// Package sky is Calma's ambient nebula: a soft sky of every app's colour,
// seeded, so each team's is its own. It lives where the suite meets you —
// the sign-in, the team's home, the front page — and never inside an app
// (AGENTS.md, decisions table, row 8).
//
// A sky is generated from its ID (generate.go): a far haze and near clouds
// of every app's colour, a fine grain, and a few white filaments. Its
// colour — the haze and clouds — is painted as the placeholder
// (placeholder.go), 50x20 pixels inline, so it is there in the first frame;
// the grain and filaments are two static SVGs served by Handler. Handler
// still serves the haze and clouds SVGs too, for pages cached before they
// stopped being linked. Light and dark each get their own placeholder;
// sky.css shows the one matching the page's scheme, drifts the sky slowly,
// and holds it still for anyone who asks their system for less motion.
// No script.
package sky

import (
	"embed"
	"html/template"
	"net/http"
	"regexp"
	"strings"
)

//go:embed sky.css
var css []byte

//go:embed templates/*.html
var templateFS embed.FS

// CSS styles [estilo-sky] and [estilo-sky-host].
func CSS() []byte { return append([]byte(nil), css...) }

// Templates holds "estilo-sky"; parse it into the page's template set.
func Templates() embed.FS { return templateFS }

// Prefix is the URL path Handler serves under. The version is the
// generator's: a change to what a given ID draws is a new version, so
// the year-long cache below never serves a stale sky. TestSkyGoldens holds
// one sky's six files still; when it fails, the drawing changed — bump
// the version here, then update the goldens.
const Prefix = "/static/estilo/sky/v1/"

// SuiteSeed is the sky of pages that belong to no one team: the front
// page and the sign-in.
const SuiteSeed = "oficina"

// Layer is one image of a sky.
type Layer struct {
	Name string // placeholder, grain or filaments (haze and clouds are served, no longer linked)
	Tone string // light, dark, or "" for both
	Src  string // a URL under Prefix; empty for the placeholder
	// Inline is the placeholder's data: URI, typed so html/template
	// passes it (a data: URI in a plain string is sanitised away). Built
	// only by Placeholder, never from input.
	Inline template.URL
}

// View is the data for the "estilo-sky" template.
type View struct{ Layers []Layer }

// For is the sky for seed — a team's id, or SuiteSeed.
func For(seed string) View {
	id := ID(seed)
	base := Prefix + id + "/"
	return View{Layers: []Layer{
		{Name: "placeholder", Tone: "light", Inline: template.URL(Placeholder(id, Light))},
		{Name: "placeholder", Tone: "dark", Inline: template.URL(Placeholder(id, Dark))},
		{Name: "grain", Src: base + "grain.svg"},
		{Name: "filaments", Src: base + "filaments.svg"},
	}}
}

var idShape = regexp.MustCompile(`^[0-9a-f]{16}$`)

// draw returns the SVG for one layer file of sky id, or "" when the name
// is not one of the six.
func draw(id, file string) string {
	switch file {
	case "haze-light.svg":
		return Haze(id, Light)
	case "haze-dark.svg":
		return Haze(id, Dark)
	case "clouds-light.svg":
		return Clouds(id, Light)
	case "clouds-dark.svg":
		return Clouds(id, Dark)
	case "grain.svg":
		return Grain(id)
	case "filaments.svg":
		return Filaments(id)
	}
	return ""
}

// Handler serves GET and HEAD for Prefix + "<id>/<layer>.svg". Mount it
// at Prefix. The ID must be the shape ID produces and the layer one of
// the five; anything else is 404 before anything is drawn. Skies are
// public artwork and need no session; an ID's drawing never changes
// within a version, so it is cached for a year.
func Handler() http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		rest, ok := strings.CutPrefix(r.URL.Path, Prefix)
		id, file, _ := strings.Cut(rest, "/")
		if !ok || !idShape.MatchString(id) || strings.Contains(file, "/") {
			http.NotFound(w, r)
			return
		}
		body := draw(id, file)
		if body == "" {
			http.NotFound(w, r)
			return
		}
		if r.Method != http.MethodGet && r.Method != http.MethodHead {
			w.Header().Set("Allow", "GET, HEAD")
			http.Error(w, http.StatusText(http.StatusMethodNotAllowed), http.StatusMethodNotAllowed)
			return
		}
		w.Header().Set("Content-Type", "image/svg+xml")
		w.Header().Set("Cache-Control", "public, max-age=31536000, immutable")
		w.Header().Set("X-Content-Type-Options", "nosniff")
		if r.Method == http.MethodHead {
			return
		}
		_, _ = w.Write([]byte(body))
	})
}