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: mod.rs
    path of file: ./repos/Replibyte/dump-parser/src/mongodb
    
    the code of the file until where you have to start completion: use bson::Document;
use crc::crc64;
use serde::{Deserialize, Serialize};
use std::collections::HashMap;
use std::io::{BufReader, Error, ErrorKind, Read};

/// Four bytes that are always present at the beginning of the archive.
const MAGIC_BYTES: [u8; 4] = [0x6d, 0xe2, 0x99, 0x81];

/// Seperator bytes, found mostly between different data blocks.
const SEPERATOR_BYTES: [u8; 4] = [0xFF, 0xFF, 0xFF, 0xFF];
/// Mongo archive header document.
///
/// Found immediately after the magic number
    
    pub fn from_reader<R: Read>(mut reader: BufReader<R>) -> Result<Archive, Error> {
        let mut buf: [u8; 4] = [0; 4];
        let mut num_blocks = 0;
        let mut vec_eofs = Vec::with_capacity(num_blocks * 2);
        let mut metadata_docs = vec![];
        let mut namespace_docs = vec![];
        let mut prefixed_collections = HashMap::new();

        // read magic bytes
        reader.read_exact(&mut buf)?;
        if buf != MAGIC_BYTES {
            return Err(Error::new(
                ErrorKind::InvalidData,
                "Stream or file does not appear to be a mongodump archive",
            ));
        }

        // read namespace header
        let header: Header = bson::from_reader(&mut reader)
            .map_err(|e| Error::new(ErrorKind::InvalidData, format!("{}", e)))?;

        // read metadata headers and seperator (seperator is read when while let fails for the 1st time)
        while let Ok(collection_metadata_doc) = bson::from_reader(&mut reader) {
            let metadata_doc = Metadata::from(collection_metadata_doc);
            metadata_docs.push(metadata_doc);
            num_blocks += 1;
        }

        if num_blocks > 0 {
            // read blocks
            loop {
                // read namespace header
                let namespace_doc: Namespace = bson::from_reader(&mut reader).map_err(|err| {
                    Error::new(
                        ErrorKind::Other,
                        format!("Error reading block header: {}", err),
                    )
                })?;
                namespace_docs.push(namespace_doc.clone()); // TODO can we avoid cloning here?
                vec_eofs.push(namespace_doc.eof);
                // read block data
                let mut collection_docs = vec![];
                while let Ok(collection_doc) = Document::from_reader(&mut reader) {
                    collection_docs.push(collection_doc.clone());
                }
                if !namespace_doc.eof {
                    // if this namespace was a footer (eof == true), that would mean the collection just ended
                    prefixed_collections.insert(
                        format!("{}.{}", namespace_doc.db, namespace_doc.collection),
                        collection_docs,
                    );
                }
                // when we've seen as much EOFs as there are blocks, we're done.
                if vec_eofs.iter().filter(|&&eof| eof).count() == num_blocks {
                    break;
                }
            }
        }
        Ok(Archive {
            header,
            metadata_docs,
            namespace_docs,
            prefixed_collections,
        })
    }