package docker

import (
	"bufio"
	"bytes"
	"errors"
	"os"
)

var (
	_dockerId     string
	dockerFeature = []byte("docker/")

	errNotMatch = errors.New("not match")
)

func SelfContainerID() (did string, err error) {
	if _dockerId != "" {
		return _dockerId, nil
	}
	var (
		pos int
		p   []byte
		fp  *os.File
	)
	if fp, err = os.Open("/proc/self/cgroup"); err != nil {
		return
	}
	defer func() {
		_ = fp.Close()
	}()
	br := bufio.NewReader(fp)
	for {
		if p, _, err = br.ReadLine(); err != nil {
			break
		}
		if pos = bytes.Index(p, dockerFeature); pos == -1 {
			continue
		}
		p = bytes.TrimSpace(p[pos+len(dockerFeature):])
		if pos = bytes.LastIndexByte(p, '/'); pos == -1 {
			_dockerId = string(p)
			return _dockerId, nil
		} else {
			_dockerId = string(p[pos+1:])
			return _dockerId, nil
		}
	}
	return "", errNotMatch
}