puppy/lib/store/src/mixin.rs

use bincode::{Decode, Encode};
/// Derive a [`Mixin`] implementation.
pub use r#macro::Mixin;

use super::{
    types::{MixinSpec, Value},
    Batch, Store, Transaction,
};
use crate::{Error, Key, Result};

/// Mixins are the simplest pieces of data in the store.
pub trait Mixin: Value<Type = MixinSpec> + Encode + Decode {}

impl Store {
    /// Get the value!
    pub fn get_mixin<M>(&self, node: Key) -> Result<Option<M>>
    where
        M: Mixin,
    {
        op::get_mixin(self, node)
    }
    /// Check if `node` has a mixin `M`.
    pub fn has_mixin<M>(&self, node: Key) -> Result<bool>
    where
        M: Mixin,
    {
        op::has_mixin::<M>(self, node)
    }
}

impl Transaction<'_> {
    /// Apply an update function to the mixin `M` of `node`.
    ///
    /// # Errors
    ///
    /// - [`Error::Missing`]: if `node` does not have a mixin of this type.
    ///
    /// [`Error::Missing`]: crate::Error::Missing
    pub fn update<M>(&self, node: Key, update: impl FnOnce(M) -> M) -> Result<()>
    where
        M: Mixin,
    {
        op::update(self, node, update)
    }
    /// Get the mixin of the specified type associated with `node`.
    pub fn get_mixin<M>(&self, node: Key) -> Result<Option<M>>
    where
        M: Mixin,
    {
        op::get_mixin(self, node)
    }
    /// Add a mixin to `node`.
    ///
    /// # Errors
    ///
    /// - [`Error::Conflict`]: if `node` already has a mixin of type `M`.
    ///
    /// [`Error::Conflict`]: crate::Error::Missing
    pub fn add_mixin<M>(&self, node: Key, mixin: M) -> Result<()>
    where
        M: Mixin,
    {
        if op::has_mixin::<M>(self, node)? {
            return Err(Error::Conflict);
        } else {
            op::add_mixin::<M>(self, node, mixin)
        }
    }
    /// Check whether `node` has an `M` defined for it.
    pub fn has_mixin<M>(&self, node: Key) -> Result<bool>
    where
        M: Mixin,
    {
        op::has_mixin::<M>(self, node)
    }
}

impl Batch {
    /// Add a mixin to the `node`.
    ///
    /// **Note**: unlike [`Transaction::add_mixin`], this will *not* return an error if the key already has a mixin
    /// of this type. This *should* not cause inconsistency.
    pub fn put_mixin<M>(&mut self, node: Key, mixin: M)
    where
        M: Mixin,
    {
        op::add_mixin(self, node, mixin).unwrap()
    }
}

mod op {
    use super::Mixin;
    use crate::{internal::*, Error, Key, Result};

    pub fn update<M>(
        cx: &(impl Query + Write),
        node: Key,
        update: impl FnOnce(M) -> M,
    ) -> Result<()>
    where
        M: Mixin,
    {
        // TODO: implement in terms of a merge operator instead of separate query and write ops.
        // this would let us remove the `Query` bound, which would in turn let us update from within
        // a batch.
        //
        // See https://github.com/facebook/rocksdb/wiki/Merge-Operator
        //
        // It looks like rocksdb allows you to specify a merge operator per column family.[^1]
        // This means we can construct our column families with a merge operator that knows how to encode and decode mixins.
        //
        // [^1]: https://github.com/facebook/rocksdb/blob/9d37408f9af15c7a1ae42f9b94d06b27d98a011a/include/rocksdb/options.h#L128
        let tree = cx.open(M::SPEC.keyspace);
        match tree.get(node.as_ref())? {
            None => Err(Error::Missing),
            Some(buf) => {
                let new = decode(buf).map(update).and_then(encode)?;
                tree.set(node, new)
            }
        }
    }

    pub fn get_mixin<M: Mixin>(cx: &impl Query, node: Key) -> Result<Option<M>> {
        cx.open(M::SPEC.keyspace).get(node)?.map(decode).transpose()
    }

    pub fn add_mixin<M: Mixin>(cx: &impl Write, node: Key, mixin: M) -> Result<()> {
        cx.open(M::SPEC.keyspace).set(node, encode(mixin)?)
    }

    pub fn has_mixin<M: Mixin>(cx: &impl Query, node: Key) -> Result<bool> {
        cx.open(M::SPEC.keyspace).has(node)
    }

    pub(super) fn encode(data: impl bincode::Encode) -> Result<Vec<u8>> {
        bincode::encode_to_vec(data, bincode::config::standard()).map_err(Error::Encoding)
    }

    fn decode<T>(data: impl AsRef<[u8]>) -> Result<T>
    where
        T: bincode::Decode,
    {
        bincode::decode_from_slice(data.as_ref(), bincode::config::standard())
            .map_err(Error::Decoding)
            .map(|(v, _)| v)
    }
}