package uevent

import (
	"bytes"
	"errors"
	"io/fs"
	"log"
	"path/filepath"
	"unsafe"
)

// Enumerate scans sysfs and emits [Synthetic] events. It returns the first
// error it encounters.
//
// The specified filesystem must present the sysfs root.
func Enumerate(
	sysfs fs.FS,
	handleWalkErr func(error) error,
	events chan<- *Message,
) error {
	if handleWalkErr == nil {
		handleWalkErr = func(err error) error {
			if errors.Is(err, fs.ErrNotExist) {
				log.Println("enumerate", err)
				return nil
			}
			return err
		}
	}

	return fs.WalkDir(sysfs, "devices", func(
		path string,
		d fs.DirEntry,
		err error,
	) error {
		if err != nil {
			return handleWalkErr(err)
		}

		if d.IsDir() || d.Name() != "uevent" {
			return nil
		}

		msg := Message{
			Action: Synthetic,

			// cleans path, appears to be compatible with kernel behaviour
			DevPath: filepath.Dir(path),
		}

		var target string
		if target, err = fs.ReadLink(
			sysfs,
			filepath.Join(msg.DevPath, "subsystem"),
		); err != nil {
			if err = handleWalkErr(err); err != nil {
				return err
			}
		} else {
			msg.Env = append(msg.Env, "SUBSYSTEM="+filepath.Base(target))
		}

		// read entire file: slicing does not copy
		var env []byte
		if env, err = fs.ReadFile(sysfs, path); err != nil {
			return handleWalkErr(err)
		}

		for _, s := range bytes.Split(env, []byte{'\n'}) {
			if len(s) == 0 {
				continue
			}
			msg.Env = append(msg.Env, unsafe.String(unsafe.SliceData(s), len(s)))
		}

		if len(msg.Env) == 0 {
			// this implies absent subsystem, its error is already handled
			return nil
		}

		if msg.DevPath != "" && msg.DevPath[0] != '/' {
			msg.DevPath = "/" + msg.DevPath
		}
		events <- &msg
		return nil
	})
}