tlv/varint: add modified bitcoin varint

This varint has the same serialization as the varint in btcd and
bitcoind, but has different behavior wrt returned errors. In order to
ensure the inner loop properly detects cleanly written records,
ReadVarInt will not only return EOF if it can't read the first byte, as
that means the reader has zero bytes left.

It also modifies the API to allow the caller to provided a static byte
array, which can be reused across all encoding and decoding and
increases performance.

tlv/varint.go

@@ -0,0 +1,109 @@
+package tlv
+
+import (
+	"encoding/binary"
+	"errors"
+	"io"
+)
+
+// ErrVarIntNotCanonical signals that the decoded varint was not minimally encoded.
+var ErrVarIntNotCanonical = errors.New("decoded varint is not canonical")
+
+// ReadVarInt reads a variable length integer from r and returns it as a uint64.
+func ReadVarInt(r io.Reader, buf *[8]byte) (uint64, error) {
+	_, err := io.ReadFull(r, buf[:1])
+	if err != nil {
+		return 0, err
+	}
+	discriminant := buf[0]
+
+	var rv uint64
+	switch {
+	case discriminant < 0xfd:
+		rv = uint64(discriminant)
+
+	case discriminant == 0xfd:
+		_, err := io.ReadFull(r, buf[:2])
+		switch {
+		case err == io.EOF:
+			return 0, io.ErrUnexpectedEOF
+		case err != nil:
+			return 0, err
+		}
+		rv = uint64(binary.BigEndian.Uint16(buf[:2]))
+
+		// The encoding is not canonical if the value could have been
+		// encoded using fewer bytes.
+		if rv < 0xfd {
+			return 0, ErrVarIntNotCanonical
+		}
+
+	case discriminant == 0xfe:
+		_, err := io.ReadFull(r, buf[:4])
+		switch {
+		case err == io.EOF:
+			return 0, io.ErrUnexpectedEOF
+		case err != nil:
+			return 0, err
+		}
+		rv = uint64(binary.BigEndian.Uint32(buf[:4]))
+
+		// The encoding is not canonical if the value could have been
+		// encoded using fewer bytes.
+		if rv <= 0xffff {
+			return 0, ErrVarIntNotCanonical
+		}
+
+	default:
+		_, err := io.ReadFull(r, buf[:])
+		switch {
+		case err == io.EOF:
+			return 0, io.ErrUnexpectedEOF
+		case err != nil:
+			return 0, err
+		}
+		rv = binary.BigEndian.Uint64(buf[:])
+
+		// The encoding is not canonical if the value could have been
+		// encoded using fewer bytes.
+		if rv <= 0xffffffff {
+			return 0, ErrVarIntNotCanonical
+		}
+	}
+
+	return rv, nil
+}
+
+// WriteVarInt serializes val to w using a variable number of bytes depending
+// on its value.
+func WriteVarInt(w io.Writer, val uint64, buf *[8]byte) error {
+	var length int
+	switch {
+	case val < 0xfd:
+		buf[0] = uint8(val)
+		length = 1
+
+	case val <= 0xffff:
+		buf[0] = uint8(0xfd)
+		binary.BigEndian.PutUint16(buf[1:3], uint16(val))
+		length = 3
+
+	case val <= 0xffffffff:
+		buf[0] = uint8(0xfe)
+		binary.BigEndian.PutUint32(buf[1:5], uint32(val))
+		length = 5
+
+	default:
+		buf[0] = uint8(0xff)
+		_, err := w.Write(buf[:1])
+		if err != nil {
+			return err
+		}
+
+		binary.BigEndian.PutUint64(buf[:], uint64(val))
+		length = 8
+	}
+
+	_, err := w.Write(buf[:length])
+	return err
+}