//! An implementation of a bao file, meaning some data blob with associated
//! outboard.
//!
//! Compared to just a pair of (data, outboard), this implementation also works
//! when both the data and the outboard is incomplete, and not even the size
//! is fully known.
//!
//! There is a full in memory implementation, and an implementation that uses
//! the file system for the data, outboard, and sizes file. There is also a
//! combined implementation that starts in memory and switches to file when
//! the memory limit is reached.
use std::{
    fs::{File, OpenOptions},
    io,
    ops::{Deref, DerefMut},
    path::{Path, PathBuf},
    sync::{Arc, RwLock, Weak},
};

use bao_tree::{
    io::{
        fsm::BaoContentItem,
        outboard::PreOrderOutboard,
        sync::{ReadAt, WriteAt},
    },
    BaoTree,
};
use bytes::{Bytes, BytesMut};
use derive_more::Debug;
use iroh_base::hash::Hash;
use iroh_io::AsyncSliceReader;

use super::mutable_mem_storage::{MutableMemStorage, SizeInfo};
use crate::{
    store::BaoBatchWriter,
    util::{get_limited_slice, MemOrFile, SparseMemFile},
    IROH_BLOCK_SIZE,
};

/// Data files are stored in 3 files. The data file, the outboard file,
/// and a sizes file. The sizes file contains the size that the remote side told us
/// when writing each data block.
///
/// For complete data files, the sizes file is not needed, since you can just
/// use the size of the data file.
///
/// For files below the chunk size, the outboard file is not needed, since
/// there is only one leaf, and the outboard file is empty.
struct DataPaths {
    /// The data file. Size is determined by the chunk with the highest offset
    /// that has been written.
    ///
    /// Gaps will be filled with zeros.
    data: PathBuf,
    /// The outboard file. This is *without* the size header, since that is not
    /// known for partial files.
    ///
    /// The size of the outboard file is therefore a multiple of a hash pair
    /// (64 bytes).
    ///
    /// The naming convention is to use obao for pre order traversal and oboa
    /// for post order traversal. The log2 of the chunk group size is appended,
    /// so for the default chunk group size in iroh of 4, the file extension
    /// is .obao4.
    outboard: PathBuf,
    /// The sizes file. This is a file with 8 byte sizes for each chunk group.
    /// The naming convention is to prepend the log2 of the chunk group size,
    /// so for the default chunk group size in iroh of 4, the file extension
    /// is .sizes4.
    ///
    /// The traversal order is not relevant for the sizes file, since it is
    /// about the data chunks, not the hash pairs.
    sizes: PathBuf,
}

/// Storage for complete blobs. There is no longer any uncertainty about the
/// size, so we don't need a sizes file.
///
/// Writing is not possible but also not needed, since the file is complete.
/// This covers all combinations of data and outboard being in memory or on
/// disk.
///
/// For the memory variant, it does reading in a zero copy way, since storage
/// is already a `Bytes`.
#[derive(Default, derive_more::Debug)]
pub struct CompleteStorage {
    /// data part, which can be in memory or on disk.
    #[debug("{:?}", data.as_ref().map_mem(|x| x.len()))]
    pub data: MemOrFile<Bytes, (File, u64)>,
    /// outboard part, which can be in memory or on disk.
    #[debug("{:?}", outboard.as_ref().map_mem(|x| x.len()))]
    pub outboard: MemOrFile<Bytes, (File, u64)>,
}

impl CompleteStorage {
    /// Read from the data file at the given offset, until end of file or max bytes.
    pub fn read_data_at(&self, offset: u64, len: usize) -> Bytes {
        match &self.data {
            MemOrFile::Mem(mem) => get_limited_slice(mem, offset, len),
            MemOrFile::File((file, _size)) => read_to_end(file, offset, len).unwrap(),
        }
    }

    /// Read from the outboard file at the given offset, until end of file or max bytes.
    pub fn read_outboard_at(&self, offset: u64, len: usize) -> Bytes {
        match &self.outboard {
            MemOrFile::Mem(mem) => get_limited_slice(mem, offset, len),
            MemOrFile::File((file, _size)) => read_to_end(file, offset, len).unwrap(),
        }
    }

    /// The size of the data file.
    pub fn data_size(&self) -> u64 {
        match &self.data {
            MemOrFile::Mem(mem) => mem.len() as u64,
            MemOrFile::File((_file, size)) => *size,
        }
    }

    /// The size of the outboard file.
    pub fn outboard_size(&self) -> u64 {
        match &self.outboard {
            MemOrFile::Mem(mem) => mem.len() as u64,
            MemOrFile::File((_file, size)) => *size,
        }
    }
}

/// Create a file for reading and writing, but *without* truncating the existing
/// file.
fn create_read_write(path: impl AsRef<Path>) -> io::Result<File> {
    OpenOptions::new()
        .read(true)
        .write(true)
        .create(true)
        .truncate(false)
        .open(path)
}

/// Read from the given file at the given offset, until end of file or max bytes.
fn read_to_end(file: impl ReadAt, offset: u64, max: usize) -> io::Result<Bytes> {
    let mut res = BytesMut::new();
    let mut buf = [0u8; 4096];
    let mut remaining = max;
    let mut offset = offset;
    while remaining > 0 {
        let end = buf.len().min(remaining);
        let read = file.read_at(offset, &mut buf[..end])?;
        if read == 0 {
            // eof
            break;
        }
        res.extend_from_slice(&buf[..read]);
        offset += read as u64;
        remaining -= read;
    }
    Ok(res.freeze())
}

fn max_offset(batch: &[BaoContentItem]) -> u64 {
    batch
        .iter()
        .filter_map(|item| match item {
            BaoContentItem::Leaf(leaf) => {
                let len = leaf.data.len().try_into().unwrap();
                let end = leaf
                    .offset
                    .checked_add(len)
                    .expect("u64 overflow for leaf end");
                Some(end)
            }
            _ => None,
        })
        .max()
        .unwrap_or(0)
}

/// A file storage for an incomplete bao file.
#[derive(Debug)]
pub struct FileStorage {
    data: std::fs::File,
    outboard: std::fs::File,
    sizes: std::fs::File,
}

impl FileStorage {
    /// Split into data, outboard and sizes files.
    pub fn into_parts(self) -> (File, File, File) {
        (self.data, self.outboard, self.sizes)
    }

    fn current_size(&self) -> io::Result<u64> {
        let len = self.sizes.metadata()?.len();
        if len < 8 {
            Ok(0)
        } else {
            // todo: use the last full u64 in case the sizes file is not a multiple of 8
            // bytes. Not sure how that would happen, but we should handle it.
            let mut buf = [0u8; 8];
            self.sizes.read_exact_at(len - 8, &mut buf)?;
            Ok(u64::from_le_bytes(buf))
        }
    }

    fn write_batch(&mut self, size: u64, batch: &[BaoContentItem]) -> io::Result<()> {
        let tree = BaoTree::new(size, IROH_BLOCK_SIZE);
        for item in batch {
            match item {
                BaoContentItem::Parent(parent) => {
                    if let Some(offset) = tree.pre_order_offset(parent.node) {
                        let o0 = offset * 64;
                        self.outboard
                            .write_all_at(o0, parent.pair.0.as_bytes().as_slice())?;
                        self.outboard
                            .write_all_at(o0 + 32, parent.pair.1.as_bytes().as_slice())?;
                    }
                }
                BaoContentItem::Leaf(leaf) => {
                    let o0 = leaf.offset;
                    // divide by chunk size, multiply by 8
                    let index = (leaf.offset >> (tree.block_size().chunk_log() + 10)) << 3;
                    tracing::trace!(
                        "write_batch f={:?} o={} l={}",
                        self.data,
                        o0,
                        leaf.data.len()
                    );
                    self.data.write_all_at(o0, leaf.data.as_ref())?;
                    let size = tree.size();
                    self.sizes.write_all_at(index, &size.to_le_bytes())?;
                }
            }
        }
        Ok(())
    }

    fn read_data_at(&self, offset: u64, len: usize) -> io::Result<Bytes> {
        read_to_end(&self.data, offset, len)
    }

    fn read_outboard_at(&self, offset: u64, len: usize) -> io::Result<Bytes> {
        read_to_end(&self.outboard, offset, len)
    }
}

/// The storage for a bao file. This can be either in memory or on disk.
#[derive(Debug)]
pub(crate) enum BaoFileStorage {
    /// The entry is incomplete and in memory.
    ///
    /// Since it is incomplete, it must be writeable.
    ///
    /// This is used mostly for tiny entries, <= 16 KiB. But in principle it
    /// can be used for larger sizes.
    ///
    /// Incomplete mem entries are *not* persisted at all. So if the store
    /// crashes they will be gone.
    IncompleteMem(MutableMemStorage),
    /// The entry is incomplete and on disk.
    IncompleteFile(FileStorage),
    /// The entry is complete. Outboard and data can come from different sources
    /// (memory or file).
    ///
    /// Writing to this is a no-op, since it is already complete.
    Complete(CompleteStorage),
}

impl Default for BaoFileStorage {
    fn default() -> Self {
        BaoFileStorage::Complete(Default::default())
    }
}

impl BaoFileStorage {
    /// Take the storage out, leaving an empty storage in its place.
    ///
    /// Be careful to put something back in its place, or you will lose data.
    #[cfg(feature = "fs-store")]
    pub fn take(&mut self) -> Self {
        std::mem::take(self)
    }

    /// Create a new mutable mem storage.
    pub fn incomplete_mem() -> Self {
        Self::IncompleteMem(Default::default())
    }

    /// Call sync_all on all the files.
    fn sync_all(&self) -> io::Result<()> {
        match self {
            Self::Complete(_) => Ok(()),
            Self::IncompleteMem(_) => Ok(()),
            Self::IncompleteFile(file) => {
                file.data.sync_all()?;
                file.outboard.sync_all()?;
                file.sizes.sync_all()?;
                Ok(())
            }
        }
    }

    /// True if the storage is in memory.
    pub fn is_mem(&self) -> bool {
        match self {
            Self::IncompleteMem(_) => true,
            Self::IncompleteFile(_) => false,
            Self::Complete(c) => c.data.is_mem() && c.outboard.is_mem(),
        }
    }
}

/// A weak reference to a bao file handle.
#[derive(Debug, Clone)]
pub struct BaoFileHandleWeak(Weak<BaoFileHandleInner>);

impl BaoFileHandleWeak {
    /// Upgrade to a strong reference if possible.
    pub fn upgrade(&self) -> Option<BaoFileHandle> {
        self.0.upgrade().map(BaoFileHandle)
    }

    /// True if the handle is still live (has strong references)
    pub fn is_live(&self) -> bool {
        self.0.strong_count() > 0
    }
}

/// The inner part of a bao file handle.
#[derive(Debug)]
pub struct BaoFileHandleInner {
    pub(crate) storage: RwLock<BaoFileStorage>,
    config: Arc<BaoFileConfig>,
    hash: Hash,
}

/// A cheaply cloneable handle to a bao file, including the hash and the configuration.
#[derive(Debug, Clone, derive_more::Deref)]
pub struct BaoFileHandle(Arc<BaoFileHandleInner>);

pub(crate) type CreateCb = Arc<dyn Fn(&Hash) -> io::Result<()> + Send + Sync>;

/// Configuration for the deferred batch writer. It will start writing to memory,
/// and then switch to a file when the memory limit is reached.
#[derive(derive_more::Debug, Clone)]
pub struct BaoFileConfig {
    /// Directory to store files in. Only used when memory limit is reached.
    dir: Arc<PathBuf>,
    /// Maximum data size (inclusive) before switching to file mode.
    max_mem: usize,
    /// Callback to call when we switch to file mode.
    ///
    /// Todo: make this async.
    #[debug("{:?}", on_file_create.as_ref().map(|_| ()))]
    on_file_create: Option<CreateCb>,
}

impl BaoFileConfig {
    /// Create a new deferred batch writer configuration.
    pub fn new(dir: Arc<PathBuf>, max_mem: usize, on_file_create: Option<CreateCb>) -> Self {
        Self {
            dir,
            max_mem,
            on_file_create,
        }
    }

    /// Get the paths for a hash.
    fn paths(&self, hash: &Hash) -> DataPaths {
        DataPaths {
            data: self.dir.join(format!("{}.data", hash.to_hex())),
            outboard: self.dir.join(format!("{}.obao4", hash.to_hex())),
            sizes: self.dir.join(format!("{}.sizes4", hash.to_hex())),
        }
    }
}

/// A reader for a bao file, reading just the data.
#[derive(Debug)]
pub struct DataReader(Option<BaoFileHandle>);

async fn with_storage<T, P, F>(opt: &mut Option<BaoFileHandle>, no_io: P, f: F) -> io::Result<T>
where
    P: Fn(&BaoFileStorage) -> bool + Send + 'static,
    F: FnOnce(&BaoFileStorage) -> io::Result<T> + Send + 'static,
    T: Send + 'static,
{
    let handle = opt
        .take()
        .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "deferred batch busy"))?;
    // if we can get the lock immediately, and we are in memory mode, we can
    // avoid spawning a task.
    if let Ok(storage) = handle.storage.try_read() {
        if no_io(&storage) {
            let res = f(&storage);
            // clone because for some reason even when we drop storage, the
            // borrow checker still thinks handle is borrowed.
            *opt = Some(handle.clone());
            return res;
        }
    };
    // otherwise, we have to spawn a task.
    let (handle, res) = tokio::task::spawn_blocking(move || {
        let storage = handle.storage.read().unwrap();
        let res = f(storage.deref());
        drop(storage);
        (handle, res)
    })
    .await
    .expect("spawn_blocking failed");
    *opt = Some(handle);
    res
}

impl AsyncSliceReader for DataReader {
    async fn read_at(&mut self, offset: u64, len: usize) -> io::Result<Bytes> {
        with_storage(
            &mut self.0,
            BaoFileStorage::is_mem,
            move |storage| match storage {
                BaoFileStorage::Complete(mem) => Ok(mem.read_data_at(offset, len)),
                BaoFileStorage::IncompleteMem(mem) => Ok(mem.read_data_at(offset, len)),
                BaoFileStorage::IncompleteFile(file) => file.read_data_at(offset, len),
            },
        )
        .await
    }

    async fn size(&mut self) -> io::Result<u64> {
        with_storage(
            &mut self.0,
            BaoFileStorage::is_mem,
            move |storage| match storage {
                BaoFileStorage::Complete(mem) => Ok(mem.data_size()),
                BaoFileStorage::IncompleteMem(mem) => Ok(mem.data.len() as u64),
                BaoFileStorage::IncompleteFile(file) => file.data.metadata().map(|m| m.len()),
            },
        )
        .await
    }
}

/// A reader for the outboard part of a bao file.
#[derive(Debug)]
pub struct OutboardReader(Option<BaoFileHandle>);

impl AsyncSliceReader for OutboardReader {
    async fn read_at(&mut self, offset: u64, len: usize) -> io::Result<Bytes> {
        with_storage(
            &mut self.0,
            BaoFileStorage::is_mem,
            move |storage| match storage {
                BaoFileStorage::Complete(mem) => Ok(mem.read_outboard_at(offset, len)),
                BaoFileStorage::IncompleteMem(mem) => Ok(mem.read_outboard_at(offset, len)),
                BaoFileStorage::IncompleteFile(file) => file.read_outboard_at(offset, len),
            },
        )
        .await
    }

    async fn size(&mut self) -> io::Result<u64> {
        with_storage(
            &mut self.0,
            BaoFileStorage::is_mem,
            move |storage| match storage {
                BaoFileStorage::Complete(mem) => Ok(mem.outboard_size()),
                BaoFileStorage::IncompleteMem(mem) => Ok(mem.outboard.len() as u64),
                BaoFileStorage::IncompleteFile(file) => file.outboard.metadata().map(|m| m.len()),
            },
        )
        .await
    }
}

enum HandleChange {
    None,
    MemToFile,
    // later: size verified
}

impl BaoFileHandle {
    /// Create a new bao file handle.
    ///
    /// This will create a new file handle with an empty memory storage.
    /// Since there are very likely to be many of these, we use an arc rwlock
    pub fn incomplete_mem(config: Arc<BaoFileConfig>, hash: Hash) -> Self {
        let storage = BaoFileStorage::incomplete_mem();
        Self(Arc::new(BaoFileHandleInner {
            storage: RwLock::new(storage),
            config,
            hash,
        }))
    }

    /// Create a new bao file handle with a partial file.
    pub fn incomplete_file(config: Arc<BaoFileConfig>, hash: Hash) -> io::Result<Self> {
        let paths = config.paths(&hash);
        let storage = BaoFileStorage::IncompleteFile(FileStorage {
            data: create_read_write(&paths.data)?,
            outboard: create_read_write(&paths.outboard)?,
            sizes: create_read_write(&paths.sizes)?,
        });
        Ok(Self(Arc::new(BaoFileHandleInner {
            storage: RwLock::new(storage),
            config,
            hash,
        })))
    }

    /// Create a new complete bao file handle.
    pub fn new_complete(
        config: Arc<BaoFileConfig>,
        hash: Hash,
        data: MemOrFile<Bytes, (File, u64)>,
        outboard: MemOrFile<Bytes, (File, u64)>,
    ) -> Self {
        let storage = BaoFileStorage::Complete(CompleteStorage { data, outboard });
        Self(Arc::new(BaoFileHandleInner {
            storage: RwLock::new(storage),
            config,
            hash,
        }))
    }

    /// Transform the storage in place. If the transform fails, the storage will
    /// be an immutable empty storage.
    #[cfg(feature = "fs-store")]
    pub(crate) fn transform(
        &self,
        f: impl FnOnce(BaoFileStorage) -> io::Result<BaoFileStorage>,
    ) -> io::Result<()> {
        let mut lock = self.storage.write().unwrap();
        let storage = lock.take();
        *lock = f(storage)?;
        Ok(())
    }

    /// True if the file is complete.
    pub fn is_complete(&self) -> bool {
        matches!(
            self.storage.read().unwrap().deref(),
            BaoFileStorage::Complete(_)
        )
    }

    /// An AsyncSliceReader for the data file.
    ///
    /// Caution: this is a reader for the unvalidated data file. Reading this
    /// can produce data that does not match the hash.
    pub fn data_reader(&self) -> DataReader {
        DataReader(Some(self.clone()))
    }

    /// An AsyncSliceReader for the outboard file.
    ///
    /// The outboard file is used to validate the data file. It is not guaranteed
    /// to be complete.
    pub fn outboard_reader(&self) -> OutboardReader {
        OutboardReader(Some(self.clone()))
    }

    /// The most precise known total size of the data file.
    pub fn current_size(&self) -> io::Result<u64> {
        match self.storage.read().unwrap().deref() {
            BaoFileStorage::Complete(mem) => Ok(mem.data_size()),
            BaoFileStorage::IncompleteMem(mem) => Ok(mem.current_size()),
            BaoFileStorage::IncompleteFile(file) => file.current_size(),
        }
    }

    /// The outboard for the file.
    pub fn outboard(&self) -> io::Result<PreOrderOutboard<OutboardReader>> {
        let root = self.hash.into();
        let tree = BaoTree::new(self.current_size()?, IROH_BLOCK_SIZE);
        let outboard = self.outboard_reader();
        Ok(PreOrderOutboard {
            root,
            tree,
            data: outboard,
        })
    }

    /// The hash of the file.
    pub fn hash(&self) -> Hash {
        self.hash
    }

    /// Create a new writer from the handle.
    pub fn writer(&self) -> BaoFileWriter {
        BaoFileWriter(Some(self.clone()))
    }

    /// This is the synchronous impl for writing a batch.
    fn write_batch(&self, size: u64, batch: &[BaoContentItem]) -> io::Result<HandleChange> {
        let mut storage = self.storage.write().unwrap();
        match storage.deref_mut() {
            BaoFileStorage::IncompleteMem(mem) => {
                // check if we need to switch to file mode, otherwise write to memory
                if max_offset(batch) <= self.config.max_mem as u64 {
                    mem.write_batch(size, batch)?;
                    Ok(HandleChange::None)
                } else {
                    // create the paths. This allocates 3 pathbufs, so we do it
                    // only when we need to.
                    let paths = self.config.paths(&self.hash);
                    // *first* switch to file mode, *then* write the batch.
                    //
                    // otherwise we might allocate a lot of memory if we get
                    // a write at the end of a very large file.
                    let mut file_batch = mem.persist(paths)?;
                    file_batch.write_batch(size, batch)?;
                    *storage = BaoFileStorage::IncompleteFile(file_batch);
                    Ok(HandleChange::MemToFile)
                }
            }
            BaoFileStorage::IncompleteFile(file) => {
                // already in file mode, just write the batch
                file.write_batch(size, batch)?;
                Ok(HandleChange::None)
            }
            BaoFileStorage::Complete(_) => {
                // we are complete, so just ignore the write
                // unless there is a bug, this would just write the exact same data
                Ok(HandleChange::None)
            }
        }
    }

    /// Downgrade to a weak reference.
    pub fn downgrade(&self) -> BaoFileHandleWeak {
        BaoFileHandleWeak(Arc::downgrade(&self.0))
    }
}

impl SizeInfo {
    /// Persist into a file where each chunk has its own slot.
    pub fn persist(&self, mut target: impl WriteAt) -> io::Result<()> {
        let size_offset = (self.offset >> IROH_BLOCK_SIZE.chunk_log()) << 3;
        target.write_all_at(size_offset, self.size.to_le_bytes().as_slice())?;
        Ok(())
    }

    /// Convert to a vec in slot format.
    pub fn to_vec(&self) -> Vec<u8> {
        let mut res = Vec::new();
        self.persist(&mut res).expect("io error writing to vec");
        res
    }
}

impl MutableMemStorage {
    /// Persist the batch to disk, creating a FileBatch.
    fn persist(&self, paths: DataPaths) -> io::Result<FileStorage> {
        let mut data = create_read_write(&paths.data)?;
        let mut outboard = create_read_write(&paths.outboard)?;
        let mut sizes = create_read_write(&paths.sizes)?;
        self.data.persist(&mut data)?;
        self.outboard.persist(&mut outboard)?;
        self.sizes.persist(&mut sizes)?;
        data.sync_all()?;
        outboard.sync_all()?;
        sizes.sync_all()?;
        Ok(FileStorage {
            data,
            outboard,
            sizes,
        })
    }

    /// Get the parts data, outboard and sizes
    pub fn into_parts(self) -> (SparseMemFile, SparseMemFile, SizeInfo) {
        (self.data, self.outboard, self.sizes)
    }
}

/// This is finally the thing for which we can implement BaoPairMut.
///
/// It is a BaoFileHandle wrapped in an Option, so that we can take it out
/// in the future.
#[derive(Debug)]
pub struct BaoFileWriter(Option<BaoFileHandle>);

impl BaoBatchWriter for BaoFileWriter {
    async fn write_batch(&mut self, size: u64, batch: Vec<BaoContentItem>) -> std::io::Result<()> {
        let Some(handle) = self.0.take() else {
            return Err(io::Error::new(io::ErrorKind::Other, "deferred batch busy"));
        };
        let (handle, change) = tokio::task::spawn_blocking(move || {
            let change = handle.write_batch(size, &batch);
            (handle, change)
        })
        .await
        .expect("spawn_blocking failed");
        match change? {
            HandleChange::None => {}
            HandleChange::MemToFile => {
                if let Some(cb) = handle.config.on_file_create.as_ref() {
                    cb(&handle.hash)?;
                }
            }
        }
        self.0 = Some(handle);
        Ok(())
    }

    async fn sync(&mut self) -> io::Result<()> {
        let Some(handle) = self.0.take() else {
            return Err(io::Error::new(io::ErrorKind::Other, "deferred batch busy"));
        };
        let (handle, res) = tokio::task::spawn_blocking(move || {
            let res = handle.storage.write().unwrap().sync_all();
            (handle, res)
        })
        .await
        .expect("spawn_blocking failed");
        self.0 = Some(handle);
        res
    }
}

#[cfg(test)]
pub mod test_support {
    use std::{future::Future, io::Cursor, ops::Range};

    use bao_tree::{
        io::{
            fsm::{ResponseDecoder, ResponseDecoderNext},
            outboard::PostOrderMemOutboard,
            round_up_to_chunks,
            sync::encode_ranges_validated,
        },
        BlockSize, ChunkRanges,
    };
    use futures_lite::{Stream, StreamExt};
    use iroh_io::AsyncStreamReader;
    use rand::RngCore;
    use range_collections::RangeSet2;

    use super::*;
    use crate::util::limited_range;

    pub const IROH_BLOCK_SIZE: BlockSize = BlockSize::from_chunk_log(4);

    /// Decode a response into a batch file writer.
    pub async fn decode_response_into_batch<R, W>(
        root: Hash,
        block_size: BlockSize,
        ranges: ChunkRanges,
        mut encoded: R,
        mut target: W,
    ) -> io::Result<()>
    where
        R: AsyncStreamReader,
        W: BaoBatchWriter,
    {
        let size = encoded.read::<8>().await?;
        let size = u64::from_le_bytes(size);
        let mut reading =
            ResponseDecoder::new(root.into(), ranges, BaoTree::new(size, block_size), encoded);
        let mut stack = Vec::new();
        loop {
            let item = match reading.next().await {
                ResponseDecoderNext::Done(_reader) => break,
                ResponseDecoderNext::More((next, item)) => {
                    reading = next;
                    item?
                }
            };
            match item {
                BaoContentItem::Parent(_) => {
                    stack.push(item);
                }
                BaoContentItem::Leaf(_) => {
                    // write a batch every time we see a leaf
                    // the last item will be a leaf.
                    stack.push(item);
                    target.write_batch(size, std::mem::take(&mut stack)).await?;
                }
            }
        }
        assert!(stack.is_empty(), "last item should be a leaf");
        Ok(())
    }

    pub fn random_test_data(size: usize) -> Vec<u8> {
        let mut rand = rand::thread_rng();
        let mut res = vec![0u8; size];
        rand.fill_bytes(&mut res);
        res
    }

    /// Take some data and encode it
    pub fn simulate_remote(data: &[u8]) -> (Hash, Cursor<Bytes>) {
        let outboard = bao_tree::io::outboard::PostOrderMemOutboard::create(data, IROH_BLOCK_SIZE);
        let size = data.len() as u64;
        let mut encoded = size.to_le_bytes().to_vec();
        bao_tree::io::sync::encode_ranges_validated(
            data,
            &outboard,
            &ChunkRanges::all(),
            &mut encoded,
        )
        .unwrap();
        let hash = outboard.root;
        (hash.into(), Cursor::new(encoded.into()))
    }

    pub fn to_ranges(ranges: &[Range<u64>]) -> RangeSet2<u64> {
        let mut range_set = RangeSet2::empty();
        for range in ranges.as_ref().iter().cloned() {
            range_set |= RangeSet2::from(range);
        }
        range_set
    }

    /// Simulate the send side, when asked to send bao encoded data for the given ranges.
    pub fn make_wire_data(
        data: &[u8],
        ranges: impl AsRef<[Range<u64>]>,
    ) -> (Hash, ChunkRanges, Vec<u8>) {
        // compute a range set from the given ranges
        let range_set = to_ranges(ranges.as_ref());
        // round up to chunks
        let chunk_ranges = round_up_to_chunks(&range_set);
        // compute the outboard
        let outboard = PostOrderMemOutboard::create(data, IROH_BLOCK_SIZE).flip();
        let size = data.len() as u64;
        let mut encoded = size.to_le_bytes().to_vec();
        encode_ranges_validated(data, &outboard, &chunk_ranges, &mut encoded).unwrap();
        (outboard.root.into(), chunk_ranges, encoded)
    }

    pub async fn validate(handle: &BaoFileHandle, original: &[u8], ranges: &[Range<u64>]) {
        let mut r = handle.data_reader();
        for range in ranges {
            let start = range.start;
            let len = (range.end - range.start).try_into().unwrap();
            let data = &original[limited_range(start, len, original.len())];
            let read = r.read_at(start, len).await.unwrap();
            assert_eq!(data.len(), read.as_ref().len());
            assert_eq!(data, read.as_ref());
        }
    }

    /// Helper to simulate a slow request.
    pub fn trickle(
        data: &[u8],
        mtu: usize,
        delay: std::time::Duration,
    ) -> impl Stream<Item = Bytes> {
        let parts = data
            .chunks(mtu)
            .map(Bytes::copy_from_slice)
            .collect::<Vec<_>>();
        futures_lite::stream::iter(parts).then(move |part| async move {
            tokio::time::sleep(delay).await;
            part
        })
    }

    pub async fn local<F>(f: F) -> F::Output
    where
        F: Future,
    {
        tokio::task::LocalSet::new().run_until(f).await
    }
}

#[cfg(test)]
mod tests {
    use std::io::Write;

    use bao_tree::{blake3, ChunkNum, ChunkRanges};
    use futures_lite::StreamExt;
    use iroh_io::TokioStreamReader;
    use tests::test_support::{
        decode_response_into_batch, local, make_wire_data, random_test_data, trickle, validate,
    };
    use tokio::task::JoinSet;

    use super::*;
    use crate::util::local_pool::LocalPool;

    #[tokio::test]
    async fn partial_downloads() {
        local(async move {
            let n = 1024 * 64u64;
            let test_data = random_test_data(n as usize);
            let temp_dir = tempfile::tempdir().unwrap();
            let hash = blake3::hash(&test_data);
            let handle = BaoFileHandle::incomplete_mem(
                Arc::new(BaoFileConfig::new(
                    Arc::new(temp_dir.as_ref().to_owned()),
                    1024 * 16,
                    None,
                )),
                hash.into(),
            );
            let mut tasks = JoinSet::new();
            for i in 1..3 {
                let file = handle.writer();
                let range = (i * (n / 4))..((i + 1) * (n / 4));
                println!("range: {:?}", range);
                let (hash, chunk_ranges, wire_data) = make_wire_data(&test_data, &[range]);
                let trickle = trickle(&wire_data, 1200, std::time::Duration::from_millis(10))
                    .map(io::Result::Ok)
                    .boxed();
                let trickle = TokioStreamReader::new(tokio_util::io::StreamReader::new(trickle));
                let _task = tasks.spawn_local(async move {
                    decode_response_into_batch(hash, IROH_BLOCK_SIZE, chunk_ranges, trickle, file)
                        .await
                });
            }
            while let Some(res) = tasks.join_next().await {
                res.unwrap().unwrap();
            }
            println!(
                "len {:?} {:?}",
                handle,
                handle.data_reader().size().await.unwrap()
            );
            #[allow(clippy::single_range_in_vec_init)]
            let ranges = [1024 * 16..1024 * 48];
            validate(&handle, &test_data, &ranges).await;

            // let ranges =
            // let full_chunks = bao_tree::io::full_chunk_groups();
            let mut encoded = Vec::new();
            let ob = handle.outboard().unwrap();
            encoded
                .write_all(ob.tree.size().to_le_bytes().as_slice())
                .unwrap();
            bao_tree::io::fsm::encode_ranges_validated(
                handle.data_reader(),
                ob,
                &ChunkRanges::from(ChunkNum(16)..ChunkNum(48)),
                encoded,
            )
            .await
            .unwrap();
        })
        .await;
    }

    #[tokio::test]
    async fn concurrent_downloads() {
        let n = 1024 * 32u64;
        let test_data = random_test_data(n as usize);
        let temp_dir = tempfile::tempdir().unwrap();
        let hash = blake3::hash(&test_data);
        let handle = BaoFileHandle::incomplete_mem(
            Arc::new(BaoFileConfig::new(
                Arc::new(temp_dir.as_ref().to_owned()),
                1024 * 16,
                None,
            )),
            hash.into(),
        );
        let local = LocalPool::default();
        let mut tasks = Vec::new();
        for i in 0..4 {
            let file = handle.writer();
            let range = (i * (n / 4))..((i + 1) * (n / 4));
            println!("range: {:?}", range);
            let (hash, chunk_ranges, wire_data) = make_wire_data(&test_data, &[range]);
            let trickle = trickle(&wire_data, 1200, std::time::Duration::from_millis(10))
                .map(io::Result::Ok)
                .boxed();
            let trickle = TokioStreamReader::new(tokio_util::io::StreamReader::new(trickle));
            let task = local.spawn(move || async move {
                decode_response_into_batch(hash, IROH_BLOCK_SIZE, chunk_ranges, trickle, file).await
            });
            tasks.push(task);
        }
        for task in tasks {
            task.await.unwrap().unwrap();
        }
        println!(
            "len {:?} {:?}",
            handle,
            handle.data_reader().size().await.unwrap()
        );
        #[allow(clippy::single_range_in_vec_init)]
        let ranges = [0..n];
        validate(&handle, &test_data, &ranges).await;

        let mut encoded = Vec::new();
        let ob = handle.outboard().unwrap();
        encoded
            .write_all(ob.tree.size().to_le_bytes().as_slice())
            .unwrap();
        bao_tree::io::fsm::encode_ranges_validated(
            handle.data_reader(),
            ob,
            &ChunkRanges::all(),
            encoded,
        )
        .await
        .unwrap();
    }

    #[tokio::test]
    async fn stay_in_mem() {
        let test_data = random_test_data(1024 * 17);
        #[allow(clippy::single_range_in_vec_init)]
        let ranges = [0..test_data.len().try_into().unwrap()];
        let (hash, chunk_ranges, wire_data) = make_wire_data(&test_data, &ranges);
        println!("file len is {:?}", chunk_ranges);
        let temp_dir = tempfile::tempdir().unwrap();
        let handle = BaoFileHandle::incomplete_mem(
            Arc::new(BaoFileConfig::new(
                Arc::new(temp_dir.as_ref().to_owned()),
                1024 * 16,
                None,
            )),
            hash,
        );
        decode_response_into_batch(
            hash,
            IROH_BLOCK_SIZE,
            chunk_ranges,
            wire_data.as_slice(),
            handle.writer(),
        )
        .await
        .unwrap();
        validate(&handle, &test_data, &ranges).await;

        let mut encoded = Vec::new();
        let ob = handle.outboard().unwrap();
        encoded
            .write_all(ob.tree.size().to_le_bytes().as_slice())
            .unwrap();
        bao_tree::io::fsm::encode_ranges_validated(
            handle.data_reader(),
            ob,
            &ChunkRanges::all(),
            encoded,
        )
        .await
        .unwrap();
        println!("{:?}", handle);
    }
}