package negentropy

import (
	"bytes"
	"encoding/hex"
	"fmt"
	"math"
	"os"
	"unsafe"

	"github.com/nbd-wtf/go-nostr"
)

const (
	protocolVersion byte = 0x61 // version 1
	maxTimestamp         = nostr.Timestamp(math.MaxInt64)
)

var infiniteBound = Bound{Item: Item{Timestamp: maxTimestamp}}

type Negentropy struct {
	storage          Storage
	sealed           bool
	frameSizeLimit   int
	isInitiator      bool
	lastTimestampIn  nostr.Timestamp
	lastTimestampOut nostr.Timestamp

	Haves    chan string
	HaveNots chan string
}

func NewNegentropy(storage Storage, frameSizeLimit int) *Negentropy {
	return &Negentropy{
		storage:        storage,
		frameSizeLimit: frameSizeLimit,
	}
}

func (n *Negentropy) Insert(evt *nostr.Event) {
	err := n.storage.Insert(evt.CreatedAt, evt.ID)
	if err != nil {
		panic(err)
	}
}

func (n *Negentropy) seal() {
	if !n.sealed {
		n.storage.Seal()
	}
	n.sealed = true
}

func (n *Negentropy) Initiate() []byte {
	n.seal()
	n.isInitiator = true

	n.Haves = make(chan string, n.storage.Size()/2)
	n.HaveNots = make(chan string, n.storage.Size()/2)

	output := bytes.NewBuffer(make([]byte, 0, 1+n.storage.Size()*32))
	output.WriteByte(protocolVersion)
	n.SplitRange(0, n.storage.Size(), infiniteBound, output)

	return output.Bytes()
}

func (n *Negentropy) Reconcile(msg []byte) (output []byte, err error) {
	n.seal()
	reader := bytes.NewReader(msg)

	output, err = n.reconcileAux(reader)
	if err != nil {
		return nil, err
	}

	if len(output) == 1 && n.isInitiator {
		close(n.Haves)
		close(n.HaveNots)
		return nil, nil
	}

	return output, nil
}

func (n *Negentropy) reconcileAux(reader *bytes.Reader) ([]byte, error) {
	n.lastTimestampIn, n.lastTimestampOut = 0, 0 // reset for each message

	fullOutput := bytes.NewBuffer(make([]byte, 0, 5000))
	fullOutput.WriteByte(protocolVersion)

	pv, err := reader.ReadByte()
	if err != nil {
		return nil, err
	}

	if pv < 0x60 || pv > 0x6f {
		return nil, fmt.Errorf("invalid protocol version byte")
	}
	if pv != protocolVersion {
		if n.isInitiator {
			return nil, fmt.Errorf("unsupported negentropy protocol version requested")
		}
		return fullOutput.Bytes(), nil
	}

	var prevBound Bound
	prevIndex := 0
	skip := false

	partialOutput := bytes.NewBuffer(make([]byte, 0, 100))
	for reader.Len() > 0 {
		partialOutput.Reset()

		doSkip := func() {
			if skip {
				skip = false
				encodedBound := n.encodeBound(prevBound)
				partialOutput.Write(encodedBound)
				partialOutput.WriteByte(SkipMode)
			}
		}

		currBound, err := n.DecodeBound(reader)
		if err != nil {
			return nil, err
		}
		modeVal, err := decodeVarInt(reader)
		if err != nil {
			return nil, err
		}
		mode := Mode(modeVal)

		lower := prevIndex
		upper := n.storage.FindLowerBound(prevIndex, n.storage.Size(), currBound)

		switch mode {
		case SkipMode:
			skip = true

		case FingerprintMode:
			var theirFingerprint [FingerprintSize]byte
			_, err := reader.Read(theirFingerprint[:])
			if err != nil {
				return nil, err
			}
			ourFingerprint, err := n.storage.Fingerprint(lower, upper)
			if err != nil {
				return nil, err
			}

			if theirFingerprint == ourFingerprint {
				skip = true
			} else {
				doSkip()
				n.SplitRange(lower, upper, currBound, partialOutput)
			}

		case IdListMode:
			numIds, err := decodeVarInt(reader)
			if err != nil {
				return nil, err
			}

			theirElems := make(map[string]struct{})
			var idb [32]byte

			for i := 0; i < numIds; i++ {
				_, err := reader.Read(idb[:])
				if err != nil {
					return nil, err
				}
				id := hex.EncodeToString(idb[:])
				theirElems[id] = struct{}{}
			}

			n.storage.Iterate(lower, upper, func(item Item, _ int) bool {
				id := item.ID
				if _, exists := theirElems[id]; !exists {
					if n.isInitiator {
						n.Haves <- id
					}
				} else {
					delete(theirElems, id)
				}
				return true
			})

			if n.isInitiator {
				skip = true
				for id := range theirElems {
					n.HaveNots <- id
				}
			} else {
				doSkip()

				responseIds := make([]byte, 0, 32*n.storage.Size())
				endBound := currBound

				n.storage.Iterate(lower, upper, func(item Item, index int) bool {
					if n.frameSizeLimit-200 < fullOutput.Len()+len(responseIds) {
						endBound = Bound{item}
						upper = index
						return false
					}

					id, _ := hex.DecodeString(item.ID)
					responseIds = append(responseIds, id...)
					return true
				})

				encodedBound := n.encodeBound(endBound)

				partialOutput.Write(encodedBound)
				partialOutput.WriteByte(IdListMode)
				partialOutput.Write(encodeVarInt(len(responseIds) / 32))
				partialOutput.Write(responseIds)

				partialOutput.WriteTo(fullOutput)
				partialOutput.Reset()
			}

		default:
			return nil, fmt.Errorf("unexpected mode %d", mode)
		}

		if n.frameSizeLimit-200 < fullOutput.Len()+partialOutput.Len() {
			// frame size limit exceeded, handle by encoding a boundary and fingerprint for the remaining range
			remainingFingerprint, err := n.storage.Fingerprint(upper, n.storage.Size())
			if err != nil {
				panic(err)
			}

			fullOutput.Write(n.encodeBound(infiniteBound))
			fullOutput.WriteByte(FingerprintMode)
			fullOutput.Write(remainingFingerprint[:])

			break // stop processing further
		} else {
			// append the constructed output for this iteration
			partialOutput.WriteTo(fullOutput)
		}

		prevIndex = upper
		prevBound = currBound
	}

	return fullOutput.Bytes(), nil
}

func (n *Negentropy) SplitRange(lower, upper int, upperBound Bound, output *bytes.Buffer) {
	numElems := upper - lower
	const buckets = 16

	if numElems < buckets*2 {
		// we just send the full ids here
		boundEncoded := n.encodeBound(upperBound)
		output.Write(boundEncoded)
		output.WriteByte(IdListMode)
		output.Write(encodeVarInt(numElems))

		n.storage.Iterate(lower, upper, func(item Item, _ int) bool {
			id, _ := hex.DecodeString(item.ID)
			output.Write(id)
			return true
		})
	} else {
		itemsPerBucket := numElems / buckets
		bucketsWithExtra := numElems % buckets
		curr := lower

		for i := 0; i < buckets; i++ {
			bucketSize := itemsPerBucket
			if i < bucketsWithExtra {
				bucketSize++
			}
			ourFingerprint, err := n.storage.Fingerprint(curr, curr+bucketSize)
			if err != nil {
				fmt.Fprintln(os.Stderr, err)
				panic(err)
			}

			curr += bucketSize

			var nextBound Bound
			if curr == upper {
				nextBound = upperBound
			} else {
				var prevItem, currItem Item

				n.storage.Iterate(curr-1, curr+1, func(item Item, index int) bool {
					if index == curr-1 {
						prevItem = item
					} else {
						currItem = item
					}
					return true
				})

				minBound := getMinimalBound(prevItem, currItem)
				nextBound = minBound
			}

			boundEncoded := n.encodeBound(nextBound)
			output.Write(boundEncoded)
			output.WriteByte(FingerprintMode)
			output.Write(ourFingerprint[:])
		}
	}
}

func (n *Negentropy) Name() string {
	p := unsafe.Pointer(n)
	return fmt.Sprintf("%d", uintptr(p)&127)
}