add serialized map

serialized-map/.gitignore

@@ -0,0 +1 @@
+/target

serialized-map/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "serialized-map"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+brotli = "5.0.0"
+thiserror = "1.0.59"

serialized-map/src/lib.rs

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+use std::collections::BTreeMap;
+pub type ByteByteMap = BTreeMap<Vec<u8>, Vec<u8>>;
+
+// File format:
+// 0xa8 0x32 (magic number)
+// 0x00      (version)
+// 0x03      (header entries)
+// (header entries)
+// 0x08      (main entries)
+
+// 0x0
+
+
+#[derive(thiserror::Error, Debug, PartialEq, Eq)]
+pub enum DeserializeError {
+    #[error("Bad magic number")]
+    BadMagicNumber,
+
+    #[error("Unexpected end of file")]
+    UnexpectedEof,
+}
+
+// TODO:
+// - [ ] Brotli compression
+// - [ ] Header data (version, format version, flavor, date, etc.)
+// - [x] Magic number
+// - [ ] Delta files
+// - [ ] Variable-length encoding of length
+
+pub fn write_length_prefixed(
+    writer: &mut impl std::io::Write,
+    value: &[u8],
+) -> std::io::Result<()> {
+    let len = value.len() as u32;
+    // write the length as a u32 in big endian
+    let mut len_bytes = [0; 4];
+    len_bytes.copy_from_slice(&len.to_be_bytes());
+    writer.write_all(&len_bytes)?;
+    writer.write_all(value)?;
+    Ok(())
+}
+
+pub fn read_length_prefixed(reader: &mut impl std::io::Read) -> Result<Vec<u8>, DeserializeError> {
+    let mut len_bytes = [0; 4];
+    reader.read_exact(&mut len_bytes).map_err(|_| DeserializeError::UnexpectedEof)?;
+    let len = u32::from_be_bytes(len_bytes);
+    let mut value = vec![0; len as usize];
+    reader.read_exact(&mut value).map_err(|_| DeserializeError::UnexpectedEof)?;
+    Ok(value)
+}
+
+pub fn serialize(map: &ByteByteMap, mut writer: impl std::io::Write) -> std::io::Result<()> {
+    let magic = [0xa8, 0x32];
+    writer.write_all(&magic)?;
+
+    let len = map.len() as u32;
+    // write the length as a u32 in big endian
+    let mut len_bytes = [0; 4];
+    len_bytes.copy_from_slice(&len.to_be_bytes());
+    writer.write_all(&len_bytes)?;
+    for (key, value) in map {
+        write_length_prefixed(&mut writer, key.as_slice())?;
+        write_length_prefixed(&mut writer, value.as_slice())?;
+    }
+    Ok(())
+}
+
+pub fn serialize_to_bytes(map: &ByteByteMap) -> Vec<u8> {
+    let mut writer = Vec::new();
+    serialize(map, &mut writer).unwrap();
+    writer
+}
+
+pub fn deserialize(mut reader: impl std::io::Read) -> Result<ByteByteMap, DeserializeError> {
+    let mut magic_buf = [0; 2];
+    reader.read_exact(&mut magic_buf).map_err(|_| DeserializeError::UnexpectedEof)?;
+
+    if magic_buf != [0xa8, 0x32] {
+        return Err(DeserializeError::BadMagicNumber)
+    }
+
+    let mut map = BTreeMap::new();
+    let mut buf = [0; 4];
+    reader.read_exact(&mut buf).map_err(|_| DeserializeError::UnexpectedEof)?;
+    let len = u32::from_be_bytes(buf);
+    for _ in 0..len {
+        let key = read_length_prefixed(&mut reader)?;
+        let value = read_length_prefixed(&mut reader)?;
+        map.insert(key, value);
+    }
+    Ok(map)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn empty_map() {
+        let map: ByteByteMap = BTreeMap::new();
+        let bytes = serialize_to_bytes(&map);
+        assert_eq!(deserialize(bytes.as_slice()).unwrap(), map);
+    }
+
+    #[test]
+    fn single_entry() {
+        let map = BTreeMap::from([(b"foo".to_vec(), b"bar".to_vec())]);
+        let bytes = serialize_to_bytes(&map);
+        assert_eq!(deserialize(bytes.as_slice()).unwrap(), map);
+    }
+
+    #[test]
+    fn multiple_entries() {
+        let map = BTreeMap::from([
+            (b"foo".to_vec(), b"bar".to_vec()),
+            (b"baz".to_vec(), b"qux".to_vec()),
+        ]);
+
+        let bytes = serialize_to_bytes(&map);
+        assert_eq!(deserialize(bytes.as_slice()).unwrap(), map);
+    }
+
+    #[test]
+    fn bad_magic_number() {
+        let c: Vec<u8> = b"1234".to_vec();
+        let d: &[u8] = &c;
+
+        let result = deserialize(d);
+
+        assert_eq!(result.unwrap_err(), DeserializeError::BadMagicNumber);
+    }
+}