merk 2.0.1

Merkle key/value store
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use crate::error::Result;
use super::Fetch;
use super::super::{Tree, Link};

/// Allows read-only traversal of a `Tree`, fetching from the given source when
/// traversing to a pruned node. The fetched nodes are then retained in memory
/// until they (possibly) get pruned on the next commit.
///
/// Only finalized trees may be walked (trees which have had `commit` called
/// since the last update).
pub struct RefWalker<'a, S>
    where S: Fetch + Sized + Clone + Send
{
    tree: &'a mut Tree,
    source: S
}

impl<'a, S> RefWalker<'a, S>
    where S: Fetch + Sized + Clone + Send
{
    /// Creates a `RefWalker` with the given tree and source.
    pub fn new(tree: &'a mut Tree, source: S) -> Self {
        // TODO: check if tree has modified links, panic if so
        RefWalker { tree, source }
    }

    /// Gets an immutable reference to the `Tree` wrapped by this `RefWalker`.
    pub fn tree(&self) -> &Tree {
        self.tree
    }

    /// Traverses to the child on the given side (if any), fetching from the
    /// source if pruned. When fetching, the link is upgraded from
    /// `Link::Pruned` to `Link::Stored`.
    pub fn walk<'b>(&'b mut self, left: bool) -> Result<Option<RefWalker<'b, S>>> {
        let link = match self.tree.link(left) {
            None => return Ok(None),
            Some(link) => link
        };

        match link {
            Link::Modified { .. } => panic!("Cannot traverse Link::Modified"),
            Link::Stored { .. } => {},
            Link::Pruned { .. } => {
                self.tree.load(left, &self.source)?;
            }
        }

        let child = self.tree.child_mut(left).unwrap();
        Ok(Some(RefWalker::new(child, self.source.clone())))
    }
}