package snmpagent

import (
	"context"
	"errors"
	"fmt"
	"os"
	"os/exec"
	"path/filepath"
	"sort"
	"strings"
	"sync"

	"tera-clone/services/tera-api/internal/storage/atomicfile"
)

const (
	DefaultConfigPath     = "/etc/snmp/snmpd.conf"
	DefaultPersistentPath = "/var/lib/net-snmp/snmpd.conf"
	DefaultInitScriptPath = "/etc/init.d/S59snmpd"
)

type Runner interface {
	Run(context.Context, string, ...string) ([]byte, error)
}

type execRunner struct{}

func (execRunner) Run(ctx context.Context, name string, args ...string) ([]byte, error) {
	return exec.CommandContext(ctx, name, args...).CombinedOutput()
}

type OSOptions struct {
	ConfigPath     string
	PersistentPath string
	InitScriptPath string
	ServiceName    string
	Runner         Runner
}

type OSController struct {
	options OSOptions
	mu      sync.Mutex
}

func NewOSController(options OSOptions) (*OSController, error) {
	if options.ConfigPath == "" {
		options.ConfigPath = DefaultConfigPath
	}
	if options.PersistentPath == "" {
		options.PersistentPath = DefaultPersistentPath
	}
	if options.InitScriptPath == "" {
		options.InitScriptPath = DefaultInitScriptPath
	}
	if options.Runner == nil {
		options.Runner = execRunner{}
	}
	for name, path := range map[string]string{"config": options.ConfigPath, "persistent": options.PersistentPath, "init script": options.InitScriptPath} {
		if !filepath.IsAbs(path) {
			return nil, fmt.Errorf("SNMP %s path must be absolute", name)
		}
	}
	return &OSController{options: options}, nil
}

func (c *OSController) Apply(ctx context.Context, config Config) error {
	if err := Validate(config); err != nil {
		return err
	}
	mainConfig, persistentConfig := render(config)
	c.mu.Lock()
	defer c.mu.Unlock()
	if err := ctx.Err(); err != nil {
		return err
	}
	mainBefore, err := snapshot(c.options.ConfigPath)
	if err != nil {
		return err
	}
	persistentBefore, err := snapshot(c.options.PersistentPath)
	if err != nil {
		return err
	}
	if err := os.MkdirAll(filepath.Dir(c.options.ConfigPath), 0o750); err != nil {
		return fmt.Errorf("prepare SNMP configuration directory: %w", err)
	}
	if err := os.MkdirAll(filepath.Dir(c.options.PersistentPath), 0o700); err != nil {
		return fmt.Errorf("prepare SNMP persistent directory: %w", err)
	}
	if err := atomicfile.Write(c.options.ConfigPath, mainConfig, 0o600); err != nil {
		return fmt.Errorf("write SNMP agent configuration: %w", err)
	}
	if err := atomicfile.Write(c.options.PersistentPath, persistentConfig, 0o600); err != nil {
		_ = restore(mainBefore)
		return fmt.Errorf("write SNMPv3 persistent configuration: %w", err)
	}
	action := "stop"
	if config.Enabled {
		action = "restart"
	}
	if output, err := c.runService(ctx, action); err != nil {
		restoreErr := errors.Join(restore(mainBefore), restore(persistentBefore))
		rollbackAction := "stop"
		if mainBefore.exists && strings.Contains(string(mainBefore.content), "agentAddress ") {
			rollbackAction = "restart"
		}
		_, restartErr := c.runService(context.Background(), rollbackAction)
		return errors.Join(fmt.Errorf("%s Net-SNMP agent: %w: %s", action, err, boundedOutput(output)), restoreErr, restartErr)
	}
	return nil
}

func (c *OSController) runService(ctx context.Context, action string) ([]byte, error) {
	args := []string{action}
	if c.options.ServiceName != "" {
		args = append(args, c.options.ServiceName)
	}
	return c.options.Runner.Run(ctx, c.options.InitScriptPath, args...)
}

type fileState struct {
	path    string
	exists  bool
	mode    os.FileMode
	content []byte
}

func snapshot(path string) (fileState, error) {
	info, err := os.Lstat(path)
	if os.IsNotExist(err) {
		return fileState{path: path}, nil
	}
	if err != nil {
		return fileState{}, fmt.Errorf("inspect %s: %w", path, err)
	}
	if !info.Mode().IsRegular() || info.Mode()&os.ModeSymlink != 0 {
		return fileState{}, fmt.Errorf("refusing non-regular SNMP configuration path %s", path)
	}
	content, err := os.ReadFile(path)
	if err != nil {
		return fileState{}, fmt.Errorf("read %s: %w", path, err)
	}
	return fileState{path: path, exists: true, mode: info.Mode().Perm(), content: content}, nil
}

func restore(state fileState) error {
	if !state.exists {
		if err := os.Remove(state.path); err != nil && !os.IsNotExist(err) {
			return err
		}
		return nil
	}
	return atomicfile.Write(state.path, state.content, state.mode)
}

func render(config Config) ([]byte, []byte) {
	var main strings.Builder
	main.WriteString("# Generated by tera-snmpd. Manual changes are overwritten.\n")
	if config.Enabled {
		main.WriteString("agentAddress udp:161\n")
	}
	main.WriteString("sysLocation \"")
	main.WriteString(quote(config.Location))
	main.WriteString("\"\nsysContact \"")
	main.WriteString(quote(config.Contact))
	main.WriteString("\"\nview tera_all included .1 80\n")
	if config.V1Enabled || config.V2CEnabled {
		writeCommunity(&main, "tera_ro", config.ReadOnlyCommunity, "read", config.V1Enabled, config.V2CEnabled)
		if config.ReadWriteCommunity != "" {
			writeCommunity(&main, "tera_rw", config.ReadWriteCommunity, "write", config.V1Enabled, config.V2CEnabled)
		}
	}
	users := append([]User(nil), config.V3Users...)
	sort.Slice(users, func(i, j int) bool { return users[i].Username < users[j].Username })
	var persistent strings.Builder
	persistent.WriteString("# Generated by tera-snmpd. Contains protected SNMPv3 bootstrap credentials.\n")
	if config.V3Enabled {
		for _, user := range users {
			if !user.Enabled {
				continue
			}
			level := "noauth"
			if user.AuthProtocol != "none" {
				level = "auth"
			}
			if user.PrivacyProtocol != "none" {
				level = "priv"
			}
			directive := "rouser"
			if user.Access == "write" {
				directive = "rwuser"
			}
			fmt.Fprintf(&main, "%s %s %s .1\n", directive, user.Username, level)
			fmt.Fprintf(&persistent, "createUser %s", user.Username)
			if user.AuthProtocol != "none" {
				fmt.Fprintf(&persistent, " %s \"%s\"", netSNMPAuth(user.AuthProtocol), quote(user.AuthKey))
			}
			if user.PrivacyProtocol != "none" {
				fmt.Fprintf(&persistent, " %s \"%s\"", netSNMPPrivacy(user.PrivacyProtocol), quote(user.PrivacyKey))
			}
			persistent.WriteByte('\n')
		}
	}
	return []byte(main.String()), []byte(persistent.String())
}

func writeCommunity(builder *strings.Builder, securityName, community, access string, v1, v2c bool) {
	fmt.Fprintf(builder, "com2sec %s default \"%s\"\n", securityName, quote(community))
	if v1 {
		fmt.Fprintf(builder, "group %s_v1 v1 %s\naccess %s_v1 \"\" any noauth exact tera_all %s none\n", securityName, securityName, securityName, viewForAccess(access))
	}
	if v2c {
		fmt.Fprintf(builder, "group %s_v2c v2c %s\naccess %s_v2c \"\" any noauth exact tera_all %s none\n", securityName, securityName, securityName, viewForAccess(access))
	}
}

func viewForAccess(access string) string {
	if access == "write" {
		return "tera_all"
	}
	return "none"
}

func netSNMPAuth(value string) string {
	switch value {
	case "md5":
		return "MD5"
	case "sha256":
		return "SHA-256"
	default:
		return "SHA"
	}
}

func netSNMPPrivacy(value string) string {
	if value == "des" {
		return "DES"
	}
	return "AES"
}

func quote(value string) string {
	return strings.NewReplacer("\\", "\\\\", "\"", "\\\"").Replace(value)
}

func boundedOutput(value []byte) string {
	value = []byte(strings.TrimSpace(string(value)))
	if len(value) > 512 {
		value = value[:512]
	}
	return string(value)
}