haematite 0.3.0

Content-addressed, branchable, actor-native storage engine
Documentation 
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// CORE-008/CORE-009: Shard router — stable hash-based key-to-shard mapping.

use crate::shard::actor::ShardHandle;

/// Private database router over shard handles in shard-index order.
#[derive(Clone, Debug)]
pub struct ShardRouter {
    handles: Vec<ShardHandle>,
}

impl ShardRouter {
    pub(crate) fn new(handles: Vec<ShardHandle>) -> Option<Self> {
        if handles.is_empty() {
            None
        } else {
            Some(Self { handles })
        }
    }

    pub(crate) fn shard_for(&self, key: &[u8]) -> usize {
        let digest = blake3::hash(key);
        let mut prefix = [0_u8; 8];
        for (target, source) in prefix.iter_mut().zip(digest.as_bytes().iter()) {
            *target = *source;
        }
        let value = u64::from_be_bytes(prefix);
        (value % self.handles.len() as u64) as usize
    }

    pub(crate) fn handle_for(&self, key: &[u8]) -> Option<&ShardHandle> {
        self.handles.get(self.shard_for(key))
    }

    /// Route directly to a shard by its index (AA-3-2 election: a `Prepare`
    /// carries the target shard index, not a key, so the acceptor must select the
    /// owning shard by id rather than by hashing a key).
    pub(crate) fn handle_for_shard(&self, shard_id: usize) -> Option<&ShardHandle> {
        self.handles.get(shard_id)
    }

    pub(crate) fn handles_in_order(&self) -> &[ShardHandle] {
        &self.handles
    }
}