package systemd

import (
	"bufio"
	"bytes"
	"context"
	"errors"
	"fmt"
	"os/exec"
	"path/filepath"
	"strings"
)

const (
	// loadStateProperty systemctl show 输出的加载状态属性名。
	loadStateProperty = "LoadState"
	// activeStateProperty systemctl show 输出的活动状态属性名。
	activeStateProperty = "ActiveState"
	// subStateProperty systemctl show 输出的子状态属性名。
	subStateProperty = "SubState"
	// loadedState 表示单元已加载。
	loadedState = "loaded"
	// notFoundState 表示单元未找到。
	notFoundState = "not-found"
)

// Systemctl 直接调用一个精确的 systemctl 可执行文件，绝不经过 shell。
type Systemctl struct {
	// executable systemctl 的绝对路径。
	executable string
}

// NewSystemctl 要求传入本地守护进程配置提供的可执行文件绝对路径。
func NewSystemctl(executable string) (*Systemctl, error) {
	if !filepath.IsAbs(executable) {
		return nil, errors.New("systemctl executable path must be absolute")
	}
	return &Systemctl{executable: executable}, nil
}

// Inspect 查询指定单元的关键状态属性并返回其状态。
// 未找到单元时错误包装 ErrUnitNotFound；加载状态异常时返回错误。
func (s *Systemctl) Inspect(ctx context.Context, unitName string) (Unit, error) {
	if err := validateUnitName(unitName); err != nil {
		return Unit{}, err
	}
	command := exec.CommandContext(ctx, s.executable,
		"show",
		"--no-pager",
		"--property="+loadStateProperty,
		"--property="+activeStateProperty,
		"--property="+subStateProperty,
		"--",
		unitName,
	)
	output, err := command.CombinedOutput()
	if err != nil {
		return Unit{}, commandError("inspect systemd unit "+unitName, output, err)
	}
	unit, err := parseUnitProperties(unitName, output)
	if err != nil {
		return Unit{}, err
	}
	if unit.LoadState == notFoundState {
		return Unit{}, fmt.Errorf("inspect systemd unit %s: %w", unitName, ErrUnitNotFound)
	}
	if unit.LoadState != loadedState {
		return Unit{}, fmt.Errorf("systemd unit %s has unsupported load state %q", unitName, unit.LoadState)
	}
	return unit, nil
}

// Start 启动指定单元。
func (s *Systemctl) Start(ctx context.Context, unitName string) error {
	return s.changeState(ctx, "start", unitName)
}

// Stop 停止指定单元。
func (s *Systemctl) Stop(ctx context.Context, unitName string) error {
	return s.changeState(ctx, "stop", unitName)
}

// changeState 通过 systemctl 执行给定动作（start 或 stop）。
func (s *Systemctl) changeState(ctx context.Context, action, unitName string) error {
	if err := validateUnitName(unitName); err != nil {
		return err
	}
	output, err := exec.CommandContext(ctx, s.executable, action, "--", unitName).CombinedOutput()
	if err != nil {
		return commandError(action+" systemd unit "+unitName, output, err)
	}
	return nil
}

// parseUnitProperties 解析 systemctl show 的输出，
// 严格要求恰好包含三个预期属性且不重复、不多余。
func parseUnitProperties(unitName string, output []byte) (Unit, error) {
	values := make(map[string]string, 3)
	scanner := bufio.NewScanner(bytes.NewReader(output))
	for scanner.Scan() {
		line := scanner.Text()
		key, value, found := strings.Cut(line, "=")
		if !found {
			return Unit{}, fmt.Errorf("decode systemd unit %s property line %q", unitName, line)
		}
		switch key {
		case loadStateProperty, activeStateProperty, subStateProperty:
			if _, duplicate := values[key]; duplicate {
				return Unit{}, fmt.Errorf("decode systemd unit %s duplicate property %s", unitName, key)
			}
			values[key] = value
		default:
			return Unit{}, fmt.Errorf("decode systemd unit %s unexpected property %q", unitName, key)
		}
	}
	if err := scanner.Err(); err != nil {
		return Unit{}, fmt.Errorf("decode systemd unit %s properties: %w", unitName, err)
	}
	for _, key := range []string{loadStateProperty, activeStateProperty, subStateProperty} {
		if _, found := values[key]; !found {
			return Unit{}, fmt.Errorf("decode systemd unit %s missing property %s", unitName, key)
		}
	}
	return Unit{
		Name:        unitName,
		LoadState:   values[loadStateProperty],
		ActiveState: values[activeStateProperty],
		SubState:    values[subStateProperty],
	}, nil
}

// validateUnitName 校验单元名称必须非空且不含首尾空白。
func validateUnitName(unitName string) error {
	if unitName == "" || strings.TrimSpace(unitName) != unitName {
		return errors.New("exact systemd unit name is required")
	}
	return nil
}

// commandError 包装命令执行错误，并附带去除首尾空白的命令输出作为诊断细节。
func commandError(action string, output []byte, err error) error {
	detail := strings.TrimSpace(string(output))
	if detail == "" {
		return fmt.Errorf("%s: %w", action, err)
	}
	return fmt.Errorf("%s: %w: %s", action, err, detail)
}

var _ Manager = (*Systemctl)(nil)