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    //! IO utilities.

    use base64::{
        Engine,
        engine::general_purpose::{URL_SAFE_NO_PAD as base64_engine},
    };
    use flate2::Compression;
    use flate2::read::ZlibDecoder;
    use flate2::write::ZlibEncoder;
    use std::ffi::OsStr;
    use std::io::{Read, Write};
    use std::path::{Path, PathBuf};
    use tempfile::NamedTempFile;

    use crate::error::Error;

    /// Abstracts a file system.
    pub trait FileSystem {
        /// File that calculates the hash of its contents.
        type HashedFileOut: HashedFileOut;
        /// File whose contents can be verified with the hash.
        type HashedFileIn: HashedFileIn;

        /// Creates a file that calculates the hash of its contents.
        fn create_hashed_file(&self) -> Result<Self::HashedFileOut, Error>;

        /// Creates a hashed file in a given directory.
        fn create_hashed_file_in(
            &self,
            path: impl AsRef<str>,
        ) -> Result<Self::HashedFileOut, Error>;

        /// Opens a file whose contents can be verified with a hash.
        fn open_hashed_file(
            &self,
            path: impl AsRef<str>,
        ) -> Result<Self::HashedFileIn, Error>;

        /// Creates a compressed file that calculates the hash of its contents.
        fn create_compressed_hashed_file(
            &self,
        ) -> Result<CompressedHashedFileOut<Self::HashedFileOut>, Error> {
            let file = self.create_hashed_file()?;
            Ok(CompressedHashedFileOut::new(file))
        }

        /// Creates a compressed hashed file in a given directory.
        fn create_compressed_hashed_file_in(
            &self,
            path: impl AsRef<str>,
        ) -> Result<CompressedHashedFileOut<Self::HashedFileOut>, Error> {
            let file = self.create_hashed_file_in(path)?;
            Ok(CompressedHashedFileOut::new(file))
        }

        /// Opens a compressed file whose contents can be verified with a hash.
        fn open_compressed_hashed_file(
            &self,
            path: impl AsRef<str>,
        ) -> Result<CompressedHashedFileIn<Self::HashedFileIn>, Error> {
            let file = self.open_hashed_file(path)?;
            Ok(CompressedHashedFileIn::new(file))
        }
    }

    /// File whose name will be the hash of its contents.
    pub trait HashedFileOut: Write {
        /// Persists the file.
        ///
        /// Finishes the calculation of the hash and persists the file.
        /// You should flush the stream before calling this function.
        ///
        /// Returns the encoded hash value that is supposed to be a URS-safe Base64
        /// encoded SHA256 digest.
        fn persist(self, extension: impl AsRef<str>) -> Result<String, Error>;
    }

    /// File whose name is the hash of its contents.
    pub trait HashedFileIn: Read {
        /// Verifies the file.
        ///
        /// Finishes the calculation of the hash and verifies the file.
        /// You should call this function after the entire file has been read.
        ///
        /// File name is supposed to be a Base64 encoded URL-safe SHA256 digest.
        fn verify(self) -> Result<(), Error>;
    }

    /// Compressed file that calculates the hash of its contents.
    pub struct CompressedHashedFileOut<W>
    where
        W: std::io::Write,
    {
        encoder: ZlibEncoder<W>,
    }

    impl<W> CompressedHashedFileOut<W>
    where
        W: std::io::Write,
    {
        /// Writes compressed data to a given [`Write`].
        pub fn new(w: W) -> Self {
            Self {
                encoder: ZlibEncoder::new(w, Compression::default()),
            }
        }
    }

    impl<W> Write for CompressedHashedFileOut<W>
    where
        W: std::io::Write,
    {
        fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
            self.encoder.write(buf)
        }

        fn flush(&mut self) -> std::io::Result<()> {
            self.encoder.flush()
        }
    }

    impl<W> HashedFileOut for CompressedHashedFileOut<W>
    where
        W: HashedFileOut
    {
        fn persist(self, extension: impl AsRef<str>) -> Result<String, Error> {
            self.encoder.finish()?.persist(extension)
        }
    }

    /// Compressed file whose contents can be verified with a hash.
    pub struct CompressedHashedFileIn<R>
    where
        R: std::io::Read,
    {
        decoder: ZlibDecoder<R>,
    }

    impl<R> CompressedHashedFileIn<R>
    where
        R: std::io::Read,
    {
        /// Reads compressed data from a given [`Read`].
        pub fn new(r: R) -> Self {
            Self {
                decoder: ZlibDecoder::new(r),
            }
        }
    }

    impl<R> Read for CompressedHashedFileIn<R>
    where
        R: std::io::Read,
    {
        fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
            self.decoder.read(buf)
        }
    }

    impl<R> HashedFileIn for CompressedHashedFileIn<R>
    where
        R: HashedFileIn,
    {
        fn verify(self) -> Result<(), Error> {
            self.decoder.into_inner().verify()
        }
    }

    /// File system uses the local file system.
    pub struct LocalFileSystem {
        // Base path.
        base_path: PathBuf,
    }

    impl LocalFileSystem {
        /// Creates a local file system working under a given base path.
        pub fn new(base_path: impl AsRef<Path>) -> Self {
            Self {
                base_path: base_path.as_ref().to_path_buf(),
            }
        }
    }

    impl FileSystem for LocalFileSystem {
        type HashedFileOut = LocalHashedFileOut;
        type HashedFileIn = LocalHashedFileIn;

        fn create_hashed_file(&self) -> Result<Self::HashedFileOut, Error> {
            LocalHashedFileOut::create(self.base_path.clone())
        }

        fn create_hashed_file_in(
            &self,
            path: impl AsRef<str>,
        ) -> Result<Self::HashedFileOut, Error> {
            LocalHashedFileOut::create(self.base_path.join(path.as_ref()))
        }

        fn open_hashed_file(
            &self,
            path: impl AsRef<str>,
        ) -> Result<Self::HashedFileIn, Error> {
            LocalHashedFileIn::open(self.base_path.join(path.as_ref()))
        }
    }

    /// Writable file in the local file system.
    ///
    /// Created as a temporary file and renamed to the hash of its contents.
    pub struct LocalHashedFileOut {
        // Temporary file.
        tempfile: NamedTempFile,
        // Persisted path.
        base_path: PathBuf,
        // Context to calculate an SHA-256 digest.
        context: ring::digest::Context,
    }

    impl LocalHashedFileOut {
        /// Creates a temporary file to be persisted under a given path.
        fn create(base_path: PathBuf) -> Result<Self, Error> {
            let tempfile = NamedTempFile::new()?;
            Ok(LocalHashedFileOut {
                tempfile,
                base_path,
                context: ring::digest::Context::new(&ring::digest::SHA256),
            })
        }
    }

    impl Write for LocalHashedFileOut {
        fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
            self.context.update(buf);
            self.tempfile.write(buf)
        }

        fn flush(&mut self) -> std::io::Result<()> {
            self.tempfile.flush()
        }
    }

    impl HashedFileOut for LocalHashedFileOut {
        fn persist(mut self, extension: impl AsRef<str>) -> Result<String, Error> {
            self.flush()?;
            if !self.base_path.exists() {
                std::fs::create_dir_all(&self.base_path)?;
            }
            let hash = self.context.finish();
            let hash = base64_engine.encode(&hash);
            let path = self.base_path
                .join(&hash)
                .with_extension(extension.as_ref());
            self.tempfile.persist(path)?;
            Ok(hash)
        }
    }

    /// Readable file in the local file system.
    pub struct LocalHashedFileIn {
        file: std::fs::File,
        path: PathBuf,
        // Context to calculate an SHA-256 digest.
        context: ring::digest::Context,
    }

    impl LocalHashedFileIn {
        /// Opens a file whose name is the hash of its contents.
        fn open(path: PathBuf) -> Result<Self, Error> {
            let file = std::fs::File::open(&path)?;
            Ok(LocalHashedFileIn {
                file,
                path,
                context: ring::digest::Context::new(&ring::digest::SHA256),
            })
        }
    }

    impl Read for LocalHashedFileIn {
        fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
            let n = self.file.read(buf)?;
            self.context.update(&buf[..n]);
            Ok(n)
        }
    }

    impl HashedFileIn for LocalHashedFileIn {
        fn verify(self) -> Result<(), Error> {
            let hash = self.context.finish();
            let hash = base64_engine.encode(&hash);
            if hash.as_str() == self.path.file_stem().unwrap_or(OsStr::new("")) {
                Ok(())
            } else {
                Err(Error::VerificationFailure(format!(
                    "Expected hash {:?}, but got {}",
                    self.path.file_stem(),
                    hash,
                )))
            }
        }
    }