diff --git a/.golangci.yml b/.golangci.yml
index 3212fdc..08dd609 100644
--- a/.golangci.yml
+++ b/.golangci.yml
@@ -12,6 +12,7 @@ linters:
         - goimports
         - lll
         - golint
+        - gocyclo
 linters-settings:
         lll:
                 line-length: 100
diff --git a/addbatch.go b/addbatch.go
index 345d080..475b3db 100644
--- a/addbatch.go
+++ b/addbatch.go
@@ -170,28 +170,16 @@ func (t *Tree) AddBatchOpt(keys, values [][]byte) ([]int, error) {
 	// nCPU
 	nCPU := highestPowerOfTwo(runtime.NumCPU())
 	l := int(math.Log2(float64(nCPU)))
+	var invalids []int
 
 	// CASE A: if nLeafs==0 (root==0)
 	if bytes.Equal(t.root, t.emptyHash) {
-		// if len(kvs) is not a power of 2, cut at the bigger power
-		// of two under len(kvs), build the tree with that, and add
-		// later the excedents
-		kvsP2, kvsNonP2 := cutPowerOfTwo(kvs)
-		invalids, err := t.buildTreeBottomUp(nCPU, kvsP2)
+		invalids, err = t.caseA(nCPU, kvs)
 		if err != nil {
 			return nil, err
 		}
-		for i := 0; i < len(kvsNonP2); i++ {
-			err = t.add(0, kvsNonP2[i].k, kvsNonP2[i].v)
-			if err != nil {
-				invalids = append(invalids, kvsNonP2[i].pos)
-			}
-		}
-		// store root to db
-		if err := t.tx.Put(dbKeyRoot, t.root); err != nil {
-			return nil, err
-		}
-		if err = t.tx.Commit(); err != nil {
+
+		if err = t.finalizeAddBatch(); err != nil {
 			return nil, err
 		}
 		return invalids, nil
@@ -203,7 +191,8 @@ func (t *Tree) AddBatchOpt(keys, values [][]byte) ([]int, error) {
 		return nil, err
 	}
 	if nLeafs < minLeafsThreshold { // CASE B
-		invalids, excedents, err := t.caseB(nCPU, 0, kvs)
+		var excedents []kv
+		invalids, excedents, err = t.caseB(nCPU, 0, kvs)
 		if err != nil {
 			return nil, err
 		}
@@ -214,11 +203,8 @@ func (t *Tree) AddBatchOpt(keys, values [][]byte) ([]int, error) {
 				invalids = append(invalids, excedents[i].pos)
 			}
 		}
-		// store root to db
-		if err := t.tx.Put(dbKeyRoot, t.root); err != nil {
-			return nil, err
-		}
-		if err = t.tx.Commit(); err != nil {
+
+		if err = t.finalizeAddBatch(); err != nil {
 			return nil, err
 		}
 		return invalids, nil
@@ -231,95 +217,40 @@ func (t *Tree) AddBatchOpt(keys, values [][]byte) ([]int, error) {
 
 	// CASE C: if nLeafs>=minLeafsThreshold && (nLeafs/nBuckets) < minLeafsThreshold
 	// available parallelization, will need to be a power of 2 (2**n)
-	var excedents []kv
 	if nLeafs >= minLeafsThreshold &&
 		(nLeafs/nCPU) < minLeafsThreshold &&
 		len(keysAtL) == nCPU {
-		// TODO move to own function
-		// 1. go down until level L (L=log2(nBuckets))
-
-		buckets := splitInBuckets(kvs, nCPU)
-
-		// 2. use keys at level L as roots of the subtrees under each one
-		excedentsInBucket := make([][]kv, nCPU)
-		subRoots := make([][]byte, nCPU)
-		txs := make([]db.Tx, nCPU)
-		var wg sync.WaitGroup
-		wg.Add(nCPU)
-		for i := 0; i < nCPU; i++ {
-			go func(cpu int) {
-				var err error
-				txs[cpu], err = t.db.NewTx()
-				if err != nil {
-					panic(err) // TODO WIP
-				}
-				bucketTree := Tree{tx: txs[cpu], db: t.db, maxLevels: t.maxLevels,
-					hashFunction: t.hashFunction, root: keysAtL[cpu]}
-
-				// 3. and do CASE B (with 1 cpu) for each
-				_, bucketExcedents, err := bucketTree.caseB(1, l, buckets[cpu])
-				if err != nil {
-					panic(err)
-					// return nil, err
-				}
-				excedentsInBucket[cpu] = bucketExcedents
-				subRoots[cpu] = bucketTree.root
-				wg.Done()
-			}(i)
-		}
-		wg.Wait()
-
-		// merge buckets txs into Tree.tx
-		for i := 0; i < len(txs); i++ {
-			if err := t.tx.Add(txs[i]); err != nil {
-				return nil, err
-			}
-		}
-		for i := 0; i < len(excedentsInBucket); i++ {
-			excedents = append(excedents, excedentsInBucket[i]...)
-		}
-
-		// 4. go upFromKeys from the new roots of the subtrees
-		newRoot, err := t.upFromKeys(subRoots)
+		invalids, err = t.caseC(nCPU, l, keysAtL, kvs)
 		if err != nil {
 			return nil, err
 		}
-		t.root = newRoot
-
-		// add the key-values that have not been used yet
-		var invalids []int
-		for i := 0; i < len(excedents); i++ {
-			// Add until the level L
-			err = t.add(0, excedents[i].k, excedents[i].v)
-			if err != nil {
-				invalids = append(invalids, excedents[i].pos) // TODO WIP
-			}
-		}
-		// store root to db
-		if err := t.tx.Put(dbKeyRoot, t.root); err != nil {
-			return nil, err
-		}
 
-		if err = t.tx.Commit(); err != nil {
+		if err = t.finalizeAddBatch(); err != nil {
 			return nil, err
 		}
 		return invalids, nil
 	}
 
-	var invalids []int
 	// CASE E
 	if len(keysAtL) != nCPU {
 		// CASE E: add one key at each bucket, and then do CASE D
 		buckets := splitInBuckets(kvs, nCPU)
 		kvs = []kv{}
 		for i := 0; i < len(buckets); i++ {
-			err = t.add(0, buckets[i][0].k, buckets[i][0].v)
-			if err != nil {
-				invalids = append(invalids, buckets[i][0].pos)
-				// TODO if err, add another key-value from the
-				// same bucket
+			// add one leaf of the bucket, if there is an error when
+			// adding the k-v, try to add the next one of the bucket
+			// (until one is added)
+			var inserted int
+			for j := 0; j < len(buckets[i]); j++ {
+				if err := t.add(0, buckets[i][j].k, buckets[i][j].v); err == nil {
+					inserted = j
+					break
+				}
 			}
-			kvs = append(kvs, buckets[i][1:]...)
+
+			// put the buckets elements except the inserted one
+			kvs = append(kvs, buckets[i][:inserted]...)
+			kvs = append(kvs, buckets[i][inserted+1:]...)
 		}
 		keysAtL, err = t.getKeysAtLevel(l + 1)
 		if err != nil {
@@ -327,24 +258,6 @@ func (t *Tree) AddBatchOpt(keys, values [][]byte) ([]int, error) {
 		}
 	}
 
-	if nCPU == 1 { // CASE D, but with 1 cpu
-		for i := 0; i < len(keys); i++ {
-			err = t.add(0, keys[i], values[i])
-			if err != nil {
-				invalids = append(invalids, i)
-			}
-		}
-		// store root to db
-		if err := t.tx.Put(dbKeyRoot, t.root); err != nil {
-			return nil, err
-		}
-
-		if err = t.tx.Commit(); err != nil {
-			return nil, err
-		}
-		return invalids, nil
-	}
-
 	// CASE D
 	if len(keysAtL) == nCPU { // enter in CASE D if len(keysAtL)=nCPU, if not, CASE E
 		invalidsCaseD, err := t.caseD(nCPU, l, keysAtL, kvs)
@@ -352,12 +265,8 @@ func (t *Tree) AddBatchOpt(keys, values [][]byte) ([]int, error) {
 			return nil, err
 		}
 		invalids = append(invalids, invalidsCaseD...)
-		// store root to db
-		if err := t.tx.Put(dbKeyRoot, t.root); err != nil {
-			return nil, err
-		}
 
-		if err = t.tx.Commit(); err != nil {
+		if err = t.finalizeAddBatch(); err != nil {
 			return nil, err
 		}
 		return invalids, nil
@@ -368,6 +277,35 @@ func (t *Tree) AddBatchOpt(keys, values [][]byte) ([]int, error) {
 	return nil, fmt.Errorf("UNIMPLEMENTED")
 }
 
+func (t *Tree) finalizeAddBatch() error {
+	// store root to db
+	if err := t.tx.Put(dbKeyRoot, t.root); err != nil {
+		return err
+	}
+	// commit db tx
+	if err := t.tx.Commit(); err != nil {
+		return err
+	}
+	return nil
+}
+
+func (t *Tree) caseA(nCPU int, kvs []kv) ([]int, error) {
+	// if len(kvs) is not a power of 2, cut at the bigger power
+	// of two under len(kvs), build the tree with that, and add
+	// later the excedents
+	kvsP2, kvsNonP2 := cutPowerOfTwo(kvs)
+	invalids, err := t.buildTreeBottomUp(nCPU, kvsP2)
+	if err != nil {
+		return nil, err
+	}
+	for i := 0; i < len(kvsNonP2); i++ {
+		if err = t.add(0, kvsNonP2[i].k, kvsNonP2[i].v); err != nil {
+			invalids = append(invalids, kvsNonP2[i].pos)
+		}
+	}
+	return invalids, nil
+}
+
 func (t *Tree) caseB(nCPU, l int, kvs []kv) ([]int, []kv, error) {
 	// get already existing keys
 	aKs, aVs, err := t.getLeafs(t.root)
@@ -402,7 +340,80 @@ func (t *Tree) caseB(nCPU, l int, kvs []kv) ([]int, []kv, error) {
 	return invalids, kvsNonP2, nil
 }
 
+func (t *Tree) caseC(nCPU, l int, keysAtL [][]byte, kvs []kv) ([]int, error) {
+	// 1. go down until level L (L=log2(nBuckets)): keysAtL
+
+	var excedents []kv
+	buckets := splitInBuckets(kvs, nCPU)
+
+	// 2. use keys at level L as roots of the subtrees under each one
+	excedentsInBucket := make([][]kv, nCPU)
+	subRoots := make([][]byte, nCPU)
+	txs := make([]db.Tx, nCPU)
+	var wg sync.WaitGroup
+	wg.Add(nCPU)
+	for i := 0; i < nCPU; i++ {
+		go func(cpu int) {
+			var err error
+			txs[cpu], err = t.db.NewTx()
+			if err != nil {
+				panic(err) // TODO WIP
+			}
+			bucketTree := Tree{tx: txs[cpu], db: t.db, maxLevels: t.maxLevels,
+				hashFunction: t.hashFunction, root: keysAtL[cpu]}
+
+			// 3. do CASE B (with 1 cpu) for each key at level L
+			_, bucketExcedents, err := bucketTree.caseB(1, l, buckets[cpu])
+			if err != nil {
+				panic(err)
+				// return nil, err
+			}
+			excedentsInBucket[cpu] = bucketExcedents
+			subRoots[cpu] = bucketTree.root
+			wg.Done()
+		}(i)
+	}
+	wg.Wait()
+
+	// merge buckets txs into Tree.tx
+	for i := 0; i < len(txs); i++ {
+		if err := t.tx.Add(txs[i]); err != nil {
+			return nil, err
+		}
+	}
+	for i := 0; i < len(excedentsInBucket); i++ {
+		excedents = append(excedents, excedentsInBucket[i]...)
+	}
+
+	// 4. go upFromKeys from the new roots of the subtrees
+	newRoot, err := t.upFromKeys(subRoots)
+	if err != nil {
+		return nil, err
+	}
+	t.root = newRoot
+
+	// add the key-values that have not been used yet
+	var invalids []int
+	for i := 0; i < len(excedents); i++ {
+		// Add until the level L
+		if err = t.add(0, excedents[i].k, excedents[i].v); err != nil {
+			invalids = append(invalids, excedents[i].pos) // TODO WIP
+		}
+	}
+	return invalids, nil
+}
+
 func (t *Tree) caseD(nCPU, l int, keysAtL [][]byte, kvs []kv) ([]int, error) {
+	if nCPU == 1 { // CASE D, but with 1 cpu
+		var invalids []int
+		for i := 0; i < len(kvs); i++ {
+			if err := t.add(0, kvs[i].k, kvs[i].v); err != nil {
+				invalids = append(invalids, kvs[i].pos)
+			}
+		}
+		return invalids, nil
+	}
+
 	buckets := splitInBuckets(kvs, nCPU)
 
 	subRoots := make([][]byte, nCPU)
diff --git a/addbatch_test.go b/addbatch_test.go
index a90d042..11a0fa8 100644
--- a/addbatch_test.go
+++ b/addbatch_test.go
@@ -517,3 +517,4 @@ func TestHighestPowerOfTwo(t *testing.T) {
 // adding the rest of keys with loop over normal Add, and with AddBatch
 
 // TODO test adding batch with repeated keys in the batch
+// TODO test adding batch with multiple invalid keys