package clique

import (
	"bytes"
	"fmt"
	"time"

	libcommon "github.com/ledgerwatch/erigon-lib/common"
	"github.com/ledgerwatch/erigon-lib/common/length"

	"github.com/ledgerwatch/erigon/consensus"
	"github.com/ledgerwatch/erigon/consensus/misc"
	"github.com/ledgerwatch/erigon/core/types"
	"github.com/ledgerwatch/erigon/params"
)

// verifyHeader checks whether a header conforms to the consensus rules.The
// caller may optionally pass in a batch of parents (ascending order) to avoid
// looking those up from the database. This is useful for concurrently verifying
// a batch of new headers.
func (c *Clique) verifyHeader(chain consensus.ChainHeaderReader, header *types.Header, parents []*types.Header) error {
	if header.Number == nil {
		return errUnknownBlock
	}
	number := header.Number.Uint64()

	now := time.Now()
	nowUnix := now.Unix()

	// Don't waste time checking blocks from the future
	if header.Time > uint64(nowUnix) {
		return consensus.ErrFutureBlock
	}

	// Checkpoint blocks need to enforce zero beneficiary
	checkpoint := (number % c.config.Epoch) == 0
	if checkpoint && header.Coinbase != (libcommon.Address{}) {
		return errInvalidCheckpointBeneficiary
	}

	// Nonces must be 0x00..0 or 0xff..f, zeroes enforced on checkpoints
	if !bytes.Equal(header.Nonce[:], NonceAuthVote) && !bytes.Equal(header.Nonce[:], nonceDropVote) {
		return errInvalidVote
	}

	if checkpoint && !bytes.Equal(header.Nonce[:], nonceDropVote) {
		return errInvalidCheckpointVote
	}

	// Check that the extra-data contains both the vanity and signature
	if len(header.Extra) < ExtraVanity {
		return errMissingVanity
	}
	if len(header.Extra) < ExtraVanity+ExtraSeal {
		return errMissingSignature
	}
	// Ensure that the extra-data contains a signer list on checkpoint, but none otherwise
	signersBytes := len(header.Extra) - ExtraVanity - ExtraSeal
	if !checkpoint && signersBytes != 0 {
		return errExtraSigners
	}
	if checkpoint && signersBytes%length.Addr != 0 {
		return errInvalidCheckpointSigners
	}
	// Ensure that the mix digest is zero as we don't have fork protection currently
	if header.MixDigest != (libcommon.Hash{}) {
		return errInvalidMixDigest
	}
	// Ensure that the block doesn't contain any uncles which are meaningless in PoA
	if header.UncleHash != uncleHash {
		return errInvalidUncleHash
	}
	// Ensure that the block's difficulty is meaningful (may not be correct at this point)
	if number > 0 {
		if header.Difficulty == nil || (header.Difficulty.Cmp(DiffInTurn) != 0 && header.Difficulty.Cmp(diffNoTurn) != 0) {
			return errInvalidDifficulty
		}
	}

	if header.WithdrawalsHash != nil {
		return consensus.ErrUnexpectedWithdrawals
	}

	// All basic checks passed, verify cascading fields
	return c.verifyCascadingFields(chain, header, parents)
}

// verifyCascadingFields verifies all the header fields that are not standalone,
// rather depend on a batch of previous headers. The caller may optionally pass
// in a batch of parents (ascending order) to avoid looking those up from the
// database. This is useful for concurrently verifying a batch of new headers.
func (c *Clique) verifyCascadingFields(chain consensus.ChainHeaderReader, header *types.Header, parents []*types.Header) error {
	// The genesis block is the always valid dead-end
	number := header.Number.Uint64()
	if number == 0 {
		return nil
	}

	// Retrieve the snapshot needed to verify this header and cache it
	var parent *types.Header
	if len(parents) > 0 {
		parent = parents[len(parents)-1]
	} else {
		parent = chain.GetHeader(header.ParentHash, number-1)
	}
	if parent == nil || parent.Number.Uint64() != number-1 || parent.Hash() != header.ParentHash {
		return consensus.ErrUnknownAncestor
	}

	if parent.Time+c.config.Period > header.Time {
		return errInvalidTimestamp
	}
	if !chain.Config().IsLondon(header.Number.Uint64()) {
		// Verify BaseFee not present before EIP-1559 fork.
		if header.BaseFee != nil {
			return fmt.Errorf("invalid baseFee before fork: have %d, want <nil>", header.BaseFee)
		}
		if err := misc.VerifyGaslimit(parent.GasLimit, header.GasLimit); err != nil {
			return err
		}
	} else if err := misc.VerifyEip1559Header(chain.Config(), parent, header, false /*skipGasLimit*/); err != nil {
		// Verify the header's EIP-1559 attributes.
		return err
	}

	if err := misc.VerifyAbsenceOfCancunHeaderFields(header); err != nil {
		return err
	}

	// Retrieve the snapshot needed to verify this header and cache it
	snap, err := c.Snapshot(chain, number-1, header.ParentHash, parents)
	if err != nil {
		return err
	}

	// If the block is a checkpoint block, verify the signer list
	if number%c.config.Epoch == 0 {
		signers := make([]byte, len(snap.Signers)*length.Addr)
		for i, signer := range snap.GetSigners() {
			copy(signers[i*length.Addr:], signer[:])
		}

		extraSuffix := len(header.Extra) - ExtraSeal
		if !bytes.Equal(header.Extra[ExtraVanity:extraSuffix], signers) {
			return errMismatchingCheckpointSigners
		}
	}

	// All basic checks passed, verify the seal and return
	return c.verifySeal(chain, header, snap)
}

func (c *Clique) Snapshot(chain consensus.ChainHeaderReader, number uint64, hash libcommon.Hash, parents []*types.Header) (*Snapshot, error) {
	// Search for a snapshot in memory or on disk for checkpoints
	var (
		headers []*types.Header
		snap    *Snapshot
	)
	for snap == nil {
		// If an in-memory snapshot was found, use that
		if s, ok := c.recents.Get(hash); ok {
			snap = s
			break
		}
		// If an on-disk checkpoint snapshot can be found, use that
		if number%c.snapshotConfig.CheckpointInterval == 0 {
			if s, err := loadSnapshot(c.config, c.DB, number, hash); err == nil {
				c.logger.Trace("Loaded voting snapshot from disk", "number", number, "hash", hash)
				snap = s
				break
			}
		}
		// If we're at the genesis, snapshot the initial state. Alternatively if we're
		// at a checkpoint block without a parent (light client CHT), or we have piled
		// up more headers than allowed to be reorged (chain reinit from a freezer),
		// consider the checkpoint trusted and snapshot it.
		if number == 0 || (number%c.config.Epoch == 0 && (len(headers) > params.FullImmutabilityThreshold || chain.GetHeaderByNumber(number-1) == nil)) {
			checkpoint := chain.GetHeaderByNumber(number)
			if checkpoint != nil {
				hash := checkpoint.Hash()

				signers := make([]libcommon.Address, (len(checkpoint.Extra)-ExtraVanity-ExtraSeal)/length.Addr)
				for i := 0; i < len(signers); i++ {
					copy(signers[i][:], checkpoint.Extra[ExtraVanity+i*length.Addr:])
				}
				snap = newSnapshot(c.config, number, hash, signers)
				if err := snap.store(c.DB); err != nil {
					return nil, err
				}
				c.logger.Info("[Clique] Stored checkpoint snapshot to disk", "number", number, "hash", hash)
				break
			}
		}
		// No snapshot for this header, gather the header and move backward
		var header *types.Header
		if len(parents) > 0 {
			// If we have explicit parents, pick from there (enforced)
			header = parents[len(parents)-1]
			if header.Hash() != hash || header.Number.Uint64() != number {
				return nil, consensus.ErrUnknownAncestor
			}
			parents = parents[:len(parents)-1]
		} else {
			// No explicit parents (or no more left), reach out to the database
			header = chain.GetHeader(hash, number)
			if header == nil {
				return nil, consensus.ErrUnknownAncestor
			}
		}
		headers = append(headers, header)
		number, hash = number-1, header.ParentHash
	}
	// Previous snapshot found, apply any pending headers on top of it
	for i := 0; i < len(headers)/2; i++ {
		headers[i], headers[len(headers)-1-i] = headers[len(headers)-1-i], headers[i]
	}
	snap, err := snap.apply(c.signatures, c.logger, headers...)
	if err != nil {
		return nil, err
	}
	c.recents.Add(snap.Hash, snap)

	// If we've generated a new checkpoint snapshot, save to disk
	if snap.Number%c.snapshotConfig.CheckpointInterval == 0 && len(headers) > 0 {
		if err = snap.store(c.DB); err != nil {
			return nil, err
		}
		c.logger.Trace("Stored voting snapshot to disk", "number", snap.Number, "hash", snap.Hash)
	}
	return snap, err
}

// verifySeal checks whether the signature contained in the header satisfies the
// consensus protocol requirements. The method accepts an optional list of parent
// headers that aren't yet part of the local blockchain to generate the snapshots
// from.
func (c *Clique) verifySeal(chain consensus.ChainHeaderReader, header *types.Header, snap *Snapshot) error {
	// Verifying the genesis block is not supported
	number := header.Number.Uint64()
	if number == 0 {
		return errUnknownBlock
	}

	// Resolve the authorization key and check against signers
	signer, err := ecrecover(header, c.signatures)
	if err != nil {
		return err
	}

	if _, ok := snap.Signers[signer]; !ok {
		return ErrUnauthorizedSigner
	}

	for seen, recent := range snap.Recents {
		if recent == signer {
			// Signer is among RecentsRLP, only fail if the current block doesn't shift it out
			if limit := uint64(len(snap.Signers)/2 + 1); seen > number-limit {
				return ErrRecentlySigned
			}
		}
	}

	// Ensure that the difficulty corresponds to the turn-ness of the signer
	if !c.FakeDiff {
		inturn := snap.inturn(header.Number.Uint64(), signer)
		if inturn && header.Difficulty.Cmp(DiffInTurn) != 0 {
			return errWrongDifficulty
		}
		if !inturn && header.Difficulty.Cmp(diffNoTurn) != 0 {
			return errWrongDifficulty
		}
	}

	return nil
}