// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
// SPDX-License-Identifier: PolyForm-Noncommercial-1.0.0

// Package app assembles the volumen HTTP server: shared services,
// route registration, and the middleware chain.
package app

import (
	"crypto/rand"
	"encoding/hex"
	json "encoding/json/v2"
	"errors"
	"fmt"
	"log/slog"
	"net/http"
	"os"
	"path/filepath"
	"slices"
	"strings"
	"syscall"
	"time"

	"sourcedock.dev/petrbalvin/volumen/internal/admin"
	"sourcedock.dev/petrbalvin/volumen/internal/audit"
	"sourcedock.dev/petrbalvin/volumen/internal/backup"
	"sourcedock.dev/petrbalvin/volumen/internal/config"
	"sourcedock.dev/petrbalvin/volumen/internal/httpapi"
	"sourcedock.dev/petrbalvin/volumen/internal/imagefile"
	"sourcedock.dev/petrbalvin/volumen/internal/payloads"
	"sourcedock.dev/petrbalvin/volumen/internal/post"
	"sourcedock.dev/petrbalvin/volumen/internal/ratelimit"
	"sourcedock.dev/petrbalvin/volumen/internal/session"
	"sourcedock.dev/petrbalvin/volumen/internal/store"
	"sourcedock.dev/petrbalvin/volumen/internal/templates"
	"sourcedock.dev/petrbalvin/volumen/internal/tokens"
	"sourcedock.dev/petrbalvin/volumen/internal/users"
	"sourcedock.dev/petrbalvin/volumen/internal/version"
	"sourcedock.dev/petrbalvin/volumen/internal/web"
	"sourcedock.dev/petrbalvin/volumen/internal/webhooks"
)

// Server holds every long-lived service the handlers share.
type Server struct {
	Config    *config.Config
	Store     *store.Store
	Users     *users.Users
	Templates *templates.Store
	Tokens    *tokens.Store
	Audit     *audit.Log
	LoginLim  *ratelimit.LoginLimiter
	Sessions  *session.Store
	Webhooks  *webhooks.Manager
	Admin     *admin.Admin
	OnEvent   func(event string, payload map[string]any)

	// previewKey is the session secret New resolved, which the admin
	// signs preview links with and the API verifies them against.
	previewKey string
}

// New validates the configuration and builds the server with all
// file-backed stores derived from it.
func New(cfg *config.Config, st *store.Store) (*Server, error) {
	if err := cfg.Validate(); err != nil {
		return nil, err
	}
	secret, err := sessionSecret(cfg)
	if err != nil {
		return nil, err
	}
	cookieSecure := cfg.Server.CookieSecure || cfg.Server.TrustProxy
	usersPath := cfg.UsersFile
	hooks := make([]webhooks.Webhook, 0, len(cfg.Webhooks))
	for _, hook := range cfg.Webhooks {
		hooks = append(hooks, webhooks.Webhook{
			URL: hook.URL, Secret: hook.Secret,
			Events: hook.Events, Enabled: hook.Delivers(),
		})
	}
	staticHooks := slices.Clone(hooks)
	// The admin-managed hooks live beside the users file and apply
	// without a restart. A file that cannot be read is a real fault and
	// is reported, but it does not take the server down: the configured
	// hooks still deliver.
	webhooksFile := filepath.Join(filepath.Dir(usersPath), "webhooks.toml")
	fileHooks, err := webhooks.LoadFile(webhooksFile)
	if err != nil {
		slog.Warn("app: ignoring the webhook store", "path", webhooksFile, "error", err)
	} else {
		hooks = append(hooks, fileHooks...)
	}
	manager := webhooks.NewManager(hooks, version.Version())
	srv := &Server{
		Config:     cfg,
		Store:      st,
		Users:      users.New(usersPath),
		Templates:  templates.New(cfg.TemplatesFile()),
		Tokens:     tokens.New(cfg.TokensFile()),
		Audit:      audit.New(cfg.AuditLog),
		LoginLim:   ratelimit.NewLoginLimiter(),
		Sessions:   session.New(secret, time.Duration(cfg.Admin.SessionTTL)*time.Second, cookieSecure),
		Webhooks:   manager,
		previewKey: secret,
		OnEvent: func(event string, payload map[string]any) {
			manager.Fire(event, payload, false)
		},
	}
	adminHandler, err := admin.New(admin.Deps{
		Config:         cfg,
		Store:          st,
		Users:          srv.Users,
		Templates:      srv.Templates,
		Tokens:         srv.Tokens,
		Audit:          srv.Audit,
		LoginLim:       srv.LoginLim,
		Sessions:       srv.Sessions,
		Webhooks:       manager,
		PreviewKey:     secret,
		WebhooksFile:   webhooksFile,
		StaticWebhooks: staticHooks,
		Version:        version.Version(),
		OnEvent:        srv.OnEvent,
		Backup:         BackupOptions(cfg),
	})
	if err != nil {
		return nil, fmt.Errorf("init admin UI: %w", err)
	}
	srv.Admin = adminHandler
	return srv, nil
}

// Handler builds the full middleware chain and route tree.
//
// Uploaded media and the backup export are served on their own branches:
// the session middleware and the gzip wrapper buffer whole responses,
// which would hold entire files in memory. Everything else flows through
// the full chain.
func (s *Server) Handler() http.Handler {
	secure := s.Config.Server.CookieSecure || s.Config.Server.TrustProxy

	mediaMux := http.NewServeMux()
	mediaMux.HandleFunc("GET /media/{name...}", s.handleMedia)
	mediaHandler := web.SecurityHeaders(secure)(mediaMux)

	adminHandler := s.Admin.Handler()

	// The backup export streams: it keeps the admin authentication but
	// bypasses the wrappers that hold a whole response in memory, the
	// session recorder and the gzip wrapper, so the archive reaches the
	// client as it is written instead of waiting in a second copy. The
	// session attaches read-only; a GET never mutates it.
	var exportHandler http.Handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		sess := s.Sessions.Load(r)
		adminHandler.ServeHTTP(w, r.WithContext(session.WithContext(r.Context(), sess)))
	})
	exportHandler = web.CrossOrigin()(exportHandler)
	exportHandler = web.SecurityHeaders(secure)(exportHandler)

	mux := http.NewServeMux()

	api := httpapi.New(httpapi.Deps{
		Config:     s.Config,
		Store:      s.Store,
		Tokens:     s.Tokens,
		OnEvent:    s.OnEvent,
		PreviewKey: s.previewKey,
	})
	mux.Handle("/api/volumen/", api)
	mux.Handle("/admin/", adminHandler)
	mux.Handle("/admin", adminHandler)

	mux.HandleFunc("GET /{$}", func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Location", "/admin/")
		w.WriteHeader(http.StatusSeeOther)
	})
	mux.HandleFunc("GET /healthz", s.handleHealthz)
	mux.HandleFunc("GET /robots.txt", s.handleRobots)
	mux.HandleFunc("GET /sitemap.xml", func(w http.ResponseWriter, _ *http.Request) {
		w.Header().Set("Location", "/api/volumen/sitemap.xml")
		w.WriteHeader(http.StatusMovedPermanently)
	})
	mux.HandleFunc("GET /favicon.ico", s.handleFavicon)
	mux.HandleFunc("/", s.handleNotFound)

	var handler http.Handler = web.RequestLogger(mux)
	handler = s.Sessions.Middleware(handler)
	handler = s.apiRateLimit(handler)
	handler = web.SecurityHeaders(secure)(handler)
	handler = web.CrossOrigin()(handler)
	handler = web.Gzip(500)(handler)

	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.URL.Path == "/admin/settings/export" {
			exportHandler.ServeHTTP(w, r)
			return
		}
		if strings.HasPrefix(r.URL.Path, "/media/") {
			mediaHandler.ServeHTTP(w, r)
			return
		}
		handler.ServeHTTP(w, r)
	})
}

func (s *Server) apiRateLimit(next http.Handler) http.Handler {
	if s.Config.API.RateLimit <= 0 {
		return next
	}
	limiter := ratelimit.New(
		s.Config.API.RateLimit,
		time.Duration(s.Config.API.RateLimitWindow)*time.Second,
	)
	trusted, err := s.Config.TrustedProxyPrefixes()
	if err != nil || !s.Config.Server.TrustProxy {
		trusted = nil
	}
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if r.URL.Path != "/api/volumen" && !strings.HasPrefix(r.URL.Path, "/api/volumen/") {
			next.ServeHTTP(w, r)
			return
		}
		allowed, remaining, retryAfter := limiter.Check(web.ClientIP(r, trusted))
		if !allowed {
			w.Header().Set("Retry-After", fmt.Sprintf("%d", retryAfter))
			w.Header().Set("X-RateLimit-Limit", fmt.Sprintf("%d", limiter.Limit()))
			w.Header().Set("X-RateLimit-Remaining", "0")
			for key, value := range map[string]string{
				"Access-Control-Allow-Origin":  "*",
				"Access-Control-Allow-Methods": "GET, OPTIONS",
				"Access-Control-Allow-Headers": "Content-Type",
			} {
				w.Header().Set(key, value)
			}
			w.Header().Set("Content-Type", "application/json")
			w.WriteHeader(http.StatusTooManyRequests)
			_ = json.MarshalWrite(w, map[string]any{
				"error":       "rate_limited",
				"retry_after": retryAfter,
			}, json.Deterministic(true))
			return
		}
		w.Header().Set("X-RateLimit-Limit", fmt.Sprintf("%d", limiter.Limit()))
		w.Header().Set("X-RateLimit-Remaining", fmt.Sprintf("%d", remaining))
		next.ServeHTTP(w, r)
	})
}

func (s *Server) handleHealthz(w http.ResponseWriter, _ *http.Request) {
	checks := map[string]string{}
	overall := "ok"

	if info, err := os.Stat(s.Config.ContentDir); err == nil && info.IsDir() {
		checks["content_dir"] = "ok"
	} else {
		checks["content_dir"] = "missing"
		overall = "degraded"
	}

	switch err := s.Users.Health(); {
	case err != nil:
		// A file that cannot be read means nobody can sign in, which is
		// not a healthy deployment: say so rather than reporting a count
		// of zero accounts.
		slog.Error("volumen: healthz cannot read the users file", "error", err)
		checks["users_file"] = "unreadable"
		overall = "degraded"
	default:
		if info, statErr := os.Stat(s.Config.UsersFile); statErr == nil && info.Mode().IsRegular() {
			checks["users_file"] = "ok"
		} else {
			checks["users_file"] = "missing (no accounts yet; /admin runs the first-run wizard)"
		}
	}
	if err := s.Tokens.Health(); err != nil {
		slog.Error("volumen: healthz cannot read the tokens file", "error", err)
		checks["tokens_file"] = "unreadable"
		overall = "degraded"
	}
	if err := s.Templates.Health(); err != nil {
		slog.Error("volumen: healthz cannot read the templates file", "error", err)
		checks["templates_file"] = "unreadable"
		overall = "degraded"
	}
	if skipped := s.Store.Unreadable(); len(skipped) > 0 {
		checks["content_files"] = fmt.Sprintf("%d file(s) cannot be parsed", len(skipped))
		overall = "degraded"
	}

	freeMB, err := freeDiskMB(s.Config.ContentDir)
	switch {
	case err != nil:
		// The path and the OS error are logged, not published: this
		// endpoint is anonymous.
		slog.Warn("volumen: healthz cannot read the content directory", "error", err)
		checks["disk"] = "error"
	case freeMB < 100:
		checks["disk"] = fmt.Sprintf("low: %.0f MB free", freeMB)
		overall = "degraded"
	default:
		checks["disk"] = fmt.Sprintf("ok (%.0f MB free)", freeMB)
	}

	status := http.StatusOK
	if overall != "ok" {
		status = http.StatusServiceUnavailable
	}
	w.Header().Set("Cache-Control", "no-store")
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)
	_ = json.MarshalWrite(w, map[string]any{"status": overall, "checks": checks}, json.Deterministic(true))
}

func freeDiskMB(path string) (float64, error) {
	var st syscall.Statfs_t
	if err := syscall.Statfs(path, &st); err != nil {
		return 0, err
	}
	return float64(st.Bavail) * float64(st.Bsize) / (1024 * 1024), nil
}

func (s *Server) handleRobots(w http.ResponseWriter, _ *http.Request) {
	base := s.Config.Site.BaseURL
	w.Header().Set("Content-Type", "text/plain; charset=utf-8")
	fmt.Fprintf(w, "User-agent: *\nAllow: /\nSitemap: %s/api/volumen/sitemap.xml\n", base)
}

func (s *Server) handleFavicon(w http.ResponseWriter, _ *http.Request) {
	icon, err := web.StaticFile("volumen-icon.svg")
	if err != nil {
		w.WriteHeader(http.StatusNotFound)
		return
	}
	w.Header().Set("Content-Type", "image/svg+xml")
	w.Header().Set("Cache-Control", "public, max-age=86400")
	w.WriteHeader(http.StatusOK)
	_, _ = w.Write(icon)
}

func (s *Server) handleMedia(w http.ResponseWriter, r *http.Request) {
	name := r.PathValue("name")
	mediaPath, err := s.Store.MediaPath(name)
	if err != nil {
		// A name that is not an allowed image, that would escape the
		// media directory, or that names nothing, is a 404: the route is
		// public, so it says nothing about why.
		s.writeNotFound(w)
		return
	}
	// The type is set from the name's extension rather than sniffed, so a
	// file whose bytes do not match its extension is still served as an
	// image and never as a document.
	contentType := imagefile.ContentType(name)
	w.Header().Set("Cache-Control", "public, max-age=604800")
	w.Header().Set("Content-Type", contentType)
	if contentType == imagefile.MIMESVG {
		// An SVG is the one accepted image that is also a document:
		// opened at its own URL it would run on this origin. The
		// sandbox and the locked-down policy make that a dead
		// document, while the <img> uses of the file ignore both.
		w.Header().Set("Content-Security-Policy",
			"default-src 'none'; style-src 'unsafe-inline'; img-src 'self' data:; sandbox")
	}
	http.ServeFile(w, r, mediaPath)
}

func (s *Server) writeNotFound(w http.ResponseWriter) {
	w.Header().Set("Cache-Control", "no-store")
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(http.StatusNotFound)
	_ = json.MarshalWrite(w, map[string]any{"error": "not_found"}, json.Deterministic(true))
}

func (s *Server) handleNotFound(w http.ResponseWriter, _ *http.Request) {
	s.writeNotFound(w)
}

// BackupOptions names the files an archive carries, for the admin UI and
// the CLI export and import.
func BackupOptions(cfg *config.Config) backup.Options {
	return backup.Options{
		ContentDir:    cfg.ContentDir,
		UsersFile:     cfg.UsersFile,
		TemplatesFile: cfg.TemplatesFile(),
		TokensFile:    cfg.TokensFile(),
	}
}

// PublishEvent reports that a scheduled post went live, as the same
// post.published event the admin delivers, so a hook subscribed to it
// hears about a post the scheduler published.
func (s *Server) PublishEvent(p *post.Post) {
	if s.OnEvent == nil || p == nil {
		return
	}
	s.OnEvent("post.published", map[string]any{"post": payloads.BuildSummary(p)})
}

// sessionSecret resolves the cookie signing key. The [admin].session_key
// in config is an override; with nothing set the server keeps its own
// secret in secret.key beside the users file, generating one on first
// start so a fresh installation can sign in without the operator
// editing the config. Production refuses a key shorter than 64 bytes
// whatever its source; development falls back to the ephemeral secret
// of session.New when the file cannot be written.
func sessionSecret(cfg *config.Config) (string, error) {
	if key := cfg.Admin.SessionKey; key != "" {
		return checkedSecret(cfg, key, "[admin].session_key")
	}
	path := cfg.SecretKeyFile()
	raw, err := os.ReadFile(path)
	switch {
	case err == nil:
		if key := strings.TrimSpace(string(raw)); key != "" {
			return checkedSecret(cfg, key, path)
		}
		// An empty file is treated as no file: one more start and the
		// key is generated and written, so the state converges.
	case !errors.Is(err, os.ErrNotExist):
		if cfg.IsProduction() {
			return "", fmt.Errorf("read %s: %w", path, err)
		}
		slog.Warn("app: cannot read the session secret file", "path", path, "error", err)
		return "", nil
	}
	// 32 random bytes as 64 hex characters, which is the length
	// production requires. A system failure here dies inside
	// crypto/rand rather than signing sessions with less entropy than
	// the key looks like.
	buf := make([]byte, 32)
	if _, err := rand.Read(buf); err != nil {
		return "", fmt.Errorf("generate the session secret: %w", err)
	}
	key := hex.EncodeToString(buf)
	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
		if cfg.IsProduction() {
			return "", fmt.Errorf("create %s: %w", filepath.Dir(path), err)
		}
		slog.Warn("app: cannot create the session secret directory; using an ephemeral secret", "error", err)
		return "", nil
	}
	// O_EXCL so two servers racing on a fresh data directory cannot
	// each write a different key: the loser reads the winner's file.
	f, err := os.OpenFile(path, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o600)
	if errors.Is(err, os.ErrExist) {
		raw, err := os.ReadFile(path)
		if err != nil {
			if cfg.IsProduction() {
				return "", fmt.Errorf("read %s: %w", path, err)
			}
			return "", nil
		}
		return checkedSecret(cfg, strings.TrimSpace(string(raw)), path)
	}
	if err != nil {
		if cfg.IsProduction() {
			return "", fmt.Errorf("write %s: %w (or set [admin].session_key)", path, err)
		}
		slog.Warn("app: cannot write the session secret file; using an ephemeral secret", "error", err)
		return "", nil
	}
	if _, err := f.WriteString(key + "\n"); err != nil {
		f.Close()
		if cfg.IsProduction() {
			return "", fmt.Errorf("write %s: %w", path, err)
		}
		return "", nil
	}
	if err := f.Close(); err != nil && cfg.IsProduction() {
		return "", fmt.Errorf("write %s: %w", path, err)
	}
	slog.Info("app: generated the session secret", "path", path)
	return key, nil
}

// checkedSecret applies the production length rule to a key named by
// its source, which is the config field or the secret file.
func checkedSecret(cfg *config.Config, key, source string) (string, error) {
	if len(key) < 64 && cfg.IsProduction() {
		return "", fmt.Errorf(
			"%s must be at least 64 bytes in production (got %d). "+
				"Generate one with: openssl rand -hex 32", source, len(key))
	}
	return key, nil
}