op-node: Refactor to expose span batch derivation

op-node/rollup/derive/batch_test.go

@@ -172,9 +172,7 @@ func TestBatchRoundTrip(t *testing.T) {
 		err = dec.UnmarshalBinary(enc)
 		require.NoError(t, err)
 		if dec.GetBatchType() == SpanBatchType {
-			rawSpanBatch, ok := dec.inner.(*RawSpanBatch)
-			require.True(t, ok)
-			_, err := rawSpanBatch.derive(blockTime, genesisTimestamp, chainID)
+			_, err := DeriveSpanBatch(&dec, blockTime, genesisTimestamp, chainID)
 			require.NoError(t, err)
 		}
 		require.Equal(t, batch, &dec, "Batch not equal test case %v", i)
@@ -222,9 +220,7 @@ func TestBatchRoundTripRLP(t *testing.T) {
 		err = dec.DecodeRLP(s)
 		require.NoError(t, err)
 		if dec.GetBatchType() == SpanBatchType {
-			rawSpanBatch, ok := dec.inner.(*RawSpanBatch)
-			require.True(t, ok)
-			_, err := rawSpanBatch.derive(blockTime, genesisTimestamp, chainID)
+			_, err = DeriveSpanBatch(&dec, blockTime, genesisTimestamp, chainID)
 			require.NoError(t, err)
 		}
 		require.Equal(t, batch, &dec, "Batch not equal test case %v", i)

op-node/rollup/derive/channel_in_reader.go

@@ -104,16 +104,7 @@ func (cr *ChannelInReader) NextBatch(ctx context.Context) (Batch, error) {
 			// This is just for early dropping invalid batches as soon as possible.
 			return nil, NewTemporaryError(fmt.Errorf("cannot accept span batch in L1 block %s at time %d", origin, origin.Time))
 		}
-		rawSpanBatch, ok := batchData.inner.(*RawSpanBatch)
-		if !ok {
-			return nil, NewCriticalError(errors.New("failed type assertion to SpanBatch"))
-		}
-		// If the batch type is Span batch, derive block inputs from RawSpanBatch.
-		spanBatch, err := rawSpanBatch.derive(cr.cfg.BlockTime, cr.cfg.Genesis.L2Time, cr.cfg.L2ChainID)
-		if err != nil {
-			return nil, err
-		}
-		return spanBatch, nil
+		return DeriveSpanBatch(batchData, cr.cfg.BlockTime, cr.cfg.Genesis.L2Time, cr.cfg.L2ChainID)
 	default:
 		// error is bubbled up to user, but pipeline can skip the batch and continue after.
 		return nil, NewTemporaryError(fmt.Errorf("unrecognized batch type: %d", batchData.GetBatchType()))

op-node/rollup/derive/span_batch.go

@@ -416,6 +416,16 @@ func (b *RawSpanBatch) derive(blockTime, genesisTimestamp uint64, chainID *big.I
 	return &spanBatch, nil
 }
 
+// ToSpanBatch converts RawSpanBatch to SpanBatch,
+// which implements a wrapper of derive method of RawSpanBatch
+func (b *RawSpanBatch) ToSpanBatch(blockTime, genesisTimestamp uint64, chainID *big.Int) (*SpanBatch, error) {
+	spanBatch, err := b.derive(blockTime, genesisTimestamp, chainID)
+	if err != nil {
+		return nil, err
+	}
+	return spanBatch, nil
+}
+
 // spanBatchElement is a derived form of input to build a L2 block.
 // similar to SingularBatch, but does not have ParentHash and EpochHash
 // because Span batch spec does not contain parent hash and epoch hash of every block in the span.
@@ -604,6 +614,16 @@ func NewSpanBatch(singularBatches []*SingularBatch) *SpanBatch {
 	return spanBatch
 }
 
+// DeriveSpanBatch derives SpanBatch from BatchData.
+func DeriveSpanBatch(batchData *BatchData, blockTime, genesisTimestamp uint64, chainID *big.Int) (*SpanBatch, error) {
+	rawSpanBatch, ok := batchData.inner.(*RawSpanBatch)
+	if !ok {
+		return nil, NewCriticalError(errors.New("failed type assertion to SpanBatch"))
+	}
+	// If the batch type is Span batch, derive block inputs from RawSpanBatch.
+	return rawSpanBatch.ToSpanBatch(blockTime, genesisTimestamp, chainID)
+}
+
 // SpanBatchBuilder is a utility type to build a SpanBatch by adding a SingularBatch one by one.
 // makes easier to stack SingularBatches and convert to RawSpanBatch for encoding.
 type SpanBatchBuilder struct {