package watcher

import (
	"syscall"
	"unsafe"

	"github.com/pkg/errors"
)

const CrDelMask uint32 = syscall.IN_CREATE | syscall.IN_DELETE

const inotifyCount = 16

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

func NewFSWatcher() (fsw *FSWatcher, err error) {
	fsw = &FSWatcher{closed: false}

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

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

	return fsw, nil
}

func (fsw *FSWatcher) AddWatch(path string, mask uint32) error {
	wd, err := syscall.InotifyAddWatch(fsw.fd, path, mask)
	if err != nil {
		return errors.Wrapf(err, "failed to set %s on watch", path)
	}

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

	return nil
}

// WatchForMask checking for events from mask and returns inotify mask to channel.
func (fsw *FSWatcher) WatchForMask(fired chan uint32, mask uint32) {
	go func() {
		for !fsw.closed {
			buffer := make([]byte, syscall.SizeofInotifyEvent*inotifyCount)
			n, err := syscall.Read(fsw.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
				}
			}
		}
	}()
}

func (fsw *FSWatcher) Close() error {
	for _, wd := range fsw.wds {
		if _, err := syscall.InotifyRmWatch(fsw.fd, uint32(wd)); err != nil {
			return err
		}
	}

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

	fsw.closed = true

	return nil
}