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use serde::{Deserialize, Serialize};
pub trait Migrate: Serialize + for<'de> Deserialize<'de> + 'static {
/// A sequence of bytes to add at the beginning of the serialized
/// string, to identify that the data is of this version.
const VERSION_MARKER: &'static [u8] = b"";
/// The previous version of this data type, from which items of this version
/// can be migrated. Set `type Previous = NoPrevious` to indicate that this datatype
/// is the initial schema and cannot be migrated.
type Previous: Migrate;
/// This function must be filled in by implementors to migrate from a previons iteration
/// of the data format.
fn migrate(previous: Self::Previous) -> Self;
fn decode(bytes: &[u8]) -> Option<Self> {
let marker_len = Self::VERSION_MARKER.len();
if bytes.len() >= marker_len && &bytes[..marker_len] == Self::VERSION_MARKER {
if let Ok(value) = rmp_serde::decode::from_read_ref::<_, Self>(&bytes[marker_len..]) {
return Some(value);
}
}
Self::Previous::decode(bytes).map(Self::migrate)
}
fn encode(&self) -> Result<Vec<u8>, rmp_serde::encode::Error> {
let mut wr = Vec::with_capacity(128);
wr.extend_from_slice(Self::VERSION_MARKER);
let mut se = rmp_serde::Serializer::new(&mut wr)
.with_struct_map()
.with_string_variants();
self.serialize(&mut se)?;
Ok(wr)
}
}
pub trait InitialFormat: Serialize + for<'de> Deserialize<'de> + 'static {
/// A sequence of bytes to add at the beginning of the serialized
/// string, to identify that the data is of this version.
const VERSION_MARKER: &'static [u8] = b"";
}
// ----
impl<T: InitialFormat> Migrate for T {
const VERSION_MARKER: &'static [u8] = <T as InitialFormat>::VERSION_MARKER;
type Previous = NoPrevious;
fn migrate(_previous: Self::Previous) -> Self {
unreachable!();
}
}
#[derive(Serialize, Deserialize)]
pub struct NoPrevious;
impl Migrate for NoPrevious {
type Previous = NoPrevious;
fn migrate(_previous: Self::Previous) -> Self {
unreachable!();
}
fn decode(_bytes: &[u8]) -> Option<Self> {
None
}
fn encode(&self) -> Result<Vec<u8>, rmp_serde::encode::Error> {
unreachable!()
}
}
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