package parser

import (
	"bytes"
	"crypto/sha256"
	"encoding/binary"
	"log"

	"github.com/gtank/ctxd/parser/internal/bytestring"
	"github.com/pkg/errors"
)

const (
	EQUIHASH_SIZE         = 1344 // size of an Equihash solution in bytes
	SER_BLOCK_HEADER_SIZE = 1487 // size of a serialized block header
)

// 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. This
// representation is a hack allowing us to use Go's binary parsing. In contexts
// outside of Zcash this could be a variable-length field.
type EquihashSize struct {
	SizeTag byte   // always the byte value 253
	Size    uint16 // always 1344
}

func (hdr *rawBlockHeader) MarshalBinary() ([]byte, error) {
	serBytes := make([]byte, 0, SER_BLOCK_HEADER_SIZE)
	serBuf := bytes.NewBuffer(serBytes)
	err := binary.Write(serBuf, binary.LittleEndian, hdr)
	return serBytes[:SER_BLOCK_HEADER_SIZE], err
}

func (hdr *rawBlockHeader) UnmarshalBinary(data []byte) error {
	reader := bytes.NewReader(data)
	err := binary.Read(reader, binary.LittleEndian, hdr)
	if err != nil {
		return errors.Wrap(err, "failed parsing block header")
	}
	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) GetHash() []byte {
	if hdr.cachedHash != nil {
		return hdr.cachedHash
	}

	serializedHeader, err := hdr.MarshalBinary()
	if err != nil {
		log.Fatalf("error marshaling block header: %v", err)
		return nil
	}

	// SHA256d
	digest := sha256.Sum256(serializedHeader)
	digest = sha256.Sum256(digest[:])

	hdr.cachedHash = digest[:]
	return hdr.cachedHash
}