lib: Add file name compression

Allows to compress short arbitrary strings and returns a string using base64 url encoding.

Generator for tables included and a few samples has been added. Add more to init.go

Tested with fuzzing for crash resistance and symmetry, see fuzz.go

lib/encoder/filename/decode.go

@@ -0,0 +1,84 @@
+package filename
+
+import (
+	"bytes"
+	"encoding/base64"
+	"encoding/binary"
+	"errors"
+	"sync"
+
+	"github.com/klauspost/compress/huff0"
+)
+
+// ErrCorrupted is returned if a provided encoded filename cannot be decoded.
+var ErrCorrupted = errors.New("file name corrupt")
+
+// ErrUnsupported is returned if a provided encoding may come from a future version or the file name is corrupt.
+var ErrUnsupported = errors.New("file name possibly generated by future version of rclone")
+
+// Custom decoder for tableCustom types. Stateful, so must have lock.
+var customDec huff0.Scratch
+var customDecMu sync.Mutex
+
+// Decode an encoded string.
+func Decode(s string) (string, error) {
+	if len(s) < 1 {
+		return "", ErrCorrupted
+	}
+	table := decodeMap[s[0]]
+	if table == 0 {
+		return "", ErrCorrupted
+	}
+	table--
+	s = s[1:]
+
+	data := make([]byte, base64.URLEncoding.DecodedLen(len(s)))
+	n, err := base64.URLEncoding.Decode(data, ([]byte)(s))
+	if err != nil || n < 0 {
+		return "", ErrCorrupted
+	}
+	data = data[:n]
+
+	switch table {
+	case tableUncompressed:
+		return string(data), nil
+	case tableReserved:
+		return "", ErrUnsupported
+	case tableRLE:
+		if len(data) < 2 {
+			return "", ErrCorrupted
+		}
+		n, used := binary.Uvarint(data[:len(data)-1])
+		if used <= 0 || n > maxLength {
+			return "", ErrCorrupted
+		}
+		return string(bytes.Repeat(data[len(data)-1:], int(n))), nil
+	case tableCustom:
+		customDecMu.Lock()
+		defer customDecMu.Unlock()
+		_, data, err := huff0.ReadTable(data, &customDec)
+		if err != nil {
+			return "", ErrCorrupted
+		}
+		customDec.MaxDecodedSize = maxLength
+		decoded, err := customDec.Decompress1X(data)
+		if err != nil {
+			return "", ErrCorrupted
+		}
+		return string(decoded), nil
+	default:
+		if table >= byte(len(decTables)) {
+			return "", ErrCorrupted
+		}
+		dec := decTables[table]
+		if dec == nil {
+			return "", ErrUnsupported
+		}
+		var dst [maxLength]byte
+		name, err := dec.Decompress1X(dst[:0], data)
+		if err != nil {
+			return "", ErrCorrupted
+		}
+		return string(name), nil
+	}
+}