package blockchain

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"strconv"
	"strings"
	"time"

	badger "github.com/dgraph-io/badger/v4"
)

// Index key prefixes
const (
	prefixTxRecord          = "tx:"      // tx:<txid>                          → TxRecord JSON
	prefixTxByAddr          = "txaddr:"  // txaddr:<pubkey>:<block020d>:<txid> → ""  (empty value)
	prefixAddrMap           = "addrmap:" // addrmap:<DCaddr>                   → pubkey hex
	prefixNetStats          = "netstats" // netstats                           → NetStats JSON
	syntheticRewardIDPrefix = "sys-reward-"
)

// TxRecord wraps a Transaction with its on-chain context.
type TxRecord struct {
	Tx         *Transaction `json:"tx"`
	BlockIndex uint64       `json:"block_index"`
	BlockHash  string       `json:"block_hash"`
	BlockTime  time.Time    `json:"block_time"`
	GasUsed    uint64       `json:"gas_used,omitempty"`
}

// NetStats are aggregate counters updated every block.
type NetStats struct {
	TotalBlocks      uint64 `json:"total_blocks"`
	TotalTxs         uint64 `json:"total_txs"`
	TotalTransfers   uint64 `json:"total_transfers"`
	TotalRelayProofs uint64 `json:"total_relay_proofs"`
	TotalSupply      uint64 `json:"total_supply"` // µT ever minted via rewards + grants
	ValidatorCount   int    `json:"validator_count"`
	RelayCount       int    `json:"relay_count"`
}

// indexBlock is called inside AddBlock's db.Update() — indexes all transactions
// in the block and updates aggregate stats.
// gasUsed maps tx.ID → gas consumed for CALL_CONTRACT transactions.
func (c *Chain) indexBlock(txn *badger.Txn, b *Block, gasUsed map[string]uint64) error {
	// Load existing stats
	stats, err := c.readNetStats(txn)
	if err != nil {
		return err
	}
	stats.TotalBlocks = b.Index + 1
	// TotalSupply is fixed at GenesisAllocation; update it once at genesis.
	if b.Index == 0 {
		stats.TotalSupply = GenesisAllocation
	}

	for seq, tx := range b.Transactions {
		// Store full TxRecord — but never overwrite an existing record.
		// The same TX can appear in multiple gossiped blocks due to a mempool/PBFT
		// race; the first block that actually applies it (via applyTx) will have
		// gasUsed > 0.  Subsequent re-indexings with an empty gasUsedByTx map
		// would zero out the stored GasUsed.  Skip if the record already exists.
		recKey := []byte(prefixTxRecord + tx.ID)
		if _, existErr := txn.Get(recKey); existErr == nil {
			// TxRecord already written (from an earlier block or earlier call);
			// do not overwrite it.
			continue
		}
		// Chronological index entry (txchron:<block20d>:<seq04d> → tx_id).
		// Lets RecentTxs iterate tx-by-tx instead of block-by-block so chains
		// with many empty blocks still answer /api/txs/recent in O(limit).
		chronKey := fmt.Sprintf("%s%020d:%04d", prefixTxChron, b.Index, seq)
		if err := txn.Set([]byte(chronKey), []byte(tx.ID)); err != nil {
			return err
		}
		gasForTx := gasUsed[tx.ID]
		rec := TxRecord{
			Tx:         tx,
			BlockIndex: b.Index,
			BlockHash:  b.HashHex(),
			BlockTime:  b.Timestamp,
			GasUsed:    gasForTx,
		}
		val, err := json.Marshal(rec)
		if err != nil {
			return err
		}
		if err := txn.Set(recKey, val); err != nil {
			return err
		}

		// Index by sender
		if tx.From != "" {
			addrKey := txAddrKey(tx.From, b.Index, tx.ID)
			if err := txn.Set([]byte(addrKey), []byte{}); err != nil {
				return err
			}
			// Store addr → pubkey mapping
			if err := c.storeAddrMap(txn, tx.From); err != nil {
				return err
			}
		}
		// Index by recipient
		if tx.To != "" && tx.To != tx.From {
			addrKey := txAddrKey(tx.To, b.Index, tx.ID)
			if err := txn.Set([]byte(addrKey), []byte{}); err != nil {
				return err
			}
			if err := c.storeAddrMap(txn, tx.To); err != nil {
				return err
			}
		}

		// Update aggregate counters
		stats.TotalTxs++
		switch tx.Type {
		case EventTransfer:
			stats.TotalTransfers++
		case EventRelayProof:
			stats.TotalRelayProofs++
		}
	}

	// Index synthetic block reward only when the validator actually earned fees,
	// or for the genesis block (one-time allocation). Empty blocks produce no
	// state change and no income, so there is nothing useful to show.
	if b.TotalFees > 0 || b.Index == 0 {
		rewardTarget, err := c.resolveRewardTarget(txn, b.Validator)
		if err != nil {
			return err
		}
		rewardTx, err := makeBlockRewardTx(b, rewardTarget)
		if err != nil {
			return err
		}
		rewardRec := TxRecord{
			Tx:         rewardTx,
			BlockIndex: b.Index,
			BlockHash:  b.HashHex(),
			BlockTime:  b.Timestamp,
		}
		rewardVal, err := json.Marshal(rewardRec)
		if err != nil {
			return err
		}
		if err := txn.Set([]byte(prefixTxRecord+rewardTx.ID), rewardVal); err != nil {
			return err
		}
		if rewardTx.From != "" {
			if err := txn.Set([]byte(txAddrKey(rewardTx.From, b.Index, rewardTx.ID)), []byte{}); err != nil {
				return err
			}
			if err := c.storeAddrMap(txn, rewardTx.From); err != nil {
				return err
			}
		}
		if rewardTx.To != "" && rewardTx.To != rewardTx.From {
			if err := txn.Set([]byte(txAddrKey(rewardTx.To, b.Index, rewardTx.ID)), []byte{}); err != nil {
				return err
			}
			if err := c.storeAddrMap(txn, rewardTx.To); err != nil {
				return err
			}
		}
	}

	// Persist updated stats
	return c.writeNetStats(txn, stats)
}

func makeBlockRewardTx(b *Block, rewardTarget string) (*Transaction, error) {
	var memo string
	if b.Index == 0 {
		memo = fmt.Sprintf("Genesis allocation: %d µT", GenesisAllocation)
	} else {
		memo = fmt.Sprintf("Block fees: %d µT", b.TotalFees)
	}

	total := b.TotalFees
	if b.Index == 0 {
		total = GenesisAllocation
	}

	payload, err := json.Marshal(BlockRewardPayload{
		ValidatorPubKey: b.Validator,
		TargetPubKey:    rewardTarget,
		FeeReward:       b.TotalFees,
		TotalReward:     total,
	})
	if err != nil {
		return nil, err
	}
	// From is intentionally left empty: a block reward is a synthetic, freshly
	// minted allocation (fees collected by the network) rather than a transfer
	// from an actual account. Leaving From="" prevents the reward from appearing
	// as "validator paid themselves" in the explorer/client when the validator
	// has no separate wallet binding (rewardTarget == b.Validator).
	// b.Validator is still recorded inside the payload (BlockRewardPayload).
	return &Transaction{
		ID:        fmt.Sprintf("%s%020d", syntheticRewardIDPrefix, b.Index),
		Type:      EventBlockReward,
		From:      "",
		To:        rewardTarget,
		Amount:    total,
		Fee:       0,
		Memo:      memo,
		Payload:   payload,
		Timestamp: b.Timestamp,
	}, nil
}

// txAddrKey builds the composite key: txaddr:<pubkey>:<block_020d>:<txid>
func txAddrKey(pubKey string, blockIdx uint64, txID string) string {
	return fmt.Sprintf("%s%s:%020d:%s", prefixTxByAddr, pubKey, blockIdx, txID)
}

// storeAddrMap stores a DC address → pubkey mapping.
func (c *Chain) storeAddrMap(txn *badger.Txn, pubKey string) error {
	addr := pubKeyToAddr(pubKey)
	return txn.Set([]byte(prefixAddrMap+addr), []byte(pubKey))
}

// pubKeyToAddr converts a hex Ed25519 public key to a DC address.
// Replicates wallet.PubKeyToAddress without importing the wallet package.
func pubKeyToAddr(pubKeyHex string) string {
	raw, err := hex.DecodeString(pubKeyHex)
	if err != nil {
		return pubKeyHex // fallback: use pubkey as-is
	}
	h := sha256.Sum256(raw)
	return "DC" + hex.EncodeToString(h[:12])
}

// --- Public query methods ---

// TxByID returns a TxRecord by transaction ID.
func (c *Chain) TxByID(txID string) (*TxRecord, error) {
	var rec TxRecord
	err := c.db.View(func(txn *badger.Txn) error {
		item, err := txn.Get([]byte(prefixTxRecord + txID))
		if err != nil {
			return err
		}
		return item.Value(func(val []byte) error {
			return json.Unmarshal(val, &rec)
		})
	})
	if errors.Is(err, badger.ErrKeyNotFound) {
		synth, synthErr := c.syntheticTxByID(txID)
		if synthErr != nil {
			return nil, synthErr
		}
		if synth != nil {
			return synth, nil
		}
		return nil, nil
	}
	return &rec, err
}

func parseSyntheticRewardIndex(txID string) (uint64, bool) {
	if !strings.HasPrefix(txID, syntheticRewardIDPrefix) {
		return 0, false
	}
	part := strings.TrimPrefix(txID, syntheticRewardIDPrefix)
	idx, err := strconv.ParseUint(part, 10, 64)
	if err != nil {
		return 0, false
	}
	return idx, true
}

func (c *Chain) syntheticTxByID(txID string) (*TxRecord, error) {
	idx, ok := parseSyntheticRewardIndex(txID)
	if !ok {
		return nil, nil
	}
	b, err := c.GetBlock(idx)
	if errors.Is(err, badger.ErrKeyNotFound) {
		return nil, nil
	}
	if err != nil {
		return nil, err
	}

	rewardTarget := b.Validator
	binding, err := c.WalletBinding(b.Validator)
	if err == nil && binding != "" {
		rewardTarget = binding
	}
	rewardTx, err := makeBlockRewardTx(b, rewardTarget)
	if err != nil {
		return nil, err
	}
	return &TxRecord{
		Tx:         rewardTx,
		BlockIndex: b.Index,
		BlockHash:  b.HashHex(),
		BlockTime:  b.Timestamp,
	}, nil
}

// TxsByAddress returns up to limit TxRecords for a public key, newest first,
// skipping the first offset results (for pagination).
func (c *Chain) TxsByAddress(pubKey string, limit, offset int) ([]*TxRecord, error) {
	if limit <= 0 {
		limit = 50
	}
	if offset < 0 {
		offset = 0
	}
	prefix := prefixTxByAddr + pubKey + ":"

	// First: collect TxID keys for this address (newest first via reverse iter),
	// skipping `offset` entries.
	var txIDs []string
	err := c.db.View(func(txn *badger.Txn) error {
		opts := badger.DefaultIteratorOptions
		opts.Reverse = true
		opts.PrefetchValues = false
		it := txn.NewIterator(opts)
		defer it.Close()

		seekKey := prefix + "\xff\xff\xff\xff\xff\xff\xff\xff"
		skipped := 0
		for it.Seek([]byte(seekKey)); it.Valid(); it.Next() {
			key := string(it.Item().Key())
			if !strings.HasPrefix(key, prefix) {
				break
			}
			parts := strings.SplitN(key[len(prefix):], ":", 2)
			if len(parts) != 2 {
				continue
			}
			if skipped < offset {
				skipped++
				continue
			}
			txIDs = append(txIDs, parts[1])
			if len(txIDs) >= limit {
				break
			}
		}
		return nil
	})
	if err != nil {
		return nil, err
	}

	// Now fetch each TxRecord
	var records []*TxRecord
	err = c.db.View(func(txn *badger.Txn) error {
		for _, txID := range txIDs {
			item, err := txn.Get([]byte(prefixTxRecord + txID))
			if errors.Is(err, badger.ErrKeyNotFound) {
				continue
			}
			if err != nil {
				return err
			}
			var rec TxRecord
			if err := item.Value(func(val []byte) error {
				return json.Unmarshal(val, &rec)
			}); err != nil {
				return err
			}
			records = append(records, &rec)
		}
		return nil
	})
	return records, err
}

// RecentTxs returns the N most recent transactions across all blocks.
func (c *Chain) RecentTxs(limit int) ([]*TxRecord, error) {
	if limit <= 0 {
		limit = 20
	}
	// Primary path: iterate the chronological tx index in reverse. This is
	// O(limit) regardless of how many empty blocks sit between txs.
	var records []*TxRecord
	err := c.db.View(func(txn *badger.Txn) error {
		opts := badger.DefaultIteratorOptions
		opts.Reverse        = true
		opts.PrefetchValues = true
		it := txn.NewIterator(opts)
		defer it.Close()

		// Seek to the highest possible key under this prefix.
		seekKey := []byte(prefixTxChron + "\xff")
		for it.Seek(seekKey); it.ValidForPrefix([]byte(prefixTxChron)); it.Next() {
			if len(records) >= limit {
				break
			}
			var txID string
			err := it.Item().Value(func(v []byte) error {
				txID = string(v)
				return nil
			})
			if err != nil || txID == "" {
				continue
			}
			recItem, err := txn.Get([]byte(prefixTxRecord + txID))
			if err != nil {
				continue
			}
			var rec TxRecord
			if err := recItem.Value(func(v []byte) error { return json.Unmarshal(v, &rec) }); err != nil {
				continue
			}
			records = append(records, &rec)
		}
		return nil
	})
	if err == nil && len(records) >= limit {
		return records, nil
	}

	// Fallback (legacy + reward-tx injection): reverse-scan blocks.
	// Only blocks committed BEFORE the chronological index existed will be
	// found this way; we cap the scan so it can't hang.
	tipIdx := c.TipIndex()
	const maxBlockScan = 5000

	seen := make(map[string]bool, len(records))
	for _, r := range records {
		seen[r.Tx.ID] = true
	}

	scanned := 0
	for idx := int64(tipIdx); idx >= 0 && len(records) < limit && scanned < maxBlockScan; idx-- {
		scanned++
		b, err := c.GetBlock(uint64(idx))
		if err != nil {
			break
		}
		for i := len(b.Transactions) - 1; i >= 0 && len(records) < limit; i-- {
			tx := b.Transactions[i]
			if seen[tx.ID] {
				continue
			}
			records = append(records, &TxRecord{
				Tx:         tx,
				BlockIndex: b.Index,
				BlockHash:  b.HashHex(),
				BlockTime:  b.Timestamp,
			})
		}
		// Include BLOCK_REWARD only for fee-earning blocks and genesis.
		if len(records) < limit && (b.TotalFees > 0 || b.Index == 0) {
			rewardTarget := b.Validator
			if binding, err2 := c.WalletBinding(b.Validator); err2 == nil && binding != "" {
				rewardTarget = binding
			}
			if rewardTx, err2 := makeBlockRewardTx(b, rewardTarget); err2 == nil {
				records = append(records, &TxRecord{
					Tx:         rewardTx,
					BlockIndex: b.Index,
					BlockHash:  b.HashHex(),
					BlockTime:  b.Timestamp,
				})
			}
		}
	}
	return records, nil
}

// RecentBlocks returns the N most recent blocks (tip first).
func (c *Chain) RecentBlocks(limit int) ([]*Block, error) {
	if limit <= 0 {
		limit = 10
	}
	// Lock-free tip lookup so this endpoint never blocks on consensus work.
	tipIdx := c.TipIndex()
	var blocks []*Block
	for idx := int64(tipIdx); idx >= 0 && len(blocks) < limit; idx-- {
		b, err := c.GetBlock(uint64(idx))
		if err != nil {
			break
		}
		blocks = append(blocks, b)
	}
	return blocks, nil
}

// NetworkStats returns aggregate counters for the chain.
// ValidatorCount and RelayCount are always live-counted from the DB so they
// are accurate even after InitValidators replaced the set or relays registered.
func (c *Chain) NetworkStats() (NetStats, error) {
	var stats NetStats
	err := c.db.View(func(txn *badger.Txn) error {
		s, err := c.readNetStats(txn)
		if err != nil {
			return err
		}
		stats = s

		opts := badger.DefaultIteratorOptions
		opts.PrefetchValues = false
		it := txn.NewIterator(opts)
		defer it.Close()

		vPrefix := []byte(prefixValidator)
		for it.Seek(vPrefix); it.ValidForPrefix(vPrefix); it.Next() {
			stats.ValidatorCount++
		}
		rPrefix := []byte(prefixRelay)
		for it.Seek(rPrefix); it.ValidForPrefix(rPrefix); it.Next() {
			stats.RelayCount++
		}
		return nil
	})
	return stats, err
}

// AddressToPubKey resolves a DC address to a pub key.
// Returns "" if not found.
func (c *Chain) AddressToPubKey(addr string) (string, error) {
	var pubKey string
	err := c.db.View(func(txn *badger.Txn) error {
		item, err := txn.Get([]byte(prefixAddrMap + addr))
		if errors.Is(err, badger.ErrKeyNotFound) {
			return nil
		}
		if err != nil {
			return err
		}
		return item.Value(func(val []byte) error {
			pubKey = string(val)
			return nil
		})
	})
	return pubKey, err
}

// --- internal ---

func (c *Chain) readNetStats(txn *badger.Txn) (NetStats, error) {
	var s NetStats
	item, err := txn.Get([]byte(prefixNetStats))
	if errors.Is(err, badger.ErrKeyNotFound) {
		return s, nil
	}
	if err != nil {
		return s, err
	}
	err = item.Value(func(val []byte) error {
		return json.Unmarshal(val, &s)
	})
	return s, err
}

func (c *Chain) writeNetStats(txn *badger.Txn, s NetStats) error {
	val, err := json.Marshal(s)
	if err != nil {
		return err
	}
	return txn.Set([]byte(prefixNetStats), val)
}