[#63] Simplify multiObjectReader and add tests

Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>

api/layer/multi_object_reader.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"io"
 
+	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/data"
 	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
 )
 
@@ -14,54 +15,124 @@ type partObj struct {
 	Size uint64
 }
 
+type readerInitiator interface {
+	initFrostFSObjectPayloadReader(ctx context.Context, p getFrostFSParams) (io.Reader, error)
+}
+
 // implements io.Reader of payloads of the object list stored in the FrostFS network.
 type multiObjectReader struct {
 	ctx context.Context
 
-	layer *layer
+	layer readerInitiator
 
-	off, ln uint64
+	startPartOffset uint64
+	endPartLength   uint64
 
 	prm getFrostFSParams
 
+	curIndex  int
 	curReader io.Reader
 
 	parts []partObj
 }
 
+type multiObjectReaderConfig struct {
+	layer readerInitiator
+
+	// the offset of complete object and total size to read
+	off, ln uint64
+
+	bktInfo *data.BucketInfo
+	parts   []partObj
+}
+
+var (
+	errOffsetIsOutOfRange = errors.New("offset is out of payload range")
+	errLengthIsOutOfRange = errors.New("length is out of payload range")
+	errEmptyPartsList     = errors.New("empty parts list")
+	errorZeroRangeLength  = errors.New("zero range length")
+)
+
+func newMultiObjectReader(ctx context.Context, cfg multiObjectReaderConfig) (*multiObjectReader, error) {
+	if len(cfg.parts) == 0 {
+		return nil, errEmptyPartsList
+	}
+
+	r := &multiObjectReader{
+		ctx:   ctx,
+		layer: cfg.layer,
+		prm: getFrostFSParams{
+			bktInfo: cfg.bktInfo,
+		},
+		parts: cfg.parts,
+	}
+
+	if cfg.off+cfg.ln == 0 {
+		return r, nil
+	}
+
+	if cfg.off > 0 && cfg.ln == 0 {
+		return nil, errorZeroRangeLength
+	}
+
+	startPartIndex, startPartOffset := findStartPart(cfg)
+	if startPartIndex == -1 {
+		return nil, errOffsetIsOutOfRange
+	}
+	r.startPartOffset = startPartOffset
+
+	endPartIndex, endPartLength := findEndPart(cfg)
+	if endPartIndex == -1 {
+		return nil, errLengthIsOutOfRange
+	}
+	r.endPartLength = endPartLength
+
+	r.parts = cfg.parts[startPartIndex : endPartIndex+1]
+
+	return r, nil
+}
+
+func findStartPart(cfg multiObjectReaderConfig) (index int, offset uint64) {
+	return findPartByPosition(cfg.off, cfg.parts)
+}
+
+func findEndPart(cfg multiObjectReaderConfig) (index int, length uint64) {
+	return findPartByPosition(cfg.off+cfg.ln, cfg.parts)
+}
+
+func findPartByPosition(position uint64, parts []partObj) (index int, positionInPart uint64) {
+	for i, part := range parts {
+		if position <= part.Size {
+			return i, position
+		}
+		position -= part.Size
+	}
+
+	return -1, 0
+}
+
 func (x *multiObjectReader) Read(p []byte) (n int, err error) {
 	if x.curReader != nil {
 		n, err = x.curReader.Read(p)
 		if !errors.Is(err, io.EOF) {
 			return n, err
 		}
+		x.curIndex++
 	}
 
-	if len(x.parts) == 0 {
+	if x.curIndex == len(x.parts) {
 		return n, io.EOF
 	}
 
-	for x.off != 0 {
-		if x.parts[0].Size < x.off {
-			x.parts = x.parts[1:]
-			x.off -= x.parts[0].Size
-		} else {
-			x.prm.off = x.off
-			x.off = 0
-		}
+	x.prm.oid = x.parts[x.curIndex].OID
+
+	if x.curIndex == 0 {
+		x.prm.off = x.startPartOffset
+		x.prm.ln = x.parts[x.curIndex].Size - x.startPartOffset
 	}
 
-	x.prm.oid = x.parts[0].OID
-
-	if x.ln != 0 {
-		if x.parts[0].Size < x.prm.off+x.ln {
-			x.prm.ln = x.parts[0].Size - x.prm.off
-			x.ln -= x.prm.ln
-		} else {
-			x.prm.ln = x.ln
-			x.ln = 0
-			x.parts = x.parts[:1]
-		}
+	if x.curIndex == len(x.parts)-1 {
+		x.prm.ln = x.endPartLength - x.prm.off
 	}
 
 	x.curReader, err = x.layer.initFrostFSObjectPayloadReader(x.ctx, x.prm)
@@ -72,8 +143,6 @@ func (x *multiObjectReader) Read(p []byte) (n int, err error) {
 	x.prm.off = 0
 	x.prm.ln = 0
 
-	x.parts = x.parts[1:]
-
 	next, err := x.Read(p[n:])
 
 	return n + next, err