You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: walker.go
    path of file: ./repos/lakeFS/pkg/block/s3
    
    the code of the file until where you have to start completion: package s3

import (
	"context"
	"fmt"
	"net/url"
	"strings"

	"github.com/aws/aws-sdk-go-v2/aws"
	"github.com/aws/aws-sdk-go-v2/service/s3"
	"github.com/treeverse/lakefs/pkg/block"
)

type Walker struct {
	client *s3.Client
	mark   block.Mark
}

func NewS3Walker(client *s3.Client) *Walker {
	return &Walker{
		client: client,
		mark:   block.Mark{HasMore: true},
	}
}

func (s *Walker) Walk(ctx context.Context, storageURI *url.URL, op block.Walk
    
func (s *Walker) Walk(ctx context.Context, storageURI *url.URL, op block.WalkOptions, walkFn func(e block.ObjectStoreEntry) error) error {
	var continuation *string
	const maxKeys = 1000
	prefix := strings.TrimLeft(storageURI.Path, "/")

	// basePath is the path relative to which the walk is done. The key of the resulting entries will be relative to this path.
	// As the original prefix might not end with a separator, it cannot be used for the
	// trim purpose, as this will create partial "folder" names. When the basePath is
	// trimmed from the key, the remains will be the object name.
	// Example:
	// Say we have the following keys:
	// pref/object
	// pref/obj/another
	// If we specify prefix="pref/obj" (both keys will be listed) then basePath="pref/" and the trim result
	// for the keys will be:
	// object
	// obj/another
	var basePath string
	if idx := strings.LastIndex(prefix, "/"); idx != -1 {
		basePath = prefix[:idx+1]
	}
	bucket := storageURI.Host
	for {
		result, err := s.client.ListObjectsV2(ctx, &s3.ListObjectsV2Input{
			Bucket:            aws.String(bucket),
			ContinuationToken: continuation,
			MaxKeys:           aws.Int32(maxKeys),
			Prefix:            aws.String(prefix),
			StartAfter:        aws.String(op.After),
		})
		if continuation != nil {
			s.mark.ContinuationToken = *continuation
		}
		if err != nil {
			return err
		}
		for _, record := range result.Contents {
			key := aws.ToString(record.Key)
			addr := fmt.Sprintf("s3://%s/%s", bucket, key)
			ent := block.ObjectStoreEntry{
				FullKey:     key,
				RelativeKey: strings.TrimPrefix(key, basePath),
				Address:     addr,
				ETag:        strings.Trim(aws.ToString(record.ETag), "\""),
				Mtime:       aws.ToTime(record.LastModified),
				Size:        aws.ToInt64(record.Size),
			}
			s.mark.LastKey = key
			err := walkFn(ent)
			if err != nil {
				return err
			}
		}
		if !aws.ToBool(result.IsTruncated) {
			break
		}
		continuation = result.NextContinuationToken
	}
	s.mark = block.Mark{
		LastKey: "",
		HasMore: false,
	}
	return nil
}