package pages

import (
	"embed"
	"net/http"
)

// sharedFS holds this package's own templates: the error partial parsed into
// every set, and the error page used by a service that ships none.
//
//go:embed templates
var sharedFS embed.FS

const (
	sharedDir = "templates"

	// errorPartialFile defines ErrorPartial and nothing else, so that it can be
	// parsed into every page set without bringing a content block with it.
	errorPartialFile = "error.tmpl"

	// errorPageFile is the default error page: a content block that is one
	// invocation of ErrorPartial.
	errorPageFile = "error.html"
)

// ErrorPage is the name Load registers the error page under, and the name to
// pass Render for it. It is a constant because a service's `fail` names it on
// every arm of its switch.
const ErrorPage = "error"

// ErrorPartial is the shared error body: the status line, the message and the
// way back. Its dot is an ErrorData — not a view struct — so a service that
// wants its own error.html around it invokes it as
// {{template "srht-error" .Data}}.
const ErrorPartial = "srht-error"

// The messages of the error pages a surface produces on its own. They are
// constants, and shared ones, for the reason the donors gave: the visibility
// rules of these services require "somebody else's private thing" and "no such
// thing" to be indistinguishable, and two 404s that differed in their prose
// would rebuild the distinction the status code was chosen to erase.
//
// A 400 has no constant deliberately. It is the one class that describes
// something the viewer just typed — "1y is not a duration", "grants are not
// lower case" — and an error page that replaced that with a house phrase would
// send them back to the form with nothing to change.
const (
	NotFoundMessage     = "There is nothing here."
	UnauthorizedMessage = "You need to be logged in to do that."
	ForbiddenMessage    = "You may not do that."
	MethodMessage       = "That is not something you can do to this page."
	InternalMessage     = "Something went wrong on our side. It has been logged."
	UnavailableMessage  = "Something we depend on is not answering. Try again in a moment."
)

// Message is the standard message for a status, or "" for a status that has
// none — a 400 above all, whose message is the caller's own text.
func Message(status int) string {
	switch status {
	case http.StatusUnauthorized:
		return UnauthorizedMessage
	case http.StatusForbidden:
		return ForbiddenMessage
	case http.StatusNotFound:
		return NotFoundMessage
	case http.StatusMethodNotAllowed:
		return MethodMessage
	case http.StatusInternalServerError:
		return InternalMessage
	case http.StatusBadGateway, http.StatusServiceUnavailable, http.StatusGatewayTimeout:
		return UnavailableMessage
	default:
		return ""
	}
}

// A Link is a href and the text that carries it.
type Link struct {
	Href string
	Text string
}

// DefaultBack is the way back off an error page for a service that names none:
// its own root, which every one of these services answers with a landing page.
var DefaultBack = Link{Href: "/", Text: "Back to the start"}

// ErrorData is the payload of the error page. A service puts it where its view
// struct keeps page payload — the field the layout's content sees as .Data in
// all of the donors:
//
//	vd := s.view(r, http.StatusText(status))
//	vd.Data = pages.Error(status, message).BackTo("/tokens", "Back to your tokens")
//	err := s.pages.Render(w, status, pages.ErrorPage, vd)
type ErrorData struct {
	Status     int
	StatusText string

	// Message is what the viewer can act on. It is never the raw error from
	// below: those name tables, queries and paths. It goes through
	// html/template, which escapes it — which matters, because the messages
	// that are not constants quote what the caller typed.
	Message string

	// Back is the way off this page. Empty renders no link at all rather than a
	// link to nowhere, which is what a hand-built ErrorData would otherwise get.
	Back Link
}

// Error builds the payload for a status. An empty message takes the standard
// one for that status (Message), so a caller that has nothing of its own to add
// says nothing rather than inventing a phrase.
func Error(status int, message string) ErrorData {
	if message == "" {
		message = Message(status)
	}
	return ErrorData{
		Status:     status,
		StatusText: http.StatusText(status),
		Message:    message,
		Back:       DefaultBack,
	}
}

// BackTo replaces the way back, for the services whose landing page is not "/"
// or whose word for it is not "back to the start". It returns a copy, so it
// chains off Error.
func (d ErrorData) BackTo(href, text string) ErrorData {
	d.Back = Link{Href: href, Text: text}
	return d
}