package markdown

import (
	"net/url"
	"strconv"
	"strings"

	"golang.org/x/net/html"
	"golang.org/x/net/html/atom"
)

const (
	markBold   = "bold"
	markItalic = "italic"
	markLink   = "link"
	markCode   = "code"

	codeLanguagePrefix = "language-"
)

type mark struct {
	kind string
	href string
}

type inline struct {
	text  string
	marks []mark
}

type blockKind int

const (
	kindHeading blockKind = iota
	kindParagraph
	kindList
	kindBlockquote
	kindCode
	kindRule
	kindImage
)

type block struct {
	kind     blockKind
	level    int
	inlines  []inline
	ordered  bool
	items    [][]block
	children []block
	language string
	text     string
	src      string
	alt      string
	align    string
}

var transparentBlocks = map[string]bool{
	"div": true, "section": true, "article": true, "header": true, "footer": true,
	"main": true, "aside": true, "nav": true, "figure": true, "figcaption": true,
	"table": true, "thead": true, "tbody": true, "tfoot": true, "tr": true,
	"td": true, "th": true, "dl": true, "dt": true, "dd": true, "details": true,
	"summary": true, "li": true,
}

var skippedElements = map[string]bool{
	"script": true, "style": true, "template": true, "head": true,
	"noscript": true, "iframe": true, "object": true, "embed": true,
}

var alignments = map[string]bool{"center": true, "right": true, "justify": true}

var allowedLinkSchemes = map[string]bool{
	"": true, "http": true, "https": true, "mailto": true, "tel": true,
}

func parseBlocks(source string) ([]block, error) {
	context := &html.Node{Type: html.ElementNode, DataAtom: atom.Body, Data: "body"}
	nodes, err := html.ParseFragment(strings.NewReader(source), context)
	if err != nil {
		return nil, err
	}
	collector := newCollector(false, paragraphBlock)
	collector.walkAll(nodes, nil)
	return collector.finish(false), nil
}

func paragraphBlock(inlines []inline) block {
	return block{kind: kindParagraph, inlines: inlines}
}

func headingBlock(level int) func([]inline) block {
	return func(inlines []inline) block {
		return block{kind: kindHeading, level: level, inlines: inlines}
	}
}

type collector struct {
	out       []block
	current   []inline
	keepBlank bool
	build     func([]inline) block
}

func newCollector(keepBlank bool, build func([]inline) block) *collector {
	return &collector{keepBlank: keepBlank, build: build}
}

func (c *collector) walkAll(nodes []*html.Node, marks []mark) {
	for _, node := range nodes {
		c.walk(node, marks)
	}
}

func (c *collector) walkChildren(parent *html.Node, marks []mark) {
	for child := parent.FirstChild; child != nil; child = child.NextSibling {
		c.walk(child, marks)
	}
}

func (c *collector) addText(text string, marks []mark) {
	if text == "" {
		return
	}
	c.current = appendInline(c.current, inline{text: text, marks: marks})
}

func (c *collector) flush() {
	if len(c.current) == 0 {
		return
	}
	if c.keepBlank || hasVisibleText(c.current) {
		c.out = append(c.out, c.build(c.current))
	}
	c.current = nil
}

func (c *collector) finish(emitEmpty bool) []block {
	hadBlocks := len(c.out) > 0
	c.flush()
	if emitEmpty && !hadBlocks && len(c.out) == 0 {
		c.out = append(c.out, c.build(nil))
	}
	return c.out
}

func (c *collector) walk(node *html.Node, marks []mark) {
	switch node.Type {
	case html.TextNode:
		c.addText(node.Data, marks)
		return
	case html.ElementNode:
	default:
		return
	}

	tag := node.Data
	if skippedElements[tag] {
		return
	}

	switch tag {
	case "br":
		c.addText("\n", marks)
	case "strong", "b":
		c.walkChildren(node, withMark(marks, mark{kind: markBold}))
	case "em", "i":
		c.walkChildren(node, withMark(marks, mark{kind: markItalic}))
	case "code":
		c.walkChildren(node, withMark(marks, mark{kind: markCode}))
	case "a":
		href := safeHref(attr(node, "href"))
		if href == "" {
			c.walkChildren(node, marks)
			return
		}
		c.walkChildren(node, withMark(marks, mark{kind: markLink, href: href}))
	case "img":
		c.flush()
		if src := attr(node, "src"); src != "" {
			c.out = append(c.out, block{kind: kindImage, src: src, alt: attr(node, "alt")})
		}
	case "hr":
		c.flush()
		c.out = append(c.out, block{kind: kindRule})
	case "p":
		c.flush()
		c.out = append(c.out, parseTextBlock(node, paragraphBlock)...)
	case "h1", "h2", "h3", "h4", "h5", "h6":
		c.flush()
		level, _ := strconv.Atoi(tag[1:])
		c.out = append(c.out, parseTextBlock(node, headingBlock(level))...)
	case "ul", "ol":
		c.flush()
		c.out = append(c.out, parseList(node))
	case "blockquote":
		c.flush()
		c.out = append(c.out, block{kind: kindBlockquote, children: parseContainer(node)})
	case "pre":
		c.flush()
		c.out = append(c.out, parseCodeBlock(node))
	default:
		if transparentBlocks[tag] {
			c.flush()
			c.walkChildren(node, marks)
			c.flush()
			return
		}
		c.walkChildren(node, marks)
	}
}

func parseTextBlock(node *html.Node, build func([]inline) block) []block {
	inner := newCollector(true, build)
	inner.walkChildren(node, nil)
	blocks := inner.finish(true)
	if align := alignFromStyle(attr(node, "style")); align != "" {
		for index := range blocks {
			if blocks[index].kind == kindParagraph || blocks[index].kind == kindHeading {
				blocks[index].align = align
			}
		}
	}
	return blocks
}

func alignFromStyle(style string) string {
	for _, declaration := range strings.Split(style, ";") {
		name, value, found := strings.Cut(declaration, ":")
		if !found || strings.ToLower(strings.TrimSpace(name)) != "text-align" {
			continue
		}
		if align := strings.ToLower(strings.TrimSpace(value)); alignments[align] {
			return align
		}
	}
	return ""
}

func parseContainer(node *html.Node) []block {
	inner := newCollector(false, paragraphBlock)
	inner.walkChildren(node, nil)
	return inner.finish(false)
}

func parseList(node *html.Node) block {
	list := block{kind: kindList, ordered: node.Data == "ol"}
	for child := node.FirstChild; child != nil; child = child.NextSibling {
		if child.Type == html.ElementNode && child.Data == "li" {
			list.items = append(list.items, parseContainer(child))
		}
	}
	return list
}

func parseCodeBlock(node *html.Node) block {
	code := block{kind: kindCode, text: strings.TrimSuffix(textContent(node), "\n")}
	for child := node.FirstChild; child != nil; child = child.NextSibling {
		if child.Type == html.ElementNode && child.Data == "code" {
			code.language = languageFromClass(attr(child, "class"))
			break
		}
	}
	return code
}

func languageFromClass(class string) string {
	for _, name := range strings.Fields(class) {
		if strings.HasPrefix(name, codeLanguagePrefix) {
			return strings.TrimPrefix(name, codeLanguagePrefix)
		}
	}
	return ""
}

func textContent(node *html.Node) string {
	var builder strings.Builder
	var visit func(*html.Node)
	visit = func(n *html.Node) {
		switch {
		case n.Type == html.TextNode:
			builder.WriteString(n.Data)
		case n.Type == html.ElementNode && skippedElements[n.Data]:
			return
		case n.Type == html.ElementNode && n.Data == "br":
			builder.WriteString("\n")
		}
		for child := n.FirstChild; child != nil; child = child.NextSibling {
			visit(child)
		}
	}
	visit(node)
	return builder.String()
}

func attr(node *html.Node, name string) string {
	for _, a := range node.Attr {
		if a.Key == name {
			return a.Val
		}
	}
	return ""
}

func safeHref(href string) string {
	trimmed := strings.TrimSpace(href)
	parsed, err := url.Parse(trimmed)
	if err != nil || !allowedLinkSchemes[strings.ToLower(parsed.Scheme)] {
		return ""
	}
	return trimmed
}

func withMark(marks []mark, added mark) []mark {
	for _, existing := range marks {
		if existing.kind == added.kind {
			return marks
		}
	}
	next := make([]mark, 0, len(marks)+1)
	next = append(next, marks...)
	return append(next, added)
}

func hasVisibleText(inlines []inline) bool {
	for _, item := range inlines {
		if strings.TrimSpace(item.text) != "" {
			return true
		}
	}
	return false
}

func sameMarks(a, b []mark) bool {
	if len(a) != len(b) {
		return false
	}
	for index := range a {
		if a[index] != b[index] {
			return false
		}
	}
	return true
}

func appendInline(inlines []inline, next inline) []inline {
	if last := len(inlines) - 1; last >= 0 && sameMarks(inlines[last].marks, next.marks) {
		inlines[last].text += next.text
		return inlines
	}
	return append(inlines, next)
}

func withoutCodeMarks(inlines []inline) []inline {
	merged := make([]inline, 0, len(inlines))
	for _, item := range inlines {
		kept := make([]mark, 0, len(item.marks))
		for _, m := range item.marks {
			if m.kind != markCode {
				kept = append(kept, m)
			}
		}
		merged = appendInline(merged, inline{text: item.text, marks: kept})
	}
	return merged
}