consensus/clique: unexport calcDifficulty and improve comment (ethere…

…um#21619)

consensus/clique/clique.go

@@ -520,7 +520,7 @@ func (c *Clique) Prepare(chain consensus.ChainHeaderReader, header *types.Header
 		c.lock.RUnlock()
 	}
 	// Set the correct difficulty
-	header.Difficulty = CalcDifficulty(snap, c.signer)
+	header.Difficulty = calcDifficulty(snap, c.signer)
 
 	// Ensure the extra data has all its components
 	if len(header.Extra) < extraVanity {
@@ -652,20 +652,18 @@ func (c *Clique) Seal(chain consensus.ChainHeaderReader, block *types.Block, res
 }
 
 // CalcDifficulty is the difficulty adjustment algorithm. It returns the difficulty
-// that a new block should have based on the previous blocks in the chain and the
-// current signer.
+// that a new block should have:
+// * DIFF_NOTURN(2) if BLOCK_NUMBER % SIGNER_COUNT != SIGNER_INDEX
+// * DIFF_INTURN(1) if BLOCK_NUMBER % SIGNER_COUNT == SIGNER_INDEX
 func (c *Clique) CalcDifficulty(chain consensus.ChainHeaderReader, time uint64, parent *types.Header) *big.Int {
 	snap, err := c.snapshot(chain, parent.Number.Uint64(), parent.Hash(), nil)
 	if err != nil {
 		return nil
 	}
-	return CalcDifficulty(snap, c.signer)
+	return calcDifficulty(snap, c.signer)
 }
 
-// CalcDifficulty is the difficulty adjustment algorithm. It returns the difficulty
-// that a new block should have based on the previous blocks in the chain and the
-// current signer.
-func CalcDifficulty(snap *Snapshot, signer common.Address) *big.Int {
+func calcDifficulty(snap *Snapshot, signer common.Address) *big.Int {
 	if snap.inturn(snap.Number+1, signer) {
 		return new(big.Int).Set(diffInTurn)
 	}