// SPDX-License-Identifier: LGPL-2.1-only
// Copyright (C) 2026 Liberux Labs, S. L. <info@liberux.net>

//! On-disk session state: `$XDG_STATE_HOME/<app_id>/{session.json,state.bin}`, on Windows `%LOCALAPPDATA%\<app_id>\` when `XDG_STATE_HOME` is unset.
//!
//! `session.json` carries the compositor session id, a clean-exit marker
//! and the pid of the run that wrote it; `state.bin` holds the bytes the
//! application returned from `App::save_state`. Everything is best effort:
//! I/O failures are logged and never abort the application.

use std::fs;
use std::io;
use std::path::{ Path, PathBuf };
use std::time::Duration;

use serde::{ Deserialize, Serialize };

use crate::app::App;

/// How often a running app's state is saved, besides on a clean exit.
pub( crate ) const SAVE_INTERVAL: Duration = Duration::from_secs( 30 );

/// Set to `1` by a shell that relaunches the app as part of a session restore.
pub( crate ) const RESTORE_ENV: &str = "LTK_SESSION_RESTORE";

const SESSION_FILE: &str = "session.json";
const STATE_FILE: &str = "state.bin";
const FORMAT_VERSION: u32 = 1;

/// Why this process is starting, as far as session restore is concerned.
#[ derive( Clone, Copy, Debug, PartialEq, Eq ) ]
pub( crate ) enum RestoreReason
{
	/// Ordinary launch: fresh app state, compositor still restores geometry.
	Launch,
	/// The previous run of this app id did not exit cleanly.
	Recover,
	/// Relaunched by the shell as part of a session restore.
	SessionRestore,
}

/// Outcome of inspecting the state directory at startup.
#[ derive( Clone, Copy, Debug, PartialEq, Eq ) ]
pub( crate ) enum Startup
{
	Reason( RestoreReason ),
	/// Another live instance owns the directory: run without persistence.
	Concurrent,
}

#[ derive( Serialize, Deserialize, Default, Debug, Clone ) ]
pub( crate ) struct SessionFile
{
	pub version:    u32,
	pub session_id: Option<String>,
	pub clean_exit: bool,
	pub pid:        u32,
}

/// Handle on one application's state directory.
pub( crate ) struct StateStore
{
	dir:        PathBuf,
	session_id: Option<String>,
	last_saved: Option<Vec<u8>>,
}

/// The state directory for `app_id` under `base`, or `None` when the id is unusable as a path component or there is no base.
pub( crate ) fn state_dir( app_id: &str, base: Option<PathBuf> ) -> Option<PathBuf>
{
	if app_id.is_empty() || app_id.contains( '/' ) || app_id == "." || app_id == ".."
	{
		return None;
	}
	#[ cfg( windows ) ]
	if app_id.contains( [ '\\', ':' ] ) { return None; }
	Some( base?.join( app_id ) )
}

impl StateStore
{
	pub fn open( app_id: &str ) -> Option<Self>
	{
		let dir = state_dir( app_id, crate::dirs::state_home() );
		if dir.is_none()
		{
			eprintln!( "ltk: session state disabled: cannot derive a state directory for app_id {app_id:?}" );
		}
		Self::open_at( dir? )
	}

	pub fn open_at( dir: PathBuf ) -> Option<Self>
	{
		let mut builder = fs::DirBuilder::new();
		builder.recursive( true );
		#[ cfg( unix ) ]
		{
			use std::os::unix::fs::DirBuilderExt;
			builder.mode( 0o700 );
		}
		if let Err( e ) = builder.create( &dir )
		{
			eprintln!( "ltk: session state disabled: cannot create {}: {e}", dir.display() );
			return None;
		}
		let mut store = Self { dir, session_id: None, last_saved: None };
		store.session_id = store.read_session_file().and_then( |f| f.session_id );
		Some( store )
	}

	pub fn read_session_file( &self ) -> Option<SessionFile>
	{
		let bytes = fs::read( self.dir.join( SESSION_FILE ) ).ok()?;
		serde_json::from_slice::<SessionFile>( &bytes ).ok().filter( |f| f.version == FORMAT_VERSION )
	}

	pub fn decide( &self, env_restore: bool ) -> Startup
	{
		if env_restore
		{
			return Startup::Reason( RestoreReason::SessionRestore );
		}
		match self.read_session_file()
		{
			Some( f ) if !f.clean_exit && f.pid != 0 && Self::pid_alive( f.pid ) => Startup::Concurrent,
			Some( f ) if !f.clean_exit => Startup::Reason( RestoreReason::Recover ),
			_ => Startup::Reason( RestoreReason::Launch ),
		}
	}

	pub fn session_id( &self ) -> Option<String>
	{
		self.session_id.clone()
	}

	pub fn load_state( &self ) -> Option<Vec<u8>>
	{
		fs::read( self.dir.join( STATE_FILE ) ).ok().filter( |b| !b.is_empty() )
	}

	pub fn mark_running( &mut self )
	{
		self.write_session_file( false );
	}

	pub fn set_session_id( &mut self, id: String )
	{
		self.session_id = Some( id );
		self.write_session_file( false );
	}

	/// Persist `state` when it differs from the last saved bytes; `None`
	/// removes any stale state file. Returns whether the disk was touched.
	pub fn save_state_if_changed( &mut self, state: Option<Vec<u8>> ) -> bool
	{
		match state
		{
			Some( bytes ) =>
			{
				if self.last_saved.as_deref() == Some( bytes.as_slice() )
				{
					return false;
				}
				match Self::write_atomic( &self.dir.join( STATE_FILE ), &bytes )
				{
					Ok( () ) =>
					{
						self.last_saved = Some( bytes );
						true
					}
					Err( e ) =>
					{
						eprintln!( "ltk: session state: cannot write {STATE_FILE}: {e}" );
						false
					}
				}
			}
			None =>
			{
				let path = self.dir.join( STATE_FILE );
				let existed = path.exists();
				if existed
				{
					if let Err( e ) = fs::remove_file( &path )
					{
						eprintln!( "ltk: session state: cannot remove {STATE_FILE}: {e}" );
					}
				}
				self.last_saved = None;
				existed
			}
		}
	}

	pub fn mark_clean_exit( &mut self, state: Option<Vec<u8>> )
	{
		self.save_state_if_changed( state );
		self.write_session_file( true );
	}

	fn write_session_file( &self, clean_exit: bool )
	{
		let file = SessionFile {
			version:    FORMAT_VERSION,
			session_id: self.session_id.clone(),
			clean_exit,
			pid:        std::process::id(),
		};
		match serde_json::to_vec( &file )
		{
			Ok( bytes ) =>
			{
				if let Err( e ) = Self::write_atomic( &self.dir.join( SESSION_FILE ), &bytes )
				{
					eprintln!( "ltk: session state: cannot write {SESSION_FILE}: {e}" );
				}
			}
			Err( e ) => eprintln!( "ltk: session state: cannot encode {SESSION_FILE}: {e}" ),
		}
	}

	fn write_atomic( path: &Path, bytes: &[u8] ) -> io::Result<()>
	{
		use std::io::Write;

		let mut tmp = path.as_os_str().to_owned();
		tmp.push( ".tmp" );
		let tmp = PathBuf::from( tmp );
		{
			let mut options = fs::OpenOptions::new();
			options.write( true ).create( true ).truncate( true );
			#[ cfg( unix ) ]
			{
				use std::os::unix::fs::OpenOptionsExt;
				options.mode( 0o600 );
			}
			let mut f = options.open( &tmp )?;
			f.write_all( bytes )?;
			f.sync_all()?;
		}
		fs::rename( &tmp, path )
	}

	#[ cfg( unix ) ]
	fn pid_alive( pid: u32 ) -> bool
	{
		Path::new( "/proc" ).join( pid.to_string() ).exists()
	}

	#[ cfg( windows ) ]
	fn pid_alive( pid: u32 ) -> bool
	{
		use windows_sys::Win32::Foundation::{ CloseHandle, STILL_ACTIVE };
		use windows_sys::Win32::System::Threading::{ GetExitCodeProcess, OpenProcess, PROCESS_QUERY_LIMITED_INFORMATION };

		// SAFETY: the handle is checked for null and closed before returning; `code` is a place for the call to write.
		unsafe
		{
			let process = OpenProcess( PROCESS_QUERY_LIMITED_INFORMATION, 0, pid );
			if process.is_null() { return false; }
			let mut code = 0;
			let alive = GetExitCodeProcess( process, &mut code ) != 0 && code == STILL_ACTIVE as u32;
			CloseHandle( process );
			alive
		}
	}
}

/// Before any window exists: read the restore hint from the environment, decide why the app is starting, hand it its saved bytes when that calls for them, and mark the run as live. `None` for the store when persistence is off: an unusable `app_id`, no state directory, or another instance running.
pub( crate ) fn bootstrap<A: App>( app: &mut A ) -> ( Option<StateStore>, RestoreReason )
{
	let env_restore = std::env::var_os( RESTORE_ENV ).is_some_and( |v| v == "1" );
	if std::env::var_os( RESTORE_ENV ).is_some()
	{
		// SAFETY: removing an env var is sound only when no other thread
		// is reading the environment concurrently. We are still in the
		// init phase before `set_channel_sender`, so the app has had
		// no opportunity to spawn worker threads yet.
		unsafe { std::env::remove_var( RESTORE_ENV ); }
	}

	let Some( mut store ) = StateStore::open( app.app_id() ) else { return ( None, RestoreReason::Launch ) };
	let reason = match store.decide( env_restore )
	{
		Startup::Concurrent =>
		{
			eprintln!( "ltk: another instance of {} is running; session persistence disabled", app.app_id() );
			return ( None, RestoreReason::Launch );
		}
		Startup::Reason( reason ) => reason,
	};
	if reason != RestoreReason::Launch || app.restore_on_launch()
	{
		if let Some( bytes ) = store.load_state()
		{
			app.restore_state( bytes );
		}
	}
	store.mark_running();
	( Some( store ), reason )
}

#[ cfg( test ) ]
mod tests
{
	use super::*;
	use std::sync::atomic::{ AtomicU32, Ordering };

	static COUNTER: AtomicU32 = AtomicU32::new( 0 );

	fn temp_dir() -> PathBuf
	{
		let n = COUNTER.fetch_add( 1, Ordering::Relaxed );
		let dir = std::env::temp_dir().join( format!( "ltk-session-state-{}-{n}", std::process::id() ) );
		let _ = fs::remove_dir_all( &dir );
		dir
	}

	#[ test ]
	fn state_dir_is_the_id_under_the_base()
	{
		let base = || Some( PathBuf::from( "/home/u/.local/state" ) );
		assert_eq!( state_dir( "net.example.App", base() ), Some( PathBuf::from( "/home/u/.local/state/net.example.App" ) ) );
		assert_eq!( state_dir( "net.example.App", None ), None );
	}

	#[ test ]
	fn state_dir_rejects_bad_ids()
	{
		let base = || Some( PathBuf::from( "/home/u/.local/state" ) );
		assert_eq!( state_dir( "", base() ), None );
		assert_eq!( state_dir( "a/b", base() ), None );
		assert_eq!( state_dir( "..", base() ), None );
	}

	#[ test ]
	fn fresh_dir_is_a_plain_launch()
	{
		let store = StateStore::open_at( temp_dir() ).unwrap();
		assert_eq!( store.decide( false ), Startup::Reason( RestoreReason::Launch ) );
		assert_eq!( store.load_state(), None );
		assert_eq!( store.session_id(), None );
	}

	#[ test ]
	fn mark_running_then_reopen_is_concurrent()
	{
		let dir = temp_dir();
		let mut store = StateStore::open_at( dir.clone() ).unwrap();
		store.mark_running();
		let file = store.read_session_file().unwrap();
		assert!( !file.clean_exit );
		assert_eq!( file.pid, std::process::id() );
		let second = StateStore::open_at( dir ).unwrap();
		assert_eq!( second.decide( false ), Startup::Concurrent );
	}

	#[ test ]
	fn dead_pid_means_recover_and_env_wins()
	{
		let dir = temp_dir();
		let store = StateStore::open_at( dir.clone() ).unwrap();
		let file = SessionFile { version: FORMAT_VERSION, session_id: Some( "abc".into() ), clean_exit: false, pid: 4_000_000_000 };
		fs::write( dir.join( SESSION_FILE ), serde_json::to_vec( &file ).unwrap() ).unwrap();
		assert_eq!( store.decide( false ), Startup::Reason( RestoreReason::Recover ) );
		assert_eq!( store.decide( true ), Startup::Reason( RestoreReason::SessionRestore ) );
	}

	#[ test ]
	fn save_state_if_changed_dedupes_and_removes()
	{
		let dir = temp_dir();
		let mut store = StateStore::open_at( dir.clone() ).unwrap();
		assert!( store.save_state_if_changed( Some( b"one".to_vec() ) ) );
		assert_eq!( fs::read( dir.join( STATE_FILE ) ).unwrap(), b"one" );
		assert!( !store.save_state_if_changed( Some( b"one".to_vec() ) ) );
		assert!( store.save_state_if_changed( Some( b"two".to_vec() ) ) );
		assert!( !dir.join( "state.bin.tmp" ).exists() );
		assert!( store.save_state_if_changed( None ) );
		assert!( !dir.join( STATE_FILE ).exists() );
		assert!( !store.save_state_if_changed( None ) );
	}

	#[ cfg( unix ) ]
	#[ test ]
	fn file_modes_are_private()
	{
		use std::os::unix::fs::PermissionsExt;
		let dir = temp_dir();
		let mut store = StateStore::open_at( dir.clone() ).unwrap();
		store.save_state_if_changed( Some( b"x".to_vec() ) );
		assert_eq!( fs::metadata( &dir ).unwrap().permissions().mode() & 0o777, 0o700 );
		assert_eq!( fs::metadata( dir.join( STATE_FILE ) ).unwrap().permissions().mode() & 0o777, 0o600 );
	}

	#[ test ]
	fn session_id_round_trips_and_clean_exit_flips()
	{
		let dir = temp_dir();
		let mut store = StateStore::open_at( dir.clone() ).unwrap();
		store.mark_running();
		store.set_session_id( "session-1".into() );
		let reopened = StateStore::open_at( dir ).unwrap();
		assert_eq!( reopened.session_id(), Some( "session-1".to_string() ) );
		store.mark_clean_exit( Some( b"final".to_vec() ) );
		let file = store.read_session_file().unwrap();
		assert!( file.clean_exit );
		assert_eq!( file.session_id.as_deref(), Some( "session-1" ) );
		assert_eq!( store.load_state().unwrap(), b"final" );
	}
}