// Copyright 2017 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package ethash

import (
	"bytes"
	"errors"
	"fmt"
	"math/big"
	"runtime"
	"sync/atomic"
	"time"

	"github.com/ethereum/go-ethereum/common"
	"github.com/ethereum/go-ethereum/common/math"
	"github.com/ethereum/go-ethereum/consensus"
	"github.com/ethereum/go-ethereum/consensus/misc"
	"github.com/ethereum/go-ethereum/core/state"
	"github.com/ethereum/go-ethereum/core/types"
	"github.com/ethereum/go-ethereum/params"
	set "gopkg.in/fatih/set.v0"
)

// Ethash proof-of-work protocol constants.
var (
	blockReward *big.Int = big.NewInt(5e+18) // Block reward in wei for successfully mining a block
	maxUncles            = 2                 // Maximum number of uncles allowed in a single block
)

var (
	ErrInvalidChain      = errors.New("invalid header chain")
	ErrTooManyUncles     = errors.New("too many uncles")
	ErrDuplicateUncle    = errors.New("duplicate uncle")
	ErrUncleIsAncestor   = errors.New("uncle is ancestor")
	ErrDanglingUncle     = errors.New("uncle's parent is not ancestor")
	ErrNonceOutOfRange   = errors.New("nonce out of range")
	ErrInvalidDifficulty = errors.New("non-positive difficulty")
	ErrInvalidMixDigest  = errors.New("invalid mix digest")
	ErrInvalidPoW        = errors.New("invalid proof-of-work")
)

// VerifyHeader checks whether a header conforms to the consensus rules of the
// stock Ethereum ethash engine.
func (ethash *Ethash) VerifyHeader(chain consensus.ChainReader, header *types.Header, seal bool) error {
	// If we're running a full engine faking, accept any input as valid
	if ethash.fakeFull {
		return nil
	}
	// Short circuit if the header is known, or it's parent not
	number := header.Number.Uint64()
	if chain.GetHeader(header.Hash(), number) != nil {
		return nil
	}
	parent := chain.GetHeader(header.ParentHash, number-1)
	if parent == nil {
		return consensus.ErrUnknownAncestor
	}
	// Sanity checks passed, do a proper verification
	return ethash.verifyHeader(chain, header, parent, false, seal)
}

// VerifyHeaders is similar to VerifyHeader, but verifies a batch of headers
// concurrently. The method returns a quit channel to abort the operations and
// a results channel to retrieve the async verifications.
func (ethash *Ethash) VerifyHeaders(chain consensus.ChainReader, headers []*types.Header, seals []bool) (chan<- struct{}, <-chan error) {
	// If we're running a full engine faking, accept any input as valid
	if ethash.fakeFull {
		abort, results := make(chan struct{}), make(chan error, len(headers))
		for i := 0; i < len(headers); i++ {
			results <- nil
		}
		return abort, results
	}
	// Spawn as many workers as allowed threads
	workers := runtime.GOMAXPROCS(0)
	if len(headers) < workers {
		workers = len(headers)
	}
	// Create a task channel and spawn the verifiers
	type result struct {
		index int
		err   error
	}
	inputs := make(chan int, workers)
	outputs := make(chan result, len(headers))

	var badblock uint64
	for i := 0; i < workers; i++ {
		go func() {
			for index := range inputs {
				// If we've found a bad block already before this, stop validating
				if bad := atomic.LoadUint64(&badblock); bad != 0 && bad <= headers[index].Number.Uint64() {
					outputs <- result{index: index, err: ErrInvalidChain}
					continue
				}
				// We need to look up the first parent
				var parent *types.Header
				if index == 0 {
					parent = chain.GetHeader(headers[0].ParentHash, headers[0].Number.Uint64()-1)
				} else if headers[index-1].Hash() == headers[index].ParentHash {
					parent = headers[index-1]
				}
				// Ensure the validation is useful and execute it
				var failure error
				switch {
				case chain.GetHeader(headers[index].Hash(), headers[index].Number.Uint64()-1) != nil:
					outputs <- result{index: index, err: nil}
				case parent == nil:
					failure = consensus.ErrUnknownAncestor
					outputs <- result{index: index, err: failure}
				default:
					failure = ethash.verifyHeader(chain, headers[index], parent, false, seals[index])
					outputs <- result{index: index, err: failure}
				}
				// If a validation failure occurred, mark subsequent blocks invalid
				if failure != nil {
					number := headers[index].Number.Uint64()
					if prev := atomic.LoadUint64(&badblock); prev == 0 || prev > number {
						// This two step atomic op isn't thread-safe in that `badblock` might end
						// up slightly higher than the block number of the first failure (if many
						// workers try to write at the same time), but it's fine as we're mostly
						// interested to avoid large useless work, we don't care about 1-2 extra
						// runs. Doing "full thread safety" would involve mutexes, which would be
						// a noticeable sync overhead on the fast spinning worker routines.
						atomic.StoreUint64(&badblock, number)
					}
				}
			}
		}()
	}
	// Feed item indices to the workers until done, sorting and feeding the results to the caller
	dones := make([]bool, len(headers))
	errors := make([]error, len(headers))

	abort := make(chan struct{})
	returns := make(chan error, len(headers))

	go func() {
		defer close(inputs)

		input, output := 0, 0
		for i := 0; i < len(headers)*2; i++ {
			var res result

			// If there are tasks left, push to workers
			if input < len(headers) {
				select {
				case inputs <- input:
					input++
					continue
				case <-abort:
					return
				case res = <-outputs:
				}
			} else {
				// Otherwise keep waiting for results
				select {
				case <-abort:
					return
				case res = <-outputs:
				}
			}
			// A result arrived, save and propagate if next
			dones[res.index], errors[res.index] = true, res.err
			for output < len(headers) && dones[output] {
				returns <- errors[output]
				output++
			}
		}
	}()
	return abort, returns
}

// VerifyUncles verifies that the given block's uncles conform to the consensus
// rules of the stock Ethereum ethash engine.
func (ethash *Ethash) VerifyUncles(chain consensus.ChainReader, block *types.Block) error {
	// If we're running a full engine faking, accept any input as valid
	if ethash.fakeFull {
		return nil
	}
	// Verify that there are at most 2 uncles included in this block
	if len(block.Uncles()) > maxUncles {
		return ErrTooManyUncles
	}
	// Gather the set of past uncles and ancestors
	uncles, ancestors := set.New(), make(map[common.Hash]*types.Header)

	number, parent := block.NumberU64()-1, block.ParentHash()
	for i := 0; i < 7; i++ {
		ancestor := chain.GetBlock(parent, number)
		if ancestor == nil {
			break
		}
		ancestors[ancestor.Hash()] = ancestor.Header()
		for _, uncle := range ancestor.Uncles() {
			uncles.Add(uncle.Hash())
		}
		parent, number = ancestor.ParentHash(), number-1
	}
	ancestors[block.Hash()] = block.Header()
	uncles.Add(block.Hash())

	// Verify each of the uncles that it's recent, but not an ancestor
	for _, uncle := range block.Uncles() {
		// Make sure every uncle is rewarded only once
		hash := uncle.Hash()
		if uncles.Has(hash) {
			return ErrDuplicateUncle
		}
		uncles.Add(hash)

		// Make sure the uncle has a valid ancestry
		if ancestors[hash] != nil {
			return ErrUncleIsAncestor
		}
		if ancestors[uncle.ParentHash] == nil || uncle.ParentHash == block.ParentHash() {
			return ErrDanglingUncle
		}
		if err := ethash.verifyHeader(chain, uncle, ancestors[uncle.ParentHash], true, true); err != nil {
			return err
		}
	}
	return nil
}

// verifyHeader checks whether a header conforms to the consensus rules of the
// stock Ethereum ethash engine.
//
// See YP section 4.3.4. "Block Header Validity"
func (ethash *Ethash) verifyHeader(chain consensus.ChainReader, header, parent *types.Header, uncle bool, seal bool) error {
	// Ensure that the header's extra-data section is of a reasonable size
	if uint64(len(header.Extra)) > params.MaximumExtraDataSize {
		return fmt.Errorf("extra-data too long: %d > %d", len(header.Extra), params.MaximumExtraDataSize)
	}
	// Verify the header's timestamp
	if uncle {
		if header.Time.Cmp(math.MaxBig256) > 0 {
			return consensus.ErrLargeBlockTime
		}
	} else {
		if header.Time.Cmp(big.NewInt(time.Now().Unix())) > 0 {
			return consensus.ErrFutureBlock
		}
	}
	if header.Time.Cmp(parent.Time) <= 0 {
		return consensus.ErrZeroBlockTime
	}
	// Verify the block's difficulty based in it's timestamp and parent's difficulty
	expected := CalcDifficulty(chain.Config(), header.Time.Uint64(), parent.Time.Uint64(), parent.Number, parent.Difficulty)
	if expected.Cmp(header.Difficulty) != 0 {
		return fmt.Errorf("invalid difficulty: have %v, want %v", header.Difficulty, expected)
	}
	// Verify that the gas limit remains within allowed bounds
	diff := new(big.Int).Set(parent.GasLimit)
	diff = diff.Sub(diff, header.GasLimit)
	diff.Abs(diff)

	limit := new(big.Int).Set(parent.GasLimit)
	limit = limit.Div(limit, params.GasLimitBoundDivisor)

	if diff.Cmp(limit) >= 0 || header.GasLimit.Cmp(params.MinGasLimit) < 0 {
		return fmt.Errorf("invalid gas limit: have %v, want %v += %v", header.GasLimit, parent.GasLimit, limit)
	}
	// Verify that the block number is parent's +1
	if diff := new(big.Int).Sub(header.Number, parent.Number); diff.Cmp(big.NewInt(1)) != 0 {
		return consensus.ErrInvalidNumber
	}
	// Verify the engine specific seal securing the block
	if seal {
		if err := ethash.VerifySeal(chain, header); err != nil {
			return err
		}
	}
	// If all checks passed, validate any special fields for hard forks
	if err := misc.VerifyDAOHeaderExtraData(chain.Config(), header); err != nil {
		return err
	}
	if err := misc.VerifyForkHashes(chain.Config(), header, uncle); err != nil {
		return err
	}
	return nil
}

// CalcDifficulty is the difficulty adjustment algorithm. It returns the difficulty
// that a new block should have when created at time given the parent block's time
// and difficulty.
//
// TODO (karalabe): Move the chain maker into this package and make this private!
func CalcDifficulty(config *params.ChainConfig, time, parentTime uint64, parentNumber, parentDiff *big.Int) *big.Int {
	if config.IsHomestead(new(big.Int).Add(parentNumber, common.Big1)) {
		return calcDifficultyHomestead(time, parentTime, parentNumber, parentDiff)
	}
	return calcDifficultyFrontier(time, parentTime, parentNumber, parentDiff)
}

// Some weird constants to avoid constant memory allocs for them.
var (
	expDiffPeriod = big.NewInt(100000)
	big10         = big.NewInt(10)
	bigMinus99    = big.NewInt(-99)
)

// calcDifficultyHomestead is the difficulty adjustment algorithm. It returns
// the difficulty that a new block should have when created at time given the
// parent block's time and difficulty. The calculation uses the Homestead rules.
func calcDifficultyHomestead(time, parentTime uint64, parentNumber, parentDiff *big.Int) *big.Int {
	// https://github.com/ethereum/EIPs/blob/master/EIPS/eip-2.mediawiki
	// algorithm:
	// diff = (parent_diff +
	//         (parent_diff / 2048 * max(1 - (block_timestamp - parent_timestamp) // 10, -99))
	//        ) + 2^(periodCount - 2)

	bigTime := new(big.Int).SetUint64(time)
	bigParentTime := new(big.Int).SetUint64(parentTime)

	// holds intermediate values to make the algo easier to read & audit
	x := new(big.Int)
	y := new(big.Int)

	// 1 - (block_timestamp -parent_timestamp) // 10
	x.Sub(bigTime, bigParentTime)
	x.Div(x, big10)
	x.Sub(common.Big1, x)

	// max(1 - (block_timestamp - parent_timestamp) // 10, -99)))
	if x.Cmp(bigMinus99) < 0 {
		x.Set(bigMinus99)
	}
	// (parent_diff + parent_diff // 2048 * max(1 - (block_timestamp - parent_timestamp) // 10, -99))
	y.Div(parentDiff, params.DifficultyBoundDivisor)
	x.Mul(y, x)
	x.Add(parentDiff, x)

	// minimum difficulty can ever be (before exponential factor)
	if x.Cmp(params.MinimumDifficulty) < 0 {
		x.Set(params.MinimumDifficulty)
	}
	// for the exponential factor
	periodCount := new(big.Int).Add(parentNumber, common.Big1)
	periodCount.Div(periodCount, expDiffPeriod)

	// the exponential factor, commonly referred to as "the bomb"
	// diff = diff + 2^(periodCount - 2)
	if periodCount.Cmp(common.Big1) > 0 {
		y.Sub(periodCount, common.Big2)
		y.Exp(common.Big2, y, nil)
		x.Add(x, y)
	}
	return x
}

// calcDifficultyFrontier is the difficulty adjustment algorithm. It returns the
// difficulty that a new block should have when created at time given the parent
// block's time and difficulty. The calculation uses the Frontier rules.
func calcDifficultyFrontier(time, parentTime uint64, parentNumber, parentDiff *big.Int) *big.Int {
	diff := new(big.Int)
	adjust := new(big.Int).Div(parentDiff, params.DifficultyBoundDivisor)
	bigTime := new(big.Int)
	bigParentTime := new(big.Int)

	bigTime.SetUint64(time)
	bigParentTime.SetUint64(parentTime)

	if bigTime.Sub(bigTime, bigParentTime).Cmp(params.DurationLimit) < 0 {
		diff.Add(parentDiff, adjust)
	} else {
		diff.Sub(parentDiff, adjust)
	}
	if diff.Cmp(params.MinimumDifficulty) < 0 {
		diff.Set(params.MinimumDifficulty)
	}

	periodCount := new(big.Int).Add(parentNumber, common.Big1)
	periodCount.Div(periodCount, expDiffPeriod)
	if periodCount.Cmp(common.Big1) > 0 {
		// diff = diff + 2^(periodCount - 2)
		expDiff := periodCount.Sub(periodCount, common.Big2)
		expDiff.Exp(common.Big2, expDiff, nil)
		diff.Add(diff, expDiff)
		diff = math.BigMax(diff, params.MinimumDifficulty)
	}
	return diff
}

// VerifySeal implements consensus.Engine, checking whether the given block satisfies
// the PoW difficulty requirements.
func (ethash *Ethash) VerifySeal(chain consensus.ChainReader, header *types.Header) error {
	// If we're running a fake PoW, accept any seal as valid
	if ethash.fakeMode {
		time.Sleep(ethash.fakeDelay)
		if ethash.fakeFail == header.Number.Uint64() {
			return ErrInvalidPoW
		}
		return nil
	}
	// If we're running a shared PoW, delegate verification to it
	if ethash.shared != nil {
		return ethash.shared.VerifySeal(chain, header)
	}
	// Sanity check that the block number is below the lookup table size (60M blocks)
	number := header.Number.Uint64()
	if number/epochLength >= uint64(len(cacheSizes)) {
		// Go < 1.7 cannot calculate new cache/dataset sizes (no fast prime check)
		return ErrNonceOutOfRange
	}
	// Ensure that we have a valid difficulty for the block
	if header.Difficulty.Sign() <= 0 {
		return ErrInvalidDifficulty
	}
	// Recompute the digest and PoW value and verify against the header
	cache := ethash.cache(number)

	size := datasetSize(number)
	if ethash.tester {
		size = 32 * 1024
	}
	digest, result := hashimotoLight(size, cache, header.HashNoNonce().Bytes(), header.Nonce.Uint64())
	if !bytes.Equal(header.MixDigest[:], digest) {
		return ErrInvalidMixDigest
	}
	target := new(big.Int).Div(maxUint256, header.Difficulty)
	if new(big.Int).SetBytes(result).Cmp(target) > 0 {
		return ErrInvalidPoW
	}
	return nil
}

// Prepare implements consensus.Engine, initializing the difficulty field of a
// header to conform to the ethash protocol. The changes are done inline.
func (ethash *Ethash) Prepare(chain consensus.ChainReader, header *types.Header) error {
	parent := chain.GetHeader(header.ParentHash, header.Number.Uint64()-1)
	if parent == nil {
		return consensus.ErrUnknownAncestor
	}
	header.Difficulty = CalcDifficulty(chain.Config(), header.Time.Uint64(),
		parent.Time.Uint64(), parent.Number, parent.Difficulty)

	return nil
}

// Finalize implements consensus.Engine, accumulating the block and uncle rewards,
// setting the final state and assembling the block.
func (ethash *Ethash) Finalize(chain consensus.ChainReader, header *types.Header, state *state.StateDB, txs []*types.Transaction, uncles []*types.Header, receipts []*types.Receipt) (*types.Block, error) {
	// Accumulate any block and uncle rewards and commit the final state root
	AccumulateRewards(state, header, uncles)
	header.Root = state.IntermediateRoot(chain.Config().IsEIP158(header.Number))

	// Header seems complete, assemble into a block and return
	return types.NewBlock(header, txs, uncles, receipts), nil
}

// Some weird constants to avoid constant memory allocs for them.
var (
	big8  = big.NewInt(8)
	big32 = big.NewInt(32)
)

// AccumulateRewards credits the coinbase of the given block with the mining
// reward. The total reward consists of the static block reward and rewards for
// included uncles. The coinbase of each uncle block is also rewarded.
//
// TODO (karalabe): Move the chain maker into this package and make this private!
func AccumulateRewards(state *state.StateDB, header *types.Header, uncles []*types.Header) {
	reward := new(big.Int).Set(blockReward)
	r := new(big.Int)
	for _, uncle := range uncles {
		r.Add(uncle.Number, big8)
		r.Sub(r, header.Number)
		r.Mul(r, blockReward)
		r.Div(r, big8)
		state.AddBalance(uncle.Coinbase, r)

		r.Div(blockReward, big32)
		reward.Add(reward, r)
	}
	state.AddBalance(header.Coinbase, reward)
}