package layer

import (
	"context"
	"errors"
	"fmt"
	"io"

	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
)

type partObj struct {
	OID  oid.ID
	Size uint64
}

// implements io.Reader of payloads of the object list stored in the FrostFS network.
type multiObjectReader struct {
	ctx context.Context

	layer *layer

	off, ln uint64

	prm getFrostFSParams

	curReader io.Reader

	parts []partObj
}

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
		}
	}

	if len(x.parts) == 0 {
		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[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]
		}
	}

	x.curReader, err = x.layer.initFrostFSObjectPayloadReader(x.ctx, x.prm)
	if err != nil {
		return n, fmt.Errorf("init payload reader for the next part: %w", err)
	}

	x.prm.off = 0
	x.prm.ln = 0

	x.parts = x.parts[1:]

	next, err := x.Read(p[n:])

	return n + next, err
}