// Package handlers provides HTTP request handlers for the
// webhooker web UI and API.
package handlers

import (
	"bytes"
	"context"
	"encoding/json"
	"errors"
	"html/template"
	"log/slog"
	"net/http"
	"sync/atomic"

	"go.uber.org/fx"
	"sneak.berlin/go/webhooker/internal/database"
	"sneak.berlin/go/webhooker/internal/delivery"
	"sneak.berlin/go/webhooker/internal/globals"
	"sneak.berlin/go/webhooker/internal/healthcheck"
	"sneak.berlin/go/webhooker/internal/logger"
	"sneak.berlin/go/webhooker/internal/metrics"
	"sneak.berlin/go/webhooker/internal/middleware"
	"sneak.berlin/go/webhooker/internal/session"
	"sneak.berlin/go/webhooker/templates"
)

const (
	// maxBodyShift is the bit shift for 1 MB body limit.
	maxBodyShift = 20
	// recentEventLimit is the number of recent events to show.
	recentEventLimit = 50
	// paginationPerPage is the number of items per page.
	paginationPerPage = 25

	// tmplKeyError is the template data key for an error message.
	tmplKeyError = "Error"
	// tmplKeyWebhook is the template data key for a webhook.
	tmplKeyWebhook = "Webhook"
)

// errInvalidPassword is returned when a password does not match.
var errInvalidPassword = errors.New("invalid password")

// errVerificationBusy is returned when no password-verification slot
// became free before the wait elapsed, so no password was verified.
var errVerificationBusy = errors.New(
	"password verification capacity exhausted",
)

//nolint:revive // HandlersParams is a standard fx naming convention.
type HandlersParams struct {
	fx.In

	Logger       *logger.Logger
	Globals      *globals.Globals
	Database     *database.Database
	WebhookDBMgr *database.WebhookDBManager
	Healthcheck  *healthcheck.Healthcheck
	Session      *session.Session
	Middleware   *middleware.Middleware
	Notifier     delivery.Notifier
	Evictor      delivery.WebhookEvictor
	SSRFGuard    *delivery.Guard
}

// Handlers provides HTTP handler methods for all application
// routes.
type Handlers struct {
	params    *HandlersParams
	log       *slog.Logger
	hc        *healthcheck.Healthcheck
	db        *database.Database
	dbMgr     *database.WebhookDBManager
	session   *session.Session
	mw        *middleware.Middleware
	notifier  delivery.Notifier
	evictor   delivery.WebhookEvictor
	mtr       *metrics.Set
	templates map[string]*template.Template

	// ssrf validates submitted target URLs. It is the same guard
	// the delivery engine dials through, so a URL accepted here
	// is one delivery will actually attempt.
	ssrf *delivery.Guard

	// dummyVerifications counts the equivalent-cost verifications
	// charged for usernames that do not exist. It exists so a test
	// can prove that path runs without measuring wall-clock time.
	dummyVerifications atomic.Uint64
}

// parsePageTemplate parses a page-specific template set from the
// embedded FS. Each page template is combined with the shared
// base, htmlheader, and navbar templates. The page file must be
// listed first so that its root action ({{template "base" .}})
// becomes the template set's entry point.
func parsePageTemplate(pageFile string) *template.Template {
	return template.Must(
		template.ParseFS(
			templates.Templates,
			pageFile,
			"base.html",
			"htmlheader.html",
			"navbar.html",
		),
	)
}

// New creates a Handlers instance, parsing all page templates at
// startup.
func New(
	lc fx.Lifecycle,
	params HandlersParams,
) (*Handlers, error) {
	s := new(Handlers)
	s.params = &params
	s.log = params.Logger.Get()
	s.hc = params.Healthcheck
	s.db = params.Database
	s.dbMgr = params.WebhookDBMgr
	s.session = params.Session
	s.mw = params.Middleware
	s.notifier = params.Notifier
	s.evictor = params.Evictor
	s.mtr = metrics.Default()
	s.ssrf = params.SSRFGuard

	// Parse all page templates once at startup
	s.templates = map[string]*template.Template{
		"login.html":         parsePageTemplate("login.html"),
		"profile.html":       parsePageTemplate("profile.html"),
		"sources_list.html":  parsePageTemplate("sources_list.html"),
		"sources_new.html":   parsePageTemplate("sources_new.html"),
		"source_detail.html": parsePageTemplate("source_detail.html"),
		"source_edit.html":   parsePageTemplate("source_edit.html"),
		"source_logs.html":   parsePageTemplate("source_logs.html"),
		"target_edit.html":   parsePageTemplate("target_edit.html"),
		"error.html":         parsePageTemplate("error.html"),
	}

	lc.Append(fx.Hook{
		OnStart: func(_ context.Context) error {
			return nil
		},
	})

	return s, nil
}

// HandleErrorPage returns a handler that answers every request with
// the error page for status. The router uses it for unknown paths and
// the CSRF middleware for a refused form.
func (s *Handlers) HandleErrorPage(status int) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		s.renderError(w, r, status)
	}
}

func (s *Handlers) respondJSON(
	w http.ResponseWriter,
	_ *http.Request,
	data any,
	status int,
) {
	w.Header().Set("Content-Type", "application/json")
	w.WriteHeader(status)

	if data != nil {
		err := json.NewEncoder(w).Encode(data)
		if err != nil {
			s.log.Error("json encode error", "error", err)
		}
	}
}

// serverError logs an error and answers with the 500 error page.
func (s *Handlers) serverError(
	w http.ResponseWriter, r *http.Request, msg string, err error,
) {
	s.log.Error(msg, "error", err)
	s.renderError(w, r, http.StatusInternalServerError)
}

// renderError answers with status and the error page: the normal
// layout, one fixed line explaining the status, and a link back to the
// webhook list, or to sign-in when nobody is signed in.
//
// It renders the page itself rather than through renderTemplate,
// whose own failure comes here. If the error page cannot render
// either, the answer is the same status in plain text: never a second
// attempt, and never a different status.
func (s *Handlers) renderError(
	w http.ResponseWriter,
	r *http.Request,
	status int,
) {
	// The page names the signed-in user, and some error pages are
	// served outside the routes where NoCache runs.
	w.Header().Set("Cache-Control", "no-store")

	data := s.pageData(r, map[string]any{
		"Status":     status,
		"StatusText": http.StatusText(status),
		"Message":    errorPageText(status),
	})

	var buf bytes.Buffer

	err := s.templates["error.html"].Execute(&buf, data)
	if err != nil {
		s.log.Error("failed to render error page", "error", err)
		http.Error(w, http.StatusText(status), status)

		return
	}

	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	w.WriteHeader(status)

	_, err = buf.WriteTo(w)
	if err != nil {
		s.log.Error("failed to write error page", "error", err)
	}
}

// errorPageText is the line the error page shows for status. It is
// fixed per status, so the page tells the reader no more than the
// plain-text answers it replaced did.
func errorPageText(status int) string {
	switch status {
	case http.StatusBadRequest:
		return "The request could not be read."
	case http.StatusForbidden:
		return "The request was refused. If it came from a form " +
			"left open for a long time, reload the page and try " +
			"again."
	case http.StatusNotFound:
		return "There is nothing here. It may have been deleted, " +
			"or the address may be wrong."
	case http.StatusServiceUnavailable:
		return "The server is busy. Please try again in a moment."
	default: // http.StatusInternalServerError
		return "Something went wrong on the server. Please try " +
			"again."
	}
}

// UserInfo represents user information for templates
type UserInfo struct {
	ID       string
	Username string
}

// templateDataWrapper wraps non-map data with common fields.
type templateDataWrapper struct {
	User      *UserInfo
	CSRFToken string
	Version   string
	Data      any
}

// getUserInfo extracts user info from the session.
func (s *Handlers) getUserInfo(
	r *http.Request,
) *UserInfo {
	sess, err := s.session.Get(r)
	if err != nil || !s.session.IsAuthenticated(sess) {
		return nil
	}

	username, ok := s.session.GetUsername(sess)
	if !ok {
		return nil
	}

	userID, ok := s.session.GetUserID(sess)
	if !ok {
		return nil
	}

	return &UserInfo{ID: userID, Username: username}
}

// renderTemplate renders a pre-parsed template with common
// data
func (s *Handlers) renderTemplate(
	w http.ResponseWriter,
	r *http.Request,
	pageTemplate string,
	data any,
) {
	tmpl, ok := s.templates[pageTemplate]
	if !ok {
		s.log.Error(
			"template not found",
			"template", pageTemplate,
		)
		s.renderError(w, r, http.StatusInternalServerError)

		return
	}

	s.executeTemplate(w, r, tmpl, s.pageData(r, data))
}

// pageData adds the fields the shared layout renders to a page's own
// data.
func (s *Handlers) pageData(r *http.Request, data any) any {
	userInfo := s.getUserInfo(r)
	csrfToken := middleware.CSRFToken(r)

	// The footer in base.html renders .Version. Every page reaches it
	// through here, so this is the one place that has to supply it;
	// left unset, the footer falls back to its literal "dev" and the
	// UI reports a build that is not the one running.
	version := s.params.Globals.Version

	if m, ok := data.(map[string]any); ok {
		m["User"] = userInfo
		m["CSRFToken"] = csrfToken
		m["Version"] = version

		return m
	}

	return templateDataWrapper{
		User:      userInfo,
		CSRFToken: csrfToken,
		Version:   version,
		Data:      data,
	}
}

// executeTemplate renders the template into a buffer and writes to
// the response only once rendering has fully succeeded. Executing
// straight into the ResponseWriter commits a partial body and a 200
// status before a mid-render error can be reported, leaving no way
// to serve a 500. Buffering makes a page's rendered size resident
// memory per concurrent viewer, so every page owes it a bound: the
// event log caps each stored body at maxRenderedBodyBytes for exactly
// this reason.
func (s *Handlers) executeTemplate(
	w http.ResponseWriter,
	r *http.Request,
	tmpl *template.Template,
	data any,
) {
	var buf bytes.Buffer

	err := tmpl.Execute(&buf, data)
	if err != nil {
		s.log.Error(
			"failed to execute template", "error", err,
		)
		s.renderError(w, r, http.StatusInternalServerError)

		return
	}

	w.Header().Set("Content-Type", "text/html; charset=utf-8")

	_, err = buf.WriteTo(w)
	if err != nil {
		s.log.Error(
			"failed to write rendered page", "error", err,
		)
	}
}