Merge pull request 'add async feature' (#1) from async into master

Reviewed-on: #1

.drone.yml

@@ -1,4 +1,5 @@
 kind: pipeline
+type: docker
 name: default
 
 steps:
@@ -6,8 +7,8 @@ steps:
   image: rust:1-alpine
   commands:
   - apk add --no-cache musl-dev
-  - cargo build --verbose --all
-  - cargo test --verbose --all
+  - cargo build --verbose --all-features --all
+  - cargo test --verbose --all-features --all
   environment:
     CARGO_REGISTRIES_CRATES_IO_PROTOCOL: sparse
 
@@ -17,6 +18,7 @@ trigger:
 
 ---
 kind: pipeline
+type: docker
 name: publish
 
 steps:

.gitignore

@@ -7,3 +7,4 @@ json.zip
 /.vscode
 test*.bin
 db.dat
+*.pending-snap

Cargo.toml

@@ -1,6 +1,8 @@
 [workspace]
+resolver = "2"
 members = [
   "app",
+  "app_async",
   "lib"
 ]
 

app/Cargo.toml

@@ -14,7 +14,7 @@ name = "db_bench"
 harness = false
 
 [dependencies]
-chgk_ledb_lib = {path = "../lib"}
+chgk_ledb_lib = {path = "../lib", features = ["sync", "source", "convert"]}
 serde_json="1.0"
 zip="0.6"
 rand="0.8"

app_async/Cargo.toml

@@ -0,0 +1,33 @@
+[package]
+name = "chgk_ledb_async"
+version = "1.1.0"
+authors = ["Dmitry <b4tm4n@mail.ru>"]
+edition = "2021"
+repository = "https://gitea.b4tman.ru/b4tman/chgk_ledb"
+license = "MIT"
+description = "Утилита загружающая базу данных ЧГК вопросов из ZIP файла в JSON формате в базу данных."
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+chgk_ledb_lib = {path = "../lib", features = ["async", "convert_async"]}
+serde_json="1.0"
+async_zip = { git = "https://github.com/Majored/rs-async-zip", rev = "ff0d985", features = [
+    "zstd",
+    "tokio",
+    "tokio-fs"] }
+tokio = { version = "1", features = [
+    "io-util",
+    "fs",
+    "rt-multi-thread"
+] }
+tokio-stream = "0.1"
+rand="0.8"
+clap = { version = "4.2.7", features = ["derive"] }
+futures = "0.3"
+
+[dev-dependencies]
+tempfile = "3.3"
+bincode = "^2.0.0-rc.2"
+serde="1.0"
+serde_derive="1.0"

app_async/src/main.rs

@@ -0,0 +1,192 @@
+extern crate serde_json;
+use clap::{Parser, Subcommand};
+use futures::{pin_mut, Future};
+use rand::distributions::Uniform;
+use rand::seq::IteratorRandom;
+use rand::{thread_rng, Rng};
+
+use async_zip::tokio::read::seek::ZipFileReader;
+use futures::stream::{self, StreamExt};
+use std::time::Instant;
+use tokio::sync::mpsc::{self, UnboundedReceiver, UnboundedSender};
+
+use async_db::WriterOpts;
+
+use tokio::{fs, io};
+use tokio_stream::wrappers::UnboundedReceiverStream;
+
+use chgk_ledb_lib::async_db;
+use chgk_ledb_lib::questions::Question;
+use chgk_ledb_lib::questions::QuestionsConverterAsyncForStream;
+use chgk_ledb_lib::source::ReadSourceQuestionsBatchesAsync;
+
+const ZIP_FILENAME: &str = "json.zip";
+const NEW_DB_FILENAME: &str = "db.dat";
+
+#[derive(Subcommand, Debug)]
+enum Command {
+    Write,
+    Print {
+        #[clap(value_parser, default_value = "0")]
+        id: u32,
+    },
+    ZipPrint {
+        #[clap(value_parser, default_value = "0")]
+        file_num: usize,
+        #[clap(value_parser, default_value = "0")]
+        num: usize,
+    },
+}
+
+#[derive(Parser, Debug)]
+#[clap(author, version, about, long_about = None)]
+#[clap(propagate_version = true)]
+struct Cli {
+    #[clap(subcommand)]
+    command: Command,
+    #[clap(short, long, action)]
+    measure: bool,
+}
+
+async fn zip_reader_task(tx: UnboundedSender<Question>) {
+    let mut file = fs::File::open(ZIP_FILENAME).await.expect("open zip");
+    let archive = ZipFileReader::with_tokio(&mut file)
+        .await
+        .expect("open zip file reader");
+    let mut source_questions = archive.source_questions();
+    let source_questions = source_questions.stream();
+    pin_mut!(source_questions);
+
+    source_questions
+        .converter()
+        .convert()
+        .enumerate()
+        .map(|(num, mut question)| {
+            question.num = 1 + (num as u32);
+            question
+        })
+        .for_each_concurrent(None, |question| async {
+            tx.send(question).expect("send");
+        })
+        .await;
+
+    println!("read done");
+}
+
+async fn print_question_from<F>(get_q: F)
+where
+    F: Future<Output = Option<Question>>,
+{
+    let q = get_q.await.expect("question not found");
+    println!("{:#?}", q)
+}
+
+async fn read_from_zip(file_num: usize, mut num: usize) -> Option<Question> {
+    let mut rng = thread_rng();
+    let zip_file = fs::File::open(ZIP_FILENAME).await.expect("open zip file");
+    let mut zip_reader = io::BufReader::new(zip_file);
+    let archive = ZipFileReader::with_tokio(&mut zip_reader)
+        .await
+        .expect("open zip file reader");
+
+    let mut source = archive.source_questions();
+    let files_count = source.len();
+    let file_index = if file_num == 0 {
+        let files = Uniform::new(0, files_count);
+        rng.sample(files)
+    } else {
+        file_num - 1
+    };
+
+    let src = source.get(file_index).await;
+    let src = stream::once(async { src.expect("get source file") });
+    pin_mut!(src);
+    let converter = src.converter();
+    let questions: Vec<_> = converter.convert().collect().await;
+    if num == 0 {
+        num = (1..=questions.len()).choose(&mut rng).unwrap();
+    }
+    let mut question = questions.get(num - 1).expect("get question").clone();
+    question.num = num as u32;
+    Some(question)
+}
+
+// measure and return time elapsed in `fut` in seconds
+pub async fn measure<F: Future>(fut: F) -> f64 {
+    let start = Instant::now();
+    fut.await;
+    let elapsed = start.elapsed();
+    (elapsed.as_secs() as f64) + (elapsed.subsec_nanos() as f64 / 1_000_000_000.0)
+}
+
+pub async fn measure_and_print<F: Future>(fut: F) {
+    let m = measure(fut).await;
+    eprintln!("{}", m);
+}
+
+#[tokio::main]
+async fn main() {
+    let args = Cli::parse();
+
+    let mut action: Box<dyn Future<Output = _>> = match &args.command {
+        Command::Write => Box::new(write_db()),
+        Command::Print { id } => {
+            let get_question = read_from_db(*id);
+            Box::new(print_question_from(get_question))
+        }
+        Command::ZipPrint { file_num, num } => {
+            let get_question = read_from_zip(*file_num, *num);
+            Box::new(print_question_from(get_question))
+        }
+    };
+
+    if args.measure {
+        action = Box::new(measure_and_print(Box::into_pin(action)));
+    }
+
+    Box::into_pin(action).await;
+}
+
+async fn read_from_db(id: u32) -> Option<Question> {
+    let reader: async_db::Reader<Question> = async_db::Reader::new(NEW_DB_FILENAME)
+        .await
+        .expect("new db reader");
+
+    let len = reader.len();
+
+    let index = if len == 0 {
+        let mut rng = thread_rng();
+        let questions = Uniform::new(0, len);
+        rng.sample(questions)
+    } else {
+        id as usize - 1
+    };
+
+    match reader.get(index).await {
+        Ok(question) => Some(question),
+        Err(_) => None,
+    }
+}
+async fn write_db() {
+    let (tx, rx) = mpsc::unbounded_channel::<Question>();
+    tokio::try_join!(
+        tokio::spawn(zip_reader_task(tx)),
+        tokio::spawn(db_writer_task(rx))
+    )
+    .expect("tokio join");
+    println!("all done");
+}
+async fn db_writer_task(rx: UnboundedReceiver<Question>) {
+    let writer_opts = WriterOpts::default();
+    let mut writer: async_db::Writer<Question> =
+        async_db::Writer::new(NEW_DB_FILENAME, writer_opts)
+            .await
+            .unwrap_or_else(|e| panic!("db writer load, {e:#?}"));
+
+    let stream: UnboundedReceiverStream<_> = rx.into();
+    let stream = stream;
+    writer.load(stream).await.expect("load");
+    writer.finish().await.expect("db writer finish");
+
+    println!("write done");
+}

lib/Cargo.toml

@@ -9,14 +9,68 @@ description = "Библиотека для доступа к файлу базы
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[features]
+default = []
+sync = ["zstd", "memmap"]
+async = [
+    "futures",
+    "futures-core",
+    "futures-util",
+    "fmmap",
+    "tokio",
+    "async-compression",
+    "async-stream",
+    "pin-project",
+]
+source = ["zip"]
+source_async = [
+    "async_zip",
+    "tokio",
+    "futures",
+    "futures-core",
+    "futures-util",
+    "async-stream",
+]
+convert = ["zip"]
+convert_async = [
+    "futures",
+    "futures-core",
+    "futures-util",
+    "async-stream",
+    "async_zip",
+    "tokio",
+]
+
 [dependencies]
-serde="1.0"
-serde_derive="1.0"
-serde_json="1.0"
-zip="0.6"
+serde = "1.0"
+serde_derive = "1.0"
+serde_json = "1.0"
 bincode = "^2.0.0-rc.2"
-zstd = "^0.10"
-memmap = "0.7.0"
+zip = { version = "0.6", optional = true }
+async_zip = { git = "https://github.com/Majored/rs-async-zip", rev = "ff0d985", features = [
+    "zstd",
+    "tokio",
+    "tokio-fs",
+], optional = true }
+fmmap = { version = "0.3", features = ["tokio-async"], optional = true }
+tokio = { version = "1", features = [
+    "fs",
+    "io-util",
+    "rt",
+    "macros",
+], optional = true }
+futures-core = { version = "0.3", optional = true }
+futures = { version = "0.3", optional = true }
+futures-util = { version = "0.3", optional = true }
+async-compression = { git = "https://github.com/Nullus157/async-compression", rev = "4fd4c42", default-features = false, features = [
+    "zstd",
+    "tokio",
+], optional = true }
+async-stream = { version = "0.3", optional = true }
+zstd = { version = "^0.12", default-features = false, optional = true }
+memmap = { version = "0.7.0", optional = true }
+pin-project = { version = "1.1.3", optional = true }
 
 [dev-dependencies]
+insta = { version = "1.31.0", features = ["yaml"] }
 tempfile = "3.3"

lib/src/async_db.rs

@@ -0,0 +1,787 @@
+use std::marker::PhantomData;
+use std::ops::Deref;
+use std::vec;
+use std::{path::Path, sync::Arc};
+
+use async_compression::tokio::bufread::ZstdDecoder;
+use async_compression::tokio::bufread::ZstdEncoder;
+use async_compression::Level;
+use futures::sink::Sink;
+use futures::stream::StreamExt;
+use futures_core::stream::Stream;
+use futures_core::Future;
+use futures_util::pin_mut;
+
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+use tokio::{
+    fs,
+    io::{self, AsyncReadExt, AsyncWriteExt},
+};
+
+use fmmap::tokio::{AsyncMmapFile, AsyncMmapFileExt, AsyncOptions};
+
+type LSize = u32;
+const LEN_SIZE: usize = std::mem::size_of::<LSize>();
+const BINCODE_CFG: bincode::config::Configuration = bincode::config::standard();
+
+use crate::util::BincodeVecWriter;
+use crate::util::ErrorToString;
+
+pub struct WriterOpts {
+    pub compress_lvl: Level,
+    pub data_buf_size: usize,
+    pub out_buf_size: usize,
+    pub current_buf_size: usize,
+}
+
+impl Default for WriterOpts {
+    fn default() -> Self {
+        Self {
+            compress_lvl: Level::Default,
+            data_buf_size: 500 * 1024 * 1024,
+            out_buf_size: 200 * 1024 * 1024,
+            current_buf_size: 100 * 1024,
+        }
+    }
+}
+
+pub struct Writer<T>
+where
+    T: bincode::Encode,
+{
+    out: io::BufWriter<fs::File>,
+    data_buf: Vec<u8>,
+    cur_buf_item: BincodeVecWriter,
+    table: Vec<LSize>,
+    compress_lvl: Level,
+    _t: PhantomData<Arc<T>>,
+}
+
+impl<T> Writer<T>
+where
+    T: bincode::Encode,
+{
+    pub async fn new<P: AsRef<Path>>(path: P, opts: WriterOpts) -> Result<Self, String> {
+        let out = fs::File::create(path).await.str_err()?;
+        let out = io::BufWriter::with_capacity(opts.out_buf_size, out);
+        let data_buf: Vec<u8> = Vec::with_capacity(opts.data_buf_size);
+        let cur_buf_item: Vec<u8> = Vec::with_capacity(opts.current_buf_size);
+        let cur_buf_item = BincodeVecWriter::new(cur_buf_item);
+
+        let compress_lvl = opts.compress_lvl;
+
+        let table: Vec<LSize> = vec![];
+
+        Ok(Self {
+            out,
+            data_buf,
+            cur_buf_item,
+            table,
+            compress_lvl,
+            _t: PhantomData,
+        })
+    }
+
+    pub async fn push(&mut self, item: T) -> Result<(), String> {
+        self.push_by_ref(&item).await
+    }
+
+    pub async fn push_by_ref(&mut self, item: &T) -> Result<(), String> {
+        let pos: LSize = self.data_buf.len() as LSize;
+
+        bincode::encode_into_writer(item, &mut self.cur_buf_item, BINCODE_CFG).str_err()?;
+
+        let mut zencoder = ZstdEncoder::with_quality(&self.cur_buf_item[..], self.compress_lvl);
+        io::copy(&mut zencoder, &mut self.data_buf)
+            .await
+            .str_err()?;
+        self.cur_buf_item.clear();
+
+        self.table.push(pos);
+
+        // FIXME
+        // this will break WriterSink::poll_ready (will wait forever), but not Writer::load
+        // tokio::time::sleep(std::time::Duration::from_secs(1)).await;
+
+        Ok(())
+    }
+
+    pub async fn load<S>(&mut self, source: S) -> Result<(), String>
+    where
+        S: Stream<Item = T> + std::marker::Unpin,
+    {
+        let hint = source.size_hint();
+        let hint = std::cmp::max(hint.0, hint.1.unwrap_or(0));
+        if hint > 0 {
+            self.table.reserve(hint);
+        }
+
+        pin_mut!(source);
+        while let Some(item) = source.next().await {
+            self.push(item).await?;
+        }
+
+        Ok(())
+    }
+
+    pub async fn finish(mut self) -> Result<(), String> {
+        // finish tab
+        let pos: LSize = self.data_buf.len() as LSize;
+        self.table.push(pos);
+
+        // write tab
+        let tab_size = (self.table.len() * LEN_SIZE) as LSize;
+        for pos in self.table {
+            let pos_data = (pos + tab_size).to_le_bytes();
+            self.out.write_all(&pos_data).await.str_err()?;
+        }
+        // copy data
+        self.out.write_all(&self.data_buf[..]).await.str_err()?;
+
+        self.out.flush().await.str_err()?;
+        Ok(())
+    }
+
+    pub fn sink(&mut self) -> WriterSink<'_, T> {
+        WriterSink {
+            writer: self,
+            item: None,
+        }
+    }
+}
+
+use pin_project::pin_project;
+
+#[pin_project]
+/// FIXME: not really async
+/// only work when ..push.poll() returns Ready immediately
+pub struct WriterSink<'a, T>
+where
+    T: bincode::Encode,
+{
+    #[pin]
+    writer: &'a mut Writer<T>,
+    item: Option<T>,
+}
+
+impl<'a, T> Sink<T> for WriterSink<'a, T>
+where
+    T: bincode::Encode,
+{
+    type Error = String;
+
+    fn poll_ready(
+        self: std::pin::Pin<&mut Self>,
+        ctx: &mut std::task::Context<'_>,
+    ) -> Poll<Result<(), String>> {
+        let mut this = self.project();
+
+        if this.item.is_none() {
+            return Poll::Ready(Ok(()));
+        }
+
+        let item = this.item.take().unwrap();
+
+        let push_fut = this.writer.push(item); // FIXME:: how to save this future???
+        pin_mut!(push_fut);
+        push_fut.poll(ctx)
+    }
+
+    fn start_send(self: std::pin::Pin<&mut Self>, item: T) -> Result<(), Self::Error> {
+        let this = self.project();
+        *this.item = Some(item);
+        Ok(())
+    }
+
+    fn poll_flush(
+        self: std::pin::Pin<&mut Self>,
+        ctx: &mut std::task::Context<'_>,
+    ) -> Poll<Result<(), Self::Error>> {
+        self.poll_ready(ctx)
+    }
+
+    fn poll_close(
+        mut self: std::pin::Pin<&mut Self>,
+        ctx: &mut std::task::Context<'_>,
+    ) -> Poll<Result<(), Self::Error>> {
+        futures::ready!(self.as_mut().poll_ready(ctx))?;
+        Poll::Ready(Ok(()))
+    }
+}
+
+pub struct Reader<T>
+where
+    T: bincode::Decode,
+{
+    mmap: AsyncMmapFile,
+    count: usize,
+    first_pos: LSize,
+    _t: PhantomData<Arc<T>>,
+}
+
+impl<T> Reader<T>
+where
+    T: bincode::Decode,
+{
+    pub async fn new<P: AsRef<Path>>(path: P) -> Result<Self, String> {
+        let mmap = AsyncOptions::new()
+            .read(true)
+            .open_mmap_file(path)
+            .await
+            .str_err()?;
+        mmap.try_lock_shared().str_err()?;
+
+        // read first pos and records count
+        let first_data: [u8; LEN_SIZE] = mmap.bytes(0, LEN_SIZE).str_err()?.try_into().str_err()?;
+        let first_pos = LSize::from_le_bytes(first_data);
+        let tab_len = (first_pos as usize) / LEN_SIZE;
+        let count = tab_len - 1;
+
+        Ok(Self {
+            mmap,
+            count,
+            first_pos,
+            _t: PhantomData,
+        })
+    }
+
+    pub fn len(&self) -> usize {
+        self.count
+    }
+
+    pub fn is_empty(&self) -> bool {
+        0 == self.len()
+    }
+
+    /// get item at index, reuse data buffer
+    pub async fn get_with_buf(&self, index: usize, data_buf: &mut Vec<u8>) -> Result<T, String> {
+        if index >= self.len() {
+            return Err("index out of range".into());
+        }
+
+        let next_pos: usize = (index + 1) * LEN_SIZE;
+
+        // read item data pos
+        let data_pos = if 0 == index {
+            self.first_pos
+        } else {
+            let tab_pos: usize = index * LEN_SIZE;
+            let pos_curr_data: [u8; LEN_SIZE] = self
+                .mmap
+                .bytes(tab_pos, LEN_SIZE)
+                .str_err()?
+                .try_into()
+                .str_err()?;
+            LSize::from_le_bytes(pos_curr_data)
+        } as usize;
+
+        // read next item pos
+        let pos_next_data: [u8; LEN_SIZE] = self
+            .mmap
+            .bytes(next_pos, LEN_SIZE)
+            .str_err()?
+            .try_into()
+            .str_err()?;
+        let data_pos_next = LSize::from_le_bytes(pos_next_data) as usize;
+        let data_len = data_pos_next - data_pos;
+
+        // read & unpack item data
+        let mut decoder = ZstdDecoder::new(self.mmap.range_reader(data_pos, data_len).str_err()?);
+        decoder.read_to_end(data_buf).await.str_err()?;
+
+        // decode item
+        let item: (T, usize) = bincode::decode_from_slice(data_buf, BINCODE_CFG).str_err()?;
+
+        data_buf.clear();
+        Ok(item.0)
+    }
+
+    /// get item at index
+    pub async fn get(&self, index: usize) -> Result<T, String> {
+        let mut data_buf: Vec<u8> = vec![];
+        self.get_with_buf(index, &mut data_buf).await
+    }
+
+    pub fn stream(&self) -> ReaderStream<'_, T> {
+        ReaderStream::new(self)
+    }
+}
+
+pub struct ReaderStream<'a, T>
+where
+    T: bincode::Decode,
+{
+    reader: &'a Reader<T>,
+    index: Option<usize>,
+}
+
+impl<'a, T> ReaderStream<'a, T>
+where
+    T: bincode::Decode,
+{
+    fn new(reader: &'a Reader<T>) -> Self {
+        ReaderStream {
+            reader,
+            index: None,
+        }
+    }
+}
+
+impl<'a, T> Stream for ReaderStream<'a, T>
+where
+    T: bincode::Decode,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        if self.index.is_none() && !self.reader.is_empty() {
+            self.index = Some(0);
+        }
+
+        if self.index.unwrap() == self.reader.len() {
+            return Poll::Ready(None);
+        }
+
+        // FIXME: mayby work only if reader.get().poll() return Ready immediately
+        let future = self.reader.get(self.index.unwrap());
+        pin_mut!(future);
+        match Pin::new(&mut future).poll(cx) {
+            Poll::Ready(Ok(item)) => {
+                self.index = Some(self.index.unwrap() + 1);
+                Poll::Ready(Some(item))
+            }
+            Poll::Ready(Err(_)) => Poll::Ready(None),
+            Poll::Pending => Poll::Pending,
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.reader.len();
+        if self.index.is_none() {
+            return (len, Some(len));
+        }
+
+        let index = self.index.unwrap();
+        let rem = if len > index + 1 {
+            len - (index + 1)
+        } else {
+            0
+        };
+        (rem, Some(rem))
+    }
+}
+
+pub struct BufReader<T>
+where
+    T: bincode::Decode,
+{
+    inner: Reader<T>,
+    buf: Vec<u8>,
+}
+
+impl<T> BufReader<T>
+where
+    T: bincode::Decode,
+{
+    pub async fn new<P: AsRef<Path>>(path: P, buf_size: usize) -> Result<Self, String> {
+        match Reader::<T>::new(path).await {
+            Ok(inner) => Ok(Self {
+                inner,
+                buf: Vec::with_capacity(buf_size),
+            }),
+            Err(e) => Err(e),
+        }
+    }
+
+    pub async fn get(&mut self, index: usize) -> Result<T, String> {
+        self.inner.get_with_buf(index, &mut self.buf).await
+    }
+
+    pub fn into_inner(self) -> Reader<T> {
+        self.inner
+    }
+
+    pub fn stream(self) -> BufReaderStream<T> {
+        BufReaderStream::new(self)
+    }
+}
+
+impl<T> From<Reader<T>> for BufReader<T>
+where
+    T: bincode::Decode,
+{
+    fn from(inner: Reader<T>) -> Self {
+        Self {
+            inner,
+            buf: Vec::new(),
+        }
+    }
+}
+
+impl<T> From<BufReader<T>> for Reader<T>
+where
+    T: bincode::Decode,
+{
+    fn from(value: BufReader<T>) -> Self {
+        value.into_inner()
+    }
+}
+
+impl<T> Deref for BufReader<T>
+where
+    T: bincode::Decode,
+{
+    type Target = Reader<T>;
+    fn deref(&self) -> &Self::Target {
+        &self.inner
+    }
+}
+
+pub struct BufReaderStream<T>
+where
+    T: bincode::Decode,
+{
+    reader: BufReader<T>,
+    index: Option<usize>,
+}
+
+impl<T> BufReaderStream<T>
+where
+    T: bincode::Decode,
+{
+    fn new(reader: BufReader<T>) -> Self {
+        BufReaderStream {
+            reader,
+            index: None,
+        }
+    }
+
+    async fn get_next(&mut self) -> Result<T, String> {
+        match self.index {
+            None => Err("index is None".into()),
+            Some(index) => {
+                let res = self.reader.get(index).await;
+                self.index = Some(index + 1);
+
+                res
+            }
+        }
+    }
+}
+
+impl<T> Stream for BufReaderStream<T>
+where
+    T: bincode::Decode,
+{
+    type Item = T;
+
+    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<T>> {
+        if self.index.is_none() && !self.reader.is_empty() {
+            self.index = Some(0);
+        }
+
+        if self.index.unwrap() == self.reader.len() {
+            return Poll::Ready(None);
+        }
+
+        // FIXME: mayby work only if reader.get().poll() return Ready immediately
+        let future = self.get_next();
+        pin_mut!(future);
+        match Pin::new(&mut future).poll(cx) {
+            Poll::Ready(Ok(item)) => Poll::Ready(Some(item)),
+            Poll::Ready(Err(_)) => Poll::Ready(None),
+            Poll::Pending => Poll::Pending,
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.reader.len();
+        if self.index.is_none() {
+            return (len, Some(len));
+        }
+
+        let index = self.index.unwrap();
+        let rem = if len > index + 1 {
+            len - (index + 1)
+        } else {
+            0
+        };
+        (rem, Some(rem))
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use core::fmt::Debug;
+    use tempfile::tempdir;
+
+    #[derive(bincode::Encode, bincode::Decode, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+    struct TestData {
+        num: u64,
+        test: String,
+        vnum: Vec<u64>,
+        vstr: Vec<String>,
+    }
+
+    fn gen_data(count: usize) -> impl Iterator<Item = TestData> {
+        (0..count).map(|i| TestData {
+            num: i as u64,
+            test: "test".repeat(i),
+            vnum: (0..i * 120).map(|x| (x ^ 0x345FE34) as u64).collect(),
+            vstr: (0..i * 111).map(|x| "test".repeat(x)).collect(),
+        })
+    }
+
+    async fn assert_data_eq((x, y): (&TestData, TestData)) {
+        assert_eq!(*x, y);
+    }
+
+    #[tokio::test]
+    async fn test_write() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+    }
+
+    #[tokio::test]
+    async fn test_write_read() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+
+        let reader: Reader<TestData> = Reader::new(&tmpfile).await.expect("new reader");
+        assert_eq!(items.len(), reader.len());
+
+        for (idx, item) in items.iter().enumerate() {
+            let ritem = reader.get(idx).await.expect("get");
+            assert_eq!(*item, ritem);
+        }
+    }
+
+    #[tokio::test]
+    async fn test_write_sink_read() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone()).map(Ok);
+        pin_mut!(src);
+        src.forward(writer.sink()).await.expect("forward");
+        writer.finish().await.expect("finish write");
+
+        let reader: Reader<TestData> = Reader::new(&tmpfile).await.expect("new reader");
+        assert_eq!(items.len(), reader.len());
+
+        for (idx, item) in items.iter().enumerate() {
+            let ritem = reader.get(idx).await.expect("get");
+            assert_eq!(*item, ritem);
+        }
+    }
+
+    #[tokio::test]
+    async fn test_write_read_get_with_buf() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+
+        let reader: Reader<TestData> = Reader::new(&tmpfile).await.expect("new reader");
+        assert_eq!(items.len(), reader.len());
+
+        for (idx, item) in items.iter().enumerate() {
+            let mut data_buf: Vec<u8> = vec![];
+            let ritem = reader.get_with_buf(idx, &mut data_buf).await.expect("get");
+            assert_eq!(*item, ritem);
+        }
+    }
+
+    #[tokio::test]
+    async fn test_write_read_stream() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+
+        let reader: Reader<TestData> = Reader::new(&tmpfile).await.expect("new reader");
+        assert_eq!(items.len(), reader.len());
+
+        let dst_stream = reader.stream();
+        let src_stream = futures::stream::iter(items.iter());
+
+        let mut count = 0;
+        src_stream
+            .zip(dst_stream)
+            .map(|x| {
+                count += 1;
+                x
+            })
+            .for_each(assert_data_eq)
+            .await;
+        assert_eq!(count, items.len())
+    }
+    /// sharing Reader instance between threads
+    #[tokio::test]
+    async fn test_share_reader() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+
+        let reader: Reader<TestData> = Reader::new(&tmpfile).await.expect("new reader");
+        assert_eq!(items.len(), reader.len());
+
+        let reader = Arc::new(reader);
+        for _ in 0..=3 {
+            let cur_items = items.clone();
+            let cur_reader = Arc::clone(&reader);
+            tokio::spawn(async move {
+                let dst_stream = cur_reader.stream();
+                let src_stream = futures::stream::iter(cur_items.iter());
+
+                src_stream.zip(dst_stream).for_each(assert_data_eq).await;
+            });
+        }
+    }
+
+    #[tokio::test]
+    async fn test_write_bufread() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+
+        let mut reader = BufReader::<TestData>::new(&tmpfile, 4096)
+            .await
+            .expect("new buf reader");
+        assert_eq!(items.len(), reader.len());
+
+        for (idx, item) in items.iter().enumerate() {
+            let ritem = reader.get(idx).await.expect("get");
+            assert_eq!(*item, ritem);
+        }
+    }
+
+    #[tokio::test]
+    async fn test_write_bufread_stream() {
+        let dir = tempdir().expect("tempdir");
+        let tmpfile = dir.path().join("test.tmp");
+        let opts = WriterOpts {
+            data_buf_size: 10 * 1024 * 1024,
+            out_buf_size: 10 * 1024 * 1024,
+            ..Default::default()
+        };
+        let mut writer: Writer<TestData> = Writer::new(&tmpfile, opts).await.expect("new writer");
+
+        let items_iter = gen_data(5);
+        let items: Vec<TestData> = items_iter.collect();
+
+        let src = futures::stream::iter(items.clone());
+        pin_mut!(src);
+        writer.load(src).await.expect("load");
+        writer.finish().await.expect("finish write");
+
+        let reader = BufReader::<TestData>::new(&tmpfile, 4096)
+            .await
+            .expect("new buf reader");
+        assert_eq!(items.len(), reader.len());
+
+        let dst_stream = reader.stream();
+        let src_stream = futures::stream::iter(items.iter());
+
+        let mut count = 0;
+        src_stream
+            .zip(dst_stream)
+            .map(|x| {
+                count += 1;
+                x
+            })
+            .for_each(assert_data_eq)
+            .await;
+        assert_eq!(count, items.len())
+    }
+}

lib/src/db.rs

@@ -12,20 +12,8 @@ type LSize = u32;
 const LEN_SIZE: usize = std::mem::size_of::<LSize>();
 const BINCODE_CFG: bincode::config::Configuration = bincode::config::standard();
 
-trait ErrorToString {
-    type Output;
-    fn str_err(self) -> std::result::Result<Self::Output, String>;
-}
-
-impl<T, E> ErrorToString for std::result::Result<T, E>
-where
-    E: std::error::Error,
-{
-    type Output = T;
-    fn str_err(self) -> std::result::Result<Self::Output, String> {
-        self.map_err(|e| e.to_string())
-    }
-}
+use crate::util::BincodeVecWriter;
+use crate::util::ErrorToString;
 
 pub struct WriterOpts {
     pub compress_lvl: i32,
@@ -52,6 +40,7 @@ where
     out: io::BufWriter<fs::File>,
     data_buf: Cursor<Vec<u8>>,
     cur_buf_raw: Cursor<Vec<u8>>,
+    cur_buf_item: BincodeVecWriter,
     table: Vec<LSize>,
     compress_lvl: i32,
     _t: PhantomData<Arc<T>>,
@@ -69,6 +58,8 @@ where
 
         let cur_buf_raw: Vec<u8> = Vec::with_capacity(opts.current_buf_size);
         let cur_buf_raw = Cursor::new(cur_buf_raw);
+        let cur_buf_item: Vec<u8> = Vec::with_capacity(opts.current_buf_size);
+        let cur_buf_item = BincodeVecWriter::new(cur_buf_item);
 
         let compress_lvl = opts.compress_lvl;
 
@@ -78,6 +69,7 @@ where
             out,
             data_buf,
             cur_buf_raw,
+            cur_buf_item,
             table,
             compress_lvl,
             _t: PhantomData,
@@ -85,20 +77,25 @@ where
     }
 
     pub fn push(&mut self, item: T) -> Result<(), String> {
+        self.push_by_ref(&item)
+    }
+
+    pub fn push_by_ref(&mut self, item: &T) -> Result<(), String> {
         let pos: LSize = self.data_buf.position() as LSize;
 
-        let item_data = bincode::encode_to_vec(item, BINCODE_CFG).str_err()?;
+        bincode::encode_into_writer(item, &mut self.cur_buf_item, BINCODE_CFG).str_err()?;
 
         let mut zencoder = zstd::stream::raw::Encoder::new(self.compress_lvl).str_err()?;
         zencoder
-            .set_pledged_src_size(item_data.len() as u64)
+            .set_pledged_src_size(Some(self.cur_buf_item.len() as u64))
             .str_err()?;
 
         self.cur_buf_raw.set_position(0);
         let mut cur_buf_z = zstd::stream::zio::Writer::new(&mut self.cur_buf_raw, zencoder);
-        cur_buf_z.write_all(&item_data).str_err()?;
+        cur_buf_z.write_all(&self.cur_buf_item).str_err()?;
         cur_buf_z.finish().str_err()?;
         cur_buf_z.flush().str_err()?;
+        self.cur_buf_item.clear();
 
         self.table.push(pos);
         let (cur_buf_raw, _) = cur_buf_z.into_inner();
@@ -416,7 +413,7 @@ mod test {
     }
 
     fn gen_data(count: usize) -> impl Iterator<Item = TestData> {
-        (0..count).into_iter().map(|i| TestData {
+        (0..count).map(|i| TestData {
             num: i as u64,
             test: "test".repeat(i),
         })

lib/src/lib.rs

@@ -1,3 +1,13 @@
+#[cfg(feature = "async")]
+pub mod async_db;
+#[cfg(feature = "sync")]
 pub mod db;
 pub mod questions;
+#[cfg(any(
+    feature = "source",
+    feature = "source_async",
+    feature = "convert",
+    feature = "convert_async"
+))]
 pub mod source;
+pub mod util;

lib/src/questions.rs

@@ -1,110 +1,126 @@
 use serde_derive::{Deserialize, Serialize};
 
-use crate::source::{SourceQuestion, SourceQuestionsBatch};
-
-macro_rules! make {
-    ($Target:ident; by {$($field:ident),+}; from $src:expr) => {$Target {$(
-        $field: $src.$field
-    ),+}};
-    ($Target:ident; with defaults and by {$($field:ident),+}; from $src:expr) => {$Target {$(
-        $field: $src.$field
-    ),+ ,..$Target::default()}}
-}
-
-#[derive(Debug, Default, Clone, Serialize, Deserialize, bincode::Decode, bincode::Encode)]
+#[derive(
+    Debug, Default, Clone, Serialize, Deserialize, bincode::Decode, bincode::Encode, PartialEq,
+)]
 pub struct BatchInfo {
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub filename: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub description: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub author: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub comment: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub url: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub date: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub processed_by: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub redacted_by: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub copyright: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub theme: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub kind: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub source: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub rating: String,
 }
 
-#[derive(Debug, Default, Clone, Serialize, Deserialize, bincode::Decode, bincode::Encode)]
+#[derive(
+    Debug, Default, Clone, Serialize, Deserialize, bincode::Decode, bincode::Encode, PartialEq,
+)]
 pub struct Question {
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "u32_is_zero")]
     pub num: u32,
     pub id: String,
 
     pub description: String,
     pub answer: String,
 
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub author: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub comment: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub comment1: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub tour: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub url: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub date: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub processed_by: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub redacted_by: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub copyright: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub theme: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub kind: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub source: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub rating: String,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "BatchInfo::is_default")]
     pub batch_info: BatchInfo,
 }
 
-impl From<SourceQuestion> for Question {
+fn u32_is_zero(num: &u32) -> bool {
+    *num == 0
+}
+
+impl BatchInfo {
+    pub fn is_default(&self) -> bool {
+        *self == BatchInfo::default()
+    }
+}
+
+#[cfg(any(feature = "convert", feature = "convert_async"))]
+pub mod convert_common {
+    use super::{BatchInfo, Question};
+    use crate::source::{SourceQuestion, SourceQuestionsBatch};
+
+    macro_rules! make {
+        ($Target:ident; by {$($field:ident),+}; from $src:expr) => {$Target {$(
+            $field: $src.$field
+        ),+}};
+        ($Target:ident; with defaults and by {$($field:ident),+}; from $src:expr) => {$Target {$(
+            $field: $src.$field
+        ),+ ,..$Target::default()}}
+    }
+
+    impl From<SourceQuestion> for Question {
         fn from(src: SourceQuestion) -> Self {
             make! {Self; with defaults and by {
                 num, id, description, answer, author, comment, comment1, tour, url,
                 date, processed_by, redacted_by, copyright, theme, kind, source, rating
             }; from src}
         }
-}
+    }
 
-impl From<SourceQuestionsBatch> for BatchInfo {
+    impl From<SourceQuestionsBatch> for BatchInfo {
         fn from(src: SourceQuestionsBatch) -> Self {
             make! {Self; by {
                 filename, description, author, comment, url, date,
                 processed_by, redacted_by, copyright, theme, kind, source, rating
             }; from src}
         }
-}
+    }
 
-impl From<SourceQuestionsBatch> for Vec<Question> {
+    impl From<SourceQuestionsBatch> for Vec<Question> {
         fn from(src: SourceQuestionsBatch) -> Self {
-        let mut result: Vec<Question> = src
-            .questions
-            .iter()
-            .map(|item| item.clone().into())
-            .collect();
+            let mut src = src;
+            let mut questions: Vec<SourceQuestion> = vec![];
+            std::mem::swap(&mut src.questions, &mut questions);
+            let mut result: Vec<Question> = questions.into_iter().map(|item| item.into()).collect();
             let batch_info = BatchInfo::from(src);
             result.iter_mut().for_each(|question| {
                 question.batch_info = batch_info.clone();
@@ -112,16 +128,22 @@ impl From<SourceQuestionsBatch> for Vec<Question> {
 
             result
         }
+    }
 }
 
-pub trait QuestionsConverter {
+#[cfg(feature = "convert")]
+pub mod convert {
+    use super::Question;
+    use crate::source::SourceQuestionsBatch;
+
+    pub trait QuestionsConverter {
         fn convert<'a>(&'a mut self) -> Box<dyn Iterator<Item = Question> + 'a>;
-}
+    }
 
-impl<T> QuestionsConverter for T
-where
+    impl<T> QuestionsConverter for T
+    where
         T: Iterator<Item = (String, Result<SourceQuestionsBatch, serde_json::Error>)>,
-{
+    {
         fn convert<'a>(&'a mut self) -> Box<dyn Iterator<Item = Question> + 'a> {
             let iter = self
                 .filter(|(_, data)| data.is_ok())
@@ -133,4 +155,244 @@ where
                 });
             Box::new(iter)
         }
+    }
+
+    #[cfg(test)]
+    mod test {
+        use crate::questions::test::convert_common::sample_batch;
+
+        use super::*;
+        use insta::assert_yaml_snapshot;
+        use std::iter;
+
+        #[test]
+        fn test_convert() {
+            let mut source = iter::once((
+                String::from("test.json"),
+                Ok::<SourceQuestionsBatch, serde_json::Error>(sample_batch()),
+            ));
+            let converted: Vec<_> = source.convert().collect();
+            assert_yaml_snapshot!(converted, @r#"
+            ---
+            - id: Вопрос 1
+              description: Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2
+              answer: "42"
+              batch_info:
+                filename: test.json
+                description: Тестовый
+                date: 00-000-2000
+            - id: Вопрос 2
+              description: Зимой и летом одним цветом
+              answer: ёлка
+              batch_info:
+                filename: test.json
+                description: Тестовый
+                date: 00-000-2000
+              
+            "#);
+        }
+    }
+}
+#[cfg(feature = "convert")]
+pub use convert::QuestionsConverter;
+
+#[cfg(feature = "convert_async")]
+pub mod convert_async {
+    use futures::stream;
+    use futures_core::stream::Stream;
+    use futures_util::StreamExt;
+
+    use super::Question;
+    use crate::source::SourceQuestionsBatch;
+
+    pub struct QuestionsConverterAsync<T>
+    where
+        T: Stream<Item = (String, Result<SourceQuestionsBatch, serde_json::Error>)>
+            + std::marker::Unpin,
+    {
+        inner: T,
+    }
+
+    impl<T> From<T> for QuestionsConverterAsync<T>
+    where
+        T: Stream<Item = (String, Result<SourceQuestionsBatch, serde_json::Error>)>
+            + std::marker::Unpin,
+    {
+        fn from(inner: T) -> Self {
+            Self { inner }
+        }
+    }
+
+    pub trait QuestionsConverterAsyncForStream<T>
+    where
+        T: Stream<Item = (String, Result<SourceQuestionsBatch, serde_json::Error>)>
+            + std::marker::Unpin,
+    {
+        fn converter(&mut self) -> QuestionsConverterAsync<&mut T>;
+    }
+
+    impl<T> QuestionsConverterAsyncForStream<T> for T
+    where
+        T: Stream<Item = (String, Result<SourceQuestionsBatch, serde_json::Error>)>
+            + std::marker::Unpin,
+    {
+        fn converter(&mut self) -> QuestionsConverterAsync<&mut T> {
+            QuestionsConverterAsync::from(self)
+        }
+    }
+
+    impl<T> QuestionsConverterAsync<T>
+    where
+        T: Stream<Item = (String, Result<SourceQuestionsBatch, serde_json::Error>)>
+            + std::marker::Unpin,
+    {
+        pub fn convert(self) -> impl Stream<Item = Question> {
+            self.inner
+                .filter_map(|(name, res)| async move {
+                    if let Ok(item) = res {
+                        Some((name, item))
+                    } else {
+                        None
+                    }
+                })
+                .flat_map(|(filename, batch)| {
+                    stream::iter({
+                        let mut batch = batch;
+                        batch.filename = filename;
+                        let questions: Vec<Question> = batch.into();
+                        questions
+                    })
+                })
+        }
+    }
+
+    #[cfg(test)]
+    mod test {
+        use crate::questions::test::convert_common::sample_batch;
+
+        use super::*;
+        use futures_util::{pin_mut, StreamExt};
+        use insta::assert_yaml_snapshot;
+
+        #[tokio::test]
+        async fn test_convert_stream() {
+            let source = futures::stream::once(async {
+                (
+                    String::from("test.json"),
+                    Ok::<SourceQuestionsBatch, serde_json::Error>(sample_batch()),
+                )
+            });
+
+            pin_mut!(source);
+            let converter = source.converter();
+            let converter = converter.convert();
+            let converted: Vec<_> = converter.collect().await;
+            assert_yaml_snapshot!(converted, @r#"
+            ---
+            - id: Вопрос 1
+              description: Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2
+              answer: "42"
+              batch_info:
+                filename: test.json
+                description: Тестовый
+                date: 00-000-2000
+            - id: Вопрос 2
+              description: Зимой и летом одним цветом
+              answer: ёлка
+              batch_info:
+                filename: test.json
+                description: Тестовый
+                date: 00-000-2000
+              
+            "#);
+        }
+    }
+}
+#[cfg(feature = "convert_async")]
+pub use convert_async::{QuestionsConverterAsync, QuestionsConverterAsyncForStream};
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use insta::assert_yaml_snapshot;
+    use serde_json::json;
+
+    #[cfg(any(feature = "convert", feature = "convert_async"))]
+    pub mod convert_common {
+        use crate::source::{SourceQuestion, SourceQuestionsBatch};
+
+        pub fn sample_batch() -> SourceQuestionsBatch {
+            SourceQuestionsBatch {
+                description: "Тестовый".into(),
+                date: "00-000-2000".into(),
+                questions: vec![
+                    SourceQuestion {
+                        id: "Вопрос 1".into(),
+                        description: "Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2".into(),
+                        answer: "42".into(),
+                        ..Default::default()
+                    },
+                    SourceQuestion {
+                        id: "Вопрос 2".into(),
+                        description: "Зимой и летом одним цветом".into(),
+                        answer: "ёлка".into(),
+                        ..Default::default()
+                    },
+                ],
+                ..Default::default()
+            }
+        }
+    }
+
+    pub fn sample_question() -> Question {
+        Question {
+            id: "Вопрос 1".into(),
+            description: "Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2".into(),
+            answer: "42".into(),
+            batch_info: BatchInfo {
+                description: "Тестовый".into(),
+                date: "00-000-2000".into(),
+                ..Default::default()
+            },
+            ..Default::default()
+        }
+    }
+
+    #[test]
+    fn test_question_ser() {
+        assert_yaml_snapshot!(sample_question(), @r#"
+            ---
+            id: Вопрос 1
+            description: Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2
+            answer: "42"
+            batch_info:
+              description: Тестовый
+              date: 00-000-2000
+              
+            "#);
+    }
+    #[test]
+    fn test_question_de() {
+        let question_from_json: Result<Question, _> = serde_json::from_value(json!({
+            "id": "Вопрос 1",
+            "description": "Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2",
+            "answer": "42",
+            "batch_info": {
+                "description": "Тестовый",
+                "date": "00-000-2000"
+            }
+        }));
+        assert!(question_from_json.is_ok());
+
+        assert_yaml_snapshot!(question_from_json.unwrap(), @r#"
+        ---
+        id: Вопрос 1
+        description: Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2
+        answer: "42"
+        batch_info:
+          description: Тестовый
+          date: 00-000-2000
+          
+        "#);
+    }
 }

lib/src/source.rs

@@ -1,11 +1,10 @@
 use serde_derive::{Deserialize, Serialize};
-use std::io::{Read, Seek};
-use zip::ZipArchive;
 
-#[derive(Debug, Default, Clone, Serialize, Deserialize)]
+#[derive(Debug, Default, Clone, Serialize, Deserialize, PartialEq)]
 pub struct SourceQuestion {
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "u32_is_zero")]
     pub num: u32,
+    #[serde(default)]
     pub id: String,
 
     #[serde(alias = "Вопрос")]
@@ -13,122 +12,136 @@ pub struct SourceQuestion {
     #[serde(alias = "Ответ")]
     pub answer: String,
 
-    #[serde(alias = "Автор")]
-    #[serde(default)]
+    #[serde(alias = "Автор", default, skip_serializing_if = "String::is_empty")]
     pub author: String,
-    #[serde(alias = "Комментарий")]
-    #[serde(default)]
+    #[serde(
+        default,
+        alias = "Комментарий",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub comment: String,
-    #[serde(alias = "Комментарии")]
-    #[serde(alias = "Инфо")]
-    #[serde(default)]
+    #[serde(
+        default,
+        alias = "Комментарии",
+        alias = "Инфо",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub comment1: String,
-    #[serde(alias = "Тур")]
-    #[serde(default)]
+    #[serde(default, alias = "Тур", skip_serializing_if = "String::is_empty")]
     pub tour: String,
-    #[serde(alias = "Ссылка")]
-    #[serde(alias = "URL")]
-    #[serde(default)]
+    #[serde(
+        default,
+        alias = "Ссылка",
+        alias = "URL",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub url: String,
-    #[serde(alias = "Дата")]
-    #[serde(default)]
+    #[serde(default, alias = "Дата", skip_serializing_if = "String::is_empty")]
     pub date: String,
-    #[serde(alias = "Обработан")]
-    #[serde(default)]
+    #[serde(default, alias = "Обработан", skip_serializing_if = "String::is_empty")]
     pub processed_by: String,
-    #[serde(alias = "Редактор")]
-    #[serde(default)]
+    #[serde(default, alias = "Редактор", skip_serializing_if = "String::is_empty")]
     pub redacted_by: String,
-    #[serde(alias = "Копирайт")]
-    #[serde(default)]
+    #[serde(default, alias = "Копирайт", skip_serializing_if = "String::is_empty")]
     pub copyright: String,
-    #[serde(alias = "Тема")]
-    #[serde(default)]
+    #[serde(default, alias = "Тема", skip_serializing_if = "String::is_empty")]
     pub theme: String,
-    #[serde(alias = "Вид")]
-    #[serde(alias = "Тип")]
-    #[serde(default)]
+    #[serde(
+        default,
+        alias = "Вид",
+        alias = "Тип",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub kind: String,
-    #[serde(alias = "Источник")]
-    #[serde(default)]
+    #[serde(default, alias = "Источник", skip_serializing_if = "String::is_empty")]
     pub source: String,
-    #[serde(alias = "Рейтинг")]
-    #[serde(default)]
+    #[serde(default, alias = "Рейтинг", skip_serializing_if = "String::is_empty")]
     pub rating: String,
 }
 
-#[derive(Debug, Default, Clone, Serialize, Deserialize)]
+#[derive(Debug, Default, Clone, Serialize, Deserialize, PartialEq)]
 pub struct SourceQuestionsBatch {
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "String::is_empty")]
     pub filename: String,
-    #[serde(alias = "Пакет")]
-    #[serde(alias = "Чемпионат")]
+    #[serde(alias = "Пакет", alias = "Чемпионат")]
     pub description: String,
-    #[serde(alias = "Автор")]
-    #[serde(default)]
+    #[serde(default, alias = "Автор", skip_serializing_if = "String::is_empty")]
     pub author: String,
-    #[serde(alias = "Комментарий")]
-    #[serde(alias = "Комментарии")]
-    #[serde(alias = "Инфо")]
-    #[serde(default)]
+    #[serde(
+        default,
+        alias = "Комментарий",
+        alias = "Комментарии",
+        alias = "Инфо",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub comment: String,
-    #[serde(alias = "Ссылка")]
-    #[serde(alias = "URL")]
-    #[serde(default)]
+    #[serde(
+        default,
+        alias = "Ссылка",
+        alias = "URL",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub url: String,
-    #[serde(alias = "Дата")]
-    #[serde(default)]
+    #[serde(default, alias = "Дата", skip_serializing_if = "String::is_empty")]
     pub date: String,
-    #[serde(alias = "Обработан")]
-    #[serde(default)]
+    #[serde(default, alias = "Обработан", skip_serializing_if = "String::is_empty")]
     pub processed_by: String,
-    #[serde(alias = "Редактор")]
-    #[serde(default)]
+    #[serde(default, alias = "Редактор", skip_serializing_if = "String::is_empty")]
     pub redacted_by: String,
-    #[serde(alias = "Копирайт")]
-    #[serde(default)]
+    #[serde(default, alias = "Копирайт", skip_serializing_if = "String::is_empty")]
     pub copyright: String,
-    #[serde(alias = "Тема")]
-    #[serde(default)]
+    #[serde(default, alias = "Тема", skip_serializing_if = "String::is_empty")]
     pub theme: String,
-    #[serde(alias = "Вид")]
-    #[serde(alias = "Тип")]
-    #[serde(default)]
+    #[serde(
+        default,
+        alias = "Вид",
+        alias = "Тип",
+        skip_serializing_if = "String::is_empty"
+    )]
     pub kind: String,
-    #[serde(alias = "Источник")]
-    #[serde(default)]
+    #[serde(default, alias = "Источник", skip_serializing_if = "String::is_empty")]
     pub source: String,
-    #[serde(alias = "Рейтинг")]
-    #[serde(default)]
+    #[serde(default, alias = "Рейтинг", skip_serializing_if = "String::is_empty")]
     pub rating: String,
     #[serde(alias = "Вопросы")]
     pub questions: Vec<SourceQuestion>,
 }
 
-pub struct SourceQuestionsZipReader<R>
-where
-    R: Read + Seek,
-{
-    zipfile: ZipArchive<R>,
-    index: Option<usize>,
+fn u32_is_zero(num: &u32) -> bool {
+    *num == 0
 }
 
-impl<R> SourceQuestionsZipReader<R>
-where
+#[cfg(any(feature = "convert", feature = "source"))]
+pub mod reader_sync {
+    use std::io::{Read, Seek};
+    use zip::ZipArchive;
+
+    use super::SourceQuestionsBatch;
+
+    pub struct SourceQuestionsZipReader<R>
+    where
         R: Read + Seek,
-{
+    {
+        zipfile: ZipArchive<R>,
+        index: Option<usize>,
+    }
+
+    impl<R> SourceQuestionsZipReader<R>
+    where
+        R: Read + Seek,
+    {
         fn new(zipfile: ZipArchive<R>) -> Self {
             SourceQuestionsZipReader {
                 zipfile,
                 index: None,
             }
         }
-}
+    }
 
-impl<R> Iterator for SourceQuestionsZipReader<R>
-where
+    impl<R> Iterator for SourceQuestionsZipReader<R>
+    where
         R: Read + Seek,
-{
+    {
         type Item = (String, Result<SourceQuestionsBatch, serde_json::Error>);
 
         fn next(&mut self) -> Option<Self::Item> {
@@ -178,29 +191,389 @@ where
         {
             self.zipfile.len()
         }
-}
+    }
 
-impl<R> ExactSizeIterator for SourceQuestionsZipReader<R>
-where
+    impl<R> ExactSizeIterator for SourceQuestionsZipReader<R>
+    where
         R: Read + Seek,
-{
+    {
         fn len(&self) -> usize {
             self.zipfile.len()
         }
-}
+    }
 
-pub trait ReadSourceQuestionsBatches<R>
-where
+    pub trait ReadSourceQuestionsBatches<R>
+    where
         R: Read + Seek,
-{
+    {
         fn source_questions(self) -> SourceQuestionsZipReader<R>;
-}
+    }
 
-impl<R> ReadSourceQuestionsBatches<R> for ZipArchive<R>
-where
+    impl<R> ReadSourceQuestionsBatches<R> for ZipArchive<R>
+    where
         R: Read + Seek,
-{
+    {
         fn source_questions(self) -> SourceQuestionsZipReader<R> {
             SourceQuestionsZipReader::new(self)
         }
+    }
+    #[cfg(test)]
+    mod test {
+        use super::super::test::sample_batch;
+        use super::*;
+        use std::fs;
+        use std::{io::Write, iter, path::Path};
+        use tempfile::tempdir;
+
+        fn write_sample_zip<P>(path: P)
+        where
+            P: AsRef<Path>,
+        {
+            let batch = sample_batch();
+            let z_file = fs::File::create(path).expect("crerate zip file");
+            let mut zip_file = zip::ZipWriter::new(z_file);
+            let options =
+                zip::write::FileOptions::default().compression_method(zip::CompressionMethod::Zstd);
+            zip_file
+                .start_file("test.json", options)
+                .expect("zip start file");
+            let data = serde_json::to_vec(&batch).unwrap();
+            let amount = zip_file.write(data.as_slice()).expect("write entry");
+            assert_eq!(amount, data.len());
+            zip_file.finish().expect("finish zip file");
+        }
+
+        #[test]
+        fn test_source_questions_get() {
+            let expected_batch = sample_batch();
+            let dir = tempdir().expect("tempdir");
+
+            // write sample
+            let tmpfile_zip = dir.path().join("test.zip");
+            write_sample_zip(&tmpfile_zip);
+
+            let z_file = fs::File::open(tmpfile_zip).expect("open zip file");
+            let zip_file = zip::ZipArchive::new(z_file).expect("open zip file reader");
+
+            let mut source = zip_file.source_questions();
+            assert_eq!(source.len(), 1);
+
+            let actual = source.next().expect("get batch");
+            assert_eq!(actual.0, "test.json");
+            assert_eq!(actual.1.expect("parse batch"), expected_batch);
+        }
+
+        #[test]
+        fn test_source_questions_iter() {
+            let expected_batch = sample_batch();
+            let dir = tempdir().expect("tempdir");
+
+            // write sample
+            let tmpfile_zip = dir.path().join("test.zip");
+            write_sample_zip(&tmpfile_zip);
+
+            let z_file = fs::File::open(tmpfile_zip).expect("open zip file");
+            let zip_file = zip::ZipArchive::new(z_file).expect("open zip file reader");
+
+            let source = zip_file.source_questions();
+            assert_eq!(source.len(), 1);
+
+            let expected_iter = iter::once((String::from("test.json"), Ok(expected_batch)));
+
+            assert!(source
+                .map(|x| (x.0, x.1.map_err(|e| e.to_string())))
+                .eq(expected_iter));
+        }
+    }
+}
+
+#[cfg(any(feature = "convert", feature = "source"))]
+pub use reader_sync::{ReadSourceQuestionsBatches, SourceQuestionsZipReader};
+
+#[cfg(any(feature = "convert_async", feature = "source_async"))]
+pub mod reader_async {
+    use async_stream::stream;
+    use async_zip::tokio::read::seek::ZipFileReader;
+    use futures_core::stream::Stream;
+    use futures_util::AsyncReadExt;
+
+    use tokio::io::{AsyncRead, AsyncSeek};
+
+    use super::SourceQuestionsBatch;
+
+    pub struct SourceQuestionsZipReaderAsync<R>
+    where
+        R: AsyncRead + AsyncSeek + Unpin,
+    {
+        zipfile: ZipFileReader<R>,
+        index: Option<usize>,
+    }
+
+    impl<R> SourceQuestionsZipReaderAsync<R>
+    where
+        R: AsyncRead + AsyncSeek + Unpin,
+    {
+        fn new(zipfile: ZipFileReader<R>) -> Self {
+            SourceQuestionsZipReaderAsync {
+                zipfile,
+                index: None,
+            }
+        }
+
+        pub fn len(&self) -> usize {
+            self.zipfile.file().entries().len()
+        }
+
+        pub fn is_empty(&self) -> bool {
+            self.len() == 0
+        }
+
+        pub async fn get(
+            &mut self,
+            index: usize,
+        ) -> Result<(String, Result<SourceQuestionsBatch, serde_json::Error>), String>
+        where
+            R: AsyncRead + AsyncSeek + Unpin,
+        {
+            let len = self.len();
+            if index >= len {
+                return Err(format!("get index={index}, when len={len}"));
+            }
+
+            let reader = self.zipfile.reader_with_entry(index).await;
+            if let Err(error) = reader {
+                return Err(format!("reader_with_entry: {error:?}"));
+            }
+            let mut reader = reader.unwrap();
+
+            let filename = reader.entry().filename().clone().into_string().unwrap();
+            let mut data: Vec<u8> = Vec::new();
+            let readed = reader.read_to_end(&mut data).await;
+            if let Err(error) = readed {
+                return Err(format!("read_to_end: {error:?}"));
+            }
+            let parsed: Result<SourceQuestionsBatch, _> = serde_json::from_slice(&data);
+            Ok((filename, parsed))
+        }
+        pub async fn get_next(
+            &mut self,
+        ) -> Option<Result<(String, Result<SourceQuestionsBatch, serde_json::Error>), String>>
+        where
+            R: AsyncRead + AsyncSeek + Unpin,
+        {
+            if self.index.is_none() && !self.is_empty() {
+                self.index = Some(0);
+            }
+
+            if self.index.unwrap() >= self.len() {
+                return None;
+            }
+
+            let item = self.get(self.index.unwrap()).await;
+            self.index = Some(self.index.unwrap() + 1);
+
+            Some(item)
+        }
+        pub fn stream(
+            &mut self,
+        ) -> impl Stream<Item = (String, Result<SourceQuestionsBatch, serde_json::Error>)> + '_
+        {
+            stream! {
+                while let Some(Ok(item)) = self.get_next().await {
+                    yield item
+                }
+            }
+        }
+    }
+
+    pub trait ReadSourceQuestionsBatchesAsync<R>
+    where
+        R: AsyncRead + AsyncSeek + Unpin,
+    {
+        fn source_questions(self) -> SourceQuestionsZipReaderAsync<R>;
+    }
+
+    impl<R> ReadSourceQuestionsBatchesAsync<R> for ZipFileReader<R>
+    where
+        R: AsyncRead + AsyncSeek + Unpin,
+    {
+        fn source_questions(self) -> SourceQuestionsZipReaderAsync<R> {
+            SourceQuestionsZipReaderAsync::new(self)
+        }
+    }
+
+    #[cfg(test)]
+    mod test {
+        use crate::source::SourceQuestion;
+
+        use super::super::test::sample_batch;
+        use super::*;
+        use async_zip::{base::write::ZipFileWriter, ZipEntryBuilder};
+        use core::fmt::Debug;
+        use futures_util::StreamExt;
+        use std::path::Path;
+        use tempfile::tempdir;
+        use tokio::fs;
+
+        async fn write_sample_zip<P>(path: P)
+        where
+            P: AsRef<Path>,
+        {
+            let batch = sample_batch();
+            let z_file = fs::File::create(path).await.expect("crerate zip file");
+            let mut zip_file = ZipFileWriter::with_tokio(z_file);
+            let entry =
+                ZipEntryBuilder::new("test.json".into(), async_zip::Compression::Zstd).build();
+            zip_file
+                .write_entry_whole(entry, serde_json::to_vec(&batch).unwrap().as_slice())
+                .await
+                .expect("write entry");
+            zip_file.close().await.expect("close zip");
+        }
+
+        async fn assert_data_rref_eq<T>((x, y): (T, &T))
+        where
+            T: PartialEq + Debug,
+        {
+            assert_eq!(x, *y);
+        }
+
+        #[tokio::test]
+        async fn test_source_questions_stream() {
+            let expected_batch = sample_batch();
+            let dir = tempdir().expect("tempdir");
+
+            // write sample
+            let tmpfile_zip = dir.path().join("test.zip");
+            write_sample_zip(&tmpfile_zip).await;
+
+            let mut z_file = fs::File::open(tmpfile_zip).await.expect("open zip file");
+            let zip_file = ZipFileReader::with_tokio(&mut z_file)
+                .await
+                .expect("open zip file reader");
+
+            let expected_count = expected_batch.questions.len();
+            let expected_stream = futures::stream::iter(expected_batch.questions.iter());
+            let mut actual_source = zip_file.source_questions();
+            let actual_stream = actual_source.stream();
+            let mut actual_count: usize = 0;
+
+            actual_stream
+                .flat_map(|x| futures::stream::iter(x.1.expect("parse batch").questions))
+                .zip(expected_stream)
+                .map(|x| {
+                    actual_count += 1;
+                    x
+                })
+                .for_each(assert_data_rref_eq::<SourceQuestion>)
+                .await;
+            assert_eq!(actual_count, expected_count);
+        }
+
+        #[tokio::test]
+        async fn test_source_questions_get() {
+            let expected_batch = sample_batch();
+            let dir = tempdir().expect("tempdir");
+
+            // write sample
+            let tmpfile_zip = dir.path().join("test.zip");
+            write_sample_zip(&tmpfile_zip).await;
+
+            let mut z_file = fs::File::open(tmpfile_zip).await.expect("open zip file");
+            let zip_file = ZipFileReader::with_tokio(&mut z_file)
+                .await
+                .expect("open zip file reader");
+
+            let mut source = zip_file.source_questions();
+            assert_eq!(source.len(), 1);
+
+            let actual = source.get(0).await.expect("get batch");
+            assert_eq!(actual.0, "test.json");
+            assert_eq!(actual.1.expect("parse batch"), expected_batch);
+        }
+    }
+}
+#[cfg(any(feature = "convert_async", feature = "source_async"))]
+pub use reader_async::{ReadSourceQuestionsBatchesAsync, SourceQuestionsZipReaderAsync};
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use insta::assert_yaml_snapshot;
+    use serde_json::json;
+
+    pub fn sample_batch() -> SourceQuestionsBatch {
+        SourceQuestionsBatch {
+            description: "Тестовый".into(),
+            date: "00-000-2000".into(),
+            questions: vec![
+                SourceQuestion {
+                    id: "Вопрос 1".into(),
+                    description: "Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2".into(),
+                    answer: "42".into(),
+                    ..Default::default()
+                },
+                SourceQuestion {
+                    id: "Вопрос 2".into(),
+                    description: "Зимой и летом одним цветом".into(),
+                    answer: "ёлка".into(),
+                    ..Default::default()
+                },
+            ],
+            ..Default::default()
+        }
+    }
+
+    #[test]
+    fn test_batch_ser() {
+        let batch = sample_batch();
+
+        assert_yaml_snapshot!(batch, @r#"
+            ---
+            description: Тестовый
+            date: 00-000-2000
+            questions:
+              - id: Вопрос 1
+                description: Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2
+                answer: "42"
+              - id: Вопрос 2
+                description: Зимой и летом одним цветом
+                answer: ёлка
+            
+            "#);
+    }
+    #[test]
+    fn test_batch_de() {
+        let batch_from_json: Result<SourceQuestionsBatch, _> = serde_json::from_value(json!({
+            "Чемпионат": "Тестовый",
+            "Дата": "00-000-2000",
+            "Вопросы": [
+                {
+                    "id": "Вопрос 1",
+                    "Вопрос": "Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2",
+                    "Ответ": "42",
+                   },
+                   {
+                    "id": "Вопрос 2",
+                    "Вопрос": "Зимой и летом одним цветом",
+                    "Ответ": "ёлка",
+                   },
+            ]
+        }));
+        assert!(batch_from_json.is_ok());
+
+        assert_yaml_snapshot!(batch_from_json.unwrap(), @r#"
+            ---
+            description: Тестовый
+            date: 00-000-2000
+            questions:
+              - id: Вопрос 1
+                description: Сколько будет (2 * 2 * 2 + 2) * 2 * 2 + 2
+                answer: "42"
+              - id: Вопрос 2
+                description: Зимой и летом одним цветом
+                answer: ёлка
+            
+            "#);
+    }
 }

lib/src/util.rs

@@ -0,0 +1,57 @@
+pub trait ErrorToString {
+    type Output;
+    fn str_err(self) -> std::result::Result<Self::Output, String>;
+}
+
+impl<T, E> ErrorToString for std::result::Result<T, E>
+where
+    E: std::error::Error,
+{
+    type Output = T;
+    fn str_err(self) -> std::result::Result<Self::Output, String> {
+        self.map_err(|e| e.to_string())
+    }
+}
+
+#[cfg(any(feature = "sync", feature = "async"))]
+mod bincode_utils {
+    use std::ops::{Deref, DerefMut};
+
+    use bincode::enc::write::Writer;
+    use bincode::error::EncodeError;
+
+    /// struct that allows [`Vec<u8>`] to implement [bincode::enc::write::Writer] trait
+    pub struct BincodeVecWriter {
+        vec: Vec<u8>,
+    }
+
+    impl BincodeVecWriter {
+        pub fn new(vec: Vec<u8>) -> BincodeVecWriter {
+            BincodeVecWriter { vec }
+        }
+    }
+
+    impl Deref for BincodeVecWriter {
+        type Target = Vec<u8>;
+
+        fn deref(&self) -> &Self::Target {
+            &self.vec
+        }
+    }
+
+    impl DerefMut for BincodeVecWriter {
+        fn deref_mut(&mut self) -> &mut Self::Target {
+            &mut self.vec
+        }
+    }
+
+    impl Writer for BincodeVecWriter {
+        fn write(&mut self, bytes: &[u8]) -> Result<(), EncodeError> {
+            self.vec.extend_from_slice(bytes);
+            Ok(())
+        }
+    }
+}
+
+#[cfg(any(feature = "sync", feature = "async"))]
+pub use bincode_utils::BincodeVecWriter;