// Package shell renders the HTML document the browser receives before React
// runs.
//
// This is the whole reason the Go process serves HTML at all. Every campaign
// needs its own og: tags: the crawlers that build a Facebook share card do not
// execute JavaScript, so a bare SPA shell makes every shared campaign look
// identical. SPEC.md section 8 calls that a permanent self-inflicted wound and
// it is the cheapest place for us to be better than the incumbents.
//
// The seam is deliberately sharp. Go owns <head> and nothing else; React owns
// everything inside #root. No markup is produced in two places, so no markup
// can drift out of sync.
package shell

import (
	"bytes"
	"encoding/json"
	"fmt"
	"html/template"
	"net/http"
	"os"
	"path/filepath"
)

type OpenGraph struct {
	Title       string
	Description string
	Type        string
	URL         string
	Image       string
	ImageAlt    string
}

type Document struct {
	Title       string
	Description string
	Canonical   string
	OG          OpenGraph
	// Bootstrap is the data the first route would otherwise have to fetch. It
	// is already JSON; see Render for why that is safe to inline.
	Bootstrap template.JS

	Styles        []string
	Scripts       []string
	ModulePreload []string
}

type Renderer struct {
	tmpl    *template.Template
	assets  assets
	siteURL string
}

type assets struct {
	styles        []string
	scripts       []string
	modulePreload []string
}

// viteManifest is the subset of Vite's manifest.json we care about.
type viteManifest map[string]struct {
	File    string   `json:"file"`
	CSS     []string `json:"css"`
	Imports []string `json:"imports"`
	IsEntry bool     `json:"isEntry"`
}

// New reads the built frontend's manifest once, at startup. A missing or
// malformed manifest is a boot failure rather than a blank page later.
func New(tmplText string, webDir string, siteURL string, dev bool) (*Renderer, error) {
	tmpl, err := template.New("document").Parse(tmplText)
	if err != nil {
		return nil, fmt.Errorf("parse document template: %w", err)
	}

	r := &Renderer{tmpl: tmpl, siteURL: siteURL}

	if dev {
		// The Vite dev server owns index.html in development, so the shell is
		// only exercised by the container. Point at the dev server's client
		// runtime so a stray render still works.
		r.assets = assets{scripts: []string{"/@vite/client", "/src/main.tsx"}}
		return r, nil
	}

	manifestPath := filepath.Join(webDir, ".vite", "manifest.json")
	raw, err := os.ReadFile(manifestPath)
	if err != nil {
		return nil, fmt.Errorf("read vite manifest at %s: %w", manifestPath, err)
	}
	var m viteManifest
	if err := json.Unmarshal(raw, &m); err != nil {
		return nil, fmt.Errorf("parse vite manifest: %w", err)
	}

	entry, ok := m["index.html"]
	if !ok {
		return nil, fmt.Errorf("vite manifest has no index.html entry")
	}
	r.assets.scripts = []string{"/" + entry.File}
	for _, css := range entry.CSS {
		r.assets.styles = append(r.assets.styles, "/"+css)
	}
	for _, imp := range entry.Imports {
		if chunk, ok := m[imp]; ok {
			r.assets.modulePreload = append(r.assets.modulePreload, "/"+chunk.File)
			for _, css := range chunk.CSS {
				r.assets.styles = append(r.assets.styles, "/"+css)
			}
		}
	}
	return r, nil
}

// Render writes a complete document. Bootstrap data is marshalled here rather
// than by the caller because encoding/json escapes <, > and & by default,
// which is exactly what keeps a campaign title containing "</script>" from
// closing the tag it sits in.
func (r *Renderer) Render(w http.ResponseWriter, status int, doc Document, bootstrap any) error {
	if bootstrap != nil {
		raw, err := json.Marshal(bootstrap)
		if err != nil {
			return err
		}
		doc.Bootstrap = template.JS(raw)
	}
	doc.Styles = r.assets.styles
	doc.Scripts = r.assets.scripts
	doc.ModulePreload = r.assets.modulePreload

	if doc.OG.Title == "" {
		doc.OG.Title = doc.Title
	}
	if doc.OG.Description == "" {
		doc.OG.Description = doc.Description
	}
	if doc.OG.Type == "" {
		doc.OG.Type = "website"
	}

	var buf bytes.Buffer
	if err := r.tmpl.Execute(&buf, doc); err != nil {
		return err
	}

	w.Header().Set("Content-Type", "text/html; charset=utf-8")
	w.Header().Set("Cache-Control", "no-store")
	w.WriteHeader(status)
	_, err := buf.WriteTo(w)
	return err
}

// Abs turns a site-relative path into the absolute URL crawlers require: a
// relative og:image is ignored by most of them.
func (r *Renderer) Abs(path string) string { return r.siteURL + path }