[#205] object: Initial EC implementation

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

object/erasurecode/constructor.go

@@ -0,0 +1,87 @@
+package erasurecode
+
+import (
+	"errors"
+
+	objectSDK "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object"
+	"github.com/klauspost/reedsolomon"
+)
+
+var (
+	// ErrMalformedSlice is returned when a slice of EC chunks is inconsistent.
+	ErrMalformedSlice = errors.New("inconsistent EC headers")
+	// ErrInvShardNum is returned from NewConstructor when the number of shards is invalid.
+	ErrInvShardNum = reedsolomon.ErrInvShardNum
+	// ErrMaxShardNum is returned from NewConstructor when the number of shards is too big.
+	ErrMaxShardNum = reedsolomon.ErrMaxShardNum
+)
+
+// MaxShardCount is the maximum number of shards.
+const MaxShardCount = 256
+
+// Constructor is a wrapper around encoder allowing to reconstruct objects.
+// It's methods are not thread-safe.
+type Constructor struct {
+	enc           reedsolomon.Encoder
+	headerLength  uint32
+	payloadShards [][]byte
+	headerShards  [][]byte
+}
+
+// NewConstructor returns new constructor instance.
+func NewConstructor(dataCount int, parityCount int) (*Constructor, error) {
+	// The library supports up to 65536 shards with some restrictions.
+	// This can easily result in OOM or panic, thus SDK declares it's own restriction.
+	if dataCount+parityCount > MaxShardCount {
+		return nil, ErrMaxShardNum
+	}
+
+	enc, err := reedsolomon.New(dataCount, parityCount)
+	if err != nil {
+		return nil, err
+	}
+	return &Constructor{enc: enc}, nil
+}
+
+// clear clears internal state of the constructor, so it can be reused.
+func (c *Constructor) clear() {
+	c.headerLength = 0
+	c.payloadShards = nil
+	c.headerShards = nil
+}
+
+func (c *Constructor) fillHeader(parts []*objectSDK.Object) error {
+	shards := make([][]byte, len(parts))
+	headerLength := 0
+	for i := range parts {
+		if parts[i] == nil {
+			continue
+		}
+
+		var err error
+		headerLength, err = validatePart(parts, i, headerLength)
+		if err != nil {
+			return err
+		}
+
+		shards[i] = parts[i].GetECHeader().Header()
+	}
+
+	c.headerLength = uint32(headerLength)
+	c.headerShards = shards
+	return nil
+}
+
+// fillPayload fills the payload shards.
+// Currently there is no case when it can be called without reconstructing header,
+// thus fillHeader() must be called before and this function performs no validation.
+func (c *Constructor) fillPayload(parts []*objectSDK.Object) {
+	shards := make([][]byte, len(parts))
+	for i := range parts {
+		if parts[i] == nil {
+			continue
+		}
+		shards[i] = parts[i].Payload()
+	}
+	c.payloadShards = shards
+}