core: blocks generation supports rounds (#196)

* Block proposing based on timestamp, instead of
   count of blocks generated.
* Add method to find tips of each round in blockdb.
* Block proposing based on tips of last round found
   on blockdb.

3 files changed, 403 insertions, 184 deletions

diff --git a/core/test/blocks-generator.go b/core/test/blocks-generator.go
index 904ee7a..10ecc38 100644
--- a/core/test/blocks-generator.go
+++ b/core/test/blocks-generator.go
@@ -30,9 +30,6 @@ import (
 	"github.com/dexon-foundation/dexon-consensus-core/core/types"
 )
 
-// TODO(mission): blocks generator should generate blocks based on chain,
-//                not nodes.
-
 // ErrParentNotAcked would be raised when some block doesn't
 // ack its parent block.
 var ErrParentNotAcked = errors.New("parent is not acked")
@@ -40,110 +37,137 @@ var ErrParentNotAcked = errors.New("parent is not acked")
 // nodeStatus is a state holder for each node
 // during generating blocks.
 type nodeStatus struct {
-	blocks           []*types.Block
-	lastAckingHeight map[types.NodeID]uint64
+	blocks          []*types.Block
+	genesisTime     time.Time
+	prvKey          crypto.PrivateKey
+	tip             *types.Block
+	nextAckingIndex map[types.NodeID]uint64
 }
 
 type hashBlockFn func(*types.Block) (common.Hash, error)
 
 // getAckedBlockHash would randomly pick one block between
 // last acked one to current head.
-func (vs *nodeStatus) getAckedBlockHash(
+func (ns *nodeStatus) getAckedBlockHash(
 	ackedNID types.NodeID,
 	ackedNode *nodeStatus,
 	randGen *rand.Rand) (
 	hash common.Hash, ok bool) {
-
-	baseAckingHeight, exists := vs.lastAckingHeight[ackedNID]
-	if exists {
-		// Do not ack the same block(height) twice.
-		baseAckingHeight++
-	}
+	baseAckingIndex := ns.nextAckingIndex[ackedNID]
 	totalBlockCount := uint64(len(ackedNode.blocks))
-	if totalBlockCount <= baseAckingHeight {
+	if totalBlockCount <= baseAckingIndex {
 		// There is no new block to ack.
 		return
 	}
-	ackableRange := totalBlockCount - baseAckingHeight
-	height := uint64((randGen.Uint64() % ackableRange) + baseAckingHeight)
-	vs.lastAckingHeight[ackedNID] = height
-	hash = ackedNode.blocks[height].Hash
+	ackableRange := totalBlockCount - baseAckingIndex
+	idx := uint64((randGen.Uint64() % ackableRange) + baseAckingIndex)
+	ns.nextAckingIndex[ackedNID] = idx + 1
+	hash = ackedNode.blocks[idx].Hash
 	ok = true
 	return
 }
 
+func (ns *nodeStatus) getNextBlockTime(
+	timePicker func(time.Time) time.Time) time.Time {
+	if ns.tip == nil {
+		return timePicker(ns.genesisTime)
+	}
+	return timePicker(ns.tip.Timestamp)
+}
+
 // nodeSetStatus is a state holder for all nodes
 // during generating blocks.
 type nodeSetStatus struct {
+	round         uint64
 	status        map[types.NodeID]*nodeStatus
 	proposerChain map[types.NodeID]uint32
-	timestamps    []time.Time
-	nodeIDs       []types.NodeID
+	endTime       time.Time
+	nIDs          []types.NodeID
 	randGen       *rand.Rand
+	timePicker    func(time.Time) time.Time
 	hashBlock     hashBlockFn
 }
 
 func newNodeSetStatus(
-	nIDs []types.NodeID, hashBlock hashBlockFn) *nodeSetStatus {
-
-	status := make(map[types.NodeID]*nodeStatus)
-	timestamps := make([]time.Time, 0, len(nIDs))
-	proposerChain := make(map[types.NodeID]uint32)
-	for i, nID := range nIDs {
+	numChains uint32,
+	tips map[uint32]*types.Block,
+	round uint64,
+	genesisTime, endTime time.Time,
+	timePicker func(time.Time) time.Time,
+	hashBlock hashBlockFn) *nodeSetStatus {
+	var (
+		status        = make(map[types.NodeID]*nodeStatus)
+		proposerChain = make(map[types.NodeID]uint32)
+		nIDs          = []types.NodeID{}
+	)
+	for i := uint32(0); i < numChains; i++ {
+		prvKey, err := ecdsa.NewPrivateKey()
+		if err != nil {
+			panic(err)
+		}
+		nID := types.NewNodeID(prvKey.PublicKey())
+		nIDs = append(nIDs, nID)
 		status[nID] = &nodeStatus{
-			blocks:           []*types.Block{},
-			lastAckingHeight: make(map[types.NodeID]uint64),
+			blocks:          []*types.Block{},
+			genesisTime:     genesisTime,
+			prvKey:          prvKey,
+			tip:             tips[i],
+			nextAckingIndex: make(map[types.NodeID]uint64),
 		}
-		timestamps = append(timestamps, time.Now().UTC())
-		proposerChain[nID] = uint32(i)
+		proposerChain[nID] = i
 	}
 	return &nodeSetStatus{
+		round:         round,
 		status:        status,
 		proposerChain: proposerChain,
-		timestamps:    timestamps,
-		nodeIDs:       nIDs,
+		endTime:       endTime,
+		nIDs:          nIDs,
 		randGen:       rand.New(rand.NewSource(time.Now().UnixNano())),
+		timePicker:    timePicker,
 		hashBlock:     hashBlock,
 	}
 }
 
-// findIncompleteNodes is a helper to check which node
-// doesn't generate enough blocks.
-func (vs *nodeSetStatus) findIncompleteNodes(
-	blockNum int) (nIDs []types.NodeID) {
-
-	for nID, status := range vs.status {
-		if len(status.blocks) < blockNum {
+// findIncompleteNodes is a helper to check which node doesn't generate
+// enough blocks.
+func (ns *nodeSetStatus) findIncompleteNodes() (nIDs []types.NodeID) {
+	for nID, status := range ns.status {
+		if status.tip == nil {
 			nIDs = append(nIDs, nID)
+			continue
+		}
+		if status.tip.Timestamp.After(ns.endTime) {
+			continue
 		}
+		nIDs = append(nIDs, nID)
 	}
 	return
 }
 
 // prepareAcksForNewBlock collects acks for one block.
-func (vs *nodeSetStatus) prepareAcksForNewBlock(
+func (ns *nodeSetStatus) prepareAcksForNewBlock(
 	proposerID types.NodeID, ackingCount int) (
 	acks common.Hashes, err error) {
-
 	acks = common.Hashes{}
-	if len(vs.status[proposerID].blocks) == 0 {
+	if len(ns.status[proposerID].blocks) == 0 {
 		// The 'Acks' filed of genesis blocks would always be empty.
 		return
 	}
 	// Pick nodeIDs to be acked.
-	ackingNIDs := map[types.NodeID]struct{}{
-		proposerID: struct{}{}, // Acking parent block is always required.
-	}
+	ackingNIDs := map[types.NodeID]struct{}{}
 	if ackingCount > 0 {
 		ackingCount-- // We would always include ack to parent block.
 	}
-	for _, i := range vs.randGen.Perm(len(vs.nodeIDs))[:ackingCount] {
-		ackingNIDs[vs.nodeIDs[i]] = struct{}{}
+	for _, i := range ns.randGen.Perm(len(ns.nIDs))[:ackingCount] {
+		ackingNIDs[ns.nIDs[i]] = struct{}{}
 	}
 	// Generate acks.
 	for nID := range ackingNIDs {
-		ack, ok := vs.status[proposerID].getAckedBlockHash(
-			nID, vs.status[nID], vs.randGen)
+		if nID == proposerID {
+			continue
+		}
+		ack, ok := ns.status[proposerID].getAckedBlockHash(
+			nID, ns.status[nID], ns.randGen)
 		if !ok {
 			if nID == proposerID {
 				err = ErrParentNotAcked
@@ -156,44 +180,39 @@ func (vs *nodeSetStatus) prepareAcksForNewBlock(
 }
 
 // proposeBlock propose new block and update node status.
-func (vs *nodeSetStatus) proposeBlock(
-	proposerID types.NodeID,
-	prvKey crypto.PrivateKey,
-	acks common.Hashes) (*types.Block, error) {
-
-	status := vs.status[proposerID]
+func (ns *nodeSetStatus) proposeBlock(
+	proposerID types.NodeID, acks common.Hashes) (*types.Block, error) {
+	status := ns.status[proposerID]
 	parentHash := common.Hash{}
-	if len(status.blocks) > 0 {
-		parentHash = status.blocks[len(status.blocks)-1].Hash
+	blockHeight := uint64(0)
+	if status.tip != nil {
+		parentHash = status.tip.Hash
+		blockHeight = status.tip.Position.Height + 1
+		acks = append(acks, parentHash)
 	}
-	chainID := vs.proposerChain[proposerID]
-	vs.timestamps[chainID] = vs.timestamps[chainID].Add(time.Second)
-
+	chainID := ns.proposerChain[proposerID]
 	newBlock := &types.Block{
 		ProposerID: proposerID,
 		ParentHash: parentHash,
 		Position: types.Position{
-			Height:  uint64(len(status.blocks)),
+			Round:   ns.round,
+			Height:  blockHeight,
 			ChainID: chainID,
 		},
 		Acks:      common.NewSortedHashes(acks),
-		Timestamp: vs.timestamps[chainID],
-	}
-	for i, nID := range vs.nodeIDs {
-		if nID == proposerID {
-			newBlock.Position.ChainID = uint32(i)
-		}
+		Timestamp: status.getNextBlockTime(ns.timePicker),
 	}
 	var err error
-	newBlock.Hash, err = vs.hashBlock(newBlock)
+	newBlock.Hash, err = ns.hashBlock(newBlock)
 	if err != nil {
 		return nil, err
 	}
-	newBlock.Signature, err = prvKey.Sign(newBlock.Hash)
+	newBlock.Signature, err = status.prvKey.Sign(newBlock.Hash)
 	if err != nil {
 		return nil, err
 	}
 	status.blocks = append(status.blocks, newBlock)
+	status.tip = newBlock
 	return newBlock, nil
 }
 
@@ -201,7 +220,6 @@ func (vs *nodeSetStatus) proposeBlock(
 // by a normal distribution.
 func normalAckingCountGenerator(
 	chainNum uint32, mean, deviation float64) func() int {
-
 	return func() int {
 		var expected float64
 		for {
@@ -229,99 +247,139 @@ func generateNodePicker() func([]types.NodeID) types.NodeID {
 	}
 }
 
-// BlocksGenerator could generate blocks forming valid DAGs.
-type BlocksGenerator struct {
-	nodePicker func([]types.NodeID) types.NodeID
-	hashBlock  hashBlockFn
+// defaultTimePicker would pick a time based on reference time and
+// the given minimum/maximum time range.
+func generateTimePicker(min, max time.Duration) (f func(time.Time) time.Time) {
+	privateRand := rand.New(rand.NewSource(time.Now().UnixNano()))
+	return func(ref time.Time) time.Time {
+		return ref.Add(min + time.Duration(privateRand.Int63n(int64(max-min))))
+	}
 }
 
-// NewBlocksGenerator constructs BlockGenerator.
-func NewBlocksGenerator(nodePicker func(
-	[]types.NodeID) types.NodeID,
-	hashBlock hashBlockFn) *BlocksGenerator {
+// BlocksGeneratorConfig is the configuration for BlocksGenerator.
+type BlocksGeneratorConfig struct {
+	NumChains            uint32
+	MinBlockTimeInterval time.Duration
+	MaxBlockTimeInterval time.Duration
+}
 
-	if nodePicker == nil {
-		nodePicker = generateNodePicker()
-	}
-	return &BlocksGenerator{
-		nodePicker: nodePicker,
-		hashBlock:  hashBlock,
-	}
+// BlocksGenerator could generate blocks forming valid DAGs.
+type BlocksGenerator struct {
+	config               *BlocksGeneratorConfig
+	nodePicker           func([]types.NodeID) types.NodeID
+	timePicker           func(time.Time) time.Time
+	ackingCountGenerator func() int
+	hashBlock            hashBlockFn
 }
 
-// Generate is the entry point to generate blocks. The caller is responsible
-// to provide a function to generate count of acked block for each new block.
-// The prototype of ackingCountGenerator is a function returning 'int'.
-// For example, if you need to generate a group of blocks and each of them
-// has maximum 2 acks.
+// NewBlocksGenerator constructs BlockGenerator.
+//
+// The caller is responsible to provide a function to generate count of
+// acked block for each new block. The prototype of ackingCountGenerator is
+// a function returning 'int'. For example, if you need to generate a group of
+// blocks and each of them has maximum 2 acks.
 //   func () int { return 2 }
 // The default ackingCountGenerator would randomly pick a number based on
 // the nodeCount you provided with a normal distribution.
-func (gen *BlocksGenerator) Generate(
-	chainNum uint32,
-	blockNum int,
+func NewBlocksGenerator(
+	config *BlocksGeneratorConfig,
 	ackingCountGenerator func() int,
-	writer blockdb.Writer) (
-	nodePrvKeys map[types.NodeID]crypto.PrivateKey, err error) {
-
-	var (
-		prvKey crypto.PrivateKey
-		nodes  = types.NodeIDs{}
-	)
+	hashBlock hashBlockFn) *BlocksGenerator {
 	if ackingCountGenerator == nil {
 		ackingCountGenerator = normalAckingCountGenerator(
-			chainNum,
-			float64(chainNum/2),
-			float64(chainNum/4+1))
+			config.NumChains,
+			float64(config.NumChains/2),
+			float64(config.NumChains/4+1))
+	}
+	timePicker := generateTimePicker(
+		config.MinBlockTimeInterval, config.MaxBlockTimeInterval)
+	return &BlocksGenerator{
+		config:               config,
+		nodePicker:           generateNodePicker(),
+		timePicker:           timePicker,
+		ackingCountGenerator: ackingCountGenerator,
+		hashBlock:            hashBlock,
 	}
-	nodePrvKeys = map[types.NodeID]crypto.PrivateKey{}
-	for i := uint32(0); i < chainNum; i++ {
-		if prvKey, err = ecdsa.NewPrivateKey(); err != nil {
+}
+
+// Generate is the entry point to generate blocks in one round.
+func (gen *BlocksGenerator) Generate(
+	roundID uint64,
+	roundBegin, roundEnd time.Time,
+	db blockdb.BlockDatabase) (err error) {
+	// Find tips of previous round if available.
+	tips := make(map[uint32]*types.Block)
+	if roundID > 0 {
+		tips, err = gen.findTips(roundID-1, db)
+		if err != nil {
 			return
 		}
-		id := types.NewNodeID(prvKey.PublicKey())
-		nodes = append(nodes, id)
-		nodePrvKeys[id] = prvKey
 	}
-	status := newNodeSetStatus(nodes, gen.hashBlock)
-
+	status := newNodeSetStatus(gen.config.NumChains, tips, roundID,
+		roundBegin, roundEnd, gen.timePicker, gen.hashBlock)
 	// We would record the smallest height of block that could be acked
 	// from each node's point-of-view.
 	toAck := make(map[types.NodeID]map[types.NodeID]uint64)
-	for _, nID := range nodes {
+	for _, nID := range status.nIDs {
 		toAck[nID] = make(map[types.NodeID]uint64)
 	}
-
 	for {
 		// Find nodes that doesn't propose enough blocks and
 		// pick one from them randomly.
-		notYet := status.findIncompleteNodes(blockNum)
+		notYet := status.findIncompleteNodes()
 		if len(notYet) == 0 {
 			break
 		}
-
 		// Propose a new block.
 		var (
 			proposerID = gen.nodePicker(notYet)
 			acks       common.Hashes
 		)
-		acks, err = status.prepareAcksForNewBlock(
-			proposerID, ackingCountGenerator())
-		if err != nil {
+		if acks, err = status.prepareAcksForNewBlock(
+			proposerID, gen.ackingCountGenerator()); err != nil {
 			return
 		}
 		var newBlock *types.Block
-		if newBlock, err = status.proposeBlock(
-			proposerID, nodePrvKeys[proposerID], acks); err != nil {
-
+		if newBlock, err = status.proposeBlock(proposerID, acks); err != nil {
 			return
 		}
-
 		// Persist block to db.
-		err = writer.Put(*newBlock)
-		if err != nil {
+		if err = db.Put(*newBlock); err != nil {
 			return
 		}
 	}
 	return
 }
+
+// findTips is an utility to find tips of each chain in that round in blockdb.
+func (gen *BlocksGenerator) findTips(
+	round uint64, db blockdb.Reader) (tips map[uint32]*types.Block, err error) {
+	iter, err := db.GetAll()
+	if err != nil {
+		return
+	}
+	revealer, err := NewRandomRevealer(iter)
+	if err != nil {
+		return
+	}
+	tips = make(map[uint32]*types.Block)
+	for {
+		var b types.Block
+		if b, err = revealer.Next(); err != nil {
+			if err == blockdb.ErrIterationFinished {
+				err = nil
+				break
+			}
+			return
+		}
+		if b.Position.Round != round {
+			continue
+		}
+		tip, exists := tips[b.Position.ChainID]
+		if exists && tip.Position.Height > b.Position.Height {
+			continue
+		}
+		tips[b.Position.ChainID] = &b
+	}
+	return
+}
diff --git a/core/test/blocks-generator_test.go b/core/test/blocks-generator_test.go
index 1baa9c5..0477664 100644
--- a/core/test/blocks-generator_test.go
+++ b/core/test/blocks-generator_test.go
@@ -20,6 +20,7 @@ package test
 import (
 	"sort"
 	"testing"
+	"time"
 
 	"github.com/dexon-foundation/dexon-consensus-core/common"
 	"github.com/dexon-foundation/dexon-consensus-core/core/blockdb"
@@ -27,27 +28,29 @@ import (
 	"github.com/stretchr/testify/suite"
 )
 
-type BlocksGeneratorTestCase struct {
+type BlocksGeneratorTestSuite struct {
 	suite.Suite
 }
 
-func (s *BlocksGeneratorTestCase) TestGenerate() {
+func (s *BlocksGeneratorTestSuite) TestGenerate() {
 	// This test case is to make sure the generated blocks are legimate.
 	var (
-		chainNum = uint32(19)
-		blockNum = 50
+		config = &BlocksGeneratorConfig{
+			NumChains:            19,
+			MinBlockTimeInterval: 50 * time.Millisecond,
+			MaxBlockTimeInterval: 400 * time.Millisecond,
+		}
+		gen       = NewBlocksGenerator(config, nil, stableRandomHash)
+		req       = s.Require()
+		beginTime = time.Now().UTC()
+		endTime   = beginTime.Add(time.Minute)
 	)
-	gen := NewBlocksGenerator(nil, stableRandomHash)
 	db, err := blockdb.NewMemBackedBlockDB()
-	s.Require().Nil(err)
-
-	keys, err := gen.Generate(chainNum, blockNum, nil, db)
-	s.Require().Nil(err)
-	s.Require().Len(keys, int(chainNum))
-
+	req.NoError(err)
+	req.NoError(gen.Generate(1, beginTime, endTime, db))
 	// Load all blocks in that database for further checking.
 	iter, err := db.GetAll()
-	s.Require().Nil(err)
+	req.NoError(err)
 	blocksByNode := make(map[types.NodeID][]*types.Block)
 	blocksByHash := make(map[common.Hash]*types.Block)
 	for {
@@ -55,34 +58,33 @@ func (s *BlocksGeneratorTestCase) TestGenerate() {
 		if err == blockdb.ErrIterationFinished {
 			break
 		}
-		s.Nil(err)
-
+		req.NoError(err)
 		// TODO(mission): Make sure each block is correctly signed once
 		//                we have a way to access core.hashBlock.
-		s.Require().NotEqual(block.Hash, common.Hash{})
-		s.Require().NotEmpty(block.Signature)
-
+		req.NotEqual(block.Hash, common.Hash{})
+		req.NotEmpty(block.Signature)
+		req.Equal(block.Position.Round, uint64(1))
 		blocksByNode[block.ProposerID] =
 			append(blocksByNode[block.ProposerID], &block)
 		sort.Sort(types.ByHeight(blocksByNode[block.ProposerID]))
 		blocksByHash[block.Hash] = &block
 	}
-
 	// Make sure these two rules are hold for these blocks:
 	//  - No backward acking: the later block should only ack new blocks
 	//                        compared to its parent block.
 	//  - Parent Ack: always ack its parent block.
+	//  - Timestamp: timestamp are increasing, and with valid interval to
+	//               previous block.
+	//  - The last block of each chain should pass endTime.
 	//  - No Acks in genesis bloc
 	for _, blocks := range blocksByNode {
 		lastAckingHeights := map[types.NodeID]uint64{}
-		s.Require().NotEmpty(blocks)
-
+		req.NotEmpty(blocks)
 		// Check genesis block.
 		genesisBlock := blocks[0]
-		s.Equal(genesisBlock.ParentHash, common.Hash{})
-		s.Equal(genesisBlock.Position.Height, uint64(0))
-		s.Empty(genesisBlock.Acks)
-
+		req.Equal(genesisBlock.ParentHash, common.Hash{})
+		req.Equal(genesisBlock.Position.Height, uint64(0))
+		req.Empty(genesisBlock.Acks)
 		// Check normal blocks.
 		for index, block := range blocks[1:] {
 			parentAcked := false
@@ -90,9 +92,8 @@ func (s *BlocksGeneratorTestCase) TestGenerate() {
 				if ack == block.ParentHash {
 					parentAcked = true
 				}
-
 				ackedBlock := blocksByHash[ack]
-				s.Require().NotNil(ackedBlock)
+				req.NotNil(ackedBlock)
 				prevAckingHeight, exists :=
 					lastAckingHeights[ackedBlock.ProposerID]
 				if exists {
@@ -103,61 +104,70 @@ func (s *BlocksGeneratorTestCase) TestGenerate() {
 				//
 				// Because we iterate blocks slice from 1,
 				// we need to add 1 to the index.
-				s.Equal(block.Position.Height, uint64(index+1))
+				req.Equal(block.Position.Height, uint64(index+1))
 			}
-			s.True(parentAcked)
+			req.True(parentAcked)
 		}
+		// The block time of the last block should be after end time.
+		req.True(blocks[len(blocks)-1].Timestamp.After(endTime))
 	}
 }
 
-func (s *BlocksGeneratorTestCase) TestGenerateWithMaxAckCount() {
+func (s *BlocksGeneratorTestSuite) TestGenerateWithMaxAckCount() {
 	var (
-		chainNum         = uint32(13)
-		blockNum         = 50
-		gen              = NewBlocksGenerator(nil, stableRandomHash)
+		config = &BlocksGeneratorConfig{
+			NumChains:            13,
+			MinBlockTimeInterval: 0,
+			MaxBlockTimeInterval: 500 * time.Millisecond,
+		}
 		req              = s.Require()
 		totalAckingCount = 0
 		totalBlockCount  = 0
+		genesisTime      = time.Now().UTC()
 	)
-
 	// Generate with 0 acks.
 	db, err := blockdb.NewMemBackedBlockDB()
-	req.Nil(err)
-	keys, err := gen.Generate(
-		chainNum, blockNum, MaxAckingCountGenerator(0), db)
-	req.Nil(err)
-	req.Len(keys, int(chainNum))
+	req.NoError(err)
+	gen := NewBlocksGenerator(
+		config, MaxAckingCountGenerator(0), stableRandomHash)
+	req.NoError(gen.Generate(
+		0,
+		genesisTime,
+		genesisTime.Add(50*time.Second),
+		db))
 	// Load blocks to check their acking count.
 	iter, err := db.GetAll()
-	req.Nil(err)
+	req.NoError(err)
 	for {
 		block, err := iter.Next()
 		if err == blockdb.ErrIterationFinished {
 			break
 		}
-		req.Nil(err)
+		req.NoError(err)
 		if block.IsGenesis() {
 			continue
 		}
 		req.Len(block.Acks, 1)
 	}
-
 	// Generate with acks as many as possible.
 	db, err = blockdb.NewMemBackedBlockDB()
-	req.Nil(err)
-	keys, err = gen.Generate(
-		chainNum, blockNum, MaxAckingCountGenerator(chainNum), db)
-	req.Nil(err)
-	req.Len(keys, int(chainNum))
+	req.NoError(err)
+	gen = NewBlocksGenerator(
+		config, MaxAckingCountGenerator(config.NumChains), stableRandomHash)
+	req.NoError(gen.Generate(
+		0,
+		genesisTime,
+		genesisTime.Add(50*time.Second),
+		db))
 	// Load blocks to verify the average acking count.
 	iter, err = db.GetAll()
-	req.Nil(err)
+	req.NoError(err)
 	for {
 		block, err := iter.Next()
 		if err == blockdb.ErrIterationFinished {
 			break
 		}
-		req.Nil(err)
+		req.NoError(err)
 		if block.IsGenesis() {
 			continue
 		}
@@ -165,9 +175,154 @@ func (s *BlocksGeneratorTestCase) TestGenerateWithMaxAckCount() {
 		totalBlockCount++
 	}
 	req.NotZero(totalBlockCount)
-	req.True((totalAckingCount / totalBlockCount) >= int(chainNum/2))
+	req.True((totalAckingCount / totalBlockCount) >= int(config.NumChains/2))
+}
+
+// TestFindTips make sure findTips works as expected.
+func (s *BlocksGeneratorTestSuite) TestFindTips() {
+	var (
+		config = &BlocksGeneratorConfig{
+			NumChains:            10,
+			MinBlockTimeInterval: 0,
+			MaxBlockTimeInterval: 500 * time.Millisecond,
+		}
+		req         = s.Require()
+		genesisTime = time.Now().UTC()
+		endTime     = genesisTime.Add(100 * time.Second)
+	)
+	gen := NewBlocksGenerator(config, nil, stableRandomHash)
+	db, err := blockdb.NewMemBackedBlockDB()
+	req.NoError(err)
+	req.NoError(gen.Generate(
+		0,
+		genesisTime,
+		endTime,
+		db))
+	tips, err := gen.findTips(0, db)
+	req.NoError(err)
+	req.Len(tips, int(config.NumChains))
+	for _, b := range tips {
+		req.True(b.Timestamp.After(endTime))
+	}
+}
+
+func (s *BlocksGeneratorTestSuite) TestConcateBlocksFromRounds() {
+	// This test case run these steps:
+	//  - generate blocks by round but sharing one blockdb.
+	//  - if those rounds are continuous, they should be concated.
+	var (
+		req         = s.Require()
+		genesisTime = time.Now().UTC()
+	)
+	db, err := blockdb.NewMemBackedBlockDB()
+	req.NoError(err)
+	// Generate round 0 blocks.
+	gen := NewBlocksGenerator(&BlocksGeneratorConfig{
+		NumChains:            4,
+		MinBlockTimeInterval: 0,
+		MaxBlockTimeInterval: 500 * time.Millisecond,
+	}, nil, stableRandomHash)
+	req.NoError(gen.Generate(
+		0,
+		genesisTime,
+		genesisTime.Add(10*time.Second),
+		db))
+	tips0, err := gen.findTips(0, db)
+	req.NoError(err)
+	req.Len(tips0, 4)
+	// Generate round 1 blocks.
+	gen = NewBlocksGenerator(&BlocksGeneratorConfig{
+		NumChains:            10,
+		MinBlockTimeInterval: 0,
+		MaxBlockTimeInterval: 500 * time.Millisecond,
+	}, nil, stableRandomHash)
+	req.NoError(gen.Generate(
+		1,
+		genesisTime.Add(10*time.Second),
+		genesisTime.Add(20*time.Second),
+		db))
+	tips1, err := gen.findTips(1, db)
+	req.NoError(err)
+	req.Len(tips1, 10)
+	// Generate round 2 blocks.
+	gen = NewBlocksGenerator(&BlocksGeneratorConfig{
+		NumChains:            7,
+		MinBlockTimeInterval: 0,
+		MaxBlockTimeInterval: 500 * time.Millisecond,
+	}, nil, stableRandomHash)
+	req.NoError(gen.Generate(
+		2,
+		genesisTime.Add(20*time.Second),
+		genesisTime.Add(30*time.Second),
+		db))
+	tips2, err := gen.findTips(2, db)
+	req.NoError(err)
+	req.Len(tips2, 7)
+	// Check results, make sure tips0, tips1 are acked by correct blocks.
+	iter, err := db.GetAll()
+	req.NoError(err)
+	revealer, err := NewRandomRevealer(iter)
+	req.NoError(err)
+	removeTip := func(tips map[uint32]*types.Block, b *types.Block) {
+		toRemove := []uint32{}
+		for chainID, tip := range tips {
+			if b.ParentHash == tip.Hash {
+				req.Equal(b.Position.Height, tip.Position.Height+1)
+				req.Equal(b.Position.Round, tip.Position.Round+1)
+				req.True(b.IsAcking(tip.Hash))
+				toRemove = append(toRemove, chainID)
+			}
+		}
+		for _, ID := range toRemove {
+			delete(tips, ID)
+		}
+	}
+	// Make sure all tips are acked by loading blocks from db
+	// and check them one by one.
+	for {
+		b, err := revealer.Next()
+		if err != nil {
+			if err == blockdb.ErrIterationFinished {
+				err = nil
+				break
+			}
+			req.NoError(err)
+		}
+		switch b.Position.Round {
+		case 1:
+			removeTip(tips0, &b)
+		case 2:
+			removeTip(tips1, &b)
+		}
+	}
+	req.Empty(tips0)
+	req.Len(tips1, 3)
+	req.Contains(tips1, uint32(7))
+	req.Contains(tips1, uint32(8))
+	req.Contains(tips1, uint32(9))
+	// Check the acking frequency of last round, it might be wrong.
+	totalBlockCount := 0
+	totalAckCount := 0
+	revealer.Reset()
+	for {
+		b, err := revealer.Next()
+		if err != nil {
+			if err == blockdb.ErrIterationFinished {
+				err = nil
+				break
+			}
+			req.NoError(err)
+		}
+		if b.Position.Round != 2 {
+			continue
+		}
+		totalBlockCount++
+		totalAckCount += len(b.Acks)
+	}
+	// At least all blocks can ack some non-parent block.
+	req.True(totalAckCount/totalBlockCount >= 2)
 }
 
 func TestBlocksGenerator(t *testing.T) {
-	suite.Run(t, new(BlocksGeneratorTestCase))
+	suite.Run(t, new(BlocksGeneratorTestSuite))
 }
diff --git a/core/test/revealer_test.go b/core/test/revealer_test.go
index 859438c..8bb46bc 100644
--- a/core/test/revealer_test.go
+++ b/core/test/revealer_test.go
@@ -19,6 +19,7 @@ package test
 
 import (
 	"testing"
+	"time"
 
 	"github.com/dexon-foundation/dexon-consensus-core/common"
 	"github.com/dexon-foundation/dexon-consensus-core/core/blockdb"
@@ -35,25 +36,30 @@ type RevealerTestSuite struct {
 
 func (s *RevealerTestSuite) SetupSuite() {
 	var (
-		err      error
-		chainNum = uint32(19)
-		blockNum = 50
+		err         error
+		genesisTime = time.Now().UTC()
 	)
 	// Setup block database.
 	s.db, err = blockdb.NewMemBackedBlockDB()
-	s.Require().Nil(err)
+	s.Require().NoError(err)
 
 	// Randomly generate blocks.
-	gen := NewBlocksGenerator(nil, stableRandomHash)
-	nodes, err := gen.Generate(chainNum, blockNum, nil, s.db)
-	s.Require().Nil(err)
-	s.Require().Len(nodes, int(chainNum))
-
+	config := &BlocksGeneratorConfig{
+		NumChains:            19,
+		MinBlockTimeInterval: 0,
+		MaxBlockTimeInterval: 500 * time.Millisecond,
+	}
+	gen := NewBlocksGenerator(config, nil, stableRandomHash)
+	s.Require().NoError(gen.Generate(
+		0,
+		genesisTime,
+		genesisTime.Add(30*time.Second),
+		s.db))
 	// Cache the count of total generated block.
 	iter, err := s.db.GetAll()
-	s.Require().Nil(err)
+	s.Require().NoError(err)
 	blocks, err := loadAllBlocks(iter)
-	s.Require().Nil(err)
+	s.Require().NoError(err)
 	s.totalBlockCount = len(blocks)
 }