Allow reconstructing into pre-allocated memory. (#66)

This changes the interface of Reconstruct and ReconstructData to accept
slices of zero length but sufficient capacity for shards to reconstruct,
and reslices them instead of allocating new memory.

reedsolomon.go

@@ -40,7 +40,9 @@ type Encoder interface {
 	// ones that don't have data.
 	//
 	// The length of the array must be equal to the total number of shards.
-	// You indicate that a shard is missing by setting it to nil.
+	// You indicate that a shard is missing by setting it to nil or zero-length.
+	// If a shard is zero-length but has sufficient capacity, that memory will
+	// be used, otherwise a new []byte will be allocated.
 	//
 	// If there are too few shards to reconstruct the missing
 	// ones, ErrTooFewShards will be returned.
@@ -55,7 +57,9 @@ type Encoder interface {
 	// data shards that don't have data.
 	//
 	// The length of the array must be equal to Shards.
-	// You indicate that a shard is missing by setting it to nil.
+	// You indicate that a shard is missing by setting it to nil or zero-length.
+	// If a shard is zero-length but has sufficient capacity, that memory will
+	// be used, otherwise a new []byte will be allocated.
 	//
 	// If there are too few shards to reconstruct the missing
 	// ones, ErrTooFewShards will be returned.
@@ -547,7 +551,9 @@ func shardSize(shards [][]byte) int {
 // ones that don't have data.
 //
 // The length of the array must be equal to Shards.
-// You indicate that a shard is missing by setting it to nil.
+// You indicate that a shard is missing by setting it to nil or zero-length.
+// If a shard is zero-length but has sufficient capacity, that memory will
+// be used, otherwise a new []byte will be allocated.
 //
 // If there are too few shards to reconstruct the missing
 // ones, ErrTooFewShards will be returned.
@@ -564,7 +570,9 @@ func (r reedSolomon) Reconstruct(shards [][]byte) error {
 // data shards that don't have data.
 //
 // The length of the array must be equal to Shards.
-// You indicate that a shard is missing by setting it to nil.
+// You indicate that a shard is missing by setting it to nil or zero-length.
+// If a shard is zero-length but has sufficient capacity, that memory will
+// be used, otherwise a new []byte will be allocated.
 //
 // If there are too few shards to reconstruct the missing
 // ones, ErrTooFewShards will be returned.
@@ -682,7 +690,11 @@ func (r reedSolomon) reconstruct(shards [][]byte, dataOnly bool) error {
 
 	for iShard := 0; iShard < r.DataShards; iShard++ {
 		if len(shards[iShard]) == 0 {
-			shards[iShard] = make([]byte, shardSize)
+			if cap(shards[iShard]) >= shardSize {
+				shards[iShard] = shards[iShard][0:shardSize]
+			} else {
+				shards[iShard] = make([]byte, shardSize)
+			}
 			outputs[outputCount] = shards[iShard]
 			matrixRows[outputCount] = dataDecodeMatrix[iShard]
 			outputCount++
@@ -704,7 +716,11 @@ func (r reedSolomon) reconstruct(shards [][]byte, dataOnly bool) error {
 	outputCount = 0
 	for iShard := r.DataShards; iShard < r.Shards; iShard++ {
 		if len(shards[iShard]) == 0 {
-			shards[iShard] = make([]byte, shardSize)
+			if cap(shards[iShard]) >= shardSize {
+				shards[iShard] = shards[iShard][0:shardSize]
+			} else {
+				shards[iShard] = make([]byte, shardSize)
+			}
 			outputs[outputCount] = shards[iShard]
 			matrixRows[outputCount] = r.parity[iShard-r.DataShards]
 			outputCount++

reedsolomon_test.go

@@ -313,10 +313,12 @@ func testReconstruct(t *testing.T, o ...Option) {
 		t.Fatal(err)
 	}
 
-	// Reconstruct with 10 shards present
+	// Reconstruct with 10 shards present. Use pre-allocated memory for one of them.
 	shards[0] = nil
 	shards[7] = nil
-	shards[11] = nil
+	shard11 := shards[11]
+	shards[11] = shard11[:0]
+	fillRandom(shard11)
 
 	err = r.Reconstruct(shards)
 	if err != nil {
@@ -331,6 +333,10 @@ func testReconstruct(t *testing.T, o ...Option) {
 		t.Fatal("Verification failed")
 	}
 
+	if &shard11[0] != &shards[11][0] {
+		t.Errorf("Shard was not reconstructed into pre-allocated memory")
+	}
+
 	// Reconstruct with 9 shards present (should fail)
 	shards[0] = nil
 	shards[4] = nil
@@ -386,10 +392,12 @@ func testReconstructData(t *testing.T, o ...Option) {
 		t.Fatal(err)
 	}
 
-	// Reconstruct with 10 shards present
+	// Reconstruct with 10 shards present. Use pre-allocated memory for one of them.
 	shards[0] = nil
 	shards[2] = nil
-	shards[4] = nil
+	shard4 := shards[4]
+	shards[4] = shard4[:0]
+	fillRandom(shard4)
 
 	err = r.ReconstructData(shards)
 	if err != nil {
@@ -405,6 +413,10 @@ func testReconstructData(t *testing.T, o ...Option) {
 		t.Fatal("Verification failed")
 	}
 
+	if &shard4[0] != &shards[4][0] {
+		t.Errorf("Shard was not reconstructed into pre-allocated memory")
+	}
+
 	// Reconstruct with 6 data and 4 parity shards
 	shards[0] = nil
 	shards[2] = nil