diff --git a/examples_test.go b/examples_test.go
index 7ba7407..dce8ddf 100644
--- a/examples_test.go
+++ b/examples_test.go
@@ -58,6 +58,63 @@ func ExampleEncoder() {
 	// ok
 }
 
+// Simple example of how to use all functions of the EncoderIdx.
+// Note that all error checks have been removed to keep it short.
+func ExampleEncoder_EncodeIdx() {
+	const dataShards = 7
+	const erasureShards = 3
+
+	// Create some sample data
+	var data = make([]byte, 250000)
+	fillRandom(data)
+
+	// Create an encoder with 17 data and 3 parity slices.
+	enc, _ := reedsolomon.New(dataShards, erasureShards)
+
+	// Split the data into shards
+	shards, _ := enc.Split(data)
+
+	// Zero erasure shards.
+	for i := 0; i < erasureShards; i++ {
+		clear := shards[dataShards+i]
+		for j := range clear {
+			clear[j] = 0
+		}
+	}
+
+	for i := 0; i < dataShards; i++ {
+		// Encode one shard at the time.
+		// Note how this gives linear access.
+		// There is however no requirement on shards being delivered in order.
+		// All parity shards will be updated on each run.
+		_ = enc.EncodeIdx(shards[i], i, shards[dataShards:])
+	}
+
+	// Verify the parity set
+	ok, err := enc.Verify(shards)
+	if ok {
+		fmt.Println("ok")
+	} else {
+		fmt.Println(err)
+	}
+
+	// Delete two shards
+	shards[dataShards-2], shards[dataShards-2] = nil, nil
+
+	// Reconstruct the shards
+	_ = enc.Reconstruct(shards)
+
+	// Verify the data set
+	ok, err = enc.Verify(shards)
+	if ok {
+		fmt.Println("ok")
+	} else {
+		fmt.Println(err)
+	}
+	// Output: ok
+	// ok
+}
+
 // This demonstrates that shards can be arbitrary sliced and
 // merged and still remain valid.
 func ExampleEncoder_slicing() {
diff --git a/reedsolomon.go b/reedsolomon.go
index 87f39db..0f8234f 100644
--- a/reedsolomon.go
+++ b/reedsolomon.go
@@ -32,6 +32,12 @@ type Encoder interface {
 	// data shards while this is running.
 	Encode(shards [][]byte) error
 
+	// EncodeIdx will add parity for a single data shard.
+	// Parity shards should start out as 0. The caller must zero them.
+	// Data shards must be delivered exactly once. There is no check for this.
+	// The parity shards will always be updated and the data shards will remain the same.
+	EncodeIdx(dataShard []byte, idx int, parity [][]byte) error
+
 	// Verify returns true if the parity shards contain correct data.
 	// The data is the same format as Encode. No data is modified, so
 	// you are allowed to read from data while this is running.
@@ -396,6 +402,48 @@ func (r *reedSolomon) Encode(shards [][]byte) error {
 	return nil
 }
 
+// EncodeIdx will add parity for a single data shard.
+// Parity shards should start out zeroed. The caller must zero them before first call.
+// Data shards should only be delivered once. There is no check for this.
+// The parity shards will always be updated and the data shards will remain the unchanged.
+func (r *reedSolomon) EncodeIdx(dataShard []byte, idx int, parity [][]byte) error {
+	if len(parity) != r.ParityShards {
+		return ErrTooFewShards
+	}
+	if len(parity) == 0 {
+		return nil
+	}
+	if idx < 0 || idx >= r.DataShards {
+		return ErrInvShardNum
+	}
+	err := checkShards(parity, false)
+	if err != nil {
+		return err
+	}
+	if len(parity[0]) != len(dataShard) {
+		return ErrShardSize
+	}
+
+	// Process using no goroutines for now.
+	start, end := 0, r.o.perRound
+	if end > len(dataShard) {
+		end = len(dataShard)
+	}
+
+	for start < len(dataShard) {
+		in := dataShard[start:end]
+		for iRow := 0; iRow < r.ParityShards; iRow++ {
+			galMulSliceXor(r.parity[iRow][idx], in, parity[iRow][start:end], &r.o)
+		}
+		start = end
+		end += r.o.perRound
+		if end > len(dataShard) {
+			end = len(dataShard)
+		}
+	}
+	return nil
+}
+
 // ErrInvalidInput is returned if invalid input parameter of Update.
 var ErrInvalidInput = errors.New("invalid input")
 
diff --git a/reedsolomon_test.go b/reedsolomon_test.go
index 4a51838..411d55e 100644
--- a/reedsolomon_test.go
+++ b/reedsolomon_test.go
@@ -171,10 +171,18 @@ func TestEncoding(t *testing.T) {
 	t.Run("default", func(t *testing.T) {
 		testEncoding(t, testOptions()...)
 	})
+	t.Run("default-dx", func(t *testing.T) {
+		testEncodingIdx(t, testOptions()...)
+	})
 	for i, o := range testOpts() {
 		t.Run(fmt.Sprintf("opt-%d", i), func(t *testing.T) {
 			testEncoding(t, o...)
 		})
+		if !testing.Short() {
+			t.Run(fmt.Sprintf("idx-opt-%d", i), func(t *testing.T) {
+				testEncodingIdx(t, o...)
+			})
+		}
 	}
 }
 
@@ -207,7 +215,7 @@ func testEncoding(t *testing.T, o ...Option) {
 						shards[s] = make([]byte, perShard)
 					}
 
-					for s := 0; s < data; s++ {
+					for s := 0; s < len(shards); s++ {
 						rng.Read(shards[s])
 					}
 
@@ -281,6 +289,108 @@ func testEncoding(t *testing.T, o ...Option) {
 	}
 }
 
+func testEncodingIdx(t *testing.T, o ...Option) {
+	for _, size := range testSizes {
+		data, parity := size[0], size[1]
+		rng := rand.New(rand.NewSource(0xabadc0cac01a))
+		t.Run(fmt.Sprintf("%dx%d", data, parity), func(t *testing.T) {
+			sz := testDataSizes
+			if testing.Short() {
+				sz = testDataSizesShort
+			}
+			for _, perShard := range sz {
+				t.Run(fmt.Sprint(perShard), func(t *testing.T) {
+
+					r, err := New(data, parity, testOptions(o...)...)
+					if err != nil {
+						t.Fatal(err)
+					}
+					shards := make([][]byte, data+parity)
+					for s := range shards {
+						shards[s] = make([]byte, perShard)
+					}
+					shuffle := make([]int, data)
+					for i := range shuffle {
+						shuffle[i] = i
+					}
+					rng.Shuffle(len(shuffle), func(i, j int) { shuffle[i], shuffle[j] = shuffle[j], shuffle[i] })
+
+					// Send shards in random order.
+					for s := 0; s < data; s++ {
+						s := shuffle[s]
+						rng.Read(shards[s])
+						err = r.EncodeIdx(shards[s], s, shards[data:])
+						if err != nil {
+							t.Fatal(err)
+						}
+					}
+
+					ok, err := r.Verify(shards)
+					if err != nil {
+						t.Fatal(err)
+					}
+					if !ok {
+						t.Fatal("Verification failed")
+					}
+
+					if parity == 0 {
+						// Check that Reconstruct and ReconstructData do nothing
+						err = r.ReconstructData(shards)
+						if err != nil {
+							t.Fatal(err)
+						}
+						err = r.Reconstruct(shards)
+						if err != nil {
+							t.Fatal(err)
+						}
+
+						// Skip integrity checks
+						return
+					}
+
+					// Delete one in data
+					idx := rng.Intn(data)
+					want := shards[idx]
+					shards[idx] = nil
+
+					err = r.ReconstructData(shards)
+					if err != nil {
+						t.Fatal(err)
+					}
+					if !bytes.Equal(shards[idx], want) {
+						t.Fatal("did not ReconstructData correctly")
+					}
+
+					// Delete one randomly
+					idx = rng.Intn(data + parity)
+					want = shards[idx]
+					shards[idx] = nil
+					err = r.Reconstruct(shards)
+					if err != nil {
+						t.Fatal(err)
+					}
+					if !bytes.Equal(shards[idx], want) {
+						t.Fatal("did not Reconstruct correctly")
+					}
+
+					err = r.Encode(make([][]byte, 1))
+					if err != ErrTooFewShards {
+						t.Errorf("expected %v, got %v", ErrTooFewShards, err)
+					}
+
+					// Make one too short.
+					shards[idx] = shards[idx][:perShard-1]
+					err = r.Encode(shards)
+					if err != ErrShardSize {
+						t.Errorf("expected %v, got %v", ErrShardSize, err)
+					}
+				})
+			}
+		})
+
+	}
+}
+
 func TestUpdate(t *testing.T) {
 	for i, o := range testOpts() {
 		t.Run(fmt.Sprintf("options %d", i), func(t *testing.T) {