parser: generalize API for decoding serializable types

parser/block.go

@@ -6,6 +6,7 @@ import (
 	"encoding/binary"
 	"log"
 
+	"github.com/gtank/ctxd/parser/internal/bytestring"
 	"github.com/pkg/errors"
 )
 
@@ -80,12 +81,37 @@ func (hdr *rawBlockHeader) UnmarshalBinary(data []byte) error {
 	return nil
 }
 
+type blockHeaderDecoder struct {
+	in *bytestring.String
+}
+
+func NewBlockHeaderDecoder(in *bytestring.String) Decoder {
+	return &blockHeaderDecoder{in}
+}
+
+func (dec *blockHeaderDecoder) Decode(out Serializable) error {
+	hdr, ok := out.(*BlockHeader)
+	if !ok {
+		return errors.New("unexpected Serializable for BlockHeader decoder")
+	}
+
+	if hdr.rawBlockHeader == nil {
+		hdr.rawBlockHeader = new(rawBlockHeader)
+	}
+
+	err := binary.Read(dec.in, binary.LittleEndian, hdr.rawBlockHeader)
+	if err != nil {
+		return errors.Wrap(err, "parsing block header")
+	}
+	return nil
+}
+
 type BlockHeader struct {
 	*rawBlockHeader
 	cachedHash []byte
 }
 
-func (hdr *BlockHeader) GetBlockHeaderHash() []byte {
+func (hdr *BlockHeader) GetHash() []byte {
 	if hdr.cachedHash != nil {
 		return hdr.cachedHash
 	}
@@ -103,16 +129,3 @@ func (hdr *BlockHeader) GetBlockHeaderHash() []byte {
 	hdr.cachedHash = digest[:]
 	return hdr.cachedHash
 }
-
-func (hdr *BlockHeader) GetSerializedSize() int {
-	// TODO: Make this dynamic. Low priority; it's unlikely to change.
-	return SER_BLOCK_HEADER_SIZE
-}
-
-func ReadBlockHeader(blockHeader *BlockHeader, data []byte) error {
-	if blockHeader.rawBlockHeader == nil {
-		blockHeader.rawBlockHeader = new(rawBlockHeader)
-	}
-	return blockHeader.UnmarshalBinary(data)
-}
-

parser/block_test.go

@@ -6,6 +6,8 @@ import (
 	"encoding/hex"
 	"os"
 	"testing"
+
+	"github.com/gtank/ctxd/parser/internal/bytestring"
 )
 
 func TestBlockHeader(t *testing.T) {
@@ -26,14 +28,21 @@ func TestBlockHeader(t *testing.T) {
 			continue
 		}
 
-		// Try to read the header
-		blockHeader := &BlockHeader{}
-		err = ReadBlockHeader(blockHeader, decodedBlockData)
+		s := bytestring.String(decodedBlockData)
+		startLength := len(s)
+		dec := NewBlockHeaderDecoder(&s)
+
+		blockHeader := BlockHeader{}
+		err = dec.Decode(&blockHeader)
 		if err != nil {
 			t.Error(err)
 			continue
 		}
 
+		if (startLength - len(s)) != SER_BLOCK_HEADER_SIZE {
+			t.Error("did not advance underlying bytestring")
+		}
+
 		// Some basic sanity checks
 		if blockHeader.Version != 4 {
 			t.Error("Read wrong version in a test block.")
@@ -73,7 +82,7 @@ func TestBlockHeader(t *testing.T) {
 			break
 		}
 
-		hash := blockHeader.GetBlockHeaderHash()
+		hash := blockHeader.GetHash()
 
 		// This is not necessarily true for anything but our current test cases.
 		for _, b := range hash[28:] {

parser/internal/bytestring/bytestring.go

@@ -0,0 +1,150 @@
+// Package bytestring provides a cryptobyte-inspired API specialized to the
+// needs of parsing Zcash transactions.
+package bytestring
+
+import (
+	"errors"
+	"io"
+)
+
+const MAX_COMPACT_SIZE uint64 = 0x02000000
+
+// String represents a string of bytes and provides methods for parsing values
+// from it.
+type String []byte
+
+// read advances the string by n bytes and returns them. If fewer than n bytes
+// remain, it returns nil.
+func (s *String) read(n int) []byte {
+	if len(*s) < n {
+		return nil
+	}
+
+	out := (*s)[:n]
+	(*s) = (*s)[n:]
+	return out
+}
+
+// Read reads the next len(p) bytes from the string, or the remainder of the
+// string if len(*s) < len(p). It returns the number of bytes read as n. If the
+// string is empty it returns an io.EOF error, or a nil error if len(p) == 0.
+// Read satisfies io.Reader.
+func (s *String) Read(p []byte) (n int, err error) {
+	if s.Empty() {
+		if len(p) == 0 {
+			return 0, nil
+		}
+		return 0, io.EOF
+	}
+
+	n = copy(p, *s)
+	if !s.Skip(n) {
+		return 0, errors.New("unexpected end of bytestring read")
+	}
+	return n, nil
+}
+
+// Empty reports whether or not the string is empty.
+func (s *String) Empty() bool {
+	return len(*s) == 0
+}
+
+// Skip advances the string by n bytes and reports whether it was successful.
+func (s *String) Skip(n int) bool {
+	return s.read(n) != nil
+}
+
+// ReadBytes reads n bytes into out and advances over them. It reports if the
+// read was successful.
+func (s *String) ReadBytes(out *[]byte, n int) bool {
+	v := s.read(n)
+	if v == nil {
+		return false
+	}
+	*out = v
+	return true
+}
+
+// ReadCompactSize reads and interprets a Bitcoin-custom compact integer
+// encoding used for length-prefixing and count values. If the values fall
+// outside the expected canonical ranges, it returns false.
+func (s *String) ReadCompactSize(size *uint64) bool {
+	lenBytes := s.read(1)
+	if lenBytes == nil {
+		return false
+	}
+	lenByte := lenBytes[0]
+
+	var lenLen int
+	var length, minSize uint64
+
+	switch {
+	case lenByte < 253:
+		length = uint64(lenByte)
+	case lenByte == 253:
+		lenLen = 2
+		minSize = 253
+	case lenByte == 254:
+		lenLen = 4
+		minSize = 0x10000
+	case lenByte == 255:
+		lenLen = 8
+		minSize = 0x100000000
+	}
+
+	if lenLen > 0 {
+		// expect little endian uint of varying size
+		lenBytes := s.read(lenLen)
+		for i := lenLen - 1; i >= 0; i-- {
+			length <<= 8
+			length = length | uint64(lenBytes[i])
+		}
+	}
+
+	if length > MAX_COMPACT_SIZE || length < minSize {
+		return false
+	}
+
+	*size = length
+	return true
+}
+
+// ReadCompactLengthPrefixed reads data prefixed by a CompactSize-encoded
+// length field into out. It reports whether the read was successful.
+func (s *String) ReadCompactLengthPrefixed(out *String) bool {
+	var length uint64
+	if ok := s.ReadCompactSize(&length); !ok {
+		return false
+	}
+
+	v := s.read(int(length))
+	if v == nil {
+		return false
+	}
+
+	*out = v
+	return true
+}
+
+// ReadUint32 decodes a little-endian, 32-bit value into out and advances over
+// it. It reports whether the read was successful.
+func (s *String) ReadUint32(out *uint32) bool {
+	v := s.read(4)
+	if v == nil {
+		return false
+	}
+	*out = uint32(v[0]) | uint32(v[1])<<8 | uint32(v[2])<<16 | uint32(v[3])<<24
+	return true
+}
+
+// ReadUint64 decodes a little-endian, 64-bit value into out and advances over
+// it. It reports whether the read was successful.
+func (s *String) ReadUint64(out *uint64) bool {
+	v := s.read(8)
+	if v == nil {
+		return false
+	}
+	*out = uint64(v[0]) | uint64(v[1])<<8 | uint64(v[2])<<16 | uint64(v[3])<<24 |
+		uint64(v[4])<<32 | uint64(v[5])<<40 | uint64(v[6])<<48 | uint64(v[7])<<56
+	return true
+}

parser/serializable.go

@@ -0,0 +1,12 @@
+package parser
+
+import "encoding"
+
+type Serializable interface {
+	encoding.BinaryMarshaler
+	encoding.BinaryUnmarshaler
+}
+
+type Decoder interface {
+	Decode(v Serializable) error
+}