf.len().saturating_add_signed(offset)
                }
                SeekFrom::Current(offset) => {
                    delay_buf.offset =
                        delay_buf.offset.saturating_add_signed(offset);
                }
            }
            if delay_buf.offset < delay_buf.file_size {
                self.file.seek(pos)
            } else {
                Ok(delay_buf.offset)
            }
        } else {
            self.file.seek(pos)
        }
    }

    /// Read exactly `length` bytes from the current position.
    /// Errors are the same as [`std::io::Read::read_exact`].
    pub fn read_exact(
        &mut self,
        length: usize,
    ) -> Result<Vec<u8>, std::io::Error> {
        if let Some(delay_buf) = self.delayed_buffer.as_mut() {
            let mut delay_buf = delay_buf.lock().unwrap();
            let mut buf = vec![0; length];
            let offset: isize =
                delay_buf.offset.try_into().expect("buffer too large");
            let file_size: isize =
                delay_buf.file_size.try_into().expect("file too large");
            let span: isize = offset - file_size;
            let length = length.try_into().expect("too large of a length");
            let absolute_span: u64 =
                span.unsigned_abs().try_into().expect("length too large");
            if span < 0 {
                if length <= absolute_span {
                    // We're only in the file
                    self.file.read_exact(&mut buf)?;
                } else {
                    // We're spanning file and buffer
                    self.file
                        .read_exact(&mut buf[..absolute_span as usize])?;
                    delay_buf
                        .buffer
                        .take(length - absolute_span)
                        .read_exact(&mut buf[absolute_span as usize..])?;
                }
            } else {
                // We're only in the buffer
                delay_buf.buffer[absolute_span as usize..]
                    .take(length)
                    .read_exact(&mut buf)?;
            }
            delay_buf.offset += length;
            Ok(buf.to_owned())
        } else {
            let mut buf = vec![0; length];
            self.file.read_exact(&mut buf)?;
            Ok(buf)
        }
    }

    /// Flush the in-memory changes to disk. This does *not* write the
    /// delayed buffer, only the pending file changes.
    pub fn flush(&mut self) -> Result<(), HgError> {
        self.file.flush().when_writing_file(&self.filename)
    }

    /// Return the current position in the file
    pub fn position(&mut self) -> Result<u64, HgError> {
        self.file
            .stream_position()
            .when_reading_file(&self.filename)
    }

    /// Append `data` to the file, or to the [`DelayedBuffer`], if any.
    pub fn write_all(&mut self, data: &[u8]) -> Result<(), HgError> {
        if let Some(buf) = &mut self.delayed_buffer {
            let mut delayed_buffer = buf.lock().expect("propagate the panic");
            assert_eq!(delayed_buffer.offset, delayed_buffer.len());
            delayed_buffer.buffer.extend_from_slice(data);
            delayed_buffer.offset += data.len() as u64;
            Ok(())
        } else {
            self.file
                .write_all(data)
                .when_writing_file(&self.filename)?;
            Ok(())
        }
    }
}

/// Write handles to a given revlog (index + maybe data)
#[derive(Debug)]
pub struct WriteHandles {
    /// Handle to the index file
    pub index_handle: FileHandle,
    /// Handle to the data file, if the revlog is non-inline
    pub data_handle: Option<FileHandle>,
}

#[cfg(test)]
mod tests {
    use std::io::ErrorKind;

    use crate::vfs::VfsImpl;

    use super::*;

    #[test]
    fn test_random_access_file() {
        let base = tempfile::tempdir().unwrap().into_path();
        let filename = Path::new("a");
        let file_path = base.join(filename);
        let raf = RandomAccessFile::new(
            Box::new(VfsImpl::new(base.clone(), true)),
            filename.to_owned(),
        );

        assert!(!raf.is_open());
        assert_eq!(&raf.filename, &filename);
        // Should fail to read a non-existing file
        match raf.get_read_handle().unwrap_err() {
            HgError::IoError { error, .. } => match error.kind() {
                std::io::ErrorKind::NotFound => {}
                _ => panic!("should be not found"),
            },
            e => panic!("{}", e.to_string()),
        }

        std::fs::write(file_path, b"1234567890").unwrap();

        // Should be able to open an existing file
        let mut handle = raf.get_read_handle().unwrap();
        assert!(raf.is_open());
        assert_eq!(handle.read_exact(10).unwrap(), b"1234567890".to_vec());
    }

    #[test]
    fn test_file_handle() {
        let base = tempfile::tempdir().unwrap().into_path();
        let filename = base.join("a");
        // No `create` should fail
        FileHandle::new(
            Box::new(VfsImpl::new(base.clone(), false)),
            &filename,
            false,
            false,
        )
        .unwrap_err();
        std::fs::write(&filename, b"1234567890").unwrap();

        let mut read_handle = FileHandle::new(
            Box::new(VfsImpl::new(base.clone(), true)),
            &filename,
            false,
            false,
        )
        .unwrap();
        assert_eq!(&read_handle.filename, &filename);
        assert_eq!(read_handle.position().unwrap(), 0);

        // Writing to an explicit read handle should fail
        read_handle.write_all(b"some data").unwrap_err();

        // reading exactly n bytes should work
        assert_eq!(read_handle.read_exact(3).unwrap(), b"123".to_vec());
        // and the position should be remembered
        assert_eq!(read_handle.read_exact(2).unwrap(), b"45".to_vec());

        // Seeking should work
        let position = read_handle.position().unwrap();
        read_handle.seek(SeekFrom::Current(-2)).unwrap();
        assert_eq!(position - 2, read_handle.position().unwrap());

        // Seeking too much data should fail
        read_handle.read_exact(1000).unwrap_err();

        // Open a write handle
        let mut handle = FileHandle::new(
            Box::new(VfsImpl::new(base.clone(), false)),
            &filename,
            false,
            true,
        )
        .unwrap();

        // Now writing should succeed
        handle.write_all(b"new data").unwrap();
        // Opening or writing does not seek, so we should be at the start
        assert_eq!(handle.position().unwrap(), 8);
        // We can still read
        assert_eq!(handle.read_exact(2).unwrap(), b"90".to_vec());

        let mut read_handle = FileHandle::new(
            Box::new(VfsImpl::new(base.clone(), true)),
            &filename,
            false,
            false,
        )
        .unwrap();
        read_handle.seek(SeekFrom::Start(0)).unwrap();
        // On-disk file contents should be changed
        assert_eq!(
            &read_handle.read_exact(10).unwrap(),
            &b"new data90".to_vec(),
        );
        // Flushing doesn't do anything unexpected
        handle.flush().unwrap();

        let delayed_buffer = Arc::new(Mutex::new(DelayedBuffer::default()));
        let mut handle = FileHandle::new_delayed(
            Box::new(VfsImpl::new(base.clone(), false)),
            &filename,
            false,
            delayed_buffer,
        )
        .unwrap();

        assert_eq!(
            handle
                .delayed_buffer
                .as_ref()
                .unwrap()
                .lock()
                .unwrap()
                .file_size,
            10
        );
        handle.seek(SeekFrom::End(0)).unwrap();
        handle.write_all(b"should go to buffer").unwrap();
        assert_eq!(
            handle
                .delayed_buffer
                .as_ref()
                .unwrap()
                .lock()
                .unwrap()
                .len(),
            29
        );
        read_handle.seek(SeekFrom::Start(0)).unwrap();
        // On-disk file contents should be unchanged
        assert_eq!(
            read_handle.read_exact(10).unwrap(),
            b"new data90".to_vec(),
        );

        assert_eq!(
            read_handle.read_exact(1).unwrap_err().kind(),
            ErrorKind::UnexpectedEof
        );

        handle.flush().unwrap();
        // On-disk file contents should still be unchanged after a flush
        assert_eq!(
            read_handle.read_exact(1).unwrap_err().kind(),
            ErrorKind::UnexpectedEof
        );

        // Read from the buffer only
        handle.seek(SeekFrom::End(-1)).unwrap();
        assert_eq!(handle.read_exact(1).unwrap(), b"r".to_vec());

        // Read from an overlapping section of file and buffer
        handle.seek(SeekFrom::Start(6)).unwrap();
        assert_eq!(
            handle.read_exact(20).unwrap(),
            b"ta90should go to buf".to_vec()
        );

        // Read from file only
        handle.seek(SeekFrom::Start(0)).unwrap();
        assert_eq!(handle.read_exact(8).unwrap(), b"new data".to_vec());
    }
}