// Package config holds the agent's own settings: which tenant and site this
// install belongs to, how to reach the broker, and how to launch the engine.
//
// Kept separate from the engine's YAML on purpose. That file describes
// recognition — thresholds, cameras, gates — and is edited by whoever tunes a
// site. This one describes identity and connectivity, is written by the
// installer and the login flow, and holds a secret.
package config

import (
	"crypto/rand"
	"encoding/base64"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"os"
	"path/filepath"
	"runtime"
	"strings"
)

// protectedPrefix marks a value that went through DPAPI, so a config written
// on Windows is never mistaken for a plaintext dev one and vice versa.
const protectedPrefix = "dpapi:"

// Config is the agent's on-disk settings.
type Config struct {
	// Tenant identity. The server keys everything on these.
	ClientID string `json:"client_id"`
	SiteID   string `json:"site_id"`
	SiteName string `json:"site_name"`

	// Broker.
	BrokerURL      string `json:"broker_url"`
	BrokerUsername string `json:"broker_username"`
	BrokerPassword string `json:"broker_password"` // protected at rest
	// Pins the broker's issuer. Empty uses the system roots, which is what a
	// Let's Encrypt certificate needs; a private CA is pinned by path.
	BrokerCAFile string `json:"broker_ca_file"`

	// Engine process.
	EngineExe string `json:"engine_exe"`
	// EngineDir is the working directory the engine is launched IN. Empty
	// means the install root.
	//
	// It exists because nothing set it, so the engine inherited whatever
	// launched the app - and for an app started by double-clicking its
	// bundle that is "/", not anywhere useful. The symptom on macOS was
	// `python: No module named behavision` repeating forever: the dev engine
	// is `-m behavision`, which resolves against the working directory. The
	// same app started from a terminal in the repo worked, which is exactly
	// the shape of a bug that survives every test run by a developer.
	EngineDir   string   `json:"engine_dir,omitempty"`
	EngineArgs  []string `json:"engine_args"`
	APIBase     string   `json:"api_base"`
	APIUser     string   `json:"api_user"`
	APIPassword string   `json:"api_password"` // protected at rest

	// Session, so a shop PC that reboots overnight is not a login every
	// morning. Protected at rest like every other secret here.
	SessionToken   string `json:"session_token"`
	SessionRefresh string `json:"session_refresh"`
	SessionEmail   string `json:"session_email"`

	// CloudBase is the server this site reports to; AgentToken is this PC's
	// own credential there, issued once at enrolment.
	//
	// Deliberately not the same secret as BrokerPassword: they authenticate
	// different things - one says this site may publish events, the other that
	// it may ask the API for something - so rotating either must not break the
	// other. Protected at rest like every other secret here.
	CloudBase  string `json:"cloud_base"`
	AgentToken string `json:"agent_token"`

	// Standalone marks a PC deliberately run on its own: cameras, recognition
	// and the local gallery, with nothing reported to head office.
	//
	// It exists so that "not linked yet" and "not going to be linked" are
	// different states. Without it every install was blocked on an enrolment
	// code, so a shop with one PC and no head office could not add a camera at
	// all - the software refused to do the thing it is for until a server it
	// does not need had issued it a credential.
	Standalone bool `json:"standalone"`

	// Queue.
	SpoolMax int `json:"spool_max"`

	// InstallID distinguishes THIS installation from every other one claimed
	// to the same site. See MQTTClientID.
	InstallID string `json:"install_id,omitempty"`

	path string
}

// Defaults returns a config that runs a locally installed engine.
//
// EngineExe is relative to the install root - the directory holding this
// executable - and names the installed layout: the engine is a PyInstaller
// one-FOLDER build, so it brings its own DLLs and cannot simply sit beside the
// app. Windows filenames are case-insensitive too, so `Behavision.exe` (the
// app) and `behavision.exe` (the engine) could not share a directory even if
// it were tidy to.
func Defaults() Config {
	exe := filepath.Join("engine", "behavision")
	if runtime.GOOS == "windows" {
		exe += ".exe"
	}
	return Config{
		EngineExe:  exe,
		EngineArgs: []string{"run"},
		APIBase:    "http://127.0.0.1:8010",
		SpoolMax:   50000,
	}
}

// Load reads the config, decrypting secrets. A missing file is not an error:
// a fresh install has none until the operator logs in, and failing to start
// because of that would leave them with no UI to log in from.
func Load(path string) (Config, error) {
	cfg := Defaults()
	cfg.path = path
	blob, err := os.ReadFile(path)
	if os.IsNotExist(err) {
		return cfg, nil
	}
	if err != nil {
		return cfg, err
	}
	if err := json.Unmarshal(blob, &cfg); err != nil {
		return cfg, fmt.Errorf("config %s: %w", path, err)
	}
	cfg.path = path
	// Minted on first load and written back, so an installation that predates
	// this field gets one without anybody doing anything. Best effort: a
	// read-only config still yields a working id for this run, it is simply
	// not the same one next time.
	if cfg.InstallID == "" {
		cfg.InstallID = newInstallID()
		_ = cfg.Save(path)
	}
	for _, field := range []*string{&cfg.BrokerPassword, &cfg.APIPassword,
		&cfg.SessionToken, &cfg.SessionRefresh, &cfg.AgentToken} {
		plain, err := reveal(*field)
		if err != nil {
			// A secret that cannot be decrypted usually means the config was
			// copied from another machine - DPAPI is machine-scoped. Blank it
			// rather than failing: the operator can log in again, but they
			// cannot fix a process that will not start.
			*field = ""
			continue
		}
		*field = plain
	}
	return cfg, nil
}

// Save writes the config atomically, protecting secrets on the way out.
func (c Config) Save(path string) error {
	if path == "" {
		path = c.path
	}
	if path == "" {
		return fmt.Errorf("config: no path to save to")
	}
	out := c
	out.path = ""
	for _, field := range []*string{&out.BrokerPassword, &out.APIPassword,
		&out.SessionToken, &out.SessionRefresh, &out.AgentToken} {
		hidden, err := conceal(*field)
		if err != nil {
			return err
		}
		*field = hidden
	}
	blob, err := json.MarshalIndent(out, "", "  ")
	if err != nil {
		return err
	}
	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
		return err
	}
	// Temp-then-rename: a crash mid-write must not leave a config that parses
	// as valid but is half old and half new.
	tmp := path + ".tmp"
	if err := os.WriteFile(tmp, blob, 0o600); err != nil {
		return err
	}
	return os.Rename(tmp, path)
}

// Configured reports whether this install has been claimed by a tenant yet.
// The UI shows a login screen until it has.
func (c Config) Configured() bool {
	return c.ClientID != "" && c.SiteID != "" && c.BrokerURL != ""
}

// SecretsProtected is false on a dev machine, where secrets are stored as-is.
// Surfaced rather than hidden so nobody ships a build believing otherwise.
func SecretsProtected() bool { return protectionAvailable() }

func conceal(plain string) (string, error) {
	if plain == "" || !protectionAvailable() {
		return plain, nil
	}
	blob, err := protect([]byte(plain))
	if err != nil {
		return "", err
	}
	return protectedPrefix + base64.StdEncoding.EncodeToString(blob), nil
}

func reveal(stored string) (string, error) {
	if !strings.HasPrefix(stored, protectedPrefix) {
		return stored, nil
	}
	blob, err := base64.StdEncoding.DecodeString(
		strings.TrimPrefix(stored, protectedPrefix))
	if err != nil {
		return "", err
	}
	plain, err := unprotect(blob)
	if err != nil {
		return "", err
	}
	return string(plain), nil
}

// MQTTClientID names this INSTALLATION, not this site.
//
// It was `behavision-<client>-<site>`, which is the same string on every
// computer claimed to one shop. MQTT requires client ids to be unique and a
// broker enforces it by disconnecting the older session when a new one
// arrives with the same id - so two machines on one site take turns kicking
// each other off, forever. Measured on a second Mac claimed to a live shop:
//
//	broker connected / broker connection lost: EOF / broker connected / ...
//
// The damage is not confined to the new machine. The shop's own till is the
// other half of that loop, so somebody signing in on a laptop to look at the
// product stops the shop delivering visits - and nothing at either end says
// why, because from each side it reads as an unstable network.
//
// The site stays in the id because it is what a broker log is read by, and
// the random half is short for the same reason. `CleanSession(true)` means
// there is no session state for a changed id to strand.
func (c Config) MQTTClientID() string {
	id := c.InstallID
	if id == "" {
		// A config that could not be written still has to produce a UNIQUE
		// id, or this falls straight back into the collision it exists to
		// prevent. Per-run is the right failure: the connection works and the
		// only cost is a new name in the broker's log after a restart.
		id = newInstallID()
	}
	return "behavision-" + c.ClientID + "-" + c.SiteID + "-" + id
}

func newInstallID() string {
	b := make([]byte, 4)
	if _, err := rand.Read(b); err != nil {
		return "x"
	}
	return hex.EncodeToString(b)
}