package redisstate

import (
	"context"
	"errors"
	"fmt"
	"strconv"
	"strings"
	"time"

	"galaxy/notification/internal/telemetry"

	"github.com/redis/go-redis/v9"
)

// StreamOffsetStore provides the Redis-backed storage used for persisted
// plain-XREAD consumer progress.
type StreamOffsetStore struct {
	client *redis.Client
	keys   Keyspace
}

// NewStreamOffsetStore constructs one Redis-backed stream-offset store.
func NewStreamOffsetStore(client *redis.Client) (*StreamOffsetStore, error) {
	if client == nil {
		return nil, errors.New("new notification stream offset store: nil redis client")
	}

	return &StreamOffsetStore{
		client: client,
		keys:   Keyspace{},
	}, nil
}

// Load returns the last processed entry id for stream when one is stored.
func (store *StreamOffsetStore) Load(ctx context.Context, stream string) (string, bool, error) {
	if store == nil || store.client == nil {
		return "", false, errors.New("load notification stream offset: nil store")
	}
	if ctx == nil {
		return "", false, errors.New("load notification stream offset: nil context")
	}

	payload, err := store.client.Get(ctx, store.keys.StreamOffset(stream)).Bytes()
	switch {
	case errors.Is(err, redis.Nil):
		return "", false, nil
	case err != nil:
		return "", false, fmt.Errorf("load notification stream offset: %w", err)
	}

	offset, err := UnmarshalStreamOffset(payload)
	if err != nil {
		return "", false, fmt.Errorf("load notification stream offset: %w", err)
	}

	return offset.LastProcessedEntryID, true, nil
}

// Save stores the last processed entry id for stream.
func (store *StreamOffsetStore) Save(ctx context.Context, stream string, entryID string) error {
	if store == nil || store.client == nil {
		return errors.New("save notification stream offset: nil store")
	}
	if ctx == nil {
		return errors.New("save notification stream offset: nil context")
	}

	offset := StreamOffset{
		Stream:               stream,
		LastProcessedEntryID: entryID,
		UpdatedAt:            time.Now().UTC().Truncate(time.Millisecond),
	}
	payload, err := MarshalStreamOffset(offset)
	if err != nil {
		return fmt.Errorf("save notification stream offset: %w", err)
	}
	if err := store.client.Set(ctx, store.keys.StreamOffset(stream), payload, 0).Err(); err != nil {
		return fmt.Errorf("save notification stream offset: %w", err)
	}

	return nil
}

// IntentStreamLagReader provides Redis-backed lag snapshots for one intent
// stream.
type IntentStreamLagReader struct {
	store  *StreamOffsetStore
	stream string
}

// NewIntentStreamLagReader constructs a lag reader for stream using store.
func NewIntentStreamLagReader(store *StreamOffsetStore, stream string) (*IntentStreamLagReader, error) {
	if store == nil || store.client == nil {
		return nil, errors.New("new notification intent stream lag reader: nil store")
	}
	if strings.TrimSpace(stream) == "" {
		return nil, errors.New("new notification intent stream lag reader: stream must not be empty")
	}

	return &IntentStreamLagReader{
		store:  store,
		stream: stream,
	}, nil
}

// ReadIntentStreamLagSnapshot returns the oldest stream entry that is newer
// than the persisted plain-XREAD consumer offset for the configured stream.
func (reader *IntentStreamLagReader) ReadIntentStreamLagSnapshot(ctx context.Context) (telemetry.IntentStreamLagSnapshot, error) {
	if reader == nil || reader.store == nil {
		return telemetry.IntentStreamLagSnapshot{}, errors.New("read notification intent stream lag snapshot: nil reader")
	}
	if ctx == nil {
		return telemetry.IntentStreamLagSnapshot{}, errors.New("read notification intent stream lag snapshot: nil context")
	}

	lastID, found, err := reader.store.Load(ctx, reader.stream)
	if err != nil {
		return telemetry.IntentStreamLagSnapshot{}, fmt.Errorf("read notification intent stream lag snapshot: %w", err)
	}

	minID := "-"
	if found {
		minID = "(" + lastID
	}

	messages, err := reader.store.client.XRangeN(ctx, reader.stream, minID, "+", 1).Result()
	if err != nil {
		return telemetry.IntentStreamLagSnapshot{}, fmt.Errorf("read notification intent stream lag snapshot: oldest entry: %w", err)
	}
	if len(messages) == 0 {
		return telemetry.IntentStreamLagSnapshot{}, nil
	}

	oldestAt, err := streamEntryTime(messages[0].ID)
	if err != nil {
		return telemetry.IntentStreamLagSnapshot{}, fmt.Errorf("read notification intent stream lag snapshot: oldest entry id: %w", err)
	}

	return telemetry.IntentStreamLagSnapshot{
		OldestUnprocessedAt: &oldestAt,
	}, nil
}

func streamEntryTime(entryID string) (time.Time, error) {
	timestampText, _, ok := strings.Cut(entryID, "-")
	if !ok || strings.TrimSpace(timestampText) == "" {
		return time.Time{}, fmt.Errorf("entry id %q is not a Redis Stream id", entryID)
	}

	timestampMS, err := strconv.ParseInt(timestampText, 10, 64)
	if err != nil {
		return time.Time{}, err
	}
	if timestampMS < 0 {
		return time.Time{}, fmt.Errorf("entry id %q has negative timestamp", entryID)
	}

	return time.UnixMilli(timestampMS).UTC(), nil
}