package derive

import (
	"context"
	"io"
	"slices"

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

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

type NextFrameProvider interface {
	NextFrame(ctx context.Context) (Frame, error)
	Origin() eth.L1BlockRef
}

// ChannelBank is a stateful stage that does the following:
// 1. Unmarshalls frames from L1 transaction data
// 2. Applies those frames to a channel
// 3. Attempts to read from the channel when it is ready
// 4. Prunes channels (not frames) when the channel bank is too large.
//
// Note: we prune before we ingest data.
// As we switch between ingesting data & reading, the prune step occurs at an odd point
// Specifically, the channel bank is not allowed to become too large between successive calls
// to `IngestData`. This means that we can do an ingest and then do a read while becoming too large.

// ChannelBank buffers channel frames, and emits full channel data
type ChannelBank struct {
	log     log.Logger
	spec    *rollup.ChainSpec
	metrics Metrics

	channels     map[ChannelID]*Channel // channels by ID
	channelQueue []ChannelID            // channels in FIFO order

	prev NextFrameProvider
}

var _ RawChannelProvider = (*ChannelBank)(nil)

// NewChannelBank creates a ChannelBank, which should be Reset(origin) before use.
func NewChannelBank(log log.Logger, spec *rollup.ChainSpec, prev NextFrameProvider, m Metrics) *ChannelBank {
	return &ChannelBank{
		log:          log,
		spec:         spec,
		metrics:      m,
		channels:     make(map[ChannelID]*Channel),
		channelQueue: make([]ChannelID, 0, 10),
		prev:         prev,
	}
}

func (cb *ChannelBank) Origin() eth.L1BlockRef {
	return cb.prev.Origin()
}

func (cb *ChannelBank) prune() {
	// check total size
	totalSize := uint64(0)
	for _, ch := range cb.channels {
		totalSize += ch.size
	}
	// prune until it is reasonable again. The high-priority channel failed to be read, so we start pruning there.
	for totalSize > cb.spec.MaxChannelBankSize(cb.Origin().Time) {
		id := cb.channelQueue[0]
		ch := cb.channels[id]
		cb.channelQueue = cb.channelQueue[1:]
		delete(cb.channels, id)
		cb.log.Info("pruning channel", "channel", id, "totalSize", totalSize, "channel_size", ch.size, "remaining_channel_count", len(cb.channels))
		totalSize -= ch.size
	}
}

// IngestFrame adds new L1 data to the channel bank.
// Read() should be called repeatedly first, until everything has been read, before adding new data.
func (cb *ChannelBank) IngestFrame(f Frame) {
	origin := cb.Origin()
	log := cb.log.New("origin", origin, "channel", f.ID, "length", len(f.Data), "frame_number", f.FrameNumber, "is_last", f.IsLast)
	log.Debug("channel bank got new data")

	currentCh, ok := cb.channels[f.ID]
	if !ok {
		// Only record a head channel if it can immediately be active.
		if len(cb.channelQueue) == 0 {
			cb.metrics.RecordHeadChannelOpened()
		}
		// create new channel if it doesn't exist yet
		currentCh = NewChannel(f.ID, origin, false)
		cb.channels[f.ID] = currentCh
		cb.channelQueue = append(cb.channelQueue, f.ID)
		log.Info("created new channel")
	}

	// check if the channel is not timed out
	if currentCh.OpenBlockNumber()+cb.spec.ChannelTimeout(origin.Time) < origin.Number {
		log.Warn("channel is timed out, ignore frame")
		return
	}

	log.Trace("ingesting frame")
	if err := currentCh.AddFrame(f, origin); err != nil {
		log.Warn("failed to ingest frame into channel", "err", err)
		return
	}
	cb.metrics.RecordFrame()

	// Prune after the frame is loaded.
	cb.prune()
}

// Read the raw data of the first channel, if it's timed-out or closed.
// Read returns io.EOF if there is nothing new to read.
func (cb *ChannelBank) Read() (data []byte, err error) {
	// Common Code pre/post canyon. Return io.EOF if no channels
	if len(cb.channelQueue) == 0 {
		return nil, io.EOF
	}
	// Return nil,nil on the first channel if it is timed out. There may be more timed out
	// channels at the head of the queue and we want to remove them all.
	first := cb.channelQueue[0]
	ch := cb.channels[first]
	timedOut := ch.OpenBlockNumber()+cb.spec.ChannelTimeout(cb.Origin().Time) < cb.Origin().Number
	if timedOut {
		cb.log.Info("channel timed out", "channel", first, "frames", len(ch.inputs))
		cb.metrics.RecordChannelTimedOut()
		delete(cb.channels, first)
		cb.channelQueue = cb.channelQueue[1:]
		return nil, nil // multiple different channels may all be timed out
	}

	// At the point we have removed all timed out channels from the front of the channelQueue.
	// Pre-Canyon we simply check the first index.
	// Post-Canyon we read the entire channelQueue for the first ready channel. If no channel is
	// available, we return `nil, io.EOF`.
	// Canyon is activated when the first L1 block whose time >= CanyonTime, not on the L2 timestamp.
	if !cb.spec.IsCanyon(cb.Origin().Time) {
		return cb.tryReadChannelAtIndex(0)
	}

	for i := 0; i < len(cb.channelQueue); i++ {
		if data, err := cb.tryReadChannelAtIndex(i); err == nil {
			return data, nil
		}
	}
	return nil, io.EOF
}

// tryReadChannelAtIndex attempts to read the channel at the specified index. If the channel is
// not ready (or timed out), it will return io.EOF.
// If the channel read was successful, it will remove the channel from the channelQueue.
func (cb *ChannelBank) tryReadChannelAtIndex(i int) (data []byte, err error) {
	chanID := cb.channelQueue[i]
	ch := cb.channels[chanID]
	timedOut := ch.OpenBlockNumber()+cb.spec.ChannelTimeout(cb.Origin().Time) < cb.Origin().Number
	if timedOut || !ch.IsReady() {
		return nil, io.EOF
	}
	cb.log.Info("Reading channel", "channel", chanID, "frames", len(ch.inputs))

	delete(cb.channels, chanID)
	cb.channelQueue = slices.Delete(cb.channelQueue, i, i+1)
	cb.metrics.RecordHeadChannelOpened()
	r := ch.Reader()
	// Suppress error here. io.ReadAll does return nil instead of io.EOF though.
	data, _ = io.ReadAll(r)
	return data, nil
}

// NextRawChannel pulls the next piece of data from the channel bank.
// Note that it attempts to pull data out of the channel bank prior to
// loading data in (unlike most other stages). This is to ensure maintain
// consistency around channel bank pruning which depends upon the order
// of operations.
func (cb *ChannelBank) NextRawChannel(ctx context.Context) ([]byte, error) {
	// Do the read from the channel bank first
	data, err := cb.Read()
	if err == io.EOF {
		// continue - We will attempt to load data into the channel bank
	} else if err != nil {
		return nil, err
	} else {
		return data, nil
	}

	// Then load data into the channel bank
	if frame, err := cb.prev.NextFrame(ctx); err == io.EOF {
		return nil, io.EOF
	} else if err != nil {
		return nil, err
	} else {
		cb.IngestFrame(frame)
		return nil, NotEnoughData
	}
}

func (cb *ChannelBank) Reset(ctx context.Context, base eth.L1BlockRef, _ eth.SystemConfig) error {
	cb.channels = make(map[ChannelID]*Channel)
	cb.channelQueue = make([]ChannelID, 0, 10)
	return io.EOF
}

func (bq *ChannelBank) FlushChannel() {
	// We need to implement the ChannelFlusher interface with the ChannelBank but it's never called
	// of which the ChannelMux takes care.
	panic("ChannelBank: invalid FlushChannel call")
}

type L1BlockRefByHashFetcher interface {
	L1BlockRefByHash(context.Context, common.Hash) (eth.L1BlockRef, error)
}
