chg: feat: add better magnet processing

magnet/infohash.go

@@ -0,0 +1,80 @@
+package magnet
+
+import (
+	"crypto/sha1"
+	"encoding"
+	"encoding/hex"
+	"fmt"
+)
+
+const Size = 20
+
+// 20-byte SHA1 hash used for info and pieces.
+type T [Size]byte
+
+var _ fmt.Formatter = (*T)(nil)
+
+func (t T) Format(f fmt.State, c rune) {
+	// TODO: I can't figure out a nice way to just override the 'x' rune, since it's meaningless
+	// with the "default" 'v', or .String() already returning the hex.
+	f.Write([]byte(t.HexString()))
+}
+
+func (t T) Bytes() []byte {
+	return t[:]
+}
+
+func (t T) AsString() string {
+	return string(t[:])
+}
+
+func (t T) String() string {
+	return t.HexString()
+}
+
+func (t T) HexString() string {
+	return fmt.Sprintf("%x", t[:])
+}
+
+func (t *T) FromHexString(s string) (err error) {
+	if len(s) != 2*Size {
+		err = fmt.Errorf("hash hex string has bad length: %d", len(s))
+		return
+	}
+	n, err := hex.Decode(t[:], []byte(s))
+	if err != nil {
+		return
+	}
+	if n != Size {
+		panic(n)
+	}
+	return
+}
+
+var (
+	_ encoding.TextUnmarshaler = (*T)(nil)
+	_ encoding.TextMarshaler   = T{}
+)
+
+func (t *T) UnmarshalText(b []byte) error {
+	return t.FromHexString(string(b))
+}
+
+func (t T) MarshalText() (text []byte, err error) {
+	return []byte(t.HexString()), nil
+}
+
+func FromHexString(s string) (h T) {
+	err := h.FromHexString(s)
+	if err != nil {
+		panic(err)
+	}
+	return
+}
+
+func HashBytes(b []byte) (ret T) {
+	hasher := sha1.New()
+	hasher.Write(b)
+	copy(ret[:], hasher.Sum(nil))
+	return
+}

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:]
+	}
+}