aboutsummaryrefslogtreecommitdiffstats
path: root/consensus/ethash/consensus.go
blob: 915e1f7f774be2127475efac35be38d7aa6a358c (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
// 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"
    "time"

    mapset "github.com/deckarep/golang-set"
    "github.com/dexon-foundation/dexon/common"
    "github.com/dexon-foundation/dexon/common/math"
    "github.com/dexon-foundation/dexon/consensus"
    "github.com/dexon-foundation/dexon/consensus/misc"
    "github.com/dexon-foundation/dexon/core/state"
    "github.com/dexon-foundation/dexon/core/types"
    "github.com/dexon-foundation/dexon/params"
    "github.com/dexon-foundation/dexon/rlp"
    "golang.org/x/crypto/sha3"
)

// Ethash proof-of-work protocol constants.
var (
    FrontierBlockReward       = big.NewInt(5e+18) // Block reward in wei for successfully mining a block
    ByzantiumBlockReward      = big.NewInt(3e+18) // Block reward in wei for successfully mining a block upward from Byzantium
    ConstantinopleBlockReward = big.NewInt(2e+18) // Block reward in wei for successfully mining a block upward from Constantinople
    maxUncles                 = 2                 // Maximum number of uncles allowed in a single block
    allowedFutureBlockTime    = 15 * time.Second  // Max time from current time allowed for blocks, before they're considered future blocks

    // calcDifficultyConstantinople is the difficulty adjustment algorithm for Constantinople.
    // 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 Byzantium rules, but with
    // bomb offset 5M.
    // Specification EIP-1234: https://eips.ethereum.org/EIPS/eip-1234
    calcDifficultyConstantinople = makeDifficultyCalculator(big.NewInt(5000000))

    // calcDifficultyByzantium 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 Byzantium rules.
    // Specification EIP-649: https://eips.ethereum.org/EIPS/eip-649
    calcDifficultyByzantium = makeDifficultyCalculator(big.NewInt(3000000))
)

// Various error messages to mark blocks invalid. These should be private to
// prevent engine specific errors from being referenced in the remainder of the
// codebase, inherently breaking if the engine is swapped out. Please put common
// error types into the consensus package.
var (
    errZeroBlockTime     = errors.New("timestamp equals parent's")
    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")
    errInvalidDifficulty = errors.New("non-positive difficulty")
    errInvalidMixDigest  = errors.New("invalid mix digest")
    errInvalidPoW        = errors.New("invalid proof-of-work")
)

// Author implements consensus.Engine, returning the header's coinbase as the
// proof-of-work verified author of the block.
func (ethash *Ethash) Author(header *types.Header) (common.Address, error) {
    return header.Coinbase, nil
}

// 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.config.PowMode == ModeFullFake {
        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.config.PowMode == ModeFullFake || len(headers) == 0 {
        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
    var (
        inputs = make(chan int)
        done   = make(chan int, workers)
        errors = make([]error, len(headers))
        abort  = make(chan struct{})
    )
    for i := 0; i < workers; i++ {
        go func() {
            for index := range inputs {
                errors[index] = ethash.verifyHeaderWorker(chain, headers, seals, index)
                done <- index
            }
        }()
    }

    errorsOut := make(chan error, len(headers))
    go func() {
        defer close(inputs)
        var (
            in, out = 0, 0
            checked = make([]bool, len(headers))
            inputs  = inputs
        )
        for {
            select {
            case inputs <- in:
                if in++; in == len(headers) {
                    // Reached end of headers. Stop sending to workers.
                    inputs = nil
                }
            case index := <-done:
                for checked[index] = true; checked[out]; out++ {
                    errorsOut <- errors[out]
                    if out == len(headers)-1 {
                        return
                    }
                }
            case <-abort:
                return
            }
        }
    }()
    return abort, errorsOut
}

func (ethash *Ethash) verifyHeaderWorker(chain consensus.ChainReader, headers []*types.Header, seals []bool, index int) error {
    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]
    }
    if parent == nil {
        return consensus.ErrUnknownAncestor
    }
    if chain.GetHeader(headers[index].Hash(), headers[index].Number.Uint64()) != nil {
        return nil // known block
    }
    return ethash.verifyHeader(chain, headers[index], parent, false, seals[index])
}

// 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.config.PowMode == ModeFullFake {
        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 := mapset.NewSet(), 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.Contains(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 > uint64(time.Now().Add(allowedFutureBlockTime).Unix()) {
            return consensus.ErrFutureBlock
        }
    }
    if header.Time <= parent.Time {
        return errZeroBlockTime
    }
    // Verify the block's difficulty based in it's timestamp and parent's difficulty
    expected := ethash.CalcDifficulty(chain, header.Time, parent)

    if expected.Cmp(header.Difficulty) != 0 {
        return fmt.Errorf("invalid difficulty: have %v, want %v", header.Difficulty, expected)
    }
    // Verify that the gas limit is <= 2^63-1
    cap := uint64(0x7fffffffffffffff)
    if header.GasLimit > cap {
        return fmt.Errorf("invalid gasLimit: have %v, max %v", header.GasLimit, cap)
    }
    // Verify that the gasUsed is <= gasLimit
    if header.GasUsed > header.GasLimit {
        return fmt.Errorf("invalid gasUsed: have %d, gasLimit %d", header.GasUsed, header.GasLimit)
    }

    // Verify that the gas limit remains within allowed bounds
    diff := int64(parent.GasLimit) - int64(header.GasLimit)
    if diff < 0 {
        diff *= -1
    }
    limit := parent.GasLimit / params.GasLimitBoundDivisor

    if uint64(diff) >= limit || header.GasLimit < params.MinGasLimit {
        return fmt.Errorf("invalid gas limit: have %d, want %d += %d", 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.
func (ethash *Ethash) CalcDifficulty(chain consensus.ChainReader, time uint64, parent *types.Header) *big.Int {
    return CalcDifficulty(chain.Config(), time, parent)
}

// 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.
func CalcDifficulty(config *params.ChainConfig, time uint64, parent *types.Header) *big.Int {
    next := new(big.Int).Add(parent.Number, big1)
    switch {
    case config.IsConstantinople(next):
        return calcDifficultyConstantinople(time, parent)
    case config.IsByzantium(next):
        return calcDifficultyByzantium(time, parent)
    case config.IsHomestead(next):
        return calcDifficultyHomestead(time, parent)
    default:
        return calcDifficultyFrontier(time, parent)
    }
}

// Some weird constants to avoid constant memory allocs for them.
var (
    expDiffPeriod = big.NewInt(100000)
    big1          = big.NewInt(1)
    big2          = big.NewInt(2)
    big9          = big.NewInt(9)
    big10         = big.NewInt(10)
    bigMinus99    = big.NewInt(-99)
)

// makeDifficultyCalculator creates a difficultyCalculator with the given bomb-delay.
// the difficulty is calculated with Byzantium rules, which differs from Homestead in
// how uncles affect the calculation
func makeDifficultyCalculator(bombDelay *big.Int) func(time uint64, parent *types.Header) *big.Int {
    // Note, the calculations below looks at the parent number, which is 1 below
    // the block number. Thus we remove one from the delay given
    bombDelayFromParent := new(big.Int).Sub(bombDelay, big1)
    return func(time uint64, parent *types.Header) *big.Int {
        // https://github.com/ethereum/EIPs/issues/100.
        // algorithm:
        // diff = (parent_diff +
        //         (parent_diff / 2048 * max((2 if len(parent.uncles) else 1) - ((timestamp - parent.timestamp) // 9), -99))
        //        ) + 2^(periodCount - 2)

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

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

        // (2 if len(parent_uncles) else 1) - (block_timestamp - parent_timestamp) // 9
        x.Sub(bigTime, bigParentTime)
        x.Div(x, big9)
        if parent.UncleHash == types.EmptyUncleHash {
            x.Sub(big1, x)
        } else {
            x.Sub(big2, x)
        }
        // max((2 if len(parent_uncles) else 1) - (block_timestamp - parent_timestamp) // 9, -99)
        if x.Cmp(bigMinus99) < 0 {
            x.Set(bigMinus99)
        }
        // parent_diff + (parent_diff / 2048 * max((2 if len(parent.uncles) else 1) - ((timestamp - parent.timestamp) // 9), -99))
        y.Div(parent.Difficulty, params.DifficultyBoundDivisor)
        x.Mul(y, x)
        x.Add(parent.Difficulty, x)

        // minimum difficulty can ever be (before exponential factor)
        if x.Cmp(params.MinimumDifficulty) < 0 {
            x.Set(params.MinimumDifficulty)
        }
        // calculate a fake block number for the ice-age delay
        // Specification: https://eips.ethereum.org/EIPS/eip-1234
        fakeBlockNumber := new(big.Int)
        if parent.Number.Cmp(bombDelayFromParent) >= 0 {
            fakeBlockNumber = fakeBlockNumber.Sub(parent.Number, bombDelayFromParent)
        }
        // for the exponential factor
        periodCount := fakeBlockNumber
        periodCount.Div(periodCount, expDiffPeriod)

        // the exponential factor, commonly referred to as "the bomb"
        // diff = diff + 2^(periodCount - 2)
        if periodCount.Cmp(big1) > 0 {
            y.Sub(periodCount, big2)
            y.Exp(big2, y, nil)
            x.Add(x, y)
        }
        return x
    }
}

// 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 uint64, parent *types.Header) *big.Int {
    // https://github.com/ethereum/EIPs/blob/master/EIPS/eip-2.md
    // 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(parent.Time)

    // 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(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(parent.Difficulty, params.DifficultyBoundDivisor)
    x.Mul(y, x)
    x.Add(parent.Difficulty, 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(parent.Number, big1)
    periodCount.Div(periodCount, expDiffPeriod)

    // the exponential factor, commonly referred to as "the bomb"
    // diff = diff + 2^(periodCount - 2)
    if periodCount.Cmp(big1) > 0 {
        y.Sub(periodCount, big2)
        y.Exp(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 uint64, parent *types.Header) *big.Int {
    diff := new(big.Int)
    adjust := new(big.Int).Div(parent.Difficulty, params.DifficultyBoundDivisor)
    bigTime := new(big.Int)
    bigParentTime := new(big.Int)

    bigTime.SetUint64(time)
    bigParentTime.SetUint64(parent.Time)

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

    periodCount := new(big.Int).Add(parent.Number, big1)
    periodCount.Div(periodCount, expDiffPeriod)
    if periodCount.Cmp(big1) > 0 {
        // diff = diff + 2^(periodCount - 2)
        expDiff := periodCount.Sub(periodCount, big2)
        expDiff.Exp(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 {
    return ethash.verifySeal(chain, header, false)
}

// verifySeal checks whether a block satisfies the PoW difficulty requirements,
// either using the usual ethash cache for it, or alternatively using a full DAG
// to make remote mining fast.
func (ethash *Ethash) verifySeal(chain consensus.ChainReader, header *types.Header, fulldag bool) error {
    // If we're running a fake PoW, accept any seal as valid
    if ethash.config.PowMode == ModeFake || ethash.config.PowMode == ModeFullFake {
        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, fulldag)
    }
    // Ensure that we have a valid difficulty for the block
    if header.Difficulty.Sign() <= 0 {
        return errInvalidDifficulty
    }
    // Recompute the digest and PoW values
    number := header.Number.Uint64()

    var (
        digest []byte
        result []byte
    )
    // If fast-but-heavy PoW verification was requested, use an ethash dataset
    if fulldag {
        dataset := ethash.dataset(number, true)
        if dataset.generated() {
            digest, result = hashimotoFull(dataset.dataset, ethash.SealHash(header).Bytes(), header.Nonce.Uint64())

            // Datasets are unmapped in a finalizer. Ensure that the dataset stays alive
            // until after the call to hashimotoFull so it's not unmapped while being used.
            runtime.KeepAlive(dataset)
        } else {
            // Dataset not yet generated, don't hang, use a cache instead
            fulldag = false
        }
    }
    // If slow-but-light PoW verification was requested (or DAG not yet ready), use an ethash cache
    if !fulldag {
        cache := ethash.cache(number)

        size := datasetSize(number)
        if ethash.config.PowMode == ModeTest {
            size = 32 * 1024
        }
        digest, result = hashimotoLight(size, cache.cache, ethash.SealHash(header).Bytes(), header.Nonce.Uint64())

        // Caches are unmapped in a finalizer. Ensure that the cache stays alive
        // until after the call to hashimotoLight so it's not unmapped while being used.
        runtime.KeepAlive(cache)
    }
    // Verify the calculated values against the ones provided in the header
    if !bytes.Equal(header.MixDigest[:], digest) {
        return errInvalidMixDigest
    }
    target := new(big.Int).Div(two256, 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 = ethash.CalcDifficulty(chain, header.Time, parent)
    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(chain.Config(), 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
}

// SealHash returns the hash of a block prior to it being sealed.
func (ethash *Ethash) SealHash(header *types.Header) (hash common.Hash) {
    hasher := sha3.NewLegacyKeccak256()

    rlp.Encode(hasher, []interface{}{
        header.ParentHash,
        header.UncleHash,
        header.Coinbase,
        header.Root,
        header.TxHash,
        header.ReceiptHash,
        header.Bloom,
        header.Difficulty,
        header.Number,
        header.GasLimit,
        header.GasUsed,
        header.Time,
        header.Extra,
    })
    hasher.Sum(hash[:0])
    return hash
}

// 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.
func accumulateRewards(config *params.ChainConfig, state *state.StateDB, header *types.Header, uncles []*types.Header) {
    // Select the correct block reward based on chain progression
    blockReward := FrontierBlockReward
    if config.IsByzantium(header.Number) {
        blockReward = ByzantiumBlockReward
    }
    if config.IsConstantinople(header.Number) {
        blockReward = ConstantinopleBlockReward
    }
    // Accumulate the rewards for the miner and any included uncles
    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)
}