extern crate kernel32;

use winapi::{OVERLAPPED, LPOVERLAPPED, HANDLE, INVALID_HANDLE_VALUE, INFINITE, TRUE, WAIT_OBJECT_0,
    ERROR_OPERATION_ABORTED, FILE_NOTIFY_INFORMATION, fileapi, winbase, winnt};

use std::collections::HashMap;
use std::mem;
use std::path::{Path, PathBuf};
use std::ptr;
use std::slice;
use std::sync::mpsc::{channel, Sender, Receiver};
use std::ffi::OsString;
use std::os::windows::ffi::{OsStrExt, OsStringExt};
use std::thread;
use std::os::raw::c_void;

use super::{Event, Error, op, Op, Watcher};

const BUF_SIZE: u32 = 16384;

#[derive(Clone)]
struct ReadData {
    dir: PathBuf, // directory that is being watched
    file: Option<PathBuf>, // if a file is being watched, this is its full path
    complete_sem: HANDLE,
}

struct ReadDirectoryRequest {
    tx: Sender<Event>,
    buffer: [u8; BUF_SIZE as usize],
    handle: HANDLE,
    data: ReadData
}

enum Action {
    Watch(PathBuf),
    Unwatch(PathBuf),
    Stop
}

pub enum MetaEvent {
    SingleWatchComplete,
    WatcherAwakened
}

struct WatchState {
    dir_handle: HANDLE,
    complete_sem: HANDLE,
}

struct ReadDirectoryChangesServer {
    rx: Receiver<Action>,
    tx: Sender<Event>,
    meta_tx: Sender<MetaEvent>,
    cmd_tx: Sender<Result<PathBuf,Error>>,
    watches: HashMap<PathBuf, WatchState>,
    wakeup_sem: HANDLE
}

impl ReadDirectoryChangesServer {
    fn start(event_tx: Sender<Event>, meta_tx: Sender<MetaEvent>, cmd_tx:Sender<Result<PathBuf,Error>>, wakeup_sem: HANDLE) -> Sender<Action> {

        let (action_tx, action_rx) = channel();
        // it is, in fact, ok to send the semaphore across threads
        let sem_temp = wakeup_sem as u64;
        thread::spawn(move || {
            let wakeup_sem = sem_temp as HANDLE;
            let server = ReadDirectoryChangesServer {
                tx: event_tx,
                rx: action_rx,
                meta_tx: meta_tx,
                cmd_tx: cmd_tx,
                watches: HashMap::new(),
                wakeup_sem: wakeup_sem
            };
            server.run();
        });
        action_tx
    }

    fn run(mut self) {
        loop {
            // process all available actions first
            let mut stopped = false;

            while let Ok(action) = self.rx.try_recv() {
                match action {
                    Action::Watch(path) => {
                        let res = self.add_watch(path);
                        let _ = self.cmd_tx.send(res);
                    },
                    Action::Unwatch(path) => self.remove_watch(path),
                    Action::Stop => {
                        stopped = true;
                        for (_, ws) in &self.watches {
                            stop_watch(ws, &self.meta_tx);
                        }
                        break;
                    }
                }
            };

            if stopped {
                break;
            }

            unsafe {
                // wait with alertable flag so that the completion routine fires
                let waitres = kernel32::WaitForSingleObjectEx(self.wakeup_sem, 500, TRUE);
                if waitres == WAIT_OBJECT_0 {
                    let _ = self.meta_tx.send(MetaEvent::WatcherAwakened);
                }
            }
        }

        // we have to clean this up, since the watcher may be long gone
        unsafe {
            kernel32::CloseHandle(self.wakeup_sem);
        }
    }

    fn add_watch(&mut self, path: PathBuf) -> Result<PathBuf,Error> {
        // path must exist and be either a file or directory
        if !path.is_dir() && !path.is_file() {
            return Err(Error::Generic("Input watch path is neither a file nor a directory.".to_owned()));
        }

        let (watching_file,dir_target) = {
            if path.is_dir() {
                (false,path.clone())
            } else {
                // emulate file watching by watching the parent directory
                (true,path.parent().unwrap().to_path_buf())
            }
        };

        let encoded_path: Vec<u16> = dir_target.as_os_str().encode_wide().chain(Some(0)).collect();
        let handle;
        unsafe {
            handle = kernel32::CreateFileW(
                encoded_path.as_ptr(),
                winnt::FILE_LIST_DIRECTORY,
                winnt::FILE_SHARE_READ | winnt::FILE_SHARE_DELETE | winnt::FILE_SHARE_WRITE,
                ptr::null_mut(),
                fileapi::OPEN_EXISTING,
                winbase::FILE_FLAG_BACKUP_SEMANTICS | winbase::FILE_FLAG_OVERLAPPED,
                ptr::null_mut());

            if handle == INVALID_HANDLE_VALUE {
                let err = if watching_file {
                    Err(Error::Generic("You attempted to watch a single file, but parent directory could not be opened.".to_owned()))
                } else {
                    // TODO: Call GetLastError for better error info?
                    Err(Error::PathNotFound)
                };
                return err;
            }
        }
        let wf = if watching_file {
            Some(path.clone())
        } else {
            None
        };
        // every watcher gets its own semaphore to signal completion
        let semaphore = unsafe {
            kernel32::CreateSemaphoreW(ptr::null_mut(), 0, 1, ptr::null_mut())
        };
        if semaphore == ptr::null_mut() || semaphore == INVALID_HANDLE_VALUE {
            unsafe { kernel32::CloseHandle(handle); }
            return Err(Error::Generic("Failed to create semaphore for watch.".to_owned()));
        }
        let rd = ReadData {
            dir: dir_target,
            file: wf,
            complete_sem: semaphore
        };
        let ws = WatchState {
            dir_handle: handle,
            complete_sem: semaphore
        };
        self.watches.insert(path.clone(), ws);
        start_read(&rd, &self.tx, handle);
        Ok(path.to_path_buf())
    }

    fn remove_watch(&mut self, path: PathBuf) {
        if let Some(ws) = self.watches.remove(&path) {
            stop_watch(&ws, &self.meta_tx);
        }
    }
}

fn stop_watch(ws:&WatchState,meta_tx: &Sender<MetaEvent>) {
    unsafe {
        let cio = kernel32::CancelIo(ws.dir_handle);
        let ch = kernel32::CloseHandle(ws.dir_handle);
        // have to wait for it, otherwise we leak the memory allocated for there read request
        if cio != 0 && ch != 0 {
            kernel32::WaitForSingleObjectEx(ws.complete_sem, INFINITE, TRUE);
        }
        kernel32::CloseHandle(ws.complete_sem);

    }
    let _ = meta_tx.send(MetaEvent::SingleWatchComplete);
}

fn start_read(rd: &ReadData, tx: &Sender<Event>, handle: HANDLE) {
    let mut request = Box::new(ReadDirectoryRequest {
        tx: tx.clone(),
        handle: handle,
        buffer: [0u8; BUF_SIZE as usize],
        data: rd.clone()
    });

    let flags = winnt::FILE_NOTIFY_CHANGE_FILE_NAME
              | winnt::FILE_NOTIFY_CHANGE_DIR_NAME
              | winnt::FILE_NOTIFY_CHANGE_ATTRIBUTES
              | winnt::FILE_NOTIFY_CHANGE_SIZE
              | winnt::FILE_NOTIFY_CHANGE_LAST_WRITE
              | winnt::FILE_NOTIFY_CHANGE_CREATION
              | winnt::FILE_NOTIFY_CHANGE_SECURITY;

    let monitor_subdir = if (&request.data.file).is_none() {
      1
    } else {
      0
    };

    unsafe {
        let mut overlapped: Box<OVERLAPPED> = Box::new(mem::zeroed());
        // When using callback based async requests, we are allowed to use the hEvent member
        // for our own purposes

        let req_buf = request.buffer.as_mut_ptr() as *mut c_void;
        let request_p = Box::into_raw(request) as *mut c_void;
        overlapped.hEvent = request_p;

        // This is using an asynchronous call with a completion routine for receiving notifications
        // An I/O completion port would probably be more performant
        let ret = kernel32::ReadDirectoryChangesW(
            handle,
            req_buf,
            BUF_SIZE,
            monitor_subdir,
            flags,
            &mut 0u32 as *mut u32,  // This parameter is not used for async requests
            &mut *overlapped as *mut OVERLAPPED,
            Some(handle_event));

        if ret == 0 {
            // error reading. retransmute request memory to allow drop.
            // allow overlapped to drop by omitting forget()
            let request: Box<ReadDirectoryRequest> = mem::transmute(request_p);

            kernel32::ReleaseSemaphore(request.data.complete_sem, 1, ptr::null_mut());
        } else {
            // read ok. forget overlapped to let the completion routine handle memory
            mem::forget(overlapped);
        }
    }
}

unsafe extern "system" fn handle_event(error_code: u32, _bytes_written: u32, overlapped: LPOVERLAPPED) {
    let overlapped: Box<OVERLAPPED> = Box::from_raw(overlapped);
    let request: Box<ReadDirectoryRequest> = Box::from_raw(overlapped.hEvent as *mut _);

    if error_code == ERROR_OPERATION_ABORTED {
        // received when dir is unwatched or watcher is shutdown; return and let overlapped/request
        // get drop-cleaned
        kernel32::ReleaseSemaphore(request.data.complete_sem, 1, ptr::null_mut());
        return;
    }

    // Get the next request queued up as soon as possible
    start_read(&request.data, &request.tx, request.handle);

    // The FILE_NOTIFY_INFORMATION struct has a variable length due to the variable length string
    // as its last member.  Each struct contains an offset for getting the next entry in the buffer
    let mut cur_offset: *const u8 = request.buffer.as_ptr();
    let mut cur_entry: *const FILE_NOTIFY_INFORMATION = mem::transmute(cur_offset);
    loop {
        // filename length is size in bytes, so / 2
        let len = (*cur_entry).FileNameLength as usize / 2;
        let encoded_path: &[u16] = slice::from_raw_parts((*cur_entry).FileName.as_ptr(), len);
        // prepend root to get a full path
        let path = request.data.dir.join(PathBuf::from(OsString::from_wide(encoded_path)));

        // if we are watching a single file, ignore the event unless the path is exactly
        // the watched file
        let skip = match request.data.file {
            None => false,
            Some(ref watch_path) => *watch_path != path
        };

        if !skip {
            let op = match (*cur_entry).Action {
                winnt::FILE_ACTION_ADDED => op::CREATE,
                winnt::FILE_ACTION_REMOVED => op::REMOVE,
                winnt::FILE_ACTION_MODIFIED => op::WRITE,
                winnt::FILE_ACTION_RENAMED_OLD_NAME | winnt::FILE_ACTION_RENAMED_NEW_NAME => op::RENAME,
                _ => Op::empty()
            };


            let evt = Event {
                path: Some(path),
                op: Ok(op)
            };
            let _ = request.tx.send(evt);
        }

        if (*cur_entry).NextEntryOffset == 0 {
            break;
        }
        cur_offset = cur_offset.offset((*cur_entry).NextEntryOffset as isize);
        cur_entry = mem::transmute(cur_offset);
    }
}

pub struct ReadDirectoryChangesWatcher {
    tx: Sender<Action>,
    cmd_rx: Receiver<Result<PathBuf,Error>>,
    wakeup_sem: HANDLE
}

impl ReadDirectoryChangesWatcher {
   pub fn create(event_tx: Sender<Event>, meta_tx: Sender<MetaEvent>) -> Result<ReadDirectoryChangesWatcher, Error> {
       let (cmd_tx, cmd_rx) = channel();

       let wakeup_sem = unsafe {
           kernel32::CreateSemaphoreW(ptr::null_mut(), 0, 1, ptr::null_mut())
       };
       if wakeup_sem == ptr::null_mut() || wakeup_sem == INVALID_HANDLE_VALUE {
           return Err(Error::Generic("Failed to create wakeup semaphore.".to_owned()));
       }

       let action_tx = ReadDirectoryChangesServer::start(event_tx,meta_tx,cmd_tx,wakeup_sem);

       Ok(ReadDirectoryChangesWatcher {
           tx: action_tx,
           cmd_rx: cmd_rx,
           wakeup_sem: wakeup_sem
       })
   }

   fn wakeup_server(&mut self) {
       // breaks the server out of its wait state.  right now this is really just an optimization,
       // so that if you add a watch you don't block for 500ms in watch() while the
       // server sleeps.
       unsafe { kernel32::ReleaseSemaphore(self.wakeup_sem, 1, ptr::null_mut()); }
   }

   fn send_action_require_ack(&mut self, action:Action, pb:&PathBuf) -> Result<(), Error> {
       match self.tx.send(action) {
           Err(_) => Err(Error::Generic("Error sending to internal channel".to_owned())),
           Ok(_) => {
               // wake 'em up, we don't want to wait around for the ack
               self.wakeup_server();

               match self.cmd_rx.recv() {
                   Err(_) => Err(Error::Generic("Error receiving from command channel".to_owned())),
                   Ok(ack_res) => {
                       match ack_res {
                           Err(e) => Err(Error::Generic(format!("Error in watcher: {:?}", e))),
                           Ok(ack_pb) => {
                               if pb.as_path() != ack_pb.as_path() {
                                   Err(Error::Generic(format!("Expected ack for {:?} but got ack for {:?}", pb, ack_pb)))
                               } else {
                                   Ok(())
                               }
                           }
                       }
                   }
               }
           }
       }
   }
}

impl Watcher for ReadDirectoryChangesWatcher {
    fn new(event_tx: Sender<Event>) -> Result<ReadDirectoryChangesWatcher, Error> {
        // create dummy channel for meta event
        let (meta_tx, _) = channel();
        ReadDirectoryChangesWatcher::create(event_tx, meta_tx)
    }

    fn watch<P: AsRef<Path>>(&mut self, path: P) -> Result<(), Error> {
        // path must exist and be either a file or directory
        let pb = path.as_ref().to_path_buf();
        if !pb.is_dir() && !pb.is_file() {
            return Err(Error::Generic("Input watch path is neither a file nor a directory.".to_owned()));
        }
        self.send_action_require_ack(Action::Watch(path.as_ref().to_path_buf()), &pb)
    }

    fn unwatch<P: AsRef<Path>>(&mut self, path: P) -> Result<(), Error> {
        let res = self.tx.send(Action::Unwatch(path.as_ref().to_path_buf()))
            .map_err(|_| Error::Generic("Error sending to internal channel".to_owned()));
        self.wakeup_server();
        res
    }
}

impl Drop for ReadDirectoryChangesWatcher {
    fn drop(&mut self) {
        let _ = self.tx.send(Action::Stop);
        // better wake it up
        self.wakeup_server();
    }
}