[#340] object: Introduce SplitHeader type

* Also introduce `SplitHeader` getter and `SetSplitHeader` setter
  for `Object` type.

Signed-off-by: Airat Arifullin <a.arifullin@yadro.com>

object/object.go

@@ -345,6 +345,32 @@ func (o *Object) SetAttributes(v ...Attribute) {
 	})
 }
 
+// SplitHeader returns split header of the object. If it's set, then split header
+// defines how the object relates to other objects in a split operation.
+func (o *Object) SplitHeader() (splitHeader *SplitHeader) {
+	if v2 := (*object.Object)(o).
+		GetHeader().
+		GetSplit(); v2 != nil {
+		splitHeader = NewSplitHeaderFromV2(v2)
+	}
+
+	return
+}
+
+// SetSplitHeader sets split header.
+func (o *Object) SetSplitHeader(v *SplitHeader) {
+	o.setSplitFields(func(sh *object.SplitHeader) {
+		v2 := v.ToV2()
+
+		sh.SetParent(v2.GetParent())
+		sh.SetPrevious(v2.GetPrevious())
+		sh.SetParentHeader(v2.GetParentHeader())
+		sh.SetParentSignature(v2.GetParentSignature())
+		sh.SetChildren(v2.GetChildren())
+		sh.SetSplitID(v2.GetSplitID())
+	})
+}
+
 // PreviousID returns identifier of the previous sibling object.
 func (o *Object) PreviousID() (v oid.ID, isSet bool) {
 	v2 := (*object.Object)(o)

object/split_header.go

@@ -0,0 +1,124 @@
+package object
+
+import (
+	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/api/object"
+	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/api/refs"
+	frostfscrypto "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/crypto"
+	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
+)
+
+// SplitHeader is an object's header component that defines the relationship
+// between this object and other objects if the object is part of a split operation.
+type SplitHeader object.SplitHeader
+
+// NewSplitHeaderFromV2 wraps v2 SplitHeader message to SplitHeader.
+func NewSplitHeaderFromV2(v2 *object.SplitHeader) *SplitHeader {
+	return (*SplitHeader)(v2)
+}
+
+// NewSplitHeader creates blank SplitHeader instance.
+func NewSplitHeader() *SplitHeader {
+	return NewSplitHeaderFromV2(new(object.SplitHeader))
+}
+
+func (sh *SplitHeader) ToV2() *object.SplitHeader {
+	return (*object.SplitHeader)(sh)
+}
+
+func (sh *SplitHeader) ParentID() (v oid.ID, isSet bool) {
+	v2 := (*object.SplitHeader)(sh)
+	if id := v2.GetParent(); id != nil {
+		_ = v.ReadFromV2(*id)
+		isSet = true
+	}
+
+	return
+}
+
+func (sh *SplitHeader) SetParentID(v oid.ID) {
+	v2 := new(refs.ObjectID)
+	v.WriteToV2(v2)
+	(*object.SplitHeader)(sh).SetParent(v2)
+}
+
+func (sh *SplitHeader) PreviousID() (v oid.ID, isSet bool) {
+	v2 := (*object.SplitHeader)(sh)
+	if id := v2.GetPrevious(); id != nil {
+		_ = v.ReadFromV2(*id)
+		isSet = true
+	}
+
+	return
+}
+
+func (sh *SplitHeader) SetPreviousID(v oid.ID) {
+	v2 := new(refs.ObjectID)
+	v.WriteToV2(v2)
+	(*object.SplitHeader)(sh).SetPrevious(v2)
+}
+
+func (sh *SplitHeader) ParentSignature() *frostfscrypto.Signature {
+	v2 := (*object.SplitHeader)(sh)
+	if parSigV2 := v2.GetParentSignature(); parSigV2 != nil {
+		parSig := new(frostfscrypto.Signature)
+		_ = parSig.ReadFromV2(*parSigV2)
+	}
+
+	return nil
+}
+
+func (sh *SplitHeader) SetParentSignature(v *frostfscrypto.Signature) {
+	var parSigV2 *refs.Signature
+
+	if v != nil {
+		parSigV2 = new(refs.Signature)
+		v.WriteToV2(parSigV2)
+	}
+
+	(*object.SplitHeader)(sh).SetParentSignature(parSigV2)
+}
+
+func (sh *SplitHeader) ParentHeader() (parentHeader *Object) {
+	v2 := (*object.SplitHeader)(sh)
+
+	if parHdr := v2.GetParentHeader(); parHdr != nil {
+		parentHeader = New()
+		parentHeader.setHeaderField(func(h *object.Header) {
+			*h = *v2.GetParentHeader()
+		})
+	}
+
+	return
+}
+
+func (sh *SplitHeader) SetParentHeader(parentHeader *Object) {
+	(*object.SplitHeader)(sh).SetParentHeader(parentHeader.ToV2().GetHeader())
+}
+
+func (sh *SplitHeader) Children() (res []oid.ID) {
+	v2 := (*object.SplitHeader)(sh)
+	if children := v2.GetChildren(); len(children) > 0 {
+		res = make([]oid.ID, len(children))
+		for i := range children {
+			_ = res[i].ReadFromV2(children[i])
+		}
+	}
+
+	return
+}
+
+func (sh *SplitHeader) SetChildren(children []oid.ID) {
+	v2Children := make([]refs.ObjectID, len(children))
+	for i := range children {
+		children[i].WriteToV2(&v2Children[i])
+	}
+	(*object.SplitHeader)(sh).SetChildren(v2Children)
+}
+
+func (sh *SplitHeader) SplitID() *SplitID {
+	return NewSplitIDFromV2((*object.SplitHeader)(sh).GetSplitID())
+}
+
+func (sh *SplitHeader) SetSplitID(v *SplitID) {
+	(*object.SplitHeader)(sh).SetSplitID(v.ToV2())
+}