Fix an off-by-one in the powerstore (#736)

Also fix the test.

Author: Stebalien

internal/powerstore/powerstore.go

@@ -125,8 +125,8 @@ func (ps *Store) f3PowerBase(ctx context.Context) (int64, uint64, error) {
 	baseInstance := ps.manifest.InitialInstance
 	if lastCert := ps.cs.Latest(); lastCert != nil {
 		baseInstance = lastCert.GPBFTInstance + 1
-		if lastCert.GPBFTInstance > ps.manifest.InitialInstance+ps.manifest.CommitteeLookback {
-			baseCert, err := ps.cs.Get(ctx, lastCert.GPBFTInstance-ps.manifest.CommitteeLookback)
+		if baseInstance >= ps.manifest.InitialInstance+ps.manifest.CommitteeLookback {
+			baseCert, err := ps.cs.Get(ctx, baseInstance-ps.manifest.CommitteeLookback)
 			if err != nil {
 				return 0, 0, err
 			}

internal/powerstore/powerstore_test.go

@@ -2,6 +2,7 @@ package powerstore_test
 
 import (
 	"context"
+	"math/rand/v2"
 	"slices"
 	"testing"
 	"time"
@@ -200,12 +201,12 @@ func advanceF3(t *testing.T, m *manifest.Manifest, ps *powerstore.Store, cs *cer
 	basePt, err := cs.GetPowerTable(ctx, instance)
 	require.NoError(t, err)
 	for len(gpbftChain) > 1 {
-		count := min(len(gpbftChain), 1+epochsPerCert, gpbft.ChainMaxLen)
+		count := min(len(gpbftChain), rand.IntN(epochsPerCert+1)+1, gpbft.ChainMaxLen)
 		newChain := gpbftChain[:count]
 
 		nextPt := basePt
-		if instance >= m.InitialInstance+m.CommitteeLookback {
-			ptCert, err := cs.Get(ctx, instance-m.CommitteeLookback)
+		if instance+1 >= m.InitialInstance+m.CommitteeLookback {
+			ptCert, err := cs.Get(ctx, instance+1-m.CommitteeLookback)
 			require.NoError(t, err)
 			nextPt, err = ps.GetPowerTable(ctx, ptCert.ECChain.Head().Key)
 			require.NoError(t, err)