package derive

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

	"github.com/ethereum/go-ethereum/log"

	"github.com/exchain/go-exchain/op-node/rollup"
	"github.com/exchain/go-exchain/op-service/eth"
)

// The attributes queue sits after the batch queue.
// It transforms batches into payload attributes. The outputted payload
// attributes cannot be buffered because each batch->attributes transformation
// pulls in data about the current L2 safe head.
//
// It also buffers batches that have been output because multiple batches can
// be created at once.
//
// This stage can be reset by clearing its batch buffer.
// This stage does not need to retain any references to L1 blocks.

type AttributesBuilder interface {
	PreparePayloadAttributes(ctx context.Context, l2Parent eth.L2BlockRef, epoch eth.BlockID) (attrs *eth.PayloadAttributes, err error)
}

type AttributesWithParent struct {
	Attributes *eth.PayloadAttributes
	Parent     eth.L2BlockRef
	Concluding bool // Concluding indicates that the attributes conclude the pending safe phase

	DerivedFrom eth.L1BlockRef
}

// WithDepositsOnly return a shallow clone with all non-Deposit transactions
// stripped from the transactions of its attributes. The order is preserved.
func (a *AttributesWithParent) WithDepositsOnly() *AttributesWithParent {
	clone := *a
	return &clone
}

func (a *AttributesWithParent) IsDerived() bool {
	return a.DerivedFrom != (eth.L1BlockRef{})
}

type AttributesQueue struct {
	log     log.Logger
	config  *rollup.Config
	builder AttributesBuilder
	prev    SingularBatchProvider

	batch       *SingularBatch
	concluding  bool
	lastAttribs *AttributesWithParent
}

type SingularBatchProvider interface {
	ResettableStage
	ChannelFlusher
	Origin() eth.L1BlockRef
	NextBatch(context.Context, eth.L2BlockRef) (*SingularBatch, bool, error)
}

func NewAttributesQueue(log log.Logger, cfg *rollup.Config, builder AttributesBuilder, prev SingularBatchProvider) *AttributesQueue {
	return &AttributesQueue{
		log:     log,
		config:  cfg,
		builder: builder,
		prev:    prev,
	}
}

func (aq *AttributesQueue) Origin() eth.L1BlockRef {
	return aq.prev.Origin()
}

func (aq *AttributesQueue) NextAttributes(ctx context.Context, parent eth.L2BlockRef) (*AttributesWithParent, error) {
	// Get a batch if we need it
	if aq.batch == nil {
		batch, concluding, err := aq.prev.NextBatch(ctx, parent)
		if err != nil {
			return nil, err
		}
		aq.batch = batch
		aq.concluding = concluding
	}

	// Actually generate the next attributes
	if attrs, err := aq.createNextAttributes(ctx, aq.batch, parent); err != nil {
		return nil, err
	} else {
		// Clear out the local state once we will succeed
		attr := AttributesWithParent{
			Attributes:  attrs,
			Parent:      parent,
			Concluding:  aq.concluding,
			DerivedFrom: aq.Origin(),
		}
		aq.lastAttribs = &attr
		aq.batch = nil
		aq.concluding = false
		return &attr, nil
	}
}

// createNextAttributes transforms a batch into a payload attributes. This sets `NoTxPool` and appends the batched transactions
// to the attributes transaction list
func (aq *AttributesQueue) createNextAttributes(ctx context.Context, batch *SingularBatch, l2SafeHead eth.L2BlockRef) (*eth.PayloadAttributes, error) {
	// sanity check parent hash
	if batch.ParentHash != l2SafeHead.Hash {
		return nil, NewResetError(fmt.Errorf("valid batch has bad parent hash %s, expected %s", batch.ParentHash, l2SafeHead.Hash))
	}
	// sanity check timestamp
	if expected := l2SafeHead.Time + aq.config.BlockTime; expected != batch.Timestamp {
		return nil, NewResetError(fmt.Errorf("valid batch has bad timestamp %d, expected %d", batch.Timestamp, expected))
	}
	fetchCtx, cancel := context.WithTimeout(ctx, 20*time.Second)
	defer cancel()
	attrs, err := aq.builder.PreparePayloadAttributes(fetchCtx, l2SafeHead, batch.Epoch())
	if err != nil {
		return nil, err
	}

	// we are verifying, not sequencing, we've got all transactions and do not pull from the tx-pool
	// (that would make the block derivation non-deterministic)

	// todo: vicotor implement this.
	//attrs.Transactions = append(attrs.Transactions, batch.Transactions...)
	//aq.log.Info("generated attributes in payload queue", "txs", len(attrs.Transactions), "timestamp", batch.Timestamp)

	return attrs, nil
}

func (aq *AttributesQueue) reset() {
	aq.batch = nil
	aq.concluding = false // overwritten later, but set for consistency
	aq.lastAttribs = nil
}

func (aq *AttributesQueue) Reset(ctx context.Context, _ eth.L1BlockRef, _ eth.SystemConfig) error {
	aq.reset()
	return io.EOF
}

func (aq *AttributesQueue) DepositsOnlyAttributes(parent eth.BlockID, derivedFrom eth.L1BlockRef) (*AttributesWithParent, error) {
	// Sanity checks - these cannot happen with correct deriver implementations.
	if aq.batch != nil {
		return nil, fmt.Errorf("unexpected buffered batch, parent hash: %s, epoch: %s", aq.batch.ParentHash, aq.batch.Epoch())
	} else if aq.lastAttribs == nil {
		return nil, errors.New("no attributes generated yet")
	} else if derivedFrom != aq.lastAttribs.DerivedFrom {
		return nil, fmt.Errorf(
			"unexpected derivation origin, last_origin: %s, invalid_origin: %s",
			aq.lastAttribs.DerivedFrom, derivedFrom)
	} else if parent != aq.lastAttribs.Parent.ID() {
		return nil, fmt.Errorf(
			"unexpected parent: last_parent: %s, invalid_parent: %s",
			aq.lastAttribs.Parent.ID(), parent)
	}

	aq.prev.FlushChannel() // flush all channel data in previous stages
	attrs := aq.lastAttribs.WithDepositsOnly()
	aq.lastAttribs = attrs
	return attrs, nil
}
