package transformer

import (
	"crypto/sha256"
	"fmt"
	"hash"
	"io"

	"github.com/TrueCloudLab/frostfs-sdk-go/checksum"
	"github.com/TrueCloudLab/frostfs-sdk-go/object"
	oid "github.com/TrueCloudLab/frostfs-sdk-go/object/id"
	"github.com/TrueCloudLab/tzhash/tz"
)

type payloadSizeLimiter struct {
	maxSize, written uint64

	withoutHomomorphicHash bool

	targetInit func() ObjectTarget

	target ObjectTarget

	current, parent *object.Object

	currentHashers, parentHashers []*payloadChecksumHasher

	previous []oid.ID

	chunkWriter io.Writer

	splitID *object.SplitID

	parAttrs []object.Attribute
}

type payloadChecksumHasher struct {
	hasher hash.Hash

	checksumWriter func([]byte)
}

// NewPayloadSizeLimiter returns ObjectTarget instance that restricts payload length
// of the writing object and writes generated objects to targets from initializer.
//
// Calculates and adds homomorphic hash to resulting objects only if withoutHomomorphicHash
// is false.
//
// Objects w/ payload size less or equal than max size remain untouched.
func NewPayloadSizeLimiter(maxSize uint64, withoutHomomorphicHash bool, targetInit TargetInitializer) ObjectTarget {
	return &payloadSizeLimiter{
		maxSize:                maxSize,
		withoutHomomorphicHash: withoutHomomorphicHash,
		targetInit:             targetInit,
		splitID:                object.NewSplitID(),
	}
}

func (s *payloadSizeLimiter) WriteHeader(hdr *object.Object) error {
	s.current = fromObject(hdr)

	s.initialize()

	return nil
}

func (s *payloadSizeLimiter) Write(p []byte) (int, error) {
	if err := s.writeChunk(p); err != nil {
		return 0, err
	}

	return len(p), nil
}

func (s *payloadSizeLimiter) Close() (*AccessIdentifiers, error) {
	return s.release(true)
}

func (s *payloadSizeLimiter) initialize() {
	// if it is an object after the 1st
	if ln := len(s.previous); ln > 0 {
		// initialize parent object once (after 1st object)
		if ln == 1 {
			s.detachParent()
		}

		// set previous object to the last previous identifier
		s.current.SetPreviousID(s.previous[ln-1])
	}

	s.initializeCurrent()
}

func fromObject(obj *object.Object) *object.Object {
	cnr, _ := obj.ContainerID()

	res := object.New()
	res.SetContainerID(cnr)
	res.SetOwnerID(obj.OwnerID())
	res.SetAttributes(obj.Attributes()...)
	res.SetType(obj.Type())

	// obj.SetSplitID creates splitHeader but we don't need to do it in case
	// of small objects, so we should make nil check.
	if obj.SplitID() != nil {
		res.SetSplitID(obj.SplitID())
	}

	return res
}

func (s *payloadSizeLimiter) initializeCurrent() {
	// initialize current object target
	s.target = s.targetInit()

	// create payload hashers
	s.currentHashers = payloadHashersForObject(s.current, s.withoutHomomorphicHash)

	// compose multi-writer from target and all payload hashers
	ws := make([]io.Writer, 0, 1+len(s.currentHashers)+len(s.parentHashers))

	ws = append(ws, s.target)

	for i := range s.currentHashers {
		ws = append(ws, s.currentHashers[i].hasher)
	}

	for i := range s.parentHashers {
		ws = append(ws, s.parentHashers[i].hasher)
	}

	s.chunkWriter = io.MultiWriter(ws...)
}

func payloadHashersForObject(obj *object.Object, withoutHomomorphicHash bool) []*payloadChecksumHasher {
	hashers := make([]*payloadChecksumHasher, 0, 2)

	hashers = append(hashers, &payloadChecksumHasher{
		hasher: sha256.New(),
		checksumWriter: func(binChecksum []byte) {
			if ln := len(binChecksum); ln != sha256.Size {
				panic(fmt.Sprintf("wrong checksum length: expected %d, has %d", sha256.Size, ln))
			}

			csSHA := [sha256.Size]byte{}
			copy(csSHA[:], binChecksum)

			var cs checksum.Checksum
			cs.SetSHA256(csSHA)

			obj.SetPayloadChecksum(cs)
		},
	})

	if !withoutHomomorphicHash {
		hashers = append(hashers, &payloadChecksumHasher{
			hasher: tz.New(),
			checksumWriter: func(binChecksum []byte) {
				if ln := len(binChecksum); ln != tz.Size {
					panic(fmt.Sprintf("wrong checksum length: expected %d, has %d", tz.Size, ln))
				}

				csTZ := [tz.Size]byte{}
				copy(csTZ[:], binChecksum)

				var cs checksum.Checksum
				cs.SetTillichZemor(csTZ)

				obj.SetPayloadHomomorphicHash(cs)
			},
		})
	}

	return hashers
}

func (s *payloadSizeLimiter) release(finalize bool) (*AccessIdentifiers, error) {
	// Arg finalize is true only when called from Close method.
	// We finalize parent and generate linking objects only if it is more
	// than 1 object in split-chain.
	withParent := finalize && len(s.previous) > 0

	if withParent {
		writeHashes(s.parentHashers)
		s.parent.SetPayloadSize(s.written)
		s.current.SetParent(s.parent)
	}

	// release current object
	writeHashes(s.currentHashers)

	// release current, get its id
	if err := s.target.WriteHeader(s.current); err != nil {
		return nil, fmt.Errorf("could not write header: %w", err)
	}

	ids, err := s.target.Close()
	if err != nil {
		return nil, fmt.Errorf("could not close target: %w", err)
	}

	// save identifier of the released object
	s.previous = append(s.previous, ids.SelfID)

	if withParent {
		// generate and release linking object
		s.initializeLinking(ids.ParentHeader)
		s.initializeCurrent()

		if _, err := s.release(false); err != nil {
			return nil, fmt.Errorf("could not release linking object: %w", err)
		}
	}

	return ids, nil
}

func writeHashes(hashers []*payloadChecksumHasher) {
	for i := range hashers {
		hashers[i].checksumWriter(hashers[i].hasher.Sum(nil))
	}
}

func (s *payloadSizeLimiter) initializeLinking(parHdr *object.Object) {
	s.current = fromObject(s.current)
	s.current.SetParent(parHdr)
	s.current.SetChildren(s.previous...)
	s.current.SetSplitID(s.splitID)
}

func (s *payloadSizeLimiter) writeChunk(chunk []byte) error {
	for {
		// statement is true if the previous write of bytes reached exactly the boundary.
		if s.written > 0 && s.written%s.maxSize == 0 {
			if s.written == s.maxSize {
				s.prepareFirstChild()
			}

			// we need to release current object
			if _, err := s.release(false); err != nil {
				return fmt.Errorf("could not release object: %w", err)
			}

			// initialize another object
			s.initialize()
		}

		var (
			ln         = uint64(len(chunk))
			cut        = ln
			leftToEdge = s.maxSize - s.written%s.maxSize
		)

		// write bytes no further than the boundary of the current object
		if ln > leftToEdge {
			cut = leftToEdge
		}

		if _, err := s.chunkWriter.Write(chunk[:cut]); err != nil {
			return fmt.Errorf("could not write chunk to target: %w", err)
		}

		// increase written bytes counter
		s.written += cut

		if cut == ln {
			return nil
		}
		// if there are more bytes in buffer we call method again to start filling another object
		chunk = chunk[cut:]
	}
}

func (s *payloadSizeLimiter) prepareFirstChild() {
	// initialize split header with split ID on first object in chain
	s.current.InitRelations()
	s.current.SetSplitID(s.splitID)

	// cut source attributes
	s.parAttrs = s.current.Attributes()
	s.current.SetAttributes()

	// attributes will be added to parent in detachParent
}

func (s *payloadSizeLimiter) detachParent() {
	s.parent = s.current
	s.current = fromObject(s.parent)
	s.parent.ResetRelations()
	s.parent.SetSignature(nil)
	s.parentHashers = s.currentHashers

	// return source attributes
	s.parent.SetAttributes(s.parAttrs...)
}