core: clarify code in forkchoice (#26257)

refactoring without logic change

core/forkchoice.go

@@ -74,10 +74,10 @@ func NewForkChoice(chainReader ChainReader, preserve func(header *types.Header)
 // In the td mode, the new head is chosen if the corresponding
 // total difficulty is higher. In the extern mode, the trusted
 // header is always selected as the head.
-func (f *ForkChoice) ReorgNeeded(current *types.Header, header *types.Header) (bool, error) {
+func (f *ForkChoice) ReorgNeeded(current *types.Header, extern *types.Header) (bool, error) {
 	var (
 		localTD  = f.chain.GetTd(current.Hash(), current.Number.Uint64())
-		externTd = f.chain.GetTd(header.Hash(), header.Number.Uint64())
+		externTd = f.chain.GetTd(extern.Hash(), extern.Number.Uint64())
 	)
 	if localTD == nil || externTd == nil {
 		return false, errors.New("missing td")
@@ -88,21 +88,26 @@ func (f *ForkChoice) ReorgNeeded(current *types.Header, header *types.Header) (b
 	if ttd := f.chain.Config().TerminalTotalDifficulty; ttd != nil && ttd.Cmp(externTd) <= 0 {
 		return true, nil
 	}
+
 	// If the total difficulty is higher than our known, add it to the canonical chain
+	if diff := externTd.Cmp(localTD); diff > 0 {
+		return true, nil
+	} else if diff < 0 {
+		return false, nil
+	}
+	// Local and external difficulty is identical.
 	// Second clause in the if statement reduces the vulnerability to selfish mining.
 	// Please refer to http://www.cs.cornell.edu/~ie53/publications/btcProcFC.pdf
-	reorg := externTd.Cmp(localTD) > 0
-	if !reorg && externTd.Cmp(localTD) == 0 {
-		number, headNumber := header.Number.Uint64(), current.Number.Uint64()
-		if number < headNumber {
-			reorg = true
-		} else if number == headNumber {
-			var currentPreserve, externPreserve bool
-			if f.preserve != nil {
-				currentPreserve, externPreserve = f.preserve(current), f.preserve(header)
-			}
-			reorg = !currentPreserve && (externPreserve || f.rand.Float64() < 0.5)
+	reorg := false
+	externNum, localNum := extern.Number.Uint64(), current.Number.Uint64()
+	if externNum < localNum {
+		reorg = true
+	} else if externNum == localNum {
+		var currentPreserve, externPreserve bool
+		if f.preserve != nil {
+			currentPreserve, externPreserve = f.preserve(current), f.preserve(extern)
 		}
+		reorg = !currentPreserve && (externPreserve || f.rand.Float64() < 0.5)
 	}
 	return reorg, nil
 }