package main

import (
	"bytes"
	"crypto/ecdsa"
	"encoding/hex"
	"errors"
	"fmt"
	"os"

	"github.com/boltdb/bolt"
)

const (
	dbFile              = "blockchain.db"
	blocksBucket        = "blocks"
	genesisCoinbaseData = "Arbitrary Genesis Data"
)

type Blockchain struct {
	tip []byte
	db  *bolt.DB
}

// Return whether the database file already exists
func dbExists() bool {
	if _, err := os.Stat(dbFile); os.IsNotExist(err) {
		return false
	}
	return true
}

// NewBlockchain creates a new Blockchain object from the DB
func NewBlockchain() *Blockchain {
	if dbExists() == false {
		fmt.Println("No existing blockchain found. Create one first.")
		os.Exit(1)
	}

	var tip []byte
	db, err := bolt.Open(dbFile, 0600, nil)
	if err != nil {
		exitWithError(err)
	}

	err = db.Update(func(tx *bolt.Tx) error {
		b := tx.Bucket([]byte(blocksBucket))
		tip = b.Get([]byte("1"))

		return nil
	})
	if err != nil {
		exitWithError(err)
	}

	bc := Blockchain{
		tip: tip,
		db:  db,
	}
	return &bc
}

// CreateBlockchain creates the blockchain DB and generate a new genesis block
func CreateBlockchain(address string) *Blockchain {
	if dbExists() {
		fmt.Println("Blockchain already exists.")
		os.Exit(1)
	}

	var tip []byte
	db, err := bolt.Open(dbFile, 0600, nil)
	if err != nil {
		exitWithError(err)
	}

	err = db.Update(func(tx *bolt.Tx) error {
		cbtx := NewCoinbaseTX(address, genesisCoinbaseData)
		genesis := NewGenesisBlock(cbtx)

		b, err := tx.CreateBucket([]byte(blocksBucket))
		if err != nil {
			exitWithError(err)
		}

		err = b.Put(genesis.Hash, genesis.Serialize())
		if err != nil {
			exitWithError(err)
		}

		err = b.Put([]byte("1"), genesis.Hash)
		if err != nil {
			exitWithError(err)
		}
		tip = genesis.Hash
		return nil
	})
	if err != nil {
		exitWithError(err)
	}

	bc := Blockchain{
		tip: tip,
		db:  db,
	}
	return &bc
}

// NewGenesisBlock creates a new Genesis Block to start the blockchain
func NewGenesisBlock(coinbase *Transaction) *Block {
	return NewBlock([]*Transaction{coinbase}, []byte{})
}

// FindUnspentTransactions returns a list of transactions containing unspent outputs
func (bc *Blockchain) FindUnspentTransactions(pubKeyHash []byte) []Transaction {
	var unspentTXs []Transaction
	spentTXOs := make(map[string][]int)
	bci := bc.Iterator()

	for {
		block := bci.Next()
		for _, tx := range block.Transactions {
			txID := hex.EncodeToString(tx.ID)

		Outputs:
			for outIdx, out := range tx.Vout {
				// Was the output spent?
				if spentTXOs[txID] != nil {
					for _, spentOut := range spentTXOs[txID] {
						if spentOut == outIdx {
							continue Outputs
						}
					}
				}

				if out.IsLockedWithKey(pubKeyHash) {
					unspentTXs = append(unspentTXs, *tx)
				}
			}

			if tx.IsCoinbase() == false {
				for _, in := range tx.Vin {
					if in.UsesKey(pubKeyHash) {
						inTxID := hex.EncodeToString(in.Txid)
						spentTXOs[inTxID] = append(spentTXOs[inTxID], in.Vout)
					}
				}
			}
		}

		if len(block.PrevBlockHash) == 0 {
			break
		}
	}

	return unspentTXs
}

// FindUTXO finds and returns all unspent transaction outputs
func (bc *Blockchain) FindUTXO(pubKeyHash []byte) []TXOutput {
	var UTXOs []TXOutput
	unspentTransactions := bc.FindUnspentTransactions(pubKeyHash)
	for _, tx := range unspentTransactions {
		for _, out := range tx.Vout {
			if out.IsLockedWithKey(pubKeyHash) {
				UTXOs = append(UTXOs, out)
			}
		}
	}
	return UTXOs
}

// FindSpendableOutputs finds and returns unspent outputs to reference in inputs
func (bc *Blockchain) FindSpendableOutputs(pubKeyHash []byte, amount int) (int, map[string][]int) {
	unspentOutputs := make(map[string][]int)
	unspentTXs := bc.FindUnspentTransactions(pubKeyHash)
	accumulated := 0

Work:
	for _, tx := range unspentTXs {
		txID := hex.EncodeToString(tx.ID)
		for outIdx, out := range tx.Vout {
			if out.IsLockedWithKey(pubKeyHash) && accumulated < amount {
				accumulated += out.Value
				unspentOutputs[txID] = append(unspentOutputs[txID], outIdx)
				if accumulated >= amount {
					break Work
				}
			}
		}
	}
	return accumulated, unspentOutputs
}

// MineBlock mines a new block with the provided transactions
func (bc *Blockchain) MineBlock(transactions []*Transaction) {
	var lastHash []byte

	for _, tx := range transactions {
		if !bc.VerifyTransaction(tx) {
			exitWithError(errors.New("Invalid Transaction"))
		}
	}

	err := bc.db.View(func(tx *bolt.Tx) error {
		b := tx.Bucket([]byte(blocksBucket))
		lastHash = b.Get([]byte("1"))

		return nil
	})
	if err != nil {
		exitWithError(err)
	}

	newBlock := NewBlock(transactions, lastHash)
	err = bc.db.Update(func(tx *bolt.Tx) error {
		b := tx.Bucket([]byte(blocksBucket))
		err := b.Put(newBlock.Hash, newBlock.Serialize())
		if err != nil {
			exitWithError(err)
		}

		err = b.Put([]byte("1"), newBlock.Hash)
		if err != nil {
			exitWithError(err)
		}

		bc.tip = newBlock.Hash
		return nil
	})
	if err != nil {
		exitWithError(err)
	}
}

func (bc *Blockchain) FindTransaction(ID []byte) (Transaction, error) {
	bci := bc.Iterator()
	for {
		block := bci.Next()
		for _, tx := range block.Transactions {
			if bytes.Compare(tx.ID, ID) == 0 {
				return *tx, nil
			}
		}
		if len(block.PrevBlockHash) == 0 {
			break
		}
	}
	return Transaction{}, errors.New("Transaction not found")
}

func (bc *Blockchain) SignTransaction(tx *Transaction, privKey ecdsa.PrivateKey) {
	prevTXs := make(map[string]Transaction)

	for _, vin := range tx.Vin {
		prevTX, err := bc.FindTransaction(vin.Txid)
		if err != nil {
			exitWithError(err)
		}
		prevTXs[hex.EncodeToString(prevTX.ID)] = prevTX
	}
	tx.Sign(privKey, prevTXs)
}

func (bc *Blockchain) VerifyTransaction(tx *Transaction) bool {
	prevTXs := make(map[string]Transaction)

	for _, vin := range tx.Vin {
		prevTX, err := bc.FindTransaction(vin.Txid)
		if err != nil {
			exitWithError(err)
		}
		prevTXs[hex.EncodeToString(prevTX.ID)] = prevTX
	}

	return tx.Verify(prevTXs)
}

// Iterating the Blockchain
type BlockchainIterator struct {
	currentHash []byte
	db          *bolt.DB
}

// Iterator returns a blockchain iterator
func (bc *Blockchain) Iterator() *BlockchainIterator {
	bci := &BlockchainIterator{bc.tip, bc.db}
	return bci
}

// next returns the next block (starting from the tip)
func (bci *BlockchainIterator) Next() *Block {
	var block *Block

	err := bci.db.View(func(tx *bolt.Tx) error {
		b := tx.Bucket([]byte(blocksBucket))
		encodedBlock := b.Get(bci.currentHash)
		block = DeserializeBlock(encodedBlock)
		return nil
	})
	if err != nil {
		exitWithError(err)
	}

	bci.currentHash = block.PrevBlockHash
	return block
}