package content

import (
	"bytes"
	"fmt"
	stdhtml "html"
	"image"
	_ "image/jpeg"
	_ "image/png"
	"io"
	"os"
	"path"
	"path/filepath"
	"regexp"
	"strconv"
	"strings"

	"github.com/alecthomas/chroma/v2"
	chromahtml "github.com/alecthomas/chroma/v2/formatters/html"
	"github.com/alecthomas/chroma/v2/lexers"
	"github.com/alecthomas/chroma/v2/styles"
	"github.com/yuin/goldmark"
	highlighting "github.com/yuin/goldmark-highlighting/v2"
	"github.com/yuin/goldmark/ast"
	"github.com/yuin/goldmark/extension"
	"github.com/yuin/goldmark/parser"
	"github.com/yuin/goldmark/renderer/html"
	"github.com/yuin/goldmark/text"
	"github.com/yuin/goldmark/util"

	"gitea.lab.audasmedia.com.au/sam/ai-resume/internal/stage"
)

// RendererStyle is the chroma style. Dark by default, matching the rest of
// Sam's surfaces.
const RendererStyle = "github-dark"

// Renderer turns Markdown into server-side HTML. Nothing here runs in a
// browser: goldmark for structure and tables, chroma for code, at build time.
type Renderer struct {
	md  goldmark.Markdown
	set *Set
}

// NewRenderer builds a renderer bound to a document set (needed to rewrite
// relative .md links into site URLs).
func NewRenderer(s *Set) *Renderer {
	r := &Renderer{set: s}
	r.md = goldmark.New(
		goldmark.WithExtensions(
			extension.GFM,
			extension.Footnote,
			extension.Typographer,
			highlighting.NewHighlighting(
				highlighting.WithStyle(RendererStyle),
				highlighting.WithFormatOptions(chromahtml.WithClasses(true)),
			),
		),
		goldmark.WithParserOptions(
			parser.WithAutoHeadingID(),
			parser.WithASTTransformers(util.Prioritized(r, 100)),
		),
		goldmark.WithRendererOptions(html.WithUnsafe()), // trusted, in-repo Markdown
	)
	return r
}

// Transform rewrites document-relative links into site URLs so the
// pre-rendered pages work as static files. The current document is carried in
// the parser context by Render and Highlight-free callers.
func (r *Renderer) Transform(doc *ast.Document, reader text.Reader, pc parser.Context) {
	current, ok := pc.Get(docKey).(*Document)
	if !ok || current == nil {
		return
	}
	_ = ast.Walk(doc, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
		if !entering {
			return ast.WalkContinue, nil
		}
		switch node := n.(type) {
		case *ast.Link:
			node.Destination = r.rewrite(current, string(node.Destination), false)
		case *ast.Image:
			node.Destination = r.rewrite(current, string(node.Destination), true)
			r.sizeImage(current, node)
		}
		return ast.WalkContinue, nil
	})
}

func (r *Renderer) rewrite(from *Document, dest string, image bool) []byte {
	if dest == "" || strings.Contains(dest, "://") || strings.HasPrefix(dest, "/") ||
		strings.HasPrefix(dest, "#") || strings.HasPrefix(dest, "mailto:") {
		return []byte(dest)
	}
	target := dest
	suffix := ""
	if i := strings.IndexAny(target, "#?"); i >= 0 {
		suffix = target[i:]
		target = target[:i]
	}
	rel := target
	if strings.HasPrefix(rel, "content/") {
		rel = strings.TrimPrefix(rel, "content/")
	} else {
		rel = path.Join(path.Dir(from.Path), rel)
	}
	rel = path.Clean(rel)
	if d, ok := r.set.byRel[rel]; ok {
		return []byte(d.URL + suffix)
	}
	if image || KindForFile(rel) != stage.KindLinks {
		return []byte(AssetHref(rel) + suffix)
	}
	return []byte(dest)
}

// sizeImage records an image's intrinsic size and loading hints on the node, so
// the rendered page reserves the space before the file arrives. Without this the
// text jumps as each image lands, which is the most visible performance defect a
// page of diagrams can have.
//
// Only the four attributes in goldmark's image filter are used, and only when a
// real size is known: an unknown asset keeps the browser's own behaviour.
func (r *Renderer) sizeImage(from *Document, node *ast.Image) {
	if r.set == nil || from == nil {
		return
	}
	dest := string(node.Destination)
	if strings.Contains(dest, "://") || strings.HasPrefix(dest, "data:") {
		return
	}
	rel := dest
	if strings.HasPrefix(rel, "/assets/") {
		rel = "assets/" + strings.TrimPrefix(rel, "/assets/")
	} else if i := strings.IndexAny(rel, "#?"); i >= 0 {
		rel = rel[:i]
	}
	w, h, ok := imageSize(filepath.Join(r.set.Root, filepath.FromSlash(path.Clean(rel))))
	if !ok {
		return
	}
	node.SetAttributeString("width", []byte(strconv.Itoa(w)))
	node.SetAttributeString("height", []byte(strconv.Itoa(h)))
	node.SetAttributeString("loading", []byte("lazy"))
	node.SetAttributeString("decoding", []byte("async"))
}

// svgSize reads width and height from the root element of an SVG. The diagrams
// in content/assets are authored in this repository with explicit dimensions,
// so a viewBox alone is not expected to occur.
var svgSizeRe = regexp.MustCompile(`<svg[^>]*\swidth="(\d+(?:\.\d+)?)(?:px)?"[^>]*\sheight="(\d+(?:\.\d+)?)(?:px)?"`)

// imageSize returns the intrinsic pixel size of a content asset.
func imageSize(abs string) (int, int, bool) {
	f, err := os.Open(abs)
	if err != nil {
		return 0, 0, false
	}
	defer f.Close()
	if strings.HasSuffix(strings.ToLower(abs), ".svg") {
		head := make([]byte, 4096)
		n, _ := f.Read(head)
		m := svgSizeRe.FindSubmatch(head[:n])
		if m == nil {
			return 0, 0, false
		}
		w, err1 := strconv.ParseFloat(string(m[1]), 64)
		h, err2 := strconv.ParseFloat(string(m[2]), 64)
		if err1 != nil || err2 != nil || w <= 0 || h <= 0 {
			return 0, 0, false
		}
		return int(w + 0.5), int(h + 0.5), true
	}
	cfg, _, err := image.DecodeConfig(f)
	if err != nil || cfg.Width <= 0 || cfg.Height <= 0 {
		return 0, 0, false
	}
	return cfg.Width, cfg.Height, true
}

// docKey carries the document being rendered through the parser context so
// the link rewriter knows what "relative" means.
var docKey = parser.NewContextKey()

// Render fills HTML and Plain on every document in the set.
func (r *Renderer) Render() error {
	for _, d := range r.set.Docs {
		var buf bytes.Buffer
		ctx := parser.NewContext()
		ctx.Set(docKey, d)
		if err := r.md.Convert([]byte(d.BodyMarkdown()), &buf, parser.WithContext(ctx)); err != nil {
			return fmt.Errorf("%s: render Markdown: %w", d.Path, err)
		}
		d.HTML = buf.String()
		d.Plain = PlainText(d.HTML)
	}
	return nil
}

// Highlight renders a standalone code file with chroma. Used by L3 code
// artefacts and by the make-target that keeps the code exhibits in sync.
func (r *Renderer) Highlight(w io.Writer, lang string, src []byte) error {
	lexer := lexers.Get(lang)
	if lexer == nil {
		lexer = lexers.Analyse(string(src))
	}
	if lexer == nil {
		_, err := fmt.Fprintf(w, "<pre class=\"chroma\"><code>%s</code></pre>", stdhtml.EscapeString(string(src)))
		return err
	}
	lexer = chroma.Coalesce(lexer)
	it, err := lexer.Tokenise(nil, string(src))
	if err != nil {
		return err
	}
	style := styles.Get(RendererStyle)
	if style == nil {
		return fmt.Errorf("unknown chroma style %q", RendererStyle)
	}
	formatter := chromahtml.New(chromahtml.WithClasses(true), chromahtml.WithLineNumbers(false))
	return formatter.Format(w, style, it)
}

// ChromaCSS returns the stylesheet for the highlighted classes, so the built
// site makes no colour decisions inline.
func (r *Renderer) ChromaCSS() (string, error) {
	style := styles.Get(RendererStyle)
	if style == nil {
		return "", fmt.Errorf("unknown chroma style %q", RendererStyle)
	}
	var buf bytes.Buffer
	if err := chromahtml.New(chromahtml.WithClasses(true)).WriteCSS(&buf, style); err != nil {
		return "", err
	}
	return buf.String(), nil
}

// AssetHref maps a content-relative asset path onto its published URL.
func AssetHref(rel string) string {
	return "/assets/" + strings.TrimPrefix(path.Clean(rel), "assets/")
}

var tagRe = regexp.MustCompile(`(?s)<[^>]*>`)

// PlainText reduces rendered HTML to indexable text.
func PlainText(h string) string {
	h = tagRe.ReplaceAllString(h, " ")
	h = stdhtml.UnescapeString(h)
	return strings.Join(strings.Fields(h), " ")
}