package sitemap

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"log/slog"
	"net"
	"net/http"
	"net/netip"
	"strings"
	"sync"
	"time"

	"git.golem15.com/golem15/sm-sitemap-plugin/classes"
	"git.golem15.com/golem15/sm-sitemap-plugin/models"
	"git.golem15.com/golem15/summercms/modules/backpack"
	"git.golem15.com/golem15/summercms/modules/pact"
	"git.golem15.com/golem15/summercms/modules/signpost"
	"git.golem15.com/golem15/summercms/modules/surf"
	"gorm.io/gorm"
)

// The response headers of the PHP routes (live responses).
const (
	contentTypeXML = "application/xml"
	contentTypeXSL = "text/xsl; charset=UTF-8"
	cacheControl   = "no-cache, private"
)

// BucketSitemap is the throttle bucket of GET /sitemap.xml.
const BucketSitemap = "sitemap-xml"

// CacheConfigKey is the config key of the number of seconds a generated
// sitemap is served from memory; 0 turns the cache off.
const CacheConfigKey = "golem15.sitemap.cache_seconds"

// cacheMaxEntries bounds the cache: one entry per root URL (app.url, or the
// request's scheme and Host when app.url is not set) and stored definition
// and settings. A full cache is emptied.
const cacheMaxEntries = 16

// Routes mounts GET /sitemap.xml and GET /sitemap.xsl at the application
// root without the backend middleware: search engines and browsers fetch
// them anonymously. /sitemap.xml resolves every registry item, so it is
// throttled per client IP (BucketSitemap) and its output is cached.
func (p *Plugin) Routes(r pact.Router) error {
	r.Get("/sitemap.xml", p.sitemapXML, "throttle:"+BucketSitemap)
	r.Get("/sitemap.xsl", p.sitemapXSL)
	return nil
}

// Buckets registers sitemap-xml: 60 requests a minute per client IP (behind
// the configured trusted proxies). 429 bodies are surf's
// {"message":"Too Many Attempts."}.
func (p *Plugin) Buckets() map[string]surf.Bucket {
	trusted := surf.TrustedProxies(nil)
	if p.app != nil {
		trusted = surf.TrustedProxies(p.app.Config)
	}
	return map[string]surf.Bucket{
		BucketSitemap: {
			Name:  BucketSitemap,
			Max:   60,
			Decay: time.Minute,
			Key: func(r *http.Request) string {
				return "sitemap-xml|" + surf.ClientIP(r, trusted)
			},
		},
	}
}

// sitemapCache holds generated sitemaps for the configured time. The key
// covers the root URL and the stored definition and settings, so a save of
// either is served at once; registry content (a new post) shows after the
// entry expires. gen lets one generation run at a time, so concurrent misses
// wait for it instead of each resolving every registry item.
type sitemapCache struct {
	ttl     time.Duration
	gen     sync.Mutex
	mu      sync.Mutex
	entries map[string]cachedSitemap
}

type cachedSitemap struct {
	xml     string
	expires time.Time
}

func newSitemapCache(ttl time.Duration) *sitemapCache {
	return &sitemapCache{ttl: ttl, entries: map[string]cachedSitemap{}}
}

func (c *sitemapCache) get(key string) (string, bool) {
	if c == nil || c.ttl <= 0 || key == "" {
		return "", false
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	e, ok := c.entries[key]
	if !ok || time.Now().After(e.expires) {
		return "", false
	}
	return e.xml, true
}

func (c *sitemapCache) put(key, xml string) {
	if c == nil || c.ttl <= 0 || key == "" {
		return
	}
	c.mu.Lock()
	defer c.mu.Unlock()
	if len(c.entries) >= cacheMaxEntries {
		clear(c.entries)
	}
	c.entries[key] = cachedSitemap{xml: xml, expires: time.Now().Add(c.ttl)}
}

// cacheKey is a digest of the root URL, the definition's items and the
// settings; "" (no caching) when either does not encode.
func cacheKey(root string, def models.Definition, settings models.Settings) string {
	items, err := json.Marshal(def.Items())
	if err != nil {
		return ""
	}
	set, err := json.Marshal(settings)
	if err != nil {
		return ""
	}
	h := sha256.New()
	for _, part := range [][]byte{[]byte(root), items, set} {
		h.Write(part)
		h.Write([]byte{0})
	}
	return hex.EncodeToString(h.Sum(nil))
}

// sitemapXSL serves the embedded stylesheet, the PHP file byte for byte.
func (p *Plugin) sitemapXSL(w http.ResponseWriter, _ *http.Request) {
	w.Header().Set("Content-Type", contentTypeXSL)
	w.Header().Set("Cache-Control", cacheControl)
	w.WriteHeader(http.StatusOK)
	_, _ = w.Write(sitemapXSL)
}

// sitemapXML serves the definition with the fixed theme key: 404 when there
// is none, an empty application/xml body when it has no items, else the
// generated XML with the settings' frontend_url rewrite applied.
func (p *Plugin) sitemapXML(w http.ResponseWriter, r *http.Request) {
	app := p.app
	db, err := database(app)
	if err != nil {
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	var defs []models.Definition
	if err := db.WithContext(r.Context()).Where("theme = ?", models.DefinitionTheme).Order("id ASC").Limit(1).Find(&defs).Error; err != nil {
		slog.Default().Error("golem15.sitemap: load definition", "error", err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	if len(defs) == 0 {
		slog.Default().Info("golem15.sitemap: No sitemap definition was found. Try creating one first.")
		http.NotFound(w, r)
		return
	}
	settings, err := loadSettings(db.WithContext(r.Context()))
	if err != nil {
		slog.Default().Error("golem15.sitemap: load settings", "error", err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	root := backendURL(app, r)
	xml, err := p.generate(r, root, defs[0], settings)
	if err != nil {
		slog.Default().Error("golem15.sitemap: generate", "error", err)
		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
		return
	}
	w.Header().Set("Content-Type", contentTypeXML)
	w.Header().Set("Cache-Control", cacheControl)
	w.WriteHeader(http.StatusOK)
	_, _ = w.Write([]byte(xml))
}

// generate returns the sitemap of def with the frontend_url rewrite
// applied, from the cache when it holds it.
func (p *Plugin) generate(r *http.Request, root string, def models.Definition, settings models.Settings) (string, error) {
	cache := p.cache
	key := ""
	if cache != nil && cache.ttl > 0 {
		key = cacheKey(root, def, settings)
		if xml, ok := cache.get(key); ok {
			return xml, nil
		}
		cache.gen.Lock()
		defer cache.gen.Unlock()
		if xml, ok := cache.get(key); ok {
			return xml, nil
		}
	}
	app := p.app
	gen := &classes.Generator{Root: root, Settings: settings, Tiers: p.tiers, Events: app.Events}
	if reg, ok := app.Lookup[*signpost.Registry](); ok && reg != nil {
		gen.Resolver = reg
	}
	xml, err := gen.Generate(r.Context(), def.Items())
	if err != nil {
		return "", err
	}
	xml = classes.Rewrite(xml, classes.BackendURL(root), models.Text(settings.FrontendURL))
	cache.put(key, xml)
	return xml, nil
}

// loadSettings reads the settings row; without one the PHP defaults apply.
func loadSettings(db *gorm.DB) (models.Settings, error) {
	var rows []models.Settings
	if err := db.Where("id = ?", 1).Limit(1).Find(&rows).Error; err != nil {
		return models.Settings{}, err
	}
	if len(rows) == 0 {
		return models.DefaultSettings(), nil
	}
	return rows[0], nil
}

// database returns the app's published *gorm.DB.
func database(app *backpack.App) (*gorm.DB, error) {
	if app == nil {
		return nil, errors.New("golem15.sitemap: app is not booted")
	}
	db, ok := app.Lookup[*gorm.DB]()
	if !ok || db == nil {
		return nil, errors.New("golem15.sitemap: database is not configured")
	}
	return db, nil
}

// backendURL is the root URL the sitemap's absolute URLs start with, PHP
// url('/') right-trimmed of slashes: app.url when it is set, else the
// request's scheme and Host. The scheme is https for a TLS connection, or
// when the immediate peer is in http.trusted_proxies and its last
// X-Forwarded-Proto value says https; a client that sends the header
// directly is ignored.
func backendURL(app *backpack.App, r *http.Request) string {
	if app != nil && app.Config != nil {
		if u := strings.TrimRight(strings.TrimSpace(app.Config.String("app.url")), "/"); u != "" {
			return u
		}
	}
	scheme := "http"
	if isHTTPS(app, r) {
		scheme = "https"
	}
	return strings.TrimRight(scheme+"://"+r.Host, "/")
}

func isHTTPS(app *backpack.App, r *http.Request) bool {
	if r.TLS != nil {
		return true
	}
	if app == nil || app.Config == nil {
		return false
	}
	trusted := surf.TrustedProxies(app.Config)
	if len(trusted) == 0 || !peerTrusted(r.RemoteAddr, trusted) {
		return false
	}
	values := r.Header.Values("X-Forwarded-Proto")
	if len(values) == 0 {
		return false
	}
	hops := strings.Split(values[len(values)-1], ",")
	return strings.EqualFold(strings.TrimSpace(hops[len(hops)-1]), "https")
}

func peerTrusted(remoteAddr string, trusted []netip.Prefix) bool {
	host, _, err := net.SplitHostPort(remoteAddr)
	if err != nil {
		host = remoteAddr
	}
	addr, err := netip.ParseAddr(strings.Trim(host, "[]"))
	if err != nil {
		return false
	}
	addr = addr.Unmap()
	for _, p := range trusted {
		if p.Contains(addr) {
			return true
		}
	}
	return false
}