feat(SmtForest) Skeleton out the new forest

miden-crypto/src/merkle/smt/forest/mod.rs

@@ -126,7 +126,7 @@ impl SmtForest {
     /// Inserts the specified key-value pair into an SMT with the specified root. This will also
     /// add a new root to the forest. Returns the new root.
     ///
-    /// Returns an error if an SMT with the specified root is not in the forest, these is not
+    /// Returns an error if an SMT with the specified root is not in the forest, there is not
     /// enough data in the forest to perform the insert, or if the insert would create a leaf
     /// with too many entries.
     pub fn insert(&mut self, root: Word, key: Word, value: Word) -> Result<Word, MerkleError> {
@@ -136,7 +136,7 @@ impl SmtForest {
     /// Inserts the specified key-value pairs into an SMT with the specified root. This will also
     /// add a single new root to the forest for the entire batch of inserts. Returns the new root.
     ///
-    /// Returns an error if an SMT with the specified root is not in the forest, these is not
+    /// Returns an error if an SMT with the specified root is not in the forest, there is not
     /// enough data in the forest to perform the insert, or if the insert would create a leaf
     /// with too many entries.
     pub fn batch_insert(

miden-crypto/src/merkle/smt/large_forest/backend.rs

@@ -0,0 +1,123 @@
+//! This file contains the [`Backend`] trait for the [`LargeSmtForest`] implementation and the
+//! supporting types it needs.
+
+use alloc::{boxed::Box, vec::Vec};
+use core::fmt::Debug;
+
+use thiserror::Error;
+
+use crate::{
+    Word,
+    merkle::{
+        MerkleError,
+        smt::{
+            SmtProof,
+            full::SMT_DEPTH,
+            large_forest::{
+                history::VersionId,
+                operation::{SmtForestUpdateBatch, SmtUpdateBatch},
+            },
+        },
+    },
+};
+// TYPE ALIASES
+// ================================================================================================
+
+/// The mutation set used by the forest backends.
+///
+/// At the moment this is used for _reverse_ mutations that "undo" the changes made to the tree(s),
+/// but may be harmonised with [`SmtUpdateBatch`] in the future. For more information on its use for
+/// reverse mutations, see [`crate::merkle::smt::SparseMerkleTree::apply_mutations_with_reversion`].
+pub type MutationSet = crate::merkle::smt::MutationSet<SMT_DEPTH, Word, Word>;
+
+// BACKEND
+// ================================================================================================
+
+/// The backing storage for the SMT forest, providing the necessary high-level methods for
+/// performing operations on the full trees that make up the forest, while allowing the forest
+/// itself to be storage agnostic.
+///
+/// # Backend Data Storage
+///
+/// Having a generic [`Backend`] provides no guarantees to the user about how it stores data and
+/// what patterns are used for data access under the hood. It is, however, guaranteed to store
+/// _only_ the data necessary to describe the latest state of each tree in the forest.
+pub trait Backend
+where
+    Self: Debug,
+{
+    // QUERIES
+    // ============================================================================================
+
+    /// Returns an opening for the specified `key` in the SMT with the specified `root`.
+    fn open(&self, root: Word, key: Word) -> Result<SmtProof>;
+
+    /// Returns the value associated with the provided `key` in the SMT with the provided `root`, or
+    /// [`None`] if no such value exists.
+    fn get(&self, root: Word, key: Word) -> Result<Option<Word>>;
+
+    /// Returns the version of the tree with the provided `root`.
+    fn version(&self, root: Word) -> Result<VersionId>;
+
+    /// Returns an iterator over all the tree roots and versions that the backend knows about.
+    ///
+    /// The iteration order is unspecified.
+    fn versions(&self) -> Result<impl Iterator<Item = (Word, VersionId)>>;
+
+    // SINGLE-TREE MODIFIERS
+    // ============================================================================================
+
+    /// Performs the provided `updates` on the tree with the provided `root`, returning the new
+    /// root.
+    ///
+    /// Implementations must guarantee the following behavior, with non-conforming implementations
+    /// considered to be a bug:
+    ///
+    /// - At most one new root must be added to the forest for the entire batch.
+    /// - If applying the provided `updates` results in no changes to the tree, no new tree must be
+    ///   allocated.
+    fn update_tree(
+        &mut self,
+        root: Word,
+        new_version: VersionId,
+        updates: SmtUpdateBatch,
+    ) -> Result<MutationSet>;
+
+    // MULTI-TREE MODIFIERS
+    // ============================================================================================
+
+    /// Performs the provided `updates` on the forest, setting all new tree states to have the
+    /// provided `new_version` and returning a vector of the mutation sets that reverse the changes
+    /// to each changed tree.
+    ///
+    /// Implementations must guarantee the following behaviour, with non-conforming implementations
+    /// considered to be a bug:
+    ///
+    /// - At most one new root must be added to the forest for each target root in the provided
+    ///   `updates`.
+    /// - If applying the provided `updates` results in no changes to a given lineage of trees in
+    ///   the forest, then no new tree must be allocated in that lineage.
+    fn update_forest(
+        &mut self,
+        new_version: VersionId,
+        updates: SmtForestUpdateBatch,
+    ) -> Result<Vec<MutationSet>>;
+}
+
+// BACKEND ERROR
+// ================================================================================================
+
+/// The error type for use within Backends.
+#[derive(Debug, Error)]
+pub enum BackendError {
+    /// Raised when there is an error with the merkle tree semantics within the backend.
+    #[error(transparent)]
+    Merkle(#[from] MerkleError),
+
+    /// Raised for arbitrary other errors within the backend.
+    #[error(transparent)]
+    Other(#[from] Box<dyn core::error::Error + Sync + Send>),
+}
+
+/// The result type for use with backends.
+pub type Result<T> = core::result::Result<T, BackendError>;

miden-crypto/src/merkle/smt/large_forest/error.rs

@@ -1,21 +1,51 @@
 //! This module contains the error types and helpers for working with errors from the large SMT
 //! forest.
 
+use alloc::boxed::Box;
+
 use thiserror::Error;
 
-use crate::merkle::{MerkleError, smt::large_forest::history::error::HistoryError};
+use crate::{
+    Word,
+    merkle::{
+        MerkleError,
+        smt::large_forest::{backend::BackendError, history::error::HistoryError},
+    },
+};
+
+// LARGE SMT FOREST ERROR
+// ================================================================================================
 
-/// The errors returned by operations on the large SMT forest.
-///
-/// This type primarily serves to wrap more specific error types from various subsystems into a
-/// generic interface type.
+/// The type of errors returned by operations on the large SMT forest.
 #[derive(Debug, Error)]
 pub enum LargeSmtForestError {
+    /// Errors in the history subsystem of the forest.
     #[error(transparent)]
     HistoryError(#[from] HistoryError),
 
+    /// Raised when an attempt is made to modify a frozen tree.
+    #[error("Attempted to modify non-current tree with root {0}")]
+    InvalidModification(Word),
+
+    /// Errors with the merkle tree operations of the forest.
     #[error(transparent)]
     MerkleError(#[from] MerkleError),
+
+    /// Raised for arbitrary other errors.
+    #[error(transparent)]
+    Other(#[from] Box<dyn core::error::Error + Sync + Send>),
+}
+
+/// We want to forward backend errors specifically when we can, so we manually implement the
+/// conversion.
+impl From<BackendError> for LargeSmtForestError {
+    fn from(value: BackendError) -> Self {
+        match value {
+            BackendError::Merkle(e) => LargeSmtForestError::from(e),
+            BackendError::Other(e) => LargeSmtForestError::from(e),
+        }
+    }
 }
 
-pub mod history {}
+/// The result type for use within the large SMT forest portion of the library.
+pub type Result<T> = core::result::Result<T, LargeSmtForestError>;

miden-crypto/src/merkle/smt/large_forest/history/mod.rs

@@ -39,7 +39,7 @@ use core::fmt::Debug;
 use error::{HistoryError, Result};
 
 use crate::{
-    Map, Set, Word,
+    Map, Word,
     merkle::{
         NodeIndex,
         smt::{LeafIndex, SMT_DEPTH},
@@ -136,13 +136,23 @@ impl History {
 
     /// Returns all the roots that the history knows about.
     ///
+    /// The iteration order of the roots is guaranteed to move backward in time, with earlier items
+    /// being roots from versions closer to the present.
+    ///
     /// # Complexity
     ///
-    /// Calling this method requires a traversal of all the versions and is hence linear in the
-    /// number of history versions.
-    #[must_use]
-    pub fn roots(&self) -> Set<Word> {
-        self.deltas.iter().map(|d| d.root).collect()
+    /// Calling this method provides an iterator whose consumption requires a traversal of all the
+    /// versions. The method's complexity is thus `O(n)` in the number of versions.
+    pub fn roots(&self) -> impl Iterator<Item = Word> {
+        self.deltas.iter().rev().map(|d| d.root)
+    }
+
+    /// Gets the version corresponding to the provided `root`, or returns [`None`] if the provided
+    /// `root` is not found within this history.
+    pub fn version(&self, root: Word) -> Option<VersionId> {
+        self.deltas
+            .iter()
+            .find_map(|d| if d.root == root { Some(d.version_id) } else { None })
     }
 
     /// Returns `true` if `root` is in the history and `false` otherwise.

miden-crypto/src/merkle/smt/large_forest/history/tests.rs

@@ -1,6 +1,8 @@
 #![cfg(feature = "std")]
 //! The functional tests for the history component.
 
+use alloc::vec::Vec;
+
 use p3_field::PrimeCharacteristicRing;
 
 use super::{CompactLeaf, History, LeafChanges, NodeChanges, error::Result};
@@ -32,7 +34,7 @@ fn roots() -> Result<()> {
     history.add_version(root_2, 1, nodes.clone(), leaves.clone())?;
 
     // We should be able to get all the roots.
-    let roots = history.roots();
+    let roots = history.roots().collect::<Vec<_>>();
     assert_eq!(roots.len(), 2);
     assert!(roots.contains(&root_1));
     assert!(roots.contains(&root_2));