BIP34(Block v2, Height in Coinbase)

// serializedHeightVersion is the block version which changed block
// coinbases to start with the serialized block height.
serializedHeightVersion = 2

// ShouldHaveSerializedBlockHeight determines if a block should have a
// serialized block height embedded within the scriptSig of its
// coinbase transaction. Judgement is based on the block version in the block
// header. Blocks with version 2 and above satisfy this criteria. See BIP0034
// for further information.
func ShouldHaveSerializedBlockHeight(header *wire.BlockHeader) bool {
        return header.Version >= serializedHeightVersion
}

// ExtractCoinbaseHeight attempts to extract the height of the block from the
// scriptSig of a coinbase transaction.  Coinbase heights are only present in
// blocks of version 2 or later.  This was added as part of BIP0034.
func ExtractCoinbaseHeight(coinbaseTx *btcutil.Tx) (int32, error) {
        sigScript := coinbaseTx.MsgTx().TxIn[0].SignatureScript
        if len(sigScript) < 1 {
                str := "the coinbase signature script for blocks of " +
                        "version %d or greater must start with the " +
                        "length of the serialized block height"
                str = fmt.Sprintf(str, serializedHeightVersion)
                return 0, ruleError(ErrMissingCoinbaseHeight, str)
        }

        // Detect the case when the block height is a small integer encoded with
        // as single byte.
        opcode := int(sigScript[0])
        if opcode == txscript.OP_0 {
                return 0, nil
        }
        if opcode >= txscript.OP_1 && opcode <= txscript.OP_16 {
                return int32(opcode - (txscript.OP_1 - 1)), nil
        }

        // Otherwise, the opcode is the length of the following bytes which
        // encode in the block height.
        serializedLen := int(sigScript[0])
        if len(sigScript[1:]) < serializedLen {
                str := "the coinbase signature script for blocks of " +
                        "version %d or greater must start with the " +
                        "serialized block height"
                str = fmt.Sprintf(str, serializedLen)
                return 0, ruleError(ErrMissingCoinbaseHeight, str)
        }

        serializedHeightBytes := make([]byte, 8)
        copy(serializedHeightBytes, sigScript[1:serializedLen+1])
        serializedHeight := binary.LittleEndian.Uint64(serializedHeightBytes)

        return int32(serializedHeight), nil
}