// Package render turns a bundle into bytes: Markdown to HTML, then a template set. It knows content and
// nothing about HTTP.
//
// The embedded templates and stylesheet are the reference theme (ADR-0026) — a demonstration of
// docs/theme-contract.md, not a design. Fields a template may rely on are listed there.
package render

import (
	"bytes"
	"embed"
	"fmt"
	"html/template"
	"io/fs"
	"time"

	"github.com/yuin/goldmark"

	"khosra/internal/content"
)

//go:embed templates
var themeFS embed.FS

// head is what every kind of page shares: the document shell the base template needs. Absence is the
// zero value — a template reads what exists and never fails on a missing field (invariant 1).
type head struct {
	// Title may be empty for a bundle; a listing always has one.
	Title string
	// Lang is the locale being served.
	Lang string
	// Canonical is the permalink of what was actually served, which differs from the URL requested when
	// the fallback chain supplied another language (ADR-0009).
	Canonical string
	// Alternates lists every language this key exists in, for hreflang.
	Alternates []Alternate
	// Style is the reference theme's stylesheet, inlined so a bare site root needs no asset route.
	Style template.CSS
}

// Page is one bundle rendered.
type Page struct {
	head
	// Key is the bundle's identity, useful for building links.
	Key string
	// HTML is the rendered body.
	HTML template.HTML
	// Extra carries every frontmatter key the parser does not name (ADR-0002).
	Extra map[string]any
}

// List is a collection page: the result of a Query, one page of it.
type List struct {
	head
	// Items are the entries on this page, in the Query's order.
	Items []Item
	// Page is 1-based; Pages is the total, at least 1 even when empty.
	Page, Pages int
	// PrevURL and NextURL are empty at the ends. Newer is "prev" because the order is newest first.
	PrevURL, NextURL string
	// Groups is set instead of Items when entries are grouped — a tag listing groups by section, so one
	// busy term stays readable (ADR-0018).
	Groups []Group
}

// Group is a named run of entries within a listing.
type Group struct {
	Name  string
	Items []Item
}

// Item is one entry in a listing.
type Item struct {
	Title string
	Key   string
	URL   string
	Date  time.Time
}

// Alternate is one language a bundle exists in.
type Alternate struct {
	Lang string
	URL  string
}

// Renderer holds the parsed template set and the Markdown converter. Templates are parsed once, never
// per request (conventions.md).
type Renderer struct {
	// Two sets, not one: base plus the block that kind of page defines. A single set would have two
	// definitions of "main" fighting, which is why per-type sets are the shape (ADR-0019).
	page  *template.Template
	list  *template.Template
	md    goldmark.Markdown
	style template.CSS
}

// New parses the theme and prepares the Markdown converter.
//
// siteFS may be nil, in which case only the embedded reference theme is used. A malformed template is a
// startup failure rather than a request-time one, so this returns an error the caller treats as fatal.
func New(siteFS fs.FS) (*Renderer, error) {
	page, err := parseSet(siteFS, "templates/page.html")
	if err != nil {
		return nil, fmt.Errorf("bundle templates: %w", err)
	}
	list, err := parseSet(siteFS, "templates/list.html")
	if err != nil {
		return nil, fmt.Errorf("listing templates: %w", err)
	}
	css, err := readStyle(siteFS)
	if err != nil {
		return nil, err
	}
	return &Renderer{page: page, list: list, md: goldmark.New(), style: css}, nil
}

// parseSet builds one kind of page: the embedded base and block, then the site's versions of exactly
// those two files parsed after them.
//
// Parse order is the whole mechanism — the last definition of a name wins — so a site redefines one
// named block and inherits the rest (ADR-0019). Only the same two names are overlaid: overlaying every
// site template into every set would let a listing's "main" leak into bundle pages, which is the
// collision per-kind sets exist to prevent.
func parseSet(siteFS fs.FS, kind string) (*template.Template, error) {
	set, err := template.ParseFS(themeFS, "templates/base.html", kind)
	if err != nil {
		return nil, fmt.Errorf("parse embedded: %w", err)
	}
	if siteFS == nil {
		return set, nil
	}
	for _, name := range []string{"templates/base.html", kind} {
		if _, err := fs.Stat(siteFS, name); err != nil {
			continue
		}
		if set, err = set.ParseFS(siteFS, name); err != nil {
			return nil, fmt.Errorf("parse site override %s: %w", name, err)
		}
	}
	return set, nil
}

// readStyle prefers the site's stylesheet and falls back to the reference one.
func readStyle(siteFS fs.FS) (template.CSS, error) {
	if siteFS != nil {
		if data, err := fs.ReadFile(siteFS, "templates/theme.css"); err == nil {
			return template.CSS(data), nil
		}
	}
	data, err := themeFS.ReadFile("templates/theme.css")
	if err != nil {
		return "", fmt.Errorf("read reference stylesheet: %w", err)
	}
	return template.CSS(data), nil
}

// Bundle renders one bundle into a complete page.
//
// served is the language actually chosen by the fallback chain, and variants every language the key
// exists in; both feed canonical and hreflang, which a theme must not construct itself.
func (r *Renderer) Bundle(b content.Bundle, served string, variants []string) ([]byte, error) {
	var body bytes.Buffer
	if err := r.md.Convert(b.Body, &body); err != nil {
		return nil, fmt.Errorf("markdown %s: %w", b.Path, err)
	}
	title := b.Title
	if title == "" {
		title = b.Key
	}
	p := Page{
		head:  head{Title: title, Lang: served, Canonical: content.URL(b.Key, served), Style: r.style},
		Key:   b.Key,
		HTML:  template.HTML(body.String()),
		Extra: b.Extra,
	}
	for _, l := range variants {
		p.Alternates = append(p.Alternates, Alternate{Lang: l, URL: content.URL(b.Key, l)})
	}
	return r.execute(r.page, p, b.Key)
}

// Listing renders one page of a Query result for a section.
func (r *Renderer) Listing(section, lang string, all []content.Bundle, page int) ([]byte, error) {
	l, window := r.paginate(section, lang, content.PageURL(section, lang, page), all, page,
		func(p int) string { return content.PageURL(section, lang, p) })
	for _, b := range window {
		l.Items = append(l.Items, r.item(b, lang))
	}
	return r.execute(r.list, l, section)
}

// Tag renders one page of a tag listing, grouped by section.
//
// section narrows the listing to one section and is empty for the global one.
func (r *Renderer) Tag(section, slug, lang string, all []content.Bundle, page int) ([]byte, error) {
	title := "#" + slug
	if section != "" {
		title = section + " · #" + slug
	}
	l, window := r.paginate(title, lang, content.TagURL(section, slug, lang, page), all, page,
		func(p int) string { return content.TagURL(section, slug, lang, p) })
	for _, b := range window {
		sec := b.Section()
		if n := len(l.Groups); n > 0 && l.Groups[n-1].Name == sec {
			l.Groups[n-1].Items = append(l.Groups[n-1].Items, r.item(b, lang))
			continue
		}
		l.Groups = append(l.Groups, Group{Name: sec, Items: []Item{r.item(b, lang)}})
	}
	return r.execute(r.list, l, "tag "+slug)
}

// item is one listing entry.
func (r *Renderer) item(b content.Bundle, lang string) Item {
	return Item{Title: b.Title, Key: b.Key, URL: content.URL(b.Key, lang), Date: b.Date}
}

// paginate builds the shell of a listing page and returns the slice of entries it shows.
func (r *Renderer) paginate(title, lang, canonical string, all []content.Bundle, page int, url func(int) string) (List, []content.Bundle) {
	pages := (len(all) + content.PerPage - 1) / content.PerPage
	if pages < 1 {
		pages = 1
	}
	start := (page - 1) * content.PerPage
	end := min(start+content.PerPage, len(all))
	l := List{
		head:  head{Title: title, Lang: lang, Canonical: canonical, Style: r.style},
		Page:  page,
		Pages: pages,
	}
	if page > 1 {
		l.PrevURL = url(page - 1)
	}
	if page < pages {
		l.NextURL = url(page + 1)
	}
	return l, all[start:end]
}

// execute runs a template set and wraps a failure with what was being rendered.
func (r *Renderer) execute(set *template.Template, data any, what string) ([]byte, error) {
	var out bytes.Buffer
	if err := set.ExecuteTemplate(&out, "base", data); err != nil {
		return nil, fmt.Errorf("template %s: %w", what, err)
	}
	return out.Bytes(), nil
}