package getsvc

import (
	"context"
	"errors"

	"git.frostfs.info/TrueCloudLab/frostfs-node/pkg/core/object"
	apistatus "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/client/status"
	objectSDK "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object"
	oid "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/object/id"
)

type objectGetter interface {
	GetObject(ctx context.Context, id oid.ID, rng *objectSDK.Range) (*objectSDK.Object, error)
	HeadObject(ctx context.Context, id oid.ID) (*objectSDK.Object, error)
}

var (
	errParentAddressDiffers = errors.New("parent address in child object differs")
)

type assembler struct {
	addr      oid.Address
	splitInfo *objectSDK.SplitInfo
	rng       *objectSDK.Range
	objGetter objectGetter

	currentOffset uint64

	parentObject *objectSDK.Object
}

func newAssembler(
	addr oid.Address,
	splitInfo *objectSDK.SplitInfo,
	rng *objectSDK.Range,
	objGetter objectGetter) *assembler {
	return &assembler{
		addr:      addr,
		rng:       rng,
		splitInfo: splitInfo,
		objGetter: objGetter,
	}
}

// Assemble assembles splitted large object and writes it's content to ObjectWriter.
// It returns parent object.
func (a *assembler) Assemble(ctx context.Context, writer ObjectWriter) (*objectSDK.Object, error) {
	sourceObjectID, ok := a.getLastPartOrLinkObjectID()
	if !ok {
		return nil, objectSDK.NewSplitInfoError(a.splitInfo)
	}
	previousID, childrenIDs, err := a.initializeFromSourceObjectID(ctx, sourceObjectID)
	if err != nil {
		return nil, err
	}
	if previousID == nil && len(childrenIDs) == 0 {
		return nil, objectSDK.NewSplitInfoError(a.splitInfo)
	}
	if len(childrenIDs) > 0 {
		if err := a.assembleObjectByChildrenList(ctx, childrenIDs, writer); err != nil {
			return nil, err
		}
	} else {
		if err := a.assemleObjectByPreviousIDInReverse(ctx, *previousID, writer); err != nil {
			return nil, err
		}
	}
	return a.parentObject, nil
}

func (a *assembler) getLastPartOrLinkObjectID() (oid.ID, bool) {
	sourceObjectID, ok := a.splitInfo.Link()
	if ok {
		return sourceObjectID, true
	}
	sourceObjectID, ok = a.splitInfo.LastPart()
	if ok {
		return sourceObjectID, true
	}
	return oid.ID{}, false
}

func (a *assembler) initializeFromSourceObjectID(ctx context.Context, id oid.ID) (*oid.ID, []oid.ID, error) {
	sourceObject, err := a.getChildObject(ctx, id, nil, true)
	if err != nil {
		return nil, nil, err
	}

	parentObject := sourceObject.Parent()
	if parentObject == nil {
		return nil, nil, errors.New("received child with empty parent")
	}

	a.parentObject = parentObject

	var payload []byte

	if a.rng != nil {
		seekOff := a.rng.GetOffset()
		seekLen := a.rng.GetLength()
		seekTo := seekOff + seekLen
		parentSize := parentObject.PayloadSize()

		if seekTo < seekOff || parentSize < seekOff || parentSize < seekTo {
			return nil, nil, &apistatus.ObjectOutOfRange{}
		}

		sourceSize := sourceObject.PayloadSize()

		a.currentOffset = parentSize - sourceSize

		from := uint64(0)
		if a.currentOffset < seekOff {
			from = seekOff - a.currentOffset
		}

		to := uint64(0)
		if seekOff+seekLen > a.currentOffset+from {
			to = seekOff + seekLen - a.currentOffset
		}

		payload = sourceObject.Payload()[from:to]
		a.rng.SetLength(a.rng.GetLength() - to + from)
	} else {
		payload = sourceObject.Payload()
	}

	a.parentObject.SetPayload(payload)

	idPrev, ok := sourceObject.PreviousID()
	if ok {
		return &idPrev, sourceObject.Children(), nil
	}

	return nil, sourceObject.Children(), nil
}

func (a *assembler) getChildObject(ctx context.Context, id oid.ID, rng *objectSDK.Range, verifyIsChild bool) (*objectSDK.Object, error) {
	obj, err := a.objGetter.GetObject(ctx, id, rng)
	if err != nil {
		return nil, err
	}

	if verifyIsChild && !a.isChild(obj) {
		return nil, errParentAddressDiffers
	}
	return obj, nil
}

func (a *assembler) assembleObjectByChildrenList(ctx context.Context, childrenIDs []oid.ID, writer ObjectWriter) error {
	if a.rng == nil {
		if err := writer.WriteHeader(ctx, a.parentObject.CutPayload()); err != nil {
			return err
		}
		return a.assemblePayloadByObjectIDs(ctx, writer, childrenIDs, nil, true)
	}

	if err := a.assemblePayloadInReverse(ctx, writer, childrenIDs[len(childrenIDs)-1]); err != nil {
		return err
	}
	return writer.WriteChunk(ctx, a.parentObject.Payload())
}

func (a *assembler) assemleObjectByPreviousIDInReverse(ctx context.Context, prevID oid.ID, writer ObjectWriter) error {
	if a.rng == nil {
		if err := writer.WriteHeader(ctx, a.parentObject.CutPayload()); err != nil {
			return err
		}
	}

	if err := a.assemblePayloadInReverse(ctx, writer, prevID); err != nil {
		return err
	}
	if err := writer.WriteChunk(ctx, a.parentObject.Payload()); err != nil { // last part
		return err
	}
	return nil
}

func (a *assembler) assemblePayloadByObjectIDs(ctx context.Context, writer ObjectWriter, partIDs []oid.ID, partRanges []objectSDK.Range, verifyIsChild bool) error {
	withRng := len(partRanges) > 0 && a.rng != nil

	for i := range partIDs {
		var r *objectSDK.Range
		if withRng {
			r = &partRanges[i]
		}

		child, err := a.getChildObject(ctx, partIDs[i], r, verifyIsChild)
		if err != nil {
			return err
		}

		if err := writer.WriteChunk(ctx, child.Payload()); err != nil {
			return err
		}
	}
	return nil
}

func (a *assembler) assemblePayloadInReverse(ctx context.Context, writer ObjectWriter, prevID oid.ID) error {
	chain, rngs, err := a.buildChain(ctx, prevID)
	if err != nil {
		return err
	}

	reverseRngs := len(rngs) > 0

	for left, right := 0, len(chain)-1; left < right; left, right = left+1, right-1 {
		chain[left], chain[right] = chain[right], chain[left]

		if reverseRngs {
			rngs[left], rngs[right] = rngs[right], rngs[left]
		}
	}

	return a.assemblePayloadByObjectIDs(ctx, writer, chain, rngs, false)
}

func (a *assembler) isChild(obj *objectSDK.Object) bool {
	parent := obj.Parent()
	return parent == nil || equalAddresses(a.addr, object.AddressOf(parent))
}

func (a *assembler) buildChain(ctx context.Context, prevID oid.ID) ([]oid.ID, []objectSDK.Range, error) {
	var (
		chain []oid.ID
		rngs  []objectSDK.Range
		from  = a.rng.GetOffset()
		to    = from + a.rng.GetLength()

		hasPrev = true
	)

	// fill the chain end-to-start
	for hasPrev {
		// check that only for "range" requests,
		// for `GET` it stops via the false `withPrev`
		if a.rng != nil && a.currentOffset <= from {
			break
		}

		head, err := a.objGetter.HeadObject(ctx, prevID)
		if err != nil {
			return nil, nil, err
		}
		if !a.isChild(head) {
			return nil, nil, errParentAddressDiffers
		}

		if a.rng != nil {
			sz := head.PayloadSize()

			a.currentOffset -= sz

			if a.currentOffset < to {
				off := uint64(0)
				if from > a.currentOffset {
					off = from - a.currentOffset
					sz -= from - a.currentOffset
				}

				if to < a.currentOffset+off+sz {
					sz = to - off - a.currentOffset
				}

				index := len(rngs)
				rngs = append(rngs, objectSDK.Range{})
				rngs[index].SetOffset(off)
				rngs[index].SetLength(sz)

				id, _ := head.ID()
				chain = append(chain, id)
			}
		} else {
			id, _ := head.ID()
			chain = append(chain, id)
		}

		prevID, hasPrev = head.PreviousID()
	}

	return chain, rngs, nil
}