rpc: implement getblockheader RPC

closes #711

pkg/rpc/response/result/block_header.go

@@ -0,0 +1,53 @@
+package result
+
+import (
+	"strconv"
+
+	"github.com/nspcc-dev/neo-go/pkg/core"
+	"github.com/nspcc-dev/neo-go/pkg/core/block"
+	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
+	"github.com/nspcc-dev/neo-go/pkg/io"
+	"github.com/nspcc-dev/neo-go/pkg/util"
+)
+
+type (
+	// Header wrapper used for the representation of
+	// block header on the RPC Server.
+	Header struct {
+		Hash          util.Uint256        `json:"hash"`
+		Size          int                 `json:"size"`
+		Version       uint32              `json:"version"`
+		PrevBlockHash util.Uint256        `json:"previousblockhash"`
+		MerkleRoot    util.Uint256        `json:"merkleroot"`
+		Timestamp     uint32              `json:"time"`
+		Index         uint32              `json:"index"`
+		Nonce         string              `json:"nonce"`
+		NextConsensus util.Uint160        `json:"nextconsensus"`
+		Script        transaction.Witness `json:"script"`
+		Confirmations uint32              `json:"confirmations"`
+		NextBlockHash *util.Uint256       `json:"nextblockhash,omitempty"`
+	}
+)
+
+// NewHeader creates a new Header wrapper.
+func NewHeader(h *block.Header, chain core.Blockchainer) Header {
+	res := Header{
+		Hash:          h.Hash(),
+		Size:          io.GetVarSize(h),
+		Version:       h.Version,
+		PrevBlockHash: h.PrevHash,
+		MerkleRoot:    h.MerkleRoot,
+		Timestamp:     h.Timestamp,
+		Index:         h.Index,
+		Nonce:         strconv.FormatUint(h.ConsensusData, 16),
+		NextConsensus: h.NextConsensus,
+		Script:        h.Script,
+		Confirmations: chain.BlockHeight() - h.Index + 1,
+	}
+
+	hash := chain.GetHeaderHash(int(h.Index) + 1)
+	if !hash.Equals(util.Uint256{}) {
+		res.NextBlockHash = &hash
+	}
+	return res
+}