package watcher

import (
	"syscall"
	"unsafe"

	"github.com/pkg/errors"
)

// CrDelMask predefined constant with create and delete events.
const CrDelMask uint32 = syscall.IN_CREATE | syscall.IN_DELETE

// inotifyCount is amount of InotifyEvent structures
// we read in WatchForMask(). 16 was chosen experimentally.
// Use of sparse files (with preoccupied space using Seek())
// allows us to have a little buffer. With smaller number
// events are often missing.
const inotifyCount = 16

type InotifyWatcher struct {
	fd     int
	wds    []int
	closed bool
}

// NewInotifyWatcher initialises inotify mechanism and returns
// an instance of InotifyWatcher.
func NewInotifyWatcher() (w *InotifyWatcher, err error) {
	w = &InotifyWatcher{closed: false}

	w.fd, err = syscall.InotifyInit()
	if err != nil {
		return nil, errors.Wrap(err, "failed to initialise inotify watcher")
	}

	w.wds = make([]int, 0)

	return w, nil
}

// AddWatch sets on watch file or directory specified in `path`
// for syscall.IN_XXXX events specified in `mask`.
func (w *InotifyWatcher) AddWatch(path string, mask uint32) error {
	wd, err := syscall.InotifyAddWatch(w.fd, path, mask)
	if err != nil {
		return errors.Wrapf(err, "failed to set %s on watch", path)
	}

	w.wds = append(w.wds, wd)

	return nil
}

// WatchForMask checks for events specified in `mask` and sends them
// to channel `fired`. See `man inotify` for events.
func (w *InotifyWatcher) WatchForMask(fired chan uint32, mask uint32) {
	for !w.closed {
		buffer := make([]byte, syscall.SizeofInotifyEvent*inotifyCount)
		n, err := syscall.Read(w.fd, buffer)
		if err != nil {
			break
		}

		if n < syscall.SizeofInotifyEvent {
			continue
		}

		for offset := 0; offset < len(buffer); offset += syscall.SizeofInotifyEvent {
			event := (*syscall.InotifyEvent)(unsafe.Pointer(&buffer[offset]))
			if event.Mask&mask > 0 {
				fired <- event.Mask
			}
		}
	}
}

// Close removes all watchers, closes inotify descriptor and stops all
// WatchForMask() event loops.
func (w *InotifyWatcher) Close() error {
	for _, wd := range w.wds {
		if _, err := syscall.InotifyRmWatch(w.fd, uint32(wd)); err != nil {
			return err
		}
	}

	if err := syscall.Close(w.fd); err != nil {
		return err
	}

	w.closed = true

	return nil
}