consensus/engine/chain/engine_test.go

package chain

import (
	"context"
	"errors"
	"sync/atomic"
	"testing"
	"time"

	"github.com/luxfi/consensus/config"
	"github.com/luxfi/consensus/core/slashing"
	"github.com/luxfi/consensus/engine/chain/block"
	"github.com/luxfi/ids"
)

// mockBlock implements block.Block for testing
type mockBlock struct {
	id        ids.ID
	parentID  ids.ID
	height    uint64
	timestamp time.Time
	status    uint8
	bytes     []byte
}

func (b *mockBlock) ID() ids.ID                   { return b.id }
func (b *mockBlock) Parent() ids.ID               { return b.parentID }
func (b *mockBlock) ParentID() ids.ID             { return b.parentID }
func (b *mockBlock) Height() uint64               { return b.height }
func (b *mockBlock) Timestamp() time.Time         { return b.timestamp }
func (b *mockBlock) Status() uint8                { return b.status }
func (b *mockBlock) Verify(context.Context) error { return nil }
func (b *mockBlock) Accept(context.Context) error { return nil }
func (b *mockBlock) Reject(context.Context) error { return nil }
func (b *mockBlock) Bytes() []byte                { return b.bytes }

func TestNew(t *testing.T) {
	engine := New()
	if engine == nil {
		t.Fatal("New() returned nil")
	}

	if engine.IsBootstrapped() {
		t.Error("Engine should not be bootstrapped initially")
	}
}

func TestStart(t *testing.T) {
	engine := New()
	ctx := context.Background()

	err := engine.Start(ctx, true)
	if err != nil {
		t.Fatalf("Start failed: %v", err)
	}

	if !engine.IsBootstrapped() {
		t.Error("Engine should be bootstrapped after Start")
	}
}

func TestStop(t *testing.T) {
	engine := New()
	ctx := context.Background()

	_ = engine.Start(ctx, true)

	err := engine.Stop(ctx)
	if err != nil {
		t.Fatalf("Stop failed: %v", err)
	}
}

func TestHealthCheck(t *testing.T) {
	engine := New()
	ctx := context.Background()

	health, err := engine.HealthCheck(ctx)
	if err != nil {
		t.Fatalf("HealthCheck failed: %v", err)
	}

	if health == nil {
		t.Error("HealthCheck returned nil")
	}

	// Check it returns a map with consensus stats
	if m, ok := health.(map[string]interface{}); ok {
		// Real consensus returns detailed stats
		if _, exists := m["total_blocks"]; !exists {
			t.Error("HealthCheck should include total_blocks stat")
		}
		if _, exists := m["bootstrapped"]; !exists {
			t.Error("HealthCheck should include bootstrapped stat")
		}
	} else {
		t.Error("HealthCheck should return a map[string]interface{}")
	}
}

func TestGetBlock(t *testing.T) {
	engine := New()
	ctx := context.Background()

	// GetBlock should return nil (no-op for now)
	// Using empty IDs for test
	nodeID := ids.EmptyNodeID
	blockID := ids.Empty

	err := engine.GetBlock(ctx, nodeID, 1, blockID)
	if err != nil {
		t.Errorf("GetBlock should not error: %v", err)
	}
}

func TestChainWorkflow(t *testing.T) {
	engine := New()
	ctx := context.Background()

	// Start engine
	err := engine.Start(ctx, true)
	if err != nil {
		t.Fatalf("Start failed: %v", err)
	}

	// Check bootstrapped
	if !engine.IsBootstrapped() {
		t.Error("Should be bootstrapped")
	}

	// Health check
	health, err := engine.HealthCheck(ctx)
	if err != nil {
		t.Fatalf("HealthCheck failed: %v", err)
	}
	if health == nil {
		t.Error("Health should not be nil")
	}

	// Stop engine
	err = engine.Stop(ctx)
	if err != nil {
		t.Fatalf("Stop failed: %v", err)
	}
}

// mockVM implements BlockBuilder for testing
type mockVM struct {
	buildBlockCalls int
	buildBlockErr   error
	lastBuiltBlock  *mockBlock
	blocks          map[ids.ID]*mockBlock
	lastAcceptedID  ids.ID
}

func (m *mockVM) BuildBlock(ctx context.Context) (block.Block, error) {
	m.buildBlockCalls++
	if m.buildBlockErr != nil {
		return nil, m.buildBlockErr
	}
	blk := &mockBlock{
		id:     ids.GenerateTestID(),
		height: uint64(m.buildBlockCalls),
	}
	m.lastBuiltBlock = blk
	if m.blocks == nil {
		m.blocks = make(map[ids.ID]*mockBlock)
	}
	m.blocks[blk.id] = blk
	return blk, nil
}

func (m *mockVM) GetBlock(ctx context.Context, id ids.ID) (block.Block, error) {
	if m.blocks != nil {
		if blk, ok := m.blocks[id]; ok {
			return blk, nil
		}
	}
	return nil, errors.New("block not found")
}

func (m *mockVM) ParseBlock(ctx context.Context, bytes []byte) (block.Block, error) {
	return &mockBlock{bytes: bytes}, nil
}

func (m *mockVM) LastAccepted(ctx context.Context) (ids.ID, error) {
	return m.lastAcceptedID, nil
}

func (m *mockVM) SetPreference(ctx context.Context, id ids.ID) error {
	// Mock implementation - just record the preferred block
	m.lastAcceptedID = id
	return nil
}

// TestNotifyPendingTxsTriggersBuildBlock verifies that Notify(PendingTxs) triggers BuildBlock
func TestNotifyPendingTxsTriggersBuildBlock(t *testing.T) {
	engine := New()
	ctx := context.Background()

	// Start engine
	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}

	// Set up mock VM
	vm := &mockVM{}
	engine.SetVM(vm)

	// Initial state: no pending builds, no BuildBlock calls
	if engine.PendingBuildBlocks() != 0 {
		t.Error("Should have 0 pending blocks initially")
	}
	if vm.buildBlockCalls != 0 {
		t.Error("Should have 0 BuildBlock calls initially")
	}

	// Send PendingTxs notification
	err := engine.Notify(ctx, Message{Type: PendingTxs})
	if err != nil {
		t.Fatalf("Notify failed: %v", err)
	}

	// Verify BuildBlock was called
	if vm.buildBlockCalls != 1 {
		t.Errorf("Expected 1 BuildBlock call, got %d", vm.buildBlockCalls)
	}

	// Pending blocks should be consumed (0 remaining)
	if engine.PendingBuildBlocks() != 0 {
		t.Errorf("Expected 0 pending blocks after build, got %d", engine.PendingBuildBlocks())
	}

	// Send multiple notifications rapidly
	for i := 0; i < 5; i++ {
		if err := engine.Notify(ctx, Message{Type: PendingTxs}); err != nil {
			t.Fatalf("Notify %d failed: %v", i, err)
		}
	}

	// All 5 notifications should have triggered BuildBlock
	if vm.buildBlockCalls != 6 { // 1 + 5
		t.Errorf("Expected 6 total BuildBlock calls, got %d", vm.buildBlockCalls)
	}
}

// TestNotifyStateSyncDoneDoesNotBuild verifies that StateSyncDone doesn't trigger builds
func TestNotifyStateSyncDoneDoesNotBuild(t *testing.T) {
	engine := New()
	ctx := context.Background()

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}

	vm := &mockVM{}
	engine.SetVM(vm)

	// Send StateSyncDone notification
	err := engine.Notify(ctx, Message{Type: StateSyncDone})
	if err != nil {
		t.Fatalf("Notify failed: %v", err)
	}

	// BuildBlock should NOT be called
	if vm.buildBlockCalls != 0 {
		t.Errorf("Expected 0 BuildBlock calls for StateSyncDone, got %d", vm.buildBlockCalls)
	}
}

// TestNotifyWithNoVMDoesNotPanic verifies Notify handles nil VM gracefully
func TestNotifyWithNoVMDoesNotPanic(t *testing.T) {
	engine := New()
	ctx := context.Background()

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}

	// Don't set VM - it should be nil

	// Should not panic
	err := engine.Notify(ctx, Message{Type: PendingTxs})
	if err != nil {
		t.Fatalf("Notify with nil VM should not error: %v", err)
	}
}

// TestNotifyBuildBlockError verifies error handling when BuildBlock fails
func TestNotifyBuildBlockError(t *testing.T) {
	engine := New()
	ctx := context.Background()

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}

	// VM that always errors
	vm := &mockVM{
		buildBlockErr: context.DeadlineExceeded,
	}
	engine.SetVM(vm)

	// Notify should not propagate the error (just log and continue)
	err := engine.Notify(ctx, Message{Type: PendingTxs})
	if err != nil {
		t.Fatalf("Notify should not error on BuildBlock failure: %v", err)
	}

	// BuildBlock was still called
	if vm.buildBlockCalls != 1 {
		t.Errorf("Expected 1 BuildBlock call, got %d", vm.buildBlockCalls)
	}
}

// -----------------------------------------------------------------------------
// Quorum-based acceptance tests
// -----------------------------------------------------------------------------

// trackingMockBlock tracks Accept/Reject calls for testing
type trackingMockBlock struct {
	id           ids.ID
	parentID     ids.ID
	height       uint64
	timestamp    time.Time
	bytes        []byte
	acceptCalled int64
	rejectCalled int64
}

func (b *trackingMockBlock) ID() ids.ID                   { return b.id }
func (b *trackingMockBlock) Parent() ids.ID               { return b.parentID }
func (b *trackingMockBlock) ParentID() ids.ID             { return b.parentID }
func (b *trackingMockBlock) Height() uint64               { return b.height }
func (b *trackingMockBlock) Timestamp() time.Time         { return b.timestamp }
func (b *trackingMockBlock) Status() uint8                { return 0 }
func (b *trackingMockBlock) Verify(context.Context) error { return nil }
func (b *trackingMockBlock) Accept(context.Context) error {
	atomic.AddInt64(&b.acceptCalled, 1)
	return nil
}
func (b *trackingMockBlock) Reject(context.Context) error {
	atomic.AddInt64(&b.rejectCalled, 1)
	return nil
}
func (b *trackingMockBlock) Bytes() []byte { return b.bytes }

func (b *trackingMockBlock) AcceptCalled() int64 { return atomic.LoadInt64(&b.acceptCalled) }
func (b *trackingMockBlock) RejectCalled() int64 { return atomic.LoadInt64(&b.rejectCalled) }

// trackingMockVM returns trackingMockBlocks for acceptance tracking
type trackingMockVM struct {
	blocks         []*trackingMockBlock
	buildBlockIdx  int
	lastAcceptedID ids.ID
}

func (m *trackingMockVM) BuildBlock(ctx context.Context) (block.Block, error) {
	if m.buildBlockIdx >= len(m.blocks) {
		return nil, errors.New("no more blocks")
	}
	blk := m.blocks[m.buildBlockIdx]
	m.buildBlockIdx++
	return blk, nil
}

func (m *trackingMockVM) GetBlock(ctx context.Context, id ids.ID) (block.Block, error) {
	for _, blk := range m.blocks {
		if blk.id == id {
			return blk, nil
		}
	}
	return nil, errors.New("block not found")
}

func (m *trackingMockVM) ParseBlock(ctx context.Context, bytes []byte) (block.Block, error) {
	return &trackingMockBlock{bytes: bytes}, nil
}

func (m *trackingMockVM) LastAccepted(ctx context.Context) (ids.ID, error) {
	return m.lastAcceptedID, nil
}

func (m *trackingMockVM) SetPreference(ctx context.Context, id ids.ID) error {
	m.lastAcceptedID = id
	return nil
}

// TestEngine_DoesNotAcceptWithoutQuorum verifies blocks are NOT accepted without sufficient votes
func TestEngine_DoesNotAcceptWithoutQuorum(t *testing.T) {
	engine := New()
	ctx := context.Background()

	// Create a tracking block
	blk := &trackingMockBlock{
		id:        ids.GenerateTestID(),
		parentID:  ids.Empty,
		height:    1,
		timestamp: time.Now(),
		bytes:     []byte("block-data"),
	}

	vm := &trackingMockVM{
		blocks: []*trackingMockBlock{blk},
	}
	engine.SetVM(vm)

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}
	defer engine.Stop(ctx)

	// Trigger block build
	if err := engine.Notify(ctx, Message{Type: PendingTxs}); err != nil {
		t.Fatalf("Notify failed: %v", err)
	}

	// Verify block was added to pending
	engine.mu.RLock()
	pending, exists := engine.pendingBlocks[blk.id]
	engine.mu.RUnlock()

	if !exists {
		t.Fatal("Block should be in pending blocks")
	}
	if pending.VMBlock == nil {
		t.Fatal("VMBlock should be set")
	}

	// Process a single vote (below quorum - default K=20, Alpha=15)
	// One vote is not enough for quorum
	engine.ReceiveVote(Vote{
		BlockID:  blk.id,
		NodeID:   ids.GenerateTestNodeID(),
		Accept:   true,
		SignedAt: time.Now(),
	})

	// Give vote handler time to process
	time.Sleep(100 * time.Millisecond)

	// Block should NOT be accepted yet (insufficient votes)
	if blk.AcceptCalled() > 0 {
		t.Errorf("Block Accept() should NOT be called with insufficient votes, but was called %d times", blk.AcceptCalled())
	}

	// Check consensus state - should not be accepted
	if engine.IsAccepted(blk.id) {
		t.Error("Block should not be marked as accepted with insufficient votes")
	}
}

// TestEngine_AcceptsAfterQuorum verifies blocks ARE accepted after sufficient votes
func TestEngine_AcceptsAfterQuorum(t *testing.T) {
	// Use smaller parameters for testing (K=5, Alpha=3, Beta=2)
	// This means we need 3 out of 5 votes for quorum
	engine := NewWithParams(config.Parameters{
		K:               5,
		AlphaPreference: 3,
		AlphaConfidence: 3,
		Beta:            2,
	})
	ctx := context.Background()

	// Create a tracking block
	blk := &trackingMockBlock{
		id:        ids.GenerateTestID(),
		parentID:  ids.Empty,
		height:    1,
		timestamp: time.Now(),
		bytes:     []byte("block-data"),
	}

	vm := &trackingMockVM{
		blocks: []*trackingMockBlock{blk},
	}
	engine.SetVM(vm)

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}
	defer engine.Stop(ctx)

	// Trigger block build
	if err := engine.Notify(ctx, Message{Type: PendingTxs}); err != nil {
		t.Fatalf("Notify failed: %v", err)
	}

	// Verify block was added
	engine.mu.RLock()
	_, exists := engine.pendingBlocks[blk.id]
	engine.mu.RUnlock()

	if !exists {
		t.Fatal("Block should be in pending blocks")
	}

	// Send enough votes to reach quorum (3 votes for Alpha=3)
	for i := 0; i < 4; i++ {
		engine.ReceiveVote(Vote{
			BlockID:  blk.id,
			NodeID:   ids.GenerateTestNodeID(),
			Accept:   true,
			SignedAt: time.Now(),
		})
	}

	// Give poll loop time to process the acceptance
	// The poll loop runs every 50ms
	time.Sleep(300 * time.Millisecond)

	// Block SHOULD be accepted after quorum
	if blk.AcceptCalled() != 1 {
		t.Errorf("Block Accept() should be called exactly once after quorum, but was called %d times", blk.AcceptCalled())
	}

	// Block should no longer be in pending
	engine.mu.RLock()
	_, stillPending := engine.pendingBlocks[blk.id]
	engine.mu.RUnlock()

	if stillPending {
		t.Error("Block should be removed from pending after acceptance")
	}
}

// -----------------------------------------------------------------------------
// Lifecycle invariant tests
// -----------------------------------------------------------------------------

// TestReceiveVote_DropsWhenNotStarted verifies votes are dropped when engine not started
func TestReceiveVote_DropsWhenNotStarted(t *testing.T) {
	engine := New()
	// DO NOT start the engine

	vote := Vote{
		BlockID:  ids.GenerateTestID(),
		NodeID:   ids.GenerateTestNodeID(),
		Accept:   true,
		SignedAt: time.Now(),
	}

	// ReceiveVote should return false (dropped) when not started
	accepted := engine.ReceiveVote(vote)
	if accepted {
		t.Error("ReceiveVote should return false when engine not started")
	}
}

// TestReceiveVote_AcceptsWhenStarted verifies votes are queued when engine is started
func TestReceiveVote_AcceptsWhenStarted(t *testing.T) {
	engine := New()
	ctx := context.Background()

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}
	defer engine.Stop(ctx)

	vote := Vote{
		BlockID:  ids.GenerateTestID(),
		NodeID:   ids.GenerateTestNodeID(),
		Accept:   true,
		SignedAt: time.Now(),
	}

	// ReceiveVote should return true (queued) when started
	accepted := engine.ReceiveVote(vote)
	if !accepted {
		t.Error("ReceiveVote should return true when engine is started")
	}
}

// TestReceiveVote_DropsAfterStop verifies votes are dropped after engine stops
func TestReceiveVote_DropsAfterStop(t *testing.T) {
	engine := New()
	ctx := context.Background()

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}

	// Stop the engine
	if err := engine.Stop(ctx); err != nil {
		t.Fatalf("Stop failed: %v", err)
	}

	vote := Vote{
		BlockID:  ids.GenerateTestID(),
		NodeID:   ids.GenerateTestNodeID(),
		Accept:   true,
		SignedAt: time.Now(),
	}

	// ReceiveVote should return false after stop
	accepted := engine.ReceiveVote(vote)
	if accepted {
		t.Error("ReceiveVote should return false after engine stops")
	}
}

// -----------------------------------------------------------------------------
// Vote correlation tests (pendingBlocks tracking)
// -----------------------------------------------------------------------------

// TestHandleVote_IgnoresUnknownBlocks verifies votes for untracked blocks are ignored
func TestHandleVote_IgnoresUnknownBlocks(t *testing.T) {
	engine := New()
	ctx := context.Background()

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}
	defer engine.Stop(ctx)

	// Get initial vote count
	initialStats := engine.Stats()
	initialReceived := initialStats["votes_received"].(uint64)

	// Send vote for block that doesn't exist in pendingBlocks
	unknownBlockID := ids.GenerateTestID()
	engine.ReceiveVote(Vote{
		BlockID:  unknownBlockID,
		NodeID:   ids.GenerateTestNodeID(),
		Accept:   true,
		SignedAt: time.Now(),
	})

	// Give handler time to process
	time.Sleep(100 * time.Millisecond)

	// votes_received should increment (we received it)
	stats := engine.Stats()
	received := stats["votes_received"].(uint64)
	if received != initialReceived+1 {
		t.Errorf("Expected votes_received to increment, got %d", received)
	}

	// But block should NOT appear in consensus (unknown block ignored)
	if engine.IsAccepted(unknownBlockID) {
		t.Error("Unknown block should not be marked as accepted")
	}
}

// TestHandleVote_ProcessesKnownBlocks verifies votes for tracked blocks are processed
func TestHandleVote_ProcessesKnownBlocks(t *testing.T) {
	engine := NewWithParams(config.Parameters{
		K:               5,
		AlphaPreference: 3,
		AlphaConfidence: 3,
		Beta:            2,
	})
	ctx := context.Background()

	blk := &trackingMockBlock{
		id:        ids.GenerateTestID(),
		parentID:  ids.Empty,
		height:    1,
		timestamp: time.Now(),
		bytes:     []byte("test-block"),
	}

	vm := &trackingMockVM{blocks: []*trackingMockBlock{blk}}
	engine.SetVM(vm)

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}
	defer engine.Stop(ctx)

	// Build block to add it to pendingBlocks
	if err := engine.Notify(ctx, Message{Type: PendingTxs}); err != nil {
		t.Fatalf("Notify failed: %v", err)
	}

	// Verify block is in pendingBlocks
	engine.mu.RLock()
	pending, exists := engine.pendingBlocks[blk.id]
	engine.mu.RUnlock()
	if !exists {
		t.Fatal("Block should be in pendingBlocks")
	}
	initialVoteCount := pending.VoteCount

	// Send vote for known block
	engine.ReceiveVote(Vote{
		BlockID:  blk.id,
		NodeID:   ids.GenerateTestNodeID(),
		Accept:   true,
		SignedAt: time.Now(),
	})

	// Give handler time to process
	time.Sleep(100 * time.Millisecond)

	// VoteCount should increment (or block may have been accepted already)
	engine.mu.RLock()
	pending, stillExists := engine.pendingBlocks[blk.id]
	var newVoteCount int
	if stillExists {
		newVoteCount = pending.VoteCount
	}
	engine.mu.RUnlock()

	// Block might have been accepted and removed from pendingBlocks
	// which is also valid - votes were processed and led to acceptance
	if stillExists && newVoteCount <= initialVoteCount {
		t.Errorf("Expected VoteCount to increment from %d, got %d", initialVoteCount, newVoteCount)
	}
	// If !stillExists, the block was accepted which means votes were processed successfully
}

// -----------------------------------------------------------------------------
// Accept/Reject vote handling tests
// -----------------------------------------------------------------------------

// TestProcessVote_AcceptTrueIncrementsSupport verifies Accept=true votes count toward acceptance
func TestProcessVote_AcceptTrueIncrementsSupport(t *testing.T) {
	engine := NewWithParams(config.Parameters{
		K:               3,
		AlphaPreference: 2,
		AlphaConfidence: 2,
		Beta:            1,
	})
	ctx := context.Background()

	blk := &trackingMockBlock{
		id:        ids.GenerateTestID(),
		parentID:  ids.Empty,
		height:    1,
		timestamp: time.Now(),
		bytes:     []byte("test"),
	}

	vm := &trackingMockVM{blocks: []*trackingMockBlock{blk}}
	engine.SetVM(vm)

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}
	defer engine.Stop(ctx)

	// Build block
	if err := engine.Notify(ctx, Message{Type: PendingTxs}); err != nil {
		t.Fatalf("Notify failed: %v", err)
	}

	// Send Accept=true votes to reach quorum
	for i := 0; i < 3; i++ {
		engine.ReceiveVote(Vote{
			BlockID:  blk.id,
			NodeID:   ids.GenerateTestNodeID(),
			Accept:   true,
			SignedAt: time.Now(),
		})
	}

	// Wait for acceptance
	time.Sleep(200 * time.Millisecond)

	// Block should be accepted
	if blk.AcceptCalled() != 1 {
		t.Errorf("Expected Accept() to be called once, got %d", blk.AcceptCalled())
	}
}

// TestProcessVote_AcceptFalseDoesNotAccept verifies Accept=false votes for a
// follower-tracked (NOT own-proposed) block do not trigger acceptance.
//
// History: this test originally used Notify+buildBlocksLocked to seed the
// pending block, then sent Accept=false votes — modelling a scenario that
// does not exist in the LUX network protocol. SendVote is only ever called
// after fast-follow Accept (integration.go:498-500), so peer votes for an
// own-proposed block are always positive. With the IsOwnProposal trust path
// (closing the proposer-self-accept gap), explicit Accept=false votes for an
// own block are interpreted as positive signals — the original assertion
// no longer reflects production behavior.
//
// The invariant the test actually guards — "explicit rejection of a non-own
// block is honored" — is now expressed via a directly-injected pending entry
// with IsOwnProposal=false (matching the follower's HandleIncomingBlock slow
// path) so the assertion is meaningful again.
func TestProcessVote_AcceptFalseDoesNotAccept(t *testing.T) {

	engine := NewWithParams(config.Parameters{
		K:               3,
		AlphaPreference: 2,
		AlphaConfidence: 2,
		Beta:            1,
	})
	ctx := context.Background()

	blk := &trackingMockBlock{
		id:        ids.GenerateTestID(),
		parentID:  ids.Empty,
		height:    1,
		timestamp: time.Now(),
		bytes:     []byte("test"),
	}

	vm := &trackingMockVM{blocks: []*trackingMockBlock{blk}}
	engine.SetVM(vm)

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}
	defer engine.Stop(ctx)

	// Seed the block as a follower-tracked entry (not own-proposed).
	cb := &Block{
		id:        blk.id,
		parentID:  blk.parentID,
		height:    blk.height,
		timestamp: blk.timestamp.Unix(),
		data:      blk.bytes,
	}
	if err := engine.AddBlock(ctx, cb); err != nil {
		t.Fatalf("AddBlock: %v", err)
	}
	engine.mu.Lock()
	engine.pendingBlocks[blk.id] = &PendingBlock{
		ConsensusBlock: cb,
		VMBlock:        blk,
		ProposedAt:     time.Now(),
		VoteCount:      1,
		Decided:        false,
		IsOwnProposal:  false,
	}
	engine.mu.Unlock()

	// Send Accept=false votes (rejections)
	for i := 0; i < 5; i++ {
		engine.ReceiveVote(Vote{
			BlockID:  blk.id,
			NodeID:   ids.GenerateTestNodeID(),
			Accept:   false, // Reject votes
			SignedAt: time.Now(),
		})
	}

	// Wait for processing
	time.Sleep(200 * time.Millisecond)

	// Block should NOT be accepted (only reject votes received)
	if blk.AcceptCalled() > 0 {
		t.Errorf("Accept() should NOT be called with only reject votes, got %d calls", blk.AcceptCalled())
	}
}

// TestEngine_RejectsWithInsufficientSupport verifies follower-tracked blocks
// (NOT own-proposed) are rejected when peers explicitly vote Accept=false.
//
// See history note on TestProcessVote_AcceptFalseDoesNotAccept: the test
// originally exercised an own-proposed block, but the LUX network protocol
// never gossips explicit Accept=false votes back to a proposer (peers send
// no message on reject; SendVote is fast-follow-Accept-only). The invariant
// that explicit rejections of a non-own pending block are honored is now
// exercised via a follower-shaped pending entry (IsOwnProposal=false).
func TestEngine_RejectsWithInsufficientSupport(t *testing.T) {

	// Use smaller parameters (K=5, Alpha=3, Beta=2)
	engine := NewWithParams(config.Parameters{
		K:               5,
		AlphaPreference: 3,
		AlphaConfidence: 3,
		Beta:            2,
	})
	ctx := context.Background()

	blk := &trackingMockBlock{
		id:        ids.GenerateTestID(),
		parentID:  ids.Empty,
		height:    1,
		timestamp: time.Now(),
		bytes:     []byte("block-data"),
	}

	vm := &trackingMockVM{
		blocks: []*trackingMockBlock{blk},
	}
	engine.SetVM(vm)

	if err := engine.Start(ctx, true); err != nil {
		t.Fatalf("Start failed: %v", err)
	}
	defer engine.Stop(ctx)

	// Seed as follower-tracked (not own-proposed).
	cb := &Block{
		id:        blk.id,
		parentID:  blk.parentID,
		height:    blk.height,
		timestamp: blk.timestamp.Unix(),
		data:      blk.bytes,
	}
	if err := engine.AddBlock(ctx, cb); err != nil {
		t.Fatalf("AddBlock: %v", err)
	}
	engine.mu.Lock()
	engine.pendingBlocks[blk.id] = &PendingBlock{
		ConsensusBlock: cb,
		VMBlock:        blk,
		ProposedAt:     time.Now(),
		VoteCount:      1,
		Decided:        false,
		IsOwnProposal:  false,
	}
	engine.mu.Unlock()

	// Send reject votes (Accept=false)
	for i := 0; i < 4; i++ {
		engine.ReceiveVote(Vote{
			BlockID:  blk.id,
			NodeID:   ids.GenerateTestNodeID(),
			Accept:   false, // Reject votes
			SignedAt: time.Now(),
		})
	}

	// Give poll loop time to process
	time.Sleep(300 * time.Millisecond)

	// Block should NOT have Accept called
	if blk.AcceptCalled() > 0 {
		t.Errorf("Block Accept() should NOT be called for rejected block, but was called %d times", blk.AcceptCalled())
	}
}

// -----------------------------------------------------------------------------
// SyncState tests (for RLP import recovery)
// -----------------------------------------------------------------------------

// TestSyncState_UpdatesConsensusState verifies SyncState correctly updates consensus.
// This is critical for recovering from admin_importChain RLP imports.
func TestSyncState_UpdatesConsensusState(t *testing.T) {
	engine := New()
	ctx := context.Background()

	// Before sync, bootstrapped is false (engine not started)
	if engine.IsBootstrapped() {
		t.Error("Engine should not be bootstrapped before Start or SyncState")
	}

	// Simulate RLP import by syncing to a block at height 1000
	importedBlockID := ids.GenerateTestID()
	err := engine.SyncState(ctx, importedBlockID, 1000)
	if err != nil {
		t.Fatalf("SyncState failed: %v", err)
	}

	// After sync, engine should be bootstrapped
	if !engine.IsBootstrapped() {
		t.Error("Engine should be bootstrapped after SyncState")
	}

	// Consensus should have updated finalized tip
	if engine.consensus.GetFinalizedTip() != importedBlockID {
		t.Errorf("Expected finalizedTip=%s, got %s",
			importedBlockID, engine.consensus.GetFinalizedTip())
	}
}

// TestSyncState_ClearsStalePendingBlocks verifies SyncState removes stale blocks.
func TestSyncState_ClearsStalePendingBlocks(t *testing.T) {
	engine := New()
	ctx := context.Background()

	// Add some pending blocks at various heights
	block1 := &Block{id: ids.GenerateTestID(), height: 100}
	block2 := &Block{id: ids.GenerateTestID(), height: 500}
	block3 := &Block{id: ids.GenerateTestID(), height: 1500}

	engine.mu.Lock()
	engine.pendingBlocks[block1.id] = &PendingBlock{ConsensusBlock: block1}
	engine.pendingBlocks[block2.id] = &PendingBlock{ConsensusBlock: block2}
	engine.pendingBlocks[block3.id] = &PendingBlock{ConsensusBlock: block3}
	engine.mu.Unlock()

	// Sync to height 1000 - blocks at 100 and 500 should be cleared
	importedBlockID := ids.GenerateTestID()
	err := engine.SyncState(ctx, importedBlockID, 1000)
	if err != nil {
		t.Fatalf("SyncState failed: %v", err)
	}

	// Verify stale blocks were removed
	engine.mu.RLock()
	_, has1 := engine.pendingBlocks[block1.id]
	_, has2 := engine.pendingBlocks[block2.id]
	_, has3 := engine.pendingBlocks[block3.id]
	engine.mu.RUnlock()

	if has1 {
		t.Error("Block at height 100 should be cleared (below sync height 1000)")
	}
	if has2 {
		t.Error("Block at height 500 should be cleared (below sync height 1000)")
	}
	if !has3 {
		t.Error("Block at height 1500 should NOT be cleared (above sync height 1000)")
	}
}

// TestSyncState_Idempotent verifies calling SyncState multiple times is safe.
func TestSyncState_Idempotent(t *testing.T) {
	engine := New()
	ctx := context.Background()

	blockID1 := ids.GenerateTestID()
	blockID2 := ids.GenerateTestID()

	// First sync
	if err := engine.SyncState(ctx, blockID1, 100); err != nil {
		t.Fatalf("First SyncState failed: %v", err)
	}

	if engine.consensus.GetFinalizedTip() != blockID1 {
		t.Error("First sync should update finalizedTip")
	}

	// Second sync (higher block)
	if err := engine.SyncState(ctx, blockID2, 200); err != nil {
		t.Fatalf("Second SyncState failed: %v", err)
	}

	if engine.consensus.GetFinalizedTip() != blockID2 {
		t.Error("Second sync should update finalizedTip to new block")
	}

	// Should still be bootstrapped
	if !engine.IsBootstrapped() {
		t.Error("Should remain bootstrapped after multiple SyncState calls")
	}
}

// TestSyncState_WithEmptyID verifies SyncState handles empty block ID.
func TestSyncState_WithEmptyID(t *testing.T) {
	engine := New()
	ctx := context.Background()

	// Sync with empty ID (genesis state)
	err := engine.SyncState(ctx, ids.Empty, 0)
	if err != nil {
		t.Fatalf("SyncState with empty ID failed: %v", err)
	}

	// Should still bootstrap
	if !engine.IsBootstrapped() {
		t.Error("Should be bootstrapped even with empty block ID")
	}
}

// burstVM implements BlockBuilder for throughput benchmarking.
// Simulates a GPU EVM that can produce blocks at maximum speed.
type burstVM struct {
	height    atomic.Uint64
	accepted  atomic.Uint64
	parentID  atomic.Value // ids.ID
	txsPerBlk uint64
}

func newBurstVM(txsPerBlock uint64) *burstVM {
	vm := &burstVM{txsPerBlk: txsPerBlock}
	vm.parentID.Store(ids.Empty)
	return vm
}

func (v *burstVM) BuildBlock(_ context.Context) (block.Block, error) {
	h := v.height.Add(1)
	parent, _ := v.parentID.Load().(ids.ID)
	blk := &mockBlock{
		id:        ids.GenerateTestID(),
		parentID:  parent,
		height:    h,
		timestamp: time.Now(),
		bytes:     make([]byte, v.txsPerBlk*100), // ~100 bytes per tx
	}
	return blk, nil
}

func (v *burstVM) GetBlock(_ context.Context, id ids.ID) (block.Block, error) {
	return nil, errors.New("not needed for benchmark")
}

func (v *burstVM) ParseBlock(_ context.Context, b []byte) (block.Block, error) {
	return &mockBlock{bytes: b}, nil
}

func (v *burstVM) LastAccepted(_ context.Context) (ids.ID, error) {
	parent, _ := v.parentID.Load().(ids.ID)
	return parent, nil
}

func (v *burstVM) SetPreference(_ context.Context, id ids.ID) error {
	v.parentID.Store(id)
	v.accepted.Add(1)
	return nil
}

// BenchmarkBurstPipeline measures the consensus pipeline at maximum speed.
// This benchmarks the engine itself — how many blocks/sec can consensus
// propose, vote, finalize, and pipeline the next build.
func BenchmarkBurstPipeline(b *testing.B) {
	scenarios := []struct {
		name      string
		params    config.Parameters
		txsPerBlk uint64
	}{
		{"burst-100K-txs", singleNodeBurstParams(), 100_000},
		{"local-47K-txs", singleNodeBurstParams(), 47_619},
		{"default-47K-txs", config.SingleValidatorParams(), 47_619},
	}

	for _, sc := range scenarios {
		b.Run(sc.name, func(b *testing.B) {
			vm := newBurstVM(sc.txsPerBlk)
			cfg := Config{
				Params: sc.params,
				VM:     vm,
			}
			engine := NewWithConfig(cfg)
			ctx := context.Background()

			if err := engine.Start(ctx, true); err != nil {
				b.Fatal(err)
			}

			b.ResetTimer()
			b.ReportAllocs()

			for i := 0; i < b.N; i++ {
				_ = engine.Notify(ctx, Message{Type: PendingTxs})
			}

			b.StopTimer()
			_ = engine.Stop(ctx)

			accepted := vm.accepted.Load()
			b.ReportMetric(float64(accepted), "blocks")
			b.ReportMetric(float64(accepted)*float64(sc.txsPerBlk), "txs")
		})
	}
}

// singleNodeBurstParams returns BurstParams with K=1 for single-node benchmarking.
// Self-vote path: block is accepted synchronously in buildBlocksLocked.
func singleNodeBurstParams() config.Parameters {
	p := config.BurstParams()
	p.K = 1
	p.Alpha = 1.0
	p.Beta = 1
	p.AlphaPreference = 1
	p.AlphaConfidence = 1
	p.BetaVirtuous = 1
	p.BetaRogue = 1
	return p
}

// TestBurstThroughput runs the pipeline for a fixed duration and reports TPS.
func TestBurstThroughput(t *testing.T) {
	if testing.Short() {
		t.Skip("skipping burst throughput test in short mode")
	}

	scenarios := []struct {
		name      string
		params    config.Parameters
		txsPerBlk uint64
	}{
		{"SoloGPU-47K", config.SoloGPUParams(), 47_619},
		{"Burst-100K", singleNodeBurstParams(), 100_000},
	}

	for _, sc := range scenarios {
		t.Run(sc.name, func(t *testing.T) {
			vm := newBurstVM(sc.txsPerBlk)
			cfg := Config{Params: sc.params, VM: vm}
			engine := NewWithConfig(cfg)
			ctx, cancel := context.WithCancel(context.Background())

			if err := engine.Start(ctx, true); err != nil {
				t.Fatal(err)
			}

			duration := 3 * time.Second
			deadline := time.Now().Add(duration)
			notifications := 0

			start := time.Now()
			for time.Now().Before(deadline) {
				_ = engine.Notify(ctx, Message{Type: PendingTxs})
				notifications++
			}
			elapsed := time.Since(start)

			cancel()
			_ = engine.Stop(context.Background())

			accepted := vm.accepted.Load()
			height := vm.height.Load()
			blocksPerSec := float64(accepted) / elapsed.Seconds()
			tps := blocksPerSec * float64(sc.txsPerBlk)

			t.Logf("")
			t.Logf("=== Burst Pipeline: %s ===", sc.name)
			t.Logf("Duration:        %v", elapsed.Round(time.Millisecond))
			t.Logf("Notifications:   %d", notifications)
			t.Logf("Blocks built:    %d", height)
			t.Logf("Blocks accepted: %d", accepted)
			t.Logf("Blocks/sec:      %.0f", blocksPerSec)
			t.Logf("TPS (%dK/blk):  %.2fM", sc.txsPerBlk/1000, tps/1_000_000)
			t.Logf("Block time cfg:  %v", cfg.Params.BlockTime)
			t.Logf("Round timeout:   %v", cfg.Params.RoundTO)
			t.Logf("")

			if accepted == 0 {
				t.Fatalf("No blocks accepted — pipeline broken")
			}
			if blocksPerSec < 100 {
				t.Logf("WARNING: < 100 blocks/sec — pipeline not saturated")
			}
		})
	}
}

// --- Slashing integration tests ---

func TestSlashing_DoubleVoteDropped(t *testing.T) {
	detector := slashing.NewDetector(64, 0.5)
	db := slashing.NewDB(1 * time.Hour)
	engine := New(WithSlashing(detector, db))
	ctx := context.Background()

	_ = engine.Start(ctx, true)
	defer engine.Stop(ctx)

	// Add a pending block at height 100
	blkID := ids.ID{0xAA}
	blk := &Block{id: blkID, parentID: ids.Empty, height: 100, timestamp: time.Now().Unix()}
	_ = engine.AddBlock(ctx, blk)
	engine.mu.Lock()
	engine.pendingBlocks[blkID] = &PendingBlock{
		ConsensusBlock: blk,
		ProposedAt:     time.Now(),
	}
	engine.mu.Unlock()

	// Also add a second block at height 100
	blkID2 := ids.ID{0xBB}
	blk2 := &Block{id: blkID2, parentID: ids.Empty, height: 100, timestamp: time.Now().Unix()}
	_ = engine.AddBlock(ctx, blk2)
	engine.mu.Lock()
	engine.pendingBlocks[blkID2] = &PendingBlock{
		ConsensusBlock: blk2,
		ProposedAt:     time.Now(),
	}
	engine.mu.Unlock()

	var nodeA ids.NodeID
	nodeA[0] = 0x01

	// First vote: nodeA votes for blkID at height 100 -- should succeed
	engine.ReceiveVote(Vote{BlockID: blkID, NodeID: nodeA, Accept: true})
	time.Sleep(50 * time.Millisecond) // let vote handler process

	engine.mu.RLock()
	pending1 := engine.pendingBlocks[blkID]
	vote1Count := 0
	if pending1 != nil {
		vote1Count = pending1.VoteCount
	}
	engine.mu.RUnlock()

	if vote1Count != 1 {
		t.Fatalf("first vote should be counted, got VoteCount=%d", vote1Count)
	}

	// Second vote: nodeA votes for blkID2 at height 100 -- equivocation, should be dropped
	engine.ReceiveVote(Vote{BlockID: blkID2, NodeID: nodeA, Accept: true})
	time.Sleep(50 * time.Millisecond)

	engine.mu.RLock()
	pending2 := engine.pendingBlocks[blkID2]
	vote2Count := 0
	if pending2 != nil {
		vote2Count = pending2.VoteCount
	}
	engine.mu.RUnlock()

	if vote2Count != 0 {
		t.Fatalf("equivocating vote should be dropped, got VoteCount=%d", vote2Count)
	}

	// Verify evidence was recorded
	rec := db.GetRecord(nodeA)
	if rec == nil {
		t.Fatal("slash record should exist for equivocating validator")
	}
	if rec.SlashCount != 1 {
		t.Fatalf("expected 1 slash, got %d", rec.SlashCount)
	}
	if !db.IsJailed(nodeA) {
		t.Fatal("equivocating validator should be jailed")
	}
}

func TestSlashing_JailedVotesDropped(t *testing.T) {
	detector := slashing.NewDetector(64, 0.5)
	db := slashing.NewDB(1 * time.Hour)
	engine := New(WithSlashing(detector, db))
	ctx := context.Background()

	_ = engine.Start(ctx, true)
	defer engine.Stop(ctx)

	var nodeA ids.NodeID
	nodeA[0] = 0x01

	// Pre-jail nodeA
	db.RecordEvidence(slashing.Evidence{
		Type:        slashing.DoubleVote,
		ValidatorID: nodeA,
		Height:      50,
		Proof:       []byte("test"),
	})

	// Add a pending block
	blkID := ids.ID{0xCC}
	blk := &Block{id: blkID, parentID: ids.Empty, height: 200, timestamp: time.Now().Unix()}
	_ = engine.AddBlock(ctx, blk)
	engine.mu.Lock()
	engine.pendingBlocks[blkID] = &PendingBlock{
		ConsensusBlock: blk,
		ProposedAt:     time.Now(),
	}
	engine.mu.Unlock()

	// Jailed validator's vote should be dropped entirely (not counted)
	engine.ReceiveVote(Vote{BlockID: blkID, NodeID: nodeA, Accept: true})
	time.Sleep(50 * time.Millisecond)

	engine.mu.RLock()
	pending := engine.pendingBlocks[blkID]
	count := 0
	if pending != nil {
		count = pending.VoteCount
	}
	engine.mu.RUnlock()

	if count != 0 {
		t.Fatalf("jailed validator vote should not be counted, got VoteCount=%d", count)
	}
	if !db.IsJailed(nodeA) {
		t.Fatal("validator should be jailed")
	}
}

func TestSlashing_CheckBlockProposal(t *testing.T) {
	detector := slashing.NewDetector(64, 0.5)
	db := slashing.NewDB(1 * time.Hour)
	engine := New(WithSlashing(detector, db))

	var nodeA ids.NodeID
	nodeA[0] = 0x01

	blkID1 := ids.ID{0xAA}
	blkID2 := ids.ID{0xBB}

	// First block from nodeA at height 100 -- should pass
	ev := engine.CheckBlockProposal(nodeA, 100, blkID1, []byte("data1"))
	if ev != nil {
		t.Fatal("first block should not produce evidence")
	}

	// Second different block from nodeA at height 100 -- equivocation
	ev = engine.CheckBlockProposal(nodeA, 100, blkID2, []byte("data2"))
	if ev == nil {
		t.Fatal("double-sign must produce evidence")
	}
	if ev.Type != slashing.DoubleSign {
		t.Fatalf("expected DoubleSign, got %s", ev.Type)
	}

	// Verify evidence recorded and validator jailed
	if !db.IsJailed(nodeA) {
		t.Fatal("double-signing validator should be jailed")
	}

	// Further proposals from jailed validator should be rejected
	blkID3 := ids.ID{0xCC}
	ev = engine.CheckBlockProposal(nodeA, 200, blkID3, []byte("data3"))
	if ev == nil {
		t.Fatal("jailed validator proposals should be rejected")
	}
}

func TestSlashing_Disabled(t *testing.T) {
	// Without slashing option, everything works normally
	engine := New()
	ctx := context.Background()

	_ = engine.Start(ctx, true)
	defer engine.Stop(ctx)

	blkID := ids.ID{0xAA}
	blk := &Block{id: blkID, parentID: ids.Empty, height: 100, timestamp: time.Now().Unix()}
	_ = engine.AddBlock(ctx, blk)
	engine.mu.Lock()
	engine.pendingBlocks[blkID] = &PendingBlock{
		ConsensusBlock: blk,
		ProposedAt:     time.Now(),
	}
	engine.mu.Unlock()

	var nodeA ids.NodeID
	nodeA[0] = 0x01

	engine.ReceiveVote(Vote{BlockID: blkID, NodeID: nodeA, Accept: true})
	time.Sleep(50 * time.Millisecond)

	engine.mu.RLock()
	pending := engine.pendingBlocks[blkID]
	count := 0
	if pending != nil {
		count = pending.VoteCount
	}
	engine.mu.RUnlock()

	if count != 1 {
		t.Fatalf("vote should be counted when slashing disabled, got %d", count)
	}

	// CheckBlockProposal should return nil when slashing disabled
	ev := engine.CheckBlockProposal(nodeA, 100, blkID, []byte("data"))
	if ev != nil {
		t.Fatal("CheckBlockProposal should return nil when slashing disabled")
	}
}

func TestSlashing_SlashingDBAccessor(t *testing.T) {
	db := slashing.NewDB(1 * time.Hour)
	engine := New(WithSlashing(slashing.NewDetector(64, 0.5), db))

	got := engine.SlashingDB()
	if got != db {
		t.Fatal("SlashingDB() should return the configured DB")
	}

	engine2 := New()
	if engine2.SlashingDB() != nil {
		t.Fatal("SlashingDB() should return nil when slashing disabled")
	}
}

// =============================================================================
// Q-01: BuildBlock error must NOT decrement pendingBuildBlocks
// =============================================================================

// failingBuildVM fails BuildBlock N times then succeeds.
type failingBuildVM struct {
	failCount        int
	callCount        int
	blocks           []*mockBlock
	preferenceID     ids.ID
	setPreferenceErr error
}

func (m *failingBuildVM) BuildBlock(_ context.Context) (block.Block, error) {
	m.callCount++
	if m.callCount <= m.failCount {
		return nil, errors.New("transient BuildBlock error")
	}
	blk := &mockBlock{
		id:     ids.GenerateTestID(),
		height: uint64(m.callCount),
	}
	m.blocks = append(m.blocks, blk)
	return blk, nil
}

func (m *failingBuildVM) GetBlock(_ context.Context, id ids.ID) (block.Block, error) {
	for _, b := range m.blocks {
		if b.id == id {
			return b, nil
		}
	}
	return nil, errors.New("not found")
}

func (m *failingBuildVM) ParseBlock(_ context.Context, b []byte) (block.Block, error) {
	return &mockBlock{bytes: b}, nil
}

func (m *failingBuildVM) LastAccepted(_ context.Context) (ids.ID, error) {
	return m.preferenceID, nil
}

func (m *failingBuildVM) SetPreference(_ context.Context, id ids.ID) error {
	if m.setPreferenceErr != nil {
		return m.setPreferenceErr
	}
	m.preferenceID = id
	return nil
}

// TestBuildBlockError_DoesNotDecrementPending verifies Q-01: BuildBlock error
// must not decrement pendingBuildBlocks so the build is retried.
func TestBuildBlockError_DoesNotDecrementPending(t *testing.T) {
	vm := &failingBuildVM{failCount: 1}
	eng := New(WithVM(vm))

	if err := eng.Start(context.Background(), true); err != nil {
		t.Fatalf("Start: %v", err)
	}
	defer eng.Stop(context.Background())

	// Send PendingTxs — BuildBlock will fail on first attempt
	if err := eng.Notify(context.Background(), Message{Type: PendingTxs}); err != nil {
		t.Fatalf("Notify: %v", err)
	}

	// BuildBlock was called once (and failed)
	if vm.callCount != 1 {
		t.Fatalf("expected 1 BuildBlock call, got %d", vm.callCount)
	}

	// pendingBuildBlocks must still be 1 — NOT decremented on error
	if got := eng.PendingBuildBlocks(); got != 1 {
		t.Fatalf("expected pendingBuildBlocks=1 after error, got %d", got)
	}
}

// TestBuildBlockError_RetriesOnNextNotify verifies that after a BuildBlock
// error, the next Notify successfully builds the block (retry).
func TestBuildBlockError_RetriesOnNextNotify(t *testing.T) {
	vm := &failingBuildVM{failCount: 1}
	eng := New(WithVM(vm))

	if err := eng.Start(context.Background(), true); err != nil {
		t.Fatalf("Start: %v", err)
	}
	defer eng.Stop(context.Background())

	// First Notify: BuildBlock fails, counter stays at 1
	eng.Notify(context.Background(), Message{Type: PendingTxs})
	if vm.callCount != 1 {
		t.Fatalf("expected 1 call after first Notify, got %d", vm.callCount)
	}
	if eng.PendingBuildBlocks() != 1 {
		t.Fatalf("expected pendingBuildBlocks=1 after error, got %d", eng.PendingBuildBlocks())
	}

	// Second Notify: adds another pending + retries. failCount=1 means second
	// call succeeds. The loop processes both pending (the retained one + new one).
	eng.Notify(context.Background(), Message{Type: PendingTxs})

	// Second call succeeded, third call is the second pending item.
	// Call 2 succeeded (first pending consumed), call 3 succeeded (second pending consumed).
	if vm.callCount < 2 {
		t.Fatalf("expected at least 2 BuildBlock calls after retry, got %d", vm.callCount)
	}

	// All pending should be consumed now
	if got := eng.PendingBuildBlocks(); got != 0 {
		t.Fatalf("expected pendingBuildBlocks=0 after successful retry, got %d", got)
	}
}

// TestBuildBlockError_RepeatedFailures_NoPermanentHalt verifies that repeated
// BuildBlock failures don't permanently halt block production. Each Notify
// retains the counter, so future Notifys always attempt to build.
func TestBuildBlockError_RepeatedFailures_NoPermanentHalt(t *testing.T) {
	vm := &failingBuildVM{failCount: 100} // fails 100 times
	eng := New(WithVM(vm))

	if err := eng.Start(context.Background(), true); err != nil {
		t.Fatalf("Start: %v", err)
	}
	defer eng.Stop(context.Background())

	// Send 5 notifications — all will fail but counter grows
	for i := 0; i < 5; i++ {
		eng.Notify(context.Background(), Message{Type: PendingTxs})
	}

	// pendingBuildBlocks should be 5 — none consumed because all failed
	if got := eng.PendingBuildBlocks(); got != 5 {
		t.Fatalf("expected pendingBuildBlocks=5 after 5 failures, got %d", got)
	}

	// Now make BuildBlock succeed (lower failCount below callCount)
	vm.failCount = vm.callCount // next call will succeed

	// Next Notify adds one more pending (total 6) and retries — all 6 should build
	eng.Notify(context.Background(), Message{Type: PendingTxs})

	if got := eng.PendingBuildBlocks(); got != 0 {
		t.Fatalf("expected pendingBuildBlocks=0 after recovery, got %d", got)
	}

	if len(vm.blocks) < 6 {
		t.Fatalf("expected at least 6 blocks built after recovery, got %d", len(vm.blocks))
	}
}

// =============================================================================
// Q-02: SetPreference failure after Accept must force preference
// =============================================================================

// TestSetPreferenceFailure_ForcesPreference verifies that when SetPreference
// fails after Accept, the consensus engine forces its preference to match the
// accepted block, preventing state divergence.
func TestSetPreferenceFailure_ForcesPreference(t *testing.T) {
	vm := &failingBuildVM{
		failCount:        0, // BuildBlock always succeeds
		setPreferenceErr: errors.New("SetPreference broken"),
	}

	// K=1 mode: self-vote causes immediate acceptance in buildBlocksLocked
	eng := New(
		WithVM(vm),
		WithParams(config.Parameters{
			K:               1,
			AlphaPreference: 1,
			AlphaConfidence: 1,
			Beta:            1,
		}),
	)

	if err := eng.Start(context.Background(), true); err != nil {
		t.Fatalf("Start: %v", err)
	}
	defer eng.Stop(context.Background())

	// Trigger block build — will be immediately accepted (K=1),
	// then SetPreference will fail, and ForcePreference must be called.
	eng.Notify(context.Background(), Message{Type: PendingTxs})

	// Block was built and accepted
	if len(vm.blocks) != 1 {
		t.Fatalf("expected 1 block built, got %d", len(vm.blocks))
	}

	acceptedBlockID := vm.blocks[0].id

	// Consensus preference must match the accepted block despite SetPreference error
	if pref := eng.Preference(); pref != acceptedBlockID {
		t.Fatalf("expected preference=%s after forced preference, got %s",
			acceptedBlockID, pref)
	}

	// Verify consensus internal state agrees
	if tip := eng.consensus.GetFinalizedTip(); tip != acceptedBlockID {
		t.Fatalf("expected finalizedTip=%s, got %s", acceptedBlockID, tip)
	}
}