[#53] object/range: Share object range traverser between services

Replace object range chain utilities to object/util package in order to
reuse it in other services.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/object/util/chain.go

@@ -0,0 +1,106 @@
+package util
+
+import (
+	"fmt"
+
+	objectSDK "github.com/nspcc-dev/neofs-api-go/pkg/object"
+
+	"github.com/nspcc-dev/neofs-node/pkg/core/object"
+)
+
+type RangeTraverser struct {
+	chain *rangeChain
+
+	seekBounds *rangeBounds
+}
+
+type rangeBounds struct {
+	left, right uint64
+}
+
+type rangeChain struct {
+	next, prev *rangeChain
+
+	bounds *rangeBounds
+
+	id *objectSDK.ID
+}
+
+func NewRangeTraverser(originSize uint64, rightElement *object.Object, rngSeek *objectSDK.Range) *RangeTraverser {
+	right := &rangeChain{
+		bounds: &rangeBounds{
+			left:  originSize - rightElement.GetPayloadSize(),
+			right: originSize,
+		},
+		id: rightElement.GetID(),
+	}
+
+	left := &rangeChain{
+		id: rightElement.GetPreviousID(),
+	}
+
+	left.next, right.prev = right, left
+
+	return &RangeTraverser{
+		chain: right,
+		seekBounds: &rangeBounds{
+			left:  rngSeek.GetOffset(),
+			right: rngSeek.GetOffset() + rngSeek.GetLength(),
+		},
+	}
+}
+
+func (c *RangeTraverser) Next() (id *objectSDK.ID, rng *objectSDK.Range) {
+	left := c.chain.bounds.left
+	seekLeft := c.seekBounds.left
+
+	if left > seekLeft {
+		id = c.chain.prev.id
+	} else {
+		id = c.chain.id
+		rng = objectSDK.NewRange()
+		rng.SetOffset(seekLeft - left)
+		rng.SetLength(min(c.chain.bounds.right, c.seekBounds.right) - seekLeft)
+	}
+
+	return
+}
+
+func min(a, b uint64) uint64 {
+	if a < b {
+		return a
+	}
+
+	return b
+}
+
+func (c *RangeTraverser) PushHeader(obj *object.Object) {
+	id := obj.GetID()
+	if !id.Equal(c.chain.prev.id) {
+		panic(fmt.Sprintf("(%T) unexpected identifier in header", c))
+	}
+
+	sz := obj.GetPayloadSize()
+
+	c.chain.prev.bounds = &rangeBounds{
+		left:  c.chain.bounds.left - sz,
+		right: c.chain.bounds.left,
+	}
+
+	c.chain = c.chain.prev
+
+	if prev := obj.GetPreviousID(); prev != nil {
+		c.chain.prev = &rangeChain{
+			next: c.chain,
+			id:   prev,
+		}
+	}
+}
+
+func (c *RangeTraverser) PushSuccessSize(sz uint64) {
+	c.seekBounds.left += sz
+
+	if c.seekBounds.left >= c.chain.bounds.right && c.chain.next != nil {
+		c.chain = c.chain.next
+	}
+}