type BlockHeader struct {
   // Standard Bitcoin fields.
   Version        uint32
   HashPrevBlock  []byte
   HashMerkleRoot []byte
   Timestamp      uint32
   Bits           uint32
   Nonce          uint32
}

// HeaderHash calculates the block header hash.
// This is the fixed multiplier when searching for primes.
// NOTE: When converting to an Int the hash must be reversed.
func (bh *BlockHeader) HeaderHash() []byte {
   hash := sha256.New()
   binary.Write(hash, binary.LittleEndian, bh.Version)
   hash.Write(bh.HashPrevBlock)
   hash.Write(bh.HashMerkleRoot)
   binary.Write(hash, binary.LittleEndian, bh.Timestamp)
   binary.Write(hash, binary.LittleEndian, bh.Bits)
   binary.Write(hash, binary.LittleEndian, bh.Nonce)

   data := sha256.Sum256(hash.Sum(nil))
   return data[:]
}

// hashToBig converts a hash number to an integer
func hashToBig(hash []byte) *big.Int {
   tmp := make([]byte, len(hash))
   copy(tmp, hash)
   reverse(tmp)
   return new(big.Int).SetBytes(tmp)
}