[NOD-1464] difficulty refactoring (#986)

* Refactor the Difficulty adjastment to the new design

* Add the necessary things to the factory for the DAA constructor

* Add missing dagParams to difficultymanager constructor

* Use DAGTraversal for blueBlockWindow, and don't store PowMax compactBits

domain/consensus/factory.go

@@ -90,7 +90,15 @@ func (f *factory) NewConsensus(dagParams *dagconfig.Params, db infrastructuredat
 		pastMedianTimeManager,
 		ghostdagDataStore)
 	difficultyManager := difficultymanager.New(
-		ghostdagManager)
+		dbManager,
+		ghostdagManager,
+		ghostdagDataStore,
+		blockHeaderStore,
+		dagTopologyManager,
+		dagTraversalManager,
+		dagParams.PowMax,
+		dagParams.DifficultyAdjustmentWindowSize,
+		dagParams.TargetTimePerBlock)
 	coinbaseManager := coinbasemanager.New(
 		dbManager,
 		ghostdagDataStore,

domain/consensus/processes/difficultymanager/blockwindow.go

@@ -0,0 +1,91 @@
+package difficultymanager
+
+import (
+	"github.com/kaspanet/kaspad/domain/consensus/model/externalapi"
+	"github.com/kaspanet/kaspad/util"
+	"github.com/kaspanet/kaspad/util/bigintpool"
+	"github.com/pkg/errors"
+	"math"
+	"math/big"
+	"sort"
+)
+
+type difficultyBlock struct {
+	timeInMilliseconds int64
+	Bits               uint32
+}
+
+type blockWindow []difficultyBlock
+
+func (dm *difficultyManager) getDifficultyBlock(blockHash *externalapi.DomainHash) (difficultyBlock, error) {
+	header, err := dm.headerStore.BlockHeader(dm.databaseContext, blockHash)
+	if err != nil {
+		return difficultyBlock{}, err
+	}
+	return difficultyBlock{
+		timeInMilliseconds: header.TimeInMilliseconds,
+		Bits:               header.Bits,
+	}, nil
+}
+
+// blueBlockWindow returns a blockWindow of the given size that contains the
+// blues in the past of startindNode, sorted by GHOSTDAG order.
+// If the number of blues in the past of startingNode is less then windowSize,
+// the window will be padded by genesis blocks to achieve a size of windowSize.
+func (dm *difficultyManager) blueBlockWindow(startingNode *externalapi.DomainHash, windowSize uint64) (blockWindow, error) {
+	window := make(blockWindow, 0, windowSize)
+	windowHashes, err := dm.dagTraversalManager.BlueWindow(startingNode, windowSize)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, hash := range windowHashes {
+		block, err := dm.getDifficultyBlock(hash)
+		if err != nil {
+			return nil, err
+		}
+		window = append(window, block)
+	}
+	return window, nil
+}
+
+func (window blockWindow) minMaxTimestamps() (min, max int64) {
+	min = math.MaxInt64
+	max = 0
+	for _, block := range window {
+		if block.timeInMilliseconds < min {
+			min = block.timeInMilliseconds
+		}
+		if block.timeInMilliseconds > max {
+			max = block.timeInMilliseconds
+		}
+	}
+	return
+}
+
+func (window blockWindow) averageTarget(averageTarget *big.Int) {
+	averageTarget.SetInt64(0)
+
+	target := bigintpool.Acquire(0)
+	defer bigintpool.Release(target)
+	for _, block := range window {
+		util.CompactToBigWithDestination(block.Bits, target)
+		averageTarget.Add(averageTarget, target)
+	}
+
+	windowLen := bigintpool.Acquire(int64(len(window)))
+	defer bigintpool.Release(windowLen)
+	averageTarget.Div(averageTarget, windowLen)
+}
+
+func (window blockWindow) medianTimestamp() (int64, error) {
+	if len(window) == 0 {
+		return 0, errors.New("Cannot calculate median timestamp for an empty block window")
+	}
+	timestamps := make([]int64, len(window))
+	for i, node := range window {
+		timestamps[i] = node.timeInMilliseconds
+	}
+	sort.Sort(timeSorter(timestamps))
+	return timestamps[len(timestamps)/2], nil
+}

domain/consensus/processes/difficultymanager/difficultymanager.go

@@ -3,22 +3,120 @@ package difficultymanager
 import (
 	"github.com/kaspanet/kaspad/domain/consensus/model"
 	"github.com/kaspanet/kaspad/domain/consensus/model/externalapi"
+	"github.com/kaspanet/kaspad/util"
+	"github.com/kaspanet/kaspad/util/bigintpool"
+	"math/big"
+	"time"
 )
 
 // DifficultyManager provides a method to resolve the
 // difficulty value of a block
 type difficultyManager struct {
-	ghostdagManager model.GHOSTDAGManager
+	databaseContext                model.DBReader
+	ghostdagManager                model.GHOSTDAGManager
+	ghostdagStore                  model.GHOSTDAGDataStore
+	headerStore                    model.BlockHeaderStore
+	dagTopologyManager             model.DAGTopologyManager
+	dagTraversalManager            model.DAGTraversalManager
+	powMax                         *big.Int
+	difficultyAdjustmentWindowSize uint64
+	targetTimePerBlock             time.Duration
 }
 
 // New instantiates a new DifficultyManager
-func New(ghostdagManager model.GHOSTDAGManager) model.DifficultyManager {
+func New(
+	databaseContext model.DBReader,
+	ghostdagManager model.GHOSTDAGManager,
+	ghostdagStore model.GHOSTDAGDataStore,
+	headerStore model.BlockHeaderStore,
+	dagTopologyManager model.DAGTopologyManager,
+	dagTraversalManager model.DAGTraversalManager,
+	powMax *big.Int,
+	difficultyAdjustmentWindowSize uint64,
+	targetTimePerBlock time.Duration,
+) model.DifficultyManager {
 	return &difficultyManager{
-		ghostdagManager: ghostdagManager,
+		databaseContext:                databaseContext,
+		ghostdagManager:                ghostdagManager,
+		ghostdagStore:                  ghostdagStore,
+		headerStore:                    headerStore,
+		dagTopologyManager:             dagTopologyManager,
+		dagTraversalManager:            dagTraversalManager,
+		powMax:                         powMax,
+		difficultyAdjustmentWindowSize: difficultyAdjustmentWindowSize,
+		targetTimePerBlock:             targetTimePerBlock,
 	}
 }
 
 // RequiredDifficulty returns the difficulty required for some block
 func (dm *difficultyManager) RequiredDifficulty(blockHash *externalapi.DomainHash) (uint32, error) {
-	return 0, nil
+
+	parents, err := dm.dagTopologyManager.Parents(blockHash)
+	if err != nil {
+		return 0, err
+	}
+	// Genesis block
+	if len(parents) == 0 {
+		return util.BigToCompact(dm.powMax), nil
+	}
+
+	// find bluestParent
+	bluestParent := parents[0]
+	bluestGhostDAG, err := dm.ghostdagStore.Get(dm.databaseContext, bluestParent)
+	if err != nil {
+		return 0, err
+	}
+	for i := 1; i < len(parents); i++ {
+		parentGhostDAG, err := dm.ghostdagStore.Get(dm.databaseContext, parents[i])
+		if err != nil {
+			return 0, err
+		}
+		newBluest, err := dm.ghostdagManager.ChooseSelectedParent(bluestParent, parents[i])
+		if err != nil {
+			return 0, err
+		}
+		if bluestParent != newBluest {
+			bluestParent = newBluest
+			bluestGhostDAG = parentGhostDAG
+		}
+	}
+
+	// Not enough blocks for building a difficulty window.
+	if bluestGhostDAG.BlueScore < dm.difficultyAdjustmentWindowSize+1 {
+		return util.BigToCompact(dm.powMax), nil
+	}
+
+	// Fetch window of dag.difficultyAdjustmentWindowSize + 1 so we can have dag.difficultyAdjustmentWindowSize block intervals
+	timestampsWindow, err := dm.blueBlockWindow(bluestParent, dm.difficultyAdjustmentWindowSize+1)
+	if err != nil {
+		return 0, err
+	}
+	windowMinTimestamp, windowMaxTimeStamp := timestampsWindow.minMaxTimestamps()
+
+	// Remove the last block from the window so to calculate the average target of dag.difficultyAdjustmentWindowSize blocks
+	targetsWindow := timestampsWindow[:dm.difficultyAdjustmentWindowSize]
+
+	// Calculate new target difficulty as:
+	// averageWindowTarget * (windowMinTimestamp / (targetTimePerBlock * windowSize))
+	// The result uses integer division which means it will be slightly
+	// rounded down.
+	newTarget := bigintpool.Acquire(0)
+	defer bigintpool.Release(newTarget)
+	windowTimeStampDifference := bigintpool.Acquire(windowMaxTimeStamp - windowMinTimestamp)
+	defer bigintpool.Release(windowTimeStampDifference)
+	targetTimePerBlock := bigintpool.Acquire(dm.targetTimePerBlock.Milliseconds())
+	defer bigintpool.Release(targetTimePerBlock)
+	difficultyAdjustmentWindowSize := bigintpool.Acquire(int64(dm.difficultyAdjustmentWindowSize))
+	defer bigintpool.Release(difficultyAdjustmentWindowSize)
+
+	targetsWindow.averageTarget(newTarget)
+	newTarget.
+		Mul(newTarget, windowTimeStampDifference).
+		Div(newTarget, targetTimePerBlock).
+		Div(newTarget, difficultyAdjustmentWindowSize)
+	if newTarget.Cmp(dm.powMax) > 0 {
+		return util.BigToCompact(dm.powMax), nil
+	}
+	newTargetBits := util.BigToCompact(newTarget)
+	return newTargetBits, nil
 }

domain/consensus/processes/difficultymanager/timesorter.go

@@ -0,0 +1,27 @@
+// Copyright (c) 2013-2017 The btcsuite developers
+// Use of this source code is governed by an ISC
+// license that can be found in the LICENSE file.
+
+package difficultymanager
+
+// timeSorter implements sort.Interface to allow a slice of timestamps to
+// be sorted.
+type timeSorter []int64
+
+// Len returns the number of timestamps in the slice. It is part of the
+// sort.Interface implementation.
+func (s timeSorter) Len() int {
+	return len(s)
+}
+
+// Swap swaps the timestamps at the passed indices. It is part of the
+// sort.Interface implementation.
+func (s timeSorter) Swap(i, j int) {
+	s[i], s[j] = s[j], s[i]
+}
+
+// Less returns whether the timstamp with index i should sort before the
+// timestamp with index j. It is part of the sort.Interface implementation.
+func (s timeSorter) Less(i, j int) bool {
+	return s[i] < s[j]
+}