chg: feat: add better magnet processing

magnet/magnet.go

@@ -0,0 +1,93 @@
+package magnet
+
+import (
+	"encoding/base32"
+	"encoding/hex"
+	"errors"
+	"fmt"
+	"net/url"
+	"strings"
+)
+
+// Magnet link components.
+type Magnet struct {
+	InfoHash    T          // Expected in this implementation
+	Trackers    []string   // "tr" values
+	DisplayName string     // "dn" value, if not empty
+	Params      url.Values // All other values, such as "x.pe", "as", "xs" etc.
+}
+
+const xtPrefix = "urn:btih:"
+
+// Deprecated: Use ParseMagnetUri.
+var ParseMagnetURI = ParseMagnetUri
+
+// ParseMagnetUri parses Magnet-formatted URIs into a Magnet instance
+func ParseMagnetUri(uri string) (m Magnet, err error) {
+	u, err := url.Parse(uri)
+	if err != nil {
+		err = fmt.Errorf("error parsing uri: %w", err)
+		return
+	}
+	if u.Scheme != "magnet" {
+		err = fmt.Errorf("unexpected scheme %q", u.Scheme)
+		return
+	}
+	q := u.Query()
+	xt := q.Get("xt")
+	m.InfoHash, err = parseInfohash(q.Get("xt"))
+	if err != nil {
+		err = fmt.Errorf("error parsing infohash %q: %w", xt, err)
+		return
+	}
+	dropFirst(q, "xt")
+	m.DisplayName = q.Get("dn")
+	dropFirst(q, "dn")
+	m.Trackers = q["tr"]
+	delete(q, "tr")
+	if len(q) == 0 {
+		q = nil
+	}
+	m.Params = q
+	return
+}
+
+func parseInfohash(xt string) (ih T, err error) {
+	if !strings.HasPrefix(xt, xtPrefix) {
+		err = errors.New("bad xt parameter prefix")
+		return
+	}
+	encoded := xt[len(xtPrefix):]
+	decode := func() func(dst, src []byte) (int, error) {
+		switch len(encoded) {
+		case 40:
+			return hex.Decode
+		case 32:
+			return base32.StdEncoding.Decode
+		}
+		return nil
+	}()
+	if decode == nil {
+		err = fmt.Errorf("unhandled xt parameter encoding (encoded length %d)", len(encoded))
+		return
+	}
+	n, err := decode(ih[:], []byte(encoded))
+	if err != nil {
+		err = fmt.Errorf("error decoding xt: %w", err)
+		return
+	}
+	if n != 20 {
+		panic(n)
+	}
+	return
+}
+
+func dropFirst(vs url.Values, key string) {
+	sl := vs[key]
+	switch len(sl) {
+	case 0, 1:
+		vs.Del(key)
+	default:
+		vs[key] = sl[1:]
+	}
+}