//! Native drawing edit leases and external-change fingerprints.
//!
//! The sidecar lock survives the editor's atomic temp-file replacement, while
//! the drawing lock interoperates with platform `flock` / `LockFileEx` users.
//! Neither mechanism can force every third-party Unix application to cooperate,
//! so saves also compare [`FileFingerprint`] immediately before replacement.

use std::fs::{File, OpenOptions, TryLockError};
use std::hash::{Hash, Hasher};
use std::io::{Read, Seek, SeekFrom, Write};
use std::path::{Path, PathBuf};
use std::time::UNIX_EPOCH;

const SAMPLE_BYTES: usize = 64 * 1024;

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct FileFingerprint {
    len: u64,
    modified_ns: Option<u128>,
    sample_hash: u64,
}

impl FileFingerprint {
    pub fn capture(path: &Path) -> std::io::Result<Self> {
        Self::capture_from(&mut File::open(path)?)
    }

    /// Fingerprint an already-open drawing handle.
    pub fn capture_from(file: &mut File) -> std::io::Result<Self> {
        let metadata = file.metadata()?;
        let len = metadata.len();
        let modified_ns = metadata
            .modified()
            .ok()
            .and_then(|time| time.duration_since(UNIX_EPOCH).ok())
            .map(|duration| duration.as_nanos());

        let mut offsets = vec![0];
        if len > SAMPLE_BYTES as u64 {
            offsets.push((len / 2).saturating_sub((SAMPLE_BYTES / 2) as u64));
            offsets.push(len.saturating_sub(SAMPLE_BYTES as u64));
        }
        offsets.sort_unstable();
        offsets.dedup();

        let mut hasher = std::collections::hash_map::DefaultHasher::new();
        len.hash(&mut hasher);
        for offset in offsets {
            file.seek(SeekFrom::Start(offset))?;
            let mut remaining = (len.saturating_sub(offset)).min(SAMPLE_BYTES as u64) as usize;
            let mut buffer = vec![0_u8; remaining];
            let mut read = 0;
            while remaining > 0 {
                let count = file.read(&mut buffer[read..])?;
                if count == 0 {
                    break;
                }
                read += count;
                remaining -= count;
            }
            offset.hash(&mut hasher);
            buffer[..read].hash(&mut hasher);
        }

        Ok(Self {
            len,
            modified_ns,
            sample_hash: hasher.finish(),
        })
    }
}

#[cfg(unix)]
pub(crate) fn path_matches_file(path: &Path, file: &File) -> std::io::Result<bool> {
    use std::os::unix::fs::MetadataExt;

    let path_metadata = std::fs::metadata(path)?;
    let file_metadata = file.metadata()?;
    Ok(path_metadata.dev() == file_metadata.dev() && path_metadata.ino() == file_metadata.ino())
}

#[cfg(target_os = "windows")]
pub(crate) fn path_matches_file(path: &Path, file: &File) -> std::io::Result<bool> {
    use std::os::windows::fs::OpenOptionsExt;
    use windows_sys::Win32::Storage::FileSystem::{
        FILE_SHARE_DELETE, FILE_SHARE_READ, FILE_SHARE_WRITE,
    };

    let path_file = OpenOptions::new()
        .access_mode(0)
        .share_mode(FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE)
        .open(path)?;
    Ok(windows_file_identity(&path_file)? == windows_file_identity(file)?)
}

#[cfg(target_os = "windows")]
fn windows_file_identity(file: &File) -> std::io::Result<(u32, u64)> {
    use std::os::windows::io::AsRawHandle;
    use windows_sys::Win32::Storage::FileSystem::{
        BY_HANDLE_FILE_INFORMATION, GetFileInformationByHandle,
    };

    let mut info = BY_HANDLE_FILE_INFORMATION::default();
    if unsafe { GetFileInformationByHandle(file.as_raw_handle() as _, &mut info) } == 0 {
        return Err(std::io::Error::last_os_error());
    }
    let index = (u64::from(info.nFileIndexHigh) << 32) | u64::from(info.nFileIndexLow);
    Ok((info.dwVolumeSerialNumber, index))
}

#[cfg(not(any(unix, target_os = "windows")))]
pub(crate) fn path_matches_file(path: &Path, file: &File) -> std::io::Result<bool> {
    let mut path_file = File::open(path)?;
    let mut leased_file = file.try_clone()?;
    Ok(FileFingerprint::capture_from(&mut path_file)?
        == FileFingerprint::capture_from(&mut leased_file)?)
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum EditLeaseError {
    Locked(String),
    Unavailable(String),
}

impl std::fmt::Display for EditLeaseError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Locked(message) | Self::Unavailable(message) => f.write_str(message),
        }
    }
}

#[derive(Debug)]
pub struct EditLease {
    _sidecar: File,
    drawing: Option<File>,
    lock_path: PathBuf,
    platform_warning: Option<String>,
}

impl EditLease {
    pub fn acquire(path: &Path) -> Result<Self, EditLeaseError> {
        let canonical = std::fs::canonicalize(path).unwrap_or_else(|_| path.to_path_buf());
        let lock_path = sidecar_path(&canonical);
        let mut sidecar = OpenOptions::new()
            .read(true)
            .write(true)
            .create(true)
            .open(&lock_path)
            .map_err(|error| {
                EditLeaseError::Unavailable(format!(
                    "Could not create edit lock {}: {error}",
                    lock_path.display()
                ))
            })?;

        match sidecar.try_lock() {
            Ok(()) => {}
            Err(TryLockError::WouldBlock) => {
                let owner = std::fs::read_to_string(&lock_path)
                    .ok()
                    .map(|value| value.trim().to_string())
                    .filter(|value| !value.is_empty())
                    .unwrap_or_else(|| "another Open CAD Studio session".to_string());
                return Err(EditLeaseError::Locked(format!(
                    "Drawing edit lock is held by {owner}."
                )));
            }
            Err(TryLockError::Error(error)) => {
                return Err(EditLeaseError::Unavailable(format!(
                    "Could not acquire edit lock {}: {error}",
                    lock_path.display()
                )));
            }
        }

        let _ = sidecar.set_len(0);
        let _ = sidecar.seek(SeekFrom::Start(0));
        let _ = writeln!(
            sidecar,
            "Open CAD Studio pid={} path={}",
            std::process::id(),
            canonical.display()
        );
        let _ = sidecar.flush();

        let (drawing, platform_warning) = match acquire_drawing_lock(&canonical) {
            Ok(lock) => lock,
            Err(error) => {
                let _ = std::fs::remove_file(&lock_path);
                return Err(error);
            }
        };
        Ok(Self {
            _sidecar: sidecar,
            drawing,
            lock_path,
            platform_warning,
        })
    }

    pub fn platform_warning(&self) -> Option<&str> {
        self.platform_warning.as_deref()
    }

    /// Fingerprint through the lease's drawing handle.
    pub fn fingerprint(&mut self) -> std::io::Result<Option<FileFingerprint>> {
        self.drawing
            .as_mut()
            .map(FileFingerprint::capture_from)
            .transpose()
    }

    /// Clone the leased drawing handle for a worker.
    pub fn reader(&self) -> std::io::Result<Option<File>> {
        self.drawing.as_ref().map(File::try_clone).transpose()
    }

    /// Atomic save replaces the path with a new file object. Move the platform
    /// lock to that new object; the sidecar remains locked throughout.
    pub fn refresh_drawing_lock(&mut self, path: &Path) -> Result<(), EditLeaseError> {
        let (drawing, warning) = acquire_drawing_lock(path)?;
        self.drawing = drawing;
        self.platform_warning = warning;
        Ok(())
    }

    #[allow(dead_code)]
    pub fn lock_path(&self) -> &Path {
        &self.lock_path
    }
}

impl Drop for EditLease {
    fn drop(&mut self) {
        // Remove the path while the exclusive sidecar lock is still held so
        // another editor cannot acquire it during cleanup.
        let _ = std::fs::remove_file(&self.lock_path);
    }
}

fn acquire_drawing_lock(path: &Path) -> Result<(Option<File>, Option<String>), EditLeaseError> {
    let drawing = match OpenOptions::new().read(true).write(true).open(path) {
        Ok(file) => file,
        Err(error) if error_is_lock_conflict(&error) => {
            return Err(EditLeaseError::Locked(format!(
                "Drawing is open for writing in another application: {error}"
            )));
        }
        Err(error) => {
            return Ok((
                None,
                Some(format!("Platform drawing lock unavailable: {error}")),
            ));
        }
    };

    match drawing.try_lock() {
        Ok(()) => Ok((Some(drawing), None)),
        Err(TryLockError::WouldBlock) => Err(EditLeaseError::Locked(
            "Drawing is locked by another application.".to_string(),
        )),
        Err(TryLockError::Error(error)) => Ok((
            None,
            Some(format!("Platform drawing lock unavailable: {error}")),
        )),
    }
}

#[cfg(target_os = "windows")]
fn error_is_lock_conflict(error: &std::io::Error) -> bool {
    matches!(error.raw_os_error(), Some(32 | 33))
}

#[cfg(not(target_os = "windows"))]
fn error_is_lock_conflict(_error: &std::io::Error) -> bool {
    false
}

fn sidecar_path(path: &Path) -> PathBuf {
    let name = path
        .file_name()
        .map(|value| value.to_string_lossy().into_owned())
        .unwrap_or_else(|| "drawing".to_string());
    path.parent()
        .unwrap_or_else(|| Path::new("."))
        .join(format!(".{name}.ocs.lock"))
}

#[cfg(test)]
mod tests {
    use super::*;

    fn unique_path(name: &str) -> PathBuf {
        std::env::temp_dir().join(format!(
            "ocs_edit_lock_{}_{}_{}",
            std::process::id(),
            name,
            std::time::SystemTime::now()
                .duration_since(UNIX_EPOCH)
                .unwrap()
                .as_nanos()
        ))
    }

    #[test]
    fn edit_lease_blocks_second_ocs_session() {
        let path = unique_path("lease.dwg");
        std::fs::write(&path, b"dwg").unwrap();
        let first = EditLease::acquire(&path).unwrap();
        assert!(matches!(
            EditLease::acquire(&path),
            Err(EditLeaseError::Locked(_))
        ));
        drop(first);
        assert!(EditLease::acquire(&path).is_ok());
        let _ = std::fs::remove_file(sidecar_path(&path));
        let _ = std::fs::remove_file(path);
    }

    #[test]
    fn a_lease_fingerprints_the_drawing_it_has_locked() {
        let path = unique_path("leased.dwg");
        std::fs::write(&path, b"drawing bytes").unwrap();
        let unlocked = FileFingerprint::capture(&path).unwrap();

        let mut lease = EditLease::acquire(&path).unwrap();
        let leased = lease
            .fingerprint()
            .expect("the lease handle is readable")
            .expect("the lease holds a drawing handle");
        assert_eq!(unlocked, leased);

        drop(lease);
        let _ = std::fs::remove_file(sidecar_path(&path));
        let _ = std::fs::remove_file(path);
    }

    #[test]
    fn a_lease_lends_a_reader_for_the_drawing_it_has_locked() {
        let path = unique_path("leased_reader.dwg");
        std::fs::write(&path, b"drawing bytes").unwrap();

        let lease = EditLease::acquire(&path).unwrap();
        let mut reader = lease
            .reader()
            .expect("the lease handle is cloneable")
            .expect("the lease holds a drawing handle");
        let mut bytes = Vec::new();
        reader.read_to_end(&mut bytes).unwrap();
        assert_eq!(b"drawing bytes".as_slice(), bytes.as_slice());

        drop(reader);
        drop(lease);
        let _ = std::fs::remove_file(sidecar_path(&path));
        let _ = std::fs::remove_file(path);
    }

    #[test]
    fn a_replaced_path_no_longer_matches_the_lease() {
        let path = unique_path("identity.dwg");
        let replacement = unique_path("replacement.dwg");
        std::fs::write(&path, b"old").unwrap();
        std::fs::write(&replacement, b"new").unwrap();

        let lease = EditLease::acquire(&path).unwrap();
        let reader = lease
            .reader()
            .unwrap()
            .expect("the lease holds a drawing handle");
        std::fs::remove_file(&path).unwrap();
        std::fs::rename(&replacement, &path).unwrap();
        assert!(!path_matches_file(&path, &reader).unwrap());

        drop(reader);
        drop(lease);
        let _ = std::fs::remove_file(sidecar_path(&path));
        let _ = std::fs::remove_file(path);
    }

    #[test]
    fn fingerprint_detects_same_size_content_change() {
        let path = unique_path("fingerprint.dwg");
        std::fs::write(&path, b"abcd").unwrap();
        let before = FileFingerprint::capture(&path).unwrap();
        std::fs::write(&path, b"abce").unwrap();
        let after = FileFingerprint::capture(&path).unwrap();
        assert_ne!(before, after);
        let _ = std::fs::remove_file(path);
    }
}