package surf

import (
	"bytes"
	_ "embed"
	"encoding/base64"
	"html/template"
	"net/http"
	"strconv"
	"strings"
	"unicode/utf8"

	"git.golem15.com/golem15/summercms/modules/backpack"
)

// ErrorPage describes a framework-generated error answered with an HTML page
// to a browser navigation: an unmatched route, a wrong method, a failed
// Where/WhereIn constraint, the uploads route while it is unavailable, or a
// recovered panic. An ErrorPageRenderer receives it to build its own page.
type ErrorPage struct {
	// Status is the HTTP status the response carries (404, 405 or 500).
	Status int
	// Title is the short English heading for Status, such as "Page not found".
	Title string
	// Message is one English sentence explaining Status.
	Message string
	// Home is the link back to the site, always "/".
	Home string
	// ShowDetails is true when app.debug is true or the environment is set
	// and is not production. Method, Path and RequestID are empty otherwise.
	ShowDetails bool
	// Method is the request method; empty unless ShowDetails is true.
	Method string
	// Path is the request path routing saw; empty unless ShowDetails is true.
	Path string
	// RequestID is the X-Request-Id request header, trimmed and cut to 128
	// bytes; empty unless ShowDetails is true or when the header is absent.
	RequestID string
}

// ErrorPageRenderer is an optional backpack-published service that replaces
// the built-in HTML error page. surf looks it up for every page it renders,
// so an application or plugin publishes it in Register:
//
//	app.Publish[surf.ErrorPageRenderer](myRenderer{})
//
// surf still sets the status and the page headers (Content-Type, nosniff,
// no-store, Vary and the error-page Content-Security-Policy, which allows no
// script). Returning ok=false or an empty body keeps the built-in page, and
// so does a panic inside RenderErrorPage.
type ErrorPageRenderer interface {
	// RenderErrorPage returns the complete HTML document for page.
	RenderErrorPage(r *http.Request, page ErrorPage) (body []byte, ok bool)
}

// errorPageCSP goes on every HTML error response. There is no script-src,
// so no script runs on an error page, an override's included.
const errorPageCSP = "default-src 'none'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; font-src 'self'; base-uri 'none'; form-action 'self'; frame-ancestors 'none'"

// errorPageMaxRanges caps how many Accept media ranges negotiation reads.
const errorPageMaxRanges = 32

// errorPageMaxRequestID caps the X-Request-Id shown on the page, in bytes.
const errorPageMaxRequestID = 128

//go:embed errorpage.html
var errorPageHTML string

//go:embed errorpage_sun.webp
var errorPageLogo []byte

var (
	errorPageTemplate = template.Must(template.New("errorpage").Parse(errorPageHTML))
	// errorPageLogoURI is a constant built from the embedded file, so it is
	// safe to hand html/template as a URL.
	errorPageLogoURI = template.URL("data:image/webp;base64," + base64.StdEncoding.EncodeToString(errorPageLogo))
)

// errorPageView is what the built-in template renders.
type errorPageView struct {
	ErrorPage
	Logo template.URL
}

// errorPages negotiates and writes the HTML error page. A nil *errorPages
// keeps every call site's pre-error-page behaviour.
type errorPages struct {
	app         *backpack.App
	adminPrefix string
}

// newErrorPages returns the error-page writer for app, or nil when
// http.error_pages.enabled is present and false. A nil app, a nil config or
// an absent key keep error pages on.
func newErrorPages(app *backpack.App) *errorPages {
	if app != nil && app.Config != nil {
		if _, ok := app.Config.Lookup("http.error_pages.enabled"); ok && !app.Config.Bool("http.error_pages.enabled") {
			return nil
		}
	}
	return &errorPages{app: app}
}

// setAdminPrefix excludes the mounted admin prefix (and every path under it)
// from negotiation.
func (p *errorPages) setAdminPrefix(prefix string) {
	if p == nil {
		return
	}
	p.adminPrefix = strings.TrimSuffix(prefix, "/")
}

// wants reports whether r gets the HTML page instead of today's response.
func (p *errorPages) wants(r *http.Request) bool {
	if p == nil || r == nil || r.URL == nil {
		return false
	}
	path := r.URL.Path
	if apiStylePath(path) {
		return false
	}
	if p.adminPrefix != "" && (path == p.adminPrefix || strings.HasPrefix(path, p.adminPrefix+"/")) {
		return false
	}
	return browserNavigation(r)
}

// browserNavigation reports whether r looks like a top-level browser
// navigation that prefers HTML: Sec-Fetch-Mode navigate when the header is
// present (GET or HEAD when it is absent), no X-Requested-With, and an Accept
// header that names an HTML type with a q above every JSON type it names.
func browserNavigation(r *http.Request) bool {
	if modes := r.Header.Values("Sec-Fetch-Mode"); len(modes) > 0 {
		if !strings.EqualFold(strings.TrimSpace(modes[0]), "navigate") {
			return false
		}
	} else if r.Method != http.MethodGet && r.Method != http.MethodHead {
		return false
	}
	if len(r.Header.Values("X-Requested-With")) > 0 {
		return false
	}
	return acceptPrefersHTML(r.Header.Values("Accept"))
}

// acceptPrefersHTML reads at most errorPageMaxRanges media ranges. text/html
// or application/xhtml+xml must be named with q > 0 and a q strictly above
// that of every JSON type named (application/json, text/json,
// application/*+json). Wildcards count for neither side; a range with a
// malformed q is skipped.
func acceptPrefersHTML(values []string) bool {
	htmlQ, jsonQ := -1.0, -1.0
	read := 0
	for _, line := range values {
		for _, rng := range strings.Split(line, ",") {
			if read >= errorPageMaxRanges {
				break
			}
			read++
			media, params, _ := strings.Cut(rng, ";")
			media = strings.ToLower(strings.TrimSpace(media))
			q, ok := acceptQ(params)
			if !ok {
				continue
			}
			switch {
			case media == "text/html" || media == "application/xhtml+xml":
				htmlQ = max(htmlQ, q)
			case media == "application/json" || media == "text/json" ||
				(strings.HasPrefix(media, "application/") && strings.HasSuffix(media, "+json")):
				jsonQ = max(jsonQ, q)
			}
		}
	}
	return htmlQ > 0 && htmlQ > jsonQ
}

// acceptQ returns the q parameter of a media range (1 when absent) and false
// when it is malformed or outside 0..1.
func acceptQ(params string) (float64, bool) {
	for _, param := range strings.Split(params, ";") {
		key, value, found := strings.Cut(param, "=")
		if !found || !strings.EqualFold(strings.TrimSpace(key), "q") {
			continue
		}
		q, err := strconv.ParseFloat(strings.TrimSpace(value), 64)
		if err != nil || q < 0 || q > 1 {
			return 0, false
		}
		return q, true
	}
	return 1, true
}

// apiStylePath reports whether path is an API path that never gets the page:
// /api, anything under /api/, and the /_<name>/ prefixes.
func apiStylePath(path string) bool {
	return path == "/api" || strings.HasPrefix(path, "/api/") || strings.HasPrefix(path, "/_")
}

// pageFor returns the English title and message for status.
func pageFor(status int) (string, string) {
	switch status {
	case http.StatusNotFound:
		return "Page not found", "The page you are looking for does not exist or has moved."
	case http.StatusMethodNotAllowed:
		return "Method not allowed", "This address does not accept that kind of request."
	case http.StatusInternalServerError:
		return "Something went wrong", "The server hit an unexpected error. Please try again in a moment."
	}
	title := http.StatusText(status)
	if title == "" {
		title = "Error"
	}
	return title, "The server could not complete this request."
}

// showDetails reports whether the page lists method, path and request id:
// app.debug is true, or the environment is set and is not production. A nil
// app or config, or an empty environment, counts as production.
func showDetails(app *backpack.App) bool {
	if app == nil || app.Config == nil {
		return false
	}
	if app.Config.Bool("app.debug") {
		return true
	}
	env := strings.TrimSpace(app.Config.Environment())
	return env != "" && !strings.EqualFold(env, "production")
}

// requestID returns the X-Request-Id header trimmed and cut to
// errorPageMaxRequestID bytes on a rune boundary.
func requestID(r *http.Request) string {
	id := strings.ToValidUTF8(strings.TrimSpace(r.Header.Get("X-Request-Id")), "")
	if len(id) <= errorPageMaxRequestID {
		return id
	}
	cut := errorPageMaxRequestID
	for cut > 0 && !utf8.RuneStart(id[cut]) {
		cut--
	}
	return id[:cut]
}

// pageData fills the ErrorPage for status and r.
func (p *errorPages) pageData(r *http.Request, status int) ErrorPage {
	title, message := pageFor(status)
	page := ErrorPage{Status: status, Title: title, Message: message, Home: "/"}
	if showDetails(p.app) {
		page.ShowDetails = true
		page.Method = r.Method
		page.Path = r.URL.Path
		page.RequestID = requestID(r)
	}
	return page
}

// render returns the published renderer's page, or nil when there is none,
// it declines, returns an empty body or panics.
func (p *errorPages) render(r *http.Request, page ErrorPage) (body []byte) {
	if p.app == nil {
		return nil
	}
	renderer, ok := p.app.Lookup[ErrorPageRenderer]()
	if !ok || renderer == nil {
		return nil
	}
	defer func() {
		if recover() != nil {
			body = nil
		}
	}()
	out, ok := renderer.RenderErrorPage(r, page)
	if !ok || len(out) == 0 {
		return nil
	}
	return out
}

// write answers r with the HTML error page for status. The recovered panic
// value, if any, never reaches it.
func (p *errorPages) write(w http.ResponseWriter, r *http.Request, status int) {
	page := p.pageData(r, status)
	body := p.render(r, page)
	if body == nil {
		var buf bytes.Buffer
		if err := errorPageTemplate.Execute(&buf, errorPageView{ErrorPage: page, Logo: errorPageLogoURI}); err != nil {
			http.Error(w, http.StatusText(status), status)
			return
		}
		body = buf.Bytes()
	}
	h := w.Header()
	h.Set("Content-Type", "text/html; charset=utf-8")
	h.Set("X-Content-Type-Options", "nosniff")
	h.Set("Cache-Control", "no-store")
	h.Set("Content-Security-Policy", errorPageCSP)
	addVary(h, "Accept")
	addVary(h, "Sec-Fetch-Mode")
	addVary(h, "X-Requested-With")
	h.Del("Content-Length")
	w.WriteHeader(status)
	if r.Method != http.MethodHead {
		_, _ = w.Write(body)
	}
}

// notFound answers 404: the page for a browser navigation, else http.NotFound.
func (p *errorPages) notFound(w http.ResponseWriter, r *http.Request) {
	if p.wants(r) {
		p.write(w, r, http.StatusNotFound)
		return
	}
	http.NotFound(w, r)
}

// methodNotAllowed answers 405 after the caller has set Allow: the page for a
// browser navigation, else the stdlib text.
func (p *errorPages) methodNotAllowed(w http.ResponseWriter, r *http.Request) {
	if p.wants(r) {
		p.write(w, r, http.StatusMethodNotAllowed)
		return
	}
	http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
}

// fallback wraps mux so the 404 and 405 that ServeMux writes itself become
// the page for a browser navigation. Every other request, and every request
// a route matches, goes straight to mux. A nil receiver returns mux.
func (p *errorPages) fallback(mux *http.ServeMux) http.Handler {
	if p == nil {
		return mux
	}
	return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
		if !p.wants(req) {
			mux.ServeHTTP(w, req)
			return
		}
		h, pattern := mux.Handler(req)
		if pattern != "" {
			mux.ServeHTTP(w, req)
			return
		}
		capture := &statusCapture{header: make(http.Header)}
		h.ServeHTTP(capture, req)
		switch capture.status {
		case http.StatusNotFound, http.StatusMethodNotAllowed:
		default:
			// A path-cleaning redirect also has no pattern: let mux answer.
			mux.ServeHTTP(w, req)
			return
		}
		if allow := capture.header.Get("Allow"); allow != "" {
			w.Header().Set("Allow", allow)
		}
		p.write(w, req, capture.status)
	})
}

// statusCapture records the header and status of ServeMux's own 404/405
// handler and discards the body.
type statusCapture struct {
	header http.Header
	status int
}

func (c *statusCapture) Header() http.Header { return c.header }

func (c *statusCapture) WriteHeader(status int) {
	if c.status == 0 {
		c.status = status
	}
}

func (c *statusCapture) Write(b []byte) (int, error) {
	if c.status == 0 {
		c.status = http.StatusOK
	}
	return len(b), nil
}