parser: implement container-level parsing of block headers

parser/block.go

@@ -0,0 +1,69 @@
+package parser
+
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/pkg/errors"
+)
+
+const (
+	EQUIHASH_SIZE = 1344 // size of an Equihash solution in bytes
+)
+
+// A block header as defined in version 2018.0-beta-29 of the Zcash Protocol Spec.
+type RawBlockHeader struct {
+	// The block version number indicates which set of block validation rules
+	// to follow. The current and only defined block version number for Zcash
+	// is 4.
+	Version int32
+
+	// A SHA-256d hash in internal byte order of the previous block's header. This
+	// ensures no previous block can be changed without also changing this block's
+	// header.
+	HashPrevBlock [32]byte
+
+	// A SHA-256d hash in internal byte order. The merkle root is derived from
+	// the hashes of all transactions included in this block, ensuring that
+	// none of those transactions can be modified without modifying the header.
+	HashMerkleRoot [32]byte
+
+	// [Pre-Sapling] A reserved field which should be ignored.
+	// [Sapling onward] The root LEBS2OSP_256(rt) of the Sapling note
+	// commitment tree corresponding to the final Sapling treestate of this
+	// block.
+	HashFinalSaplingRoot [32]byte
+
+	// The block time is a Unix epoch time (UTC) when the miner started hashing
+	// the header (according to the miner).
+	Time uint32
+
+	// An encoded version of the target threshold this block's header hash must
+	// be less than or equal to, in the same nBits format used by Bitcoin.
+	NBits [4]byte
+
+	// An arbitrary field that miners can change to modify the header hash in
+	// order to produce a hash less than or equal to the target threshold.
+	Nonce [32]byte
+
+	// The size of an Equihash solution in bytes (always 1344).
+	SolutionSize EquihashSize
+
+	// The Equihash solution.
+	Solution [EQUIHASH_SIZE]byte
+}
+
+// EquihashSize is a concrete instance of Bitcoin's CompactSize encoding.
+type EquihashSize struct {
+	_    byte   // always the byte value 253
+	Size uint16 // always 1344
+}
+
+func ReadBlockHeader(r io.Reader) (*RawBlockHeader, error) {
+	var blockHeader RawBlockHeader
+	err := binary.Read(r, binary.LittleEndian, &blockHeader)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed reading block header")
+	}
+	return &blockHeader, nil
+}