package linkmeta
import (
"html"
"net/url"
"regexp"
"strings"
)
var (
headRe = regexp.MustCompile(`(?is)]*>(.*?)`)
metaRe = regexp.MustCompile(`(?is)]*?>`)
attrRe = regexp.MustCompile(`(?is)\b(property|name|http-equiv|content)\s*=\s*("([^"]*)"|'([^']*)'|([^\s>"']+))`)
titleRe = regexp.MustCompile(`(?is)]*>(.*?)`)
tagRe = regexp.MustCompile(`(?s)<[^>]*>`)
wsRe = regexp.MustCompile(`\s+`)
)
// parse 从 HTML 里挑出卡片字段。优先 Open Graph，其次 Twitter Card，最后
// 退到 与 description——绝大多数站点至少满足其中一个。
func parse(doc string, base *url.URL) *Card {
c := &Card{URL: base.String(), Site: base.Hostname()}
head := doc
if m := headRe.FindStringSubmatch(doc); m != nil {
head = m[1]
} else if len(head) > 64<<10 {
head = head[:64<<10] // 没有闭合 <head> 时也别整篇扫
}
// 同一个 key 出现多次时先出现的赢（后面的往往是重复声明）
meta := map[string]string{}
for _, tag := range metaRe.FindAllString(head, -1) {
key, val := metaTag(tag)
if key == "" || val == "" {
continue
}
if _, seen := meta[key]; !seen {
meta[key] = val
}
}
c.Title = first(meta["og:title"], meta["twitter:title"])
if c.Title == "" {
if m := titleRe.FindStringSubmatch(head); m != nil {
c.Title = clean(m[1])
}
}
c.Desc = first(meta["og:description"], meta["twitter:description"], meta["description"])
if site := meta["og:site_name"]; site != "" {
c.Site = site
}
if img := first(meta["og:image"], meta["twitter:image"], meta["twitter:image:src"]); img != "" {
c.Image = resolve(img, base)
}
return c
}
// metaTag 从单个 <meta> 标签里取 (key, content)，属性顺序不限。
func metaTag(tag string) (string, string) {
var key, val string
for _, a := range attrRe.FindAllStringSubmatch(tag, -1) {
v := first(a[3], a[4], a[5])
switch strings.ToLower(a[1]) {
case "property", "name", "http-equiv":
key = strings.ToLower(v)
case "content":
val = v
}
}
return key, clean(val)
}
// clean 去标签、解实体、压空白：抓来的文本可能带内联标签或连续换行。
func clean(s string) string {
s = html.UnescapeString(tagRe.ReplaceAllString(s, " "))
return strings.TrimSpace(wsRe.ReplaceAllString(s, " "))
}
// resolve 把相对的图片地址补成绝对 URL；解析不了就丢掉这个字段。
func resolve(ref string, base *url.URL) string {
ref = strings.TrimSpace(ref)
if ref == "" || strings.HasPrefix(ref, "data:") {
return ""
}
u, err := url.Parse(ref)
if err != nil {
return ""
}
return base.ResolveReference(u).String()
}
func first(vals ...string) string {
for _, v := range vals {
if v != "" {
return v
}
}
return ""
}