package markdown

import (
	"html"
	"strings"
)

var (
	headerStart = []string{"<h1>", "<h2>", "<h3>", "<h4>", "<h5>", "<h6>"}
	headerEnd   = []string{"</h1>", "</h2>", "</h3>", "</h4>", "</h5>", "</h6>"}
)

type renderer struct {
	out            strings.Builder
	paragraphLevel int
	quoteLevel     int
	listLevel      int
}

// Render creates HTML from the supplied markdown text.
func Render(markdown string) string {
	var (
		r         renderer
		i         = 0
		lineStart = 0
	)

	for {
		if i > len(markdown) {
			r.closeAll()

			for range r.quoteLevel {
				r.out.WriteString("</blockquote>")
			}

			return r.out.String()
		}

		if i != len(markdown) && markdown[i] != '\n' {
			i++
			continue
		}

		line := markdown[lineStart:i]
		lineStart = i + 1
		i++

		r.processLine(line)
	}
}

func (r *renderer) processLine(line string) {
	newQuoteLevel := 0

	for strings.HasPrefix(line, ">") {
		line = strings.TrimSpace(line[1:])
		newQuoteLevel++
	}

	if newQuoteLevel > r.quoteLevel {
		r.closeParagraphs()

		for range newQuoteLevel - r.quoteLevel {
			r.out.WriteString("<blockquote>")
		}
	} else if newQuoteLevel < r.quoteLevel {
		r.closeParagraphs()

		for range r.quoteLevel - newQuoteLevel {
			r.out.WriteString("</blockquote>")
		}
	}

	r.quoteLevel = newQuoteLevel

	if len(line) == 0 {
		r.closeAll()
		return
	}

	switch line[0] {
	case '#':
		r.closeAll()
		space := strings.IndexByte(line, ' ')

		if space > 0 && space <= 6 {
			r.out.WriteString(headerStart[space-1])
			r.writeText(line[space+1:])
			r.out.WriteString(headerEnd[space-1])
		}

		return

	case '-', '*':
		r.closeParagraphs()
		line = strings.TrimSpace(line[1:])

		if r.listLevel == 0 {
			r.out.WriteString("<ul>")
			r.listLevel++
		}

		r.out.WriteString("<li>")
		r.writeText(line)
		r.out.WriteString("</li>")
		return
	}

	if r.paragraphLevel == 0 {
		r.out.WriteString("<p>")
		r.paragraphLevel++
		r.writeText(line)
		return
	}

	r.out.WriteByte(' ')
	r.writeText(line)
}

// closeAll closes all open tags.
func (r *renderer) closeAll() {
	r.closeLists()
	r.closeParagraphs()
}

// closeParagraphs closes open paragraphs.
func (r *renderer) closeParagraphs() {
	for range r.paragraphLevel {
		r.out.WriteString("</p>")
	}

	r.paragraphLevel = 0
}

// closeLists closes open lists.
func (r *renderer) closeLists() {
	for range r.listLevel {
		r.out.WriteString("</ul>")
	}

	r.listLevel = 0
}

// writeText converts inline markdown to HTML.
func (r *renderer) writeText(markdown string) {
	var (
		i          = 0
		tokenStart = 0
	)

	var (
		textStart   = -1
		textEnd     = -1
		urlStart    = -1
		parentheses = 0
	)

	for {
		if i == len(markdown) {
			r.out.WriteString(html.EscapeString(markdown[tokenStart:]))
			return
		}

		c := markdown[i]

		switch c {
		case '[':
			r.out.WriteString(html.EscapeString(markdown[tokenStart:i]))
			tokenStart = i
			textStart = i
		case ']':
			textEnd = i
		case '(':
			if parentheses == 0 {
				urlStart = i
			}

			parentheses++
		case ')':
			parentheses--

			if parentheses == 0 && textStart >= 0 && textEnd >= 0 && urlStart >= 0 {
				linkText := markdown[textStart+1 : textEnd]
				linkURL := markdown[urlStart+1 : i]

				r.out.WriteString("<a href=\"")
				r.out.WriteString(sanitizeURL(linkURL))
				r.out.WriteString("\">")
				r.out.WriteString(html.EscapeString(linkText))
				r.out.WriteString("</a>")

				textStart = -1
				textEnd = -1
				urlStart = -1

				tokenStart = i + 1
			}
		}

		i++
	}
}

// sanitizeURL makes a URL safe to use as the value for a `href` attribute.
func sanitizeURL(linkURL string) string {
	linkURL = strings.TrimSpace(linkURL)

	if strings.HasPrefix(strings.ToLower(linkURL), "javascript:") {
		return ""
	}

	return html.EscapeString(linkURL)
}