package continuity

import (
	"fmt"
	"os"
	"syscall"
)

// hardlinkKey provides a tuple-key for managing hardlinks. This is system-
// specific.
type hardlinkKey struct {
	dev   uint64
	inode uint64
}

// newHardlinkKey returns a hardlink key for the provided file info. If the
// resource does not represent a possible hardlink, errNotAHardLink will be
// returned.
func newHardlinkKey(fi os.FileInfo) (hardlinkKey, error) {
	sys, ok := fi.Sys().(*syscall.Stat_t)
	if !ok {
		return hardlinkKey{}, fmt.Errorf("cannot resolve (*syscall.Stat_t) from os.FileInfo")
	}

	if sys.Nlink < 2 {
		// NOTE(stevvooe): This is not always true for all filesystems. We
		// should somehow detect this and provided a slow "polyfill" that
		// leverages os.SameFile if we detect a filesystem where link counts
		// is not really supported.
		return hardlinkKey{}, errNotAHardLink
	}

	return hardlinkKey{dev: uint64(sys.Dev), inode: sys.Ino}, nil
}