// Package parser extracts title, description, text content, and links from HTML.
package parser

import (
	"path"
	"regexp"
	"strings"

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

var wsRe = regexp.MustCompile(`\s+`)

// ParseHTML parses an HTML document and returns title, meta description, body text, and href list.
func ParseHTML(body, baseURL string) (title, description, text string, hrefs []string) {
	// Determine base scheme+host
	base := baseFromURL(baseURL)
	basePath := pathFromURL(baseURL)

	doc, err := html.Parse(strings.NewReader(body))
	if err != nil {
		return
	}

	var textParts []string

	var dfs func(n *html.Node)
	dfs = func(n *html.Node) {
		if n.Type == html.ElementNode {
			tag := strings.ToLower(n.Data)
			if tag == "script" || tag == "style" || tag == "svg" {
				return
			}
			if tag == "meta" {
				name := ""
				content := ""
				for _, a := range n.Attr {
					switch strings.ToLower(a.Key) {
					case "name":
						name = strings.ToLower(a.Val)
					case "content":
						content = a.Val
					}
				}
				if name == "description" && description == "" {
					description = content
				}
			}
			if tag == "a" {
				href := attrVal(n, "href")
				if href != "" {
					href = strings.SplitN(href, "#", 2)[0]
					if href != "" {
						href = resolveURL(base, basePath, href)
						if href != "" {
							hrefs = append(hrefs, href)
						}
					}
				}
			}
		}

		if n.Type == html.TextNode && n.Parent != nil {
			parentTag := ""
			if n.Parent.Type == html.ElementNode {
				parentTag = strings.ToLower(n.Parent.Data)
			}
			if parentTag == "script" || parentTag == "style" || parentTag == "svg" {
				goto children
			}
			s := wsRe.ReplaceAllString(n.Data, " ")
			s = strings.TrimSpace(s)
			if s != "" {
				if parentTag == "title" {
					title = s
				} else {
					textParts = append(textParts, s)
				}
			}
		}

	children:
		for c := n.FirstChild; c != nil; c = c.NextSibling {
			dfs(c)
		}
	}
	dfs(doc)

	text = strings.Join(textParts, " ")
	return
}

func attrVal(n *html.Node, key string) string {
	for _, a := range n.Attr {
		if strings.ToLower(a.Key) == key {
			return a.Val
		}
	}
	return ""
}

func baseFromURL(rawURL string) string {
	idx := strings.Index(rawURL, "://")
	if idx < 0 {
		return ""
	}
	rest := rawURL[idx+3:]
	slash := strings.Index(rest, "/")
	if slash < 0 {
		return rawURL
	}
	return rawURL[:idx+3+slash]
}

func pathFromURL(rawURL string) string {
	idx := strings.Index(rawURL, "://")
	if idx < 0 {
		return "/"
	}
	rest := rawURL[idx+3:]
	slash := strings.Index(rest, "/")
	if slash < 0 {
		return "/"
	}
	p := rest[slash:]
	// strip query/fragment
	p = strings.SplitN(p, "?", 2)[0]
	p = strings.SplitN(p, "#", 2)[0]
	return p
}

func resolveURL(base, basePath, href string) string {
	// Absolute URL
	if strings.HasPrefix(href, "http://") || strings.HasPrefix(href, "https://") {
		return href
	}
	// Protocol-relative
	if strings.HasPrefix(href, "//") {
		// extract scheme from base
		idx := strings.Index(base, "://")
		if idx < 0 {
			return ""
		}
		return base[:idx+1] + href
	}
	// Absolute path
	if strings.HasPrefix(href, "/") {
		return base + href
	}
	// Relative path
	dir := path.Dir(basePath)
	return base + path.Clean(dir+"/"+href)
}