Start implementing a cas in separate crates.

cas-core/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "cas-core"
+version = "0.1.0"
+authors = ["Simon Boyé <sim.boye@gmail.com>"]
+edition = "2018"
+license = "AGPL-3.0-or-later"
+
+[dependencies]
+thiserror = "1.0.25"
+digest = { version = "0.9.0", features = ["alloc"] }
+
+[dev-dependencies]
+sha2 = "0.9.5"

cas-core/src/cas.rs

@@ -0,0 +1,40 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+
+use super::error::Result;
+use super::object_id::ObjectId;
+use super::object_type::ObjectType;
+use super::object_metadata::ObjectMetadata;
+
+
+// pub struct ObjectIdIterator;
+
+
+pub trait Cas {
+    fn object_id_from_string(&self, hex: &str) -> Result<ObjectId>;
+    fn object_id_from_partial(&self, hex: &str) -> Result<ObjectId>;
+
+    fn has_object_id(&self, oid: &ObjectId) -> Result<bool>;
+    // fn iter_object_id(&self) -> Result<ObjectIdIterator>;
+
+    fn read_object(&self, oid: &ObjectId) -> Result<(ObjectMetadata, &dyn std::io::Read)>;
+    fn write_object(&mut self, otype: ObjectType, data: &mut dyn std::io::Read) -> Result<ObjectId>;
+    fn remove_object(&mut self, oid: &ObjectId) -> Result<()>;
+
+    fn read_ref(&self, key: &str) -> Result<ObjectId>;
+    fn write_ref(&mut self, key: &str, value: &ObjectId) -> Result<()>;
+    fn remove_ref(&mut self, key: &str) -> Result<()>;
+}

cas-core/src/error.rs

@@ -0,0 +1,122 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+
+// use std::path::PathBuf;
+
+
+/// Result type used through cas-core.
+pub type Result<T> = std::result::Result<T, Box<Error>>;
+
+/// Error type used through cas-core.
+#[non_exhaustive]
+#[derive(Debug, PartialEq, Eq, thiserror::Error)]
+pub enum Error {
+    // #[error("failed to create repository: {message}")]
+    // RepositoryCreationFailed {
+    //     message: String,
+    //     source: Option<Box<dyn std::error::Error>>,
+    // },
+
+    // #[error("invalid object id: {message}")]
+    // InvalidObjectId {
+    //     message: String,
+    //     source: Option<Box<dyn std::error::Error>>,
+    // },
+
+    // #[error("invalid size (got: {size}, expected: {expected})")]
+    // InvalidSize {
+    //     size: usize,
+    //     expected: usize,
+    // },
+
+    // #[error("non-empty directory ({dir})")]
+    // NonEmptyDirectory {
+    //     dir: PathBuf
+    // },
+
+    // #[error("invalid character(s) ({characters})")]
+    // InvalidCharacters {
+    //     characters: String,
+    // },
+
+    // #[error("invalid object type ({otype:?})")]
+    // InvalidObjectType {
+    //     otype: [u8; 4],
+    // },
+
+    // #[error("invalid object size (expected {expected}, got {size})")]
+    // InvalidObjectSize {
+    //     size: u64,
+    //     expected: u64,
+    // },
+
+    // #[error("unsupported file type")]
+    // UnsupportedFileType,
+
+    // #[error("invalid path ({path})")]
+    // InvalidPath { path: PathBuf },
+
+    // #[error("io error{}", format_optional_path(path))]
+    // IoError {
+    //     source: std::io::Error,
+    //     path: Option<PathBuf>,
+    // },
+
+    #[error("{0}")]
+    Error(String),
+}
+
+impl Error {
+    // pub fn repository_creation_failed<M: Into<String>>(message: M) -> Box<Error> {
+    //     Box::new(Error::RepositoryCreationFailed {
+    //         message: message.into(),
+    //         source: None,
+    //     })
+    // }
+
+    // pub fn repository_creation_failed_from<M: Into<String>>(source: Box<dyn std::error::Error>, message: M) -> Box<Error> {
+    //     Box::new(Error::RepositoryCreationFailed {
+    //         message: message.into(),
+    //         source: Some(source),
+    //     })
+    // }
+
+    // pub fn invalid_object_id<M: Into<String>>(message: M) -> Box<Error> {
+    //     Box::new(Error::InvalidObjectId {
+    //         message: message.into(),
+    //         source: None,
+    //     })
+    // }
+
+    // pub fn invalid_object_id_from<M: Into<String>>(source: Box<dyn std::error::Error>, message: M) -> Box<Error> {
+    //     Box::new(Error::InvalidObjectId {
+    //         message: message.into(),
+    //         source: Some(source),
+    //     })
+    // }
+
+    pub fn error<M: Into<String>>(message: M) -> Box<Error> {
+        Box::new(Error::Error(message.into()))
+    }
+}
+
+
+// fn format_optional_path(maybe_path: &Option<PathBuf>) -> String {
+//     match maybe_path {
+//         Some(path) => { format!(" ({:?})", path) },
+//         None       => { String::new() }
+//     }
+// }

cas-core/src/lib.rs

@@ -0,0 +1,44 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+//! # cas-core
+//!
+//! `cas-core` provides traits and types to interface with content-addressable
+//! storage.
+
+#![feature(inherent_ascii_escape)]
+
+
+extern crate thiserror;
+extern crate digest;
+
+
+mod error;
+mod object_id;
+mod object_type;
+mod object_metadata;
+mod pipeline;
+mod cas;
+
+
+pub use crate::{
+    error::{Error, Result},
+    object_id::{ObjectId, hex, write_hex},
+    object_type::{ObjectType},
+    object_metadata::{ObjectMetadata},
+    pipeline::{Pipeline, DefaultPipeline},
+    cas::{Cas},
+};
+

cas-core/src/object_id.rs

@@ -0,0 +1,212 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+
+use std::fmt;
+use std::str::{FromStr};
+use std::sync::Arc;
+
+use super::error::*;
+
+
+/// A unique identifier for an object.
+///
+/// This is the handle used to reference an Object. This is an opaque type that uniquely identify an
+/// object. It can be compared to another ObjectId, be hashed and that's about it.
+#[derive(Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ObjectId {
+    id: Arc<Vec<u8>>,
+}
+
+impl ObjectId {
+    pub fn new(id: &[u8]) -> ObjectId {
+        ObjectId {
+            id: Arc::new(id.into()),
+        }
+    }
+
+    pub fn id(&self) -> &[u8] {
+        self.id.as_slice()
+    }
+}
+
+impl FromStr for ObjectId {
+    type Err = Box<Error>;
+
+    fn from_str(id_str: &str) -> Result<ObjectId> {
+        Ok(ObjectId {
+            id: Arc::new(
+                CharPairs::new(id_str, false)
+                    .map(parse_byte)
+                    .collect::<Result<Vec<_>>>()?
+            ),
+        })
+    }
+}
+
+impl fmt::Display for ObjectId {
+    fn fmt(&self, f: &mut fmt::Formatter) -> std::result::Result<(), fmt::Error> {
+        write_hex(f, self.id.as_slice())
+    }
+}
+
+impl fmt::Debug for ObjectId {
+    fn fmt(&self, f: &mut fmt::Formatter) -> std::result::Result<(), fmt::Error> {
+        write!(f, "ObjectId::new(\"{}\")", self)
+    }
+}
+
+
+pub fn hex(bytes: &[u8]) -> String {
+    let mut hex = String::with_capacity(bytes.len() * 2);
+    write_hex(&mut hex, bytes).expect("can't format bytes ?");
+    hex
+}
+
+pub fn write_hex<W: std::fmt::Write>(write: &mut W, bytes: &[u8])
+    -> std::result::Result<(), std::fmt::Error>
+{
+    for byte in bytes.iter() {
+        write!(write, "{:02x}", byte)?;
+    }
+    Ok(())
+}
+
+
+struct CharPairs<'a> {
+    string: &'a str,
+    chars: std::str::CharIndices<'a>,
+    char_count: usize,
+    index: usize,
+    allow_partial: bool,
+}
+
+impl<'a> CharPairs<'a> {
+    fn new(string: &'a str, allow_partial: bool) -> Self {
+        let mut char_pairs = Self {
+            string,
+            chars: string.char_indices(),
+            char_count: 0,
+            index: 0,
+            allow_partial,
+        };
+        char_pairs.next_char();
+        char_pairs
+    }
+
+    fn next_char(&mut self) -> bool {
+        if self.index == self.string.len() {
+            false
+        }
+        else {
+            match self.chars.next() {
+                Some((i, _)) => {
+                    self.char_count += 1;
+                    self.index = i;
+                },
+                None => {
+                    self.index = self.string.len();
+                }
+            }
+            true
+        }
+    }
+}
+
+impl<'a> Iterator for CharPairs<'a> {
+    type Item = Result<&'a str>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let start_index = self.index;
+        if !self.next_char() {
+            return None;
+        }
+
+        if self.next_char() || self.allow_partial {
+            Some(Ok(&self.string[start_index..self.index]))
+        }
+        else {
+            Some(Err(Error::error(&format!("invalid string: got {} characters, expected even number", self.char_count))))
+        }
+    }
+}
+
+
+fn parse_byte(maybe_str_byte: Result<&str>) -> Result<u8> {
+    match maybe_str_byte {
+        Ok(str_byte) =>
+            u8::from_str_radix(str_byte, 16)
+                .map_err(|_| Error::error("invalid character")),
+        Err(err) => Err(err),
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use std::str::{FromStr};
+
+    use crate::error::*;
+    use super::ObjectId;
+
+    #[test]
+    fn object_id_display() {
+        let obj = ObjectId::new(&[
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+        ]);
+
+        assert_eq!(format!("{}", obj), "0001020304050607");
+    }
+
+    #[test]
+    fn object_id_debug() {
+        let obj = ObjectId::new(&[
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+        ]);
+
+        assert_eq!(format!("{:?}", obj), "ObjectId::new(\"0001020304050607\")");
+    }
+
+    #[test]
+    fn str_to_object_id() {
+        let obj = ObjectId::from_str("0001020304050607")
+            .expect("object id parsing failed");
+        let obj_ref = ObjectId::new(&[
+            0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
+        ]);
+
+        assert_eq!(obj, obj_ref);
+    }
+
+    #[test]
+    fn str_to_object_id_invalid_size() {
+        let result = ObjectId::from_str("000102030405060");
+
+        assert_eq!(
+            result.expect_err("object id parsing should have failed"),
+            Error::error("invalid string: got 15 characters, expected even number"),
+        );
+    }
+
+    #[test]
+    fn str_to_object_id_invalid_character() {
+        let result = ObjectId::from_str("00010203 4050607");
+
+        assert_eq!(
+            result.expect_err("object id parsing should have failed"),
+            Error::error("invalid character"),
+        );
+    }
+}

cas-core/src/object_metadata.rs

@@ -0,0 +1,61 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+
+use super::object_type::{ObjectType};
+
+
+#[derive(Clone, Eq, PartialEq)]
+pub struct ObjectMetadata {
+    otype: ObjectType,
+    size: usize,
+}
+
+impl ObjectMetadata {
+    pub fn new(otype: ObjectType, size: usize) -> Self {
+        Self {
+            otype,
+            size,
+        }
+    }
+
+    pub fn otype(&self) -> &ObjectType {
+        &self.otype
+    }
+
+    pub fn size(&self) -> usize {
+        self.size
+    }
+}
+
+impl std::fmt::Debug for ObjectMetadata {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error> {
+        f.write_fmt(format_args!("<meta {} {}>", self.otype().id().escape_ascii().to_string(), self.size))
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn object_metadata_debug() {
+        let meta = ObjectMetadata::new(ObjectType::new(b"tree").unwrap(), 1234);
+
+        assert_eq!(meta, meta);
+        assert_eq!(format!("{:?}", meta), "<meta tree 1234>");
+    }
+}

cas-core/src/object_type.rs

@@ -0,0 +1,63 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+use super::error::*;
+
+
+#[derive(Clone, Eq, Hash, Ord, PartialEq, PartialOrd)]
+pub struct ObjectType {
+    id: [u8; 4],
+}
+
+impl ObjectType {
+    pub fn new(id: &[u8]) -> Result<Self> {
+        if id.len() != 4 {
+            Err(Error::error("Invalid object type size."))
+        }
+        else {
+            let mut buf = [0; 4];
+            for (i, b) in id.iter().enumerate() {
+                buf[i] = *b;
+            }
+            Ok(Self {
+                id: buf,
+            })
+        }
+    }
+
+    pub fn id(&self) -> &[u8; 4] {
+        &self.id
+    }
+}
+
+impl std::fmt::Debug for ObjectType {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::result::Result<(), std::fmt::Error> {
+        f.write_fmt(format_args!("<otype {}>", self.id.escape_ascii().to_string()))
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn object_type_debug() {
+        let otype = ObjectType::new(b"tree").unwrap();
+
+        assert_eq!(otype, otype);
+        assert_eq!(format!("{:?}", otype), "<otype tree>");
+    }
+}

cas-core/src/pipeline.rs

@@ -0,0 +1,252 @@
+// This file is part of bsv.
+//
+// bsv is free software: you can redistribute it and/or modify it under the
+// terms of the GNU Affero General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// cdb is distributed in the hope that it will be useful, but WITHOUT ANY
+// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+// FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
+// more details.
+//
+// You should have received a copy of the Affero GNU General Public License
+// along with cdb.  If not, see <https://www.gnu.org/licenses/>.
+
+use digest::DynDigest;
+
+use super::error::*;
+use super::object_id::ObjectId;
+
+
+pub trait Reader {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
+    fn finalize(self: Box<Self>) -> Result<ObjectId>;
+
+    fn read_all(&mut self) -> Result<Vec<u8>> {
+        let mut buffer = [0u8; 1 << 13];
+        let mut data = Vec::new();
+        let mut count = usize::MAX;
+
+        while count != 0 {
+            count = self.read(&mut buffer)?;
+            if count != 0 {
+                data.extend_from_slice(&buffer[..count]);
+            }
+        }
+
+        Ok(data)
+    }
+}
+
+pub trait Writer {
+    fn write(&mut self, buf: &[u8]) -> Result<()>;
+    fn finalize(self: Box<Self>) -> Result<ObjectId>;
+}
+
+
+pub trait Pipeline {
+    fn new_reader(&self, reader: Box<dyn Reader>) -> Box<dyn Reader>;
+    fn new_writer(&self, writer: Box<dyn Writer>) -> Box<dyn Writer>;
+}
+
+
+impl<R: std::io::Read> Reader for R {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        <Self as std::io::Read>::read(self, buf).or_else(|err|
+            Err(Error::error(format!("Read error: {}", err)))
+        )
+    }
+
+    fn finalize(self: Box<Self>) -> Result<ObjectId> {
+        Err(Error::error("reader pipline has no digest step"))
+    }
+}
+
+
+impl<W: std::io::Write> Writer for W {
+    fn write(&mut self, buf: &[u8]) -> Result<()> {
+        self.write_all(buf).or_else(|err|
+            Err(Error::error(format!("Write error: {}", err)))
+        )
+    }
+
+    fn finalize(self: Box<Self>) -> Result<ObjectId> {
+        Err(Error::error("reader pipline has no digest step"))
+    }
+}
+
+
+pub struct DigestReader<'a> {
+    reader: Box<dyn Reader + 'a>,
+    digest: Option<Box<dyn DynDigest + 'a>>,
+    oid: Option<ObjectId>,
+}
+
+impl<'a> DigestReader<'a> {
+    pub fn new(reader: Box<dyn Reader + 'a>, digest: Box<dyn DynDigest + 'a>) -> Self {
+        return DigestReader {
+            reader,
+            digest: Some(digest),
+            oid: None,
+        }
+    }
+}
+
+impl<'a> Reader for DigestReader<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        let count = if let Some(dig) = &mut self.digest {
+            let count = self.reader.read(buf)?;
+            dig.update(&buf[..count]);
+            count
+        }
+        else {
+            0
+        };
+
+        if count == 0 {
+            if let Some(digest) = self.digest.take() {
+                self.oid = Some(ObjectId::new(&digest.finalize()));
+            }
+        }
+
+        Ok(count)
+    }
+
+    fn finalize(self: Box<Self>) -> Result<ObjectId> {
+        self.oid.ok_or_else(|| Error::error("Reader not finalized"))
+    }
+}
+
+
+pub struct DigestWriter<'a> {
+    writer: Box<dyn Writer + 'a>,
+    digest: Box<dyn DynDigest + 'a>,
+}
+
+impl<'a> DigestWriter<'a> {
+    pub fn new(writer: Box<dyn Writer + 'a>, digest: Box<dyn DynDigest + 'a>) -> Self {
+        return DigestWriter {
+            writer,
+            digest,
+        }
+    }
+}
+
+impl<'a> Writer for DigestWriter<'a> {
+    fn write(&mut self, buf: &[u8]) -> Result<()> {
+        self.digest.update(buf);
+        self.writer.write(buf)?;
+        Ok(())
+    }
+
+    fn finalize(self: Box<Self>) -> Result<ObjectId> {
+        Ok(ObjectId::new(&self.digest.finalize()))
+    }
+}
+
+
+pub struct DefaultPipeline {
+    digest: Box<dyn DynDigest>,
+}
+
+impl DefaultPipeline {
+    pub fn new(digest: Box<dyn DynDigest>) -> Self {
+        Self {
+            digest,
+        }
+    }
+}
+
+impl Pipeline for DefaultPipeline {
+    fn new_reader(&self, reader: Box<dyn Reader>) -> Box<dyn Reader> {
+        Box::new(DigestReader::new(reader, self.digest.box_clone()))
+    }
+
+    fn new_writer(&self, writer: Box<dyn Writer>) -> Box<dyn Writer> {
+        Box::new(DigestWriter::new(writer, self.digest.box_clone()))
+    }
+}
+
+
+#[cfg(test)]
+mod tests {
+    extern crate sha2;
+
+    use std::str::FromStr;
+
+    use sha2::Sha256;
+
+    use super::*;
+
+    #[test]
+    fn std_read_as_reader() {
+        let data = b"Hello World!".to_vec();
+        let mut reader: Box<dyn Reader> = Box::new(data.as_slice());
+
+        let data2 = reader.read_all().expect("read failed");
+        let maybe_oid = reader.finalize();
+
+        assert_eq!(data2, data);
+        assert!(maybe_oid.is_err());
+    }
+
+    #[test]
+    fn std_write_as_writer() {
+        let data = b"Hello World!".to_vec();
+        let mut buffer = [0u8; 32];
+
+        let maybe_oid = {
+            let mut writer: Box<dyn Writer> = Box::new(&mut buffer[..]);
+            writer.write(&data).expect("write failed");
+            writer.finalize()
+        };
+
+        assert_eq!(buffer[..data.len()], data);
+        // Check that the rest of the buffer is unchanged
+        assert!(buffer[data.len()..].iter().all(|&v| v == 0));
+        assert!(maybe_oid.is_err());
+    }
+
+    #[test]
+    fn digest_reader() {
+        let data = b"Hello World!".to_vec();
+        let oid = ObjectId::from_str(
+            "7f83b1657ff1fc53b92dc18148a1d65dfc2d4b1fa3d677284addd200126d9069"
+        ).expect("invalid object id");
+
+        let mut reader = Box::new(DigestReader::new(
+            Box::new(data.as_slice()),
+            Box::new(Sha256::default()),
+        ));
+
+        let data2 = reader.read_all().expect("read failed");
+        let oid2 = reader.finalize().expect("finalize failed");
+
+        assert_eq!(data2, data);
+        assert_eq!(oid2, oid);
+    }
+
+    #[test]
+    fn digest_writer() {
+        let data = b"Hello World!".to_vec();
+        let oid = ObjectId::from_str(
+            "7f83b1657ff1fc53b92dc18148a1d65dfc2d4b1fa3d677284addd200126d9069"
+        ).expect("invalid object id");
+
+        let mut buffer = [0u8; 32];
+        let oid2 = {
+            let mut writer = Box::new(DigestWriter::new(
+                Box::new(&mut buffer[..]),
+                Box::new(Sha256::default()),
+            ));
+            writer.write(&data).expect("write failed");
+            writer.finalize().expect("finalize failed")
+        };
+
+        assert_eq!(buffer[..data.len()], data);
+        // Check that the rest of the buffer is unchanged
+        assert!(buffer[data.len()..].iter().all(|&v| v == 0));
+        assert_eq!(oid2, oid);
+    }
+}