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