perf: faster count_leaves and prove_membership_async for MMRA

With this commit the DB is only accessed once when counting leaves, whereas
earlier, a helper function that read multiple digests from the MMRA was used.

Also changes index arithmetic for getting the relevant node indices for an
MMR authentication path slightly, and then reads all the digests in one
go instead of in a loop.

Good speedup seen from this commit:
- batch-mutation benchmark is 10 % faster, regular mutation is 20 % faster.

src/util_types/mutator_set/archival_mmr.rs

@@ -1,7 +1,10 @@
 use crate::database::storage::storage_vec::traits::*;
 use crate::prelude::twenty_first;
 
+use tasm_lib::twenty_first::util_types::mmr::shared_advanced::get_authentication_path_node_indices;
+use tasm_lib::twenty_first::util_types::mmr::shared_advanced::get_peak_heights_and_peak_node_indices;
 use tasm_lib::twenty_first::util_types::mmr::shared_advanced::node_index_to_leaf_index;
+use tasm_lib::twenty_first::util_types::mmr::shared_basic::leaf_index_to_mt_index_and_peak_index;
 use tasm_lib::twenty_first::util_types::mmr::shared_basic::right_lineage_length_from_leaf_index;
 use twenty_first::shared_math::digest::Digest;
 use twenty_first::util_types::algebraic_hasher::AlgebraicHasher;
@@ -51,13 +54,10 @@ where
 
     /// Return the number of leaves in the tree
     pub async fn count_leaves(&self) -> u64 {
-        let peaks_and_heights: Vec<(_, u32)> = self.get_peaks_with_heights_async().await;
-        let mut acc = 0;
-        for (_, height) in peaks_and_heights {
-            acc += 1 << height
-        }
+        let leaf_index_last_insertion = node_index_to_leaf_index(self.digests.len().await).unwrap();
 
-        acc
+        // Add one to last index, as leaf-indices are 0-indexed
+        leaf_index_last_insertion + 1
     }
 
     /// Append an element to the archival MMR, return the membership proof of the newly added leaf.
@@ -200,45 +200,25 @@ impl<H: AlgebraicHasher, Storage: StorageVec<Digest>> ArchivalMmr<H, Storage> {
     pub async fn prove_membership_async(&self, leaf_index: u64) -> MmrMembershipProof<H> {
         // A proof consists of an authentication path
         // and a list of peaks
+        let num_leafs = self.count_leaves().await;
         assert!(
-            leaf_index < self.count_leaves().await,
+            leaf_index < num_leafs,
             "Cannot prove membership of leaf outside of range. Got leaf_index {leaf_index}. Leaf count is {}", self.count_leaves().await
         );
 
-        // Find out how long the authentication path is
         let node_index = shared_advanced::leaf_index_to_node_index(leaf_index);
-        let mut top_height: i32 = -1;
-        let mut parent_index = node_index;
-        while parent_index < self.digests.len().await {
-            parent_index = shared_advanced::parent(parent_index);
-            top_height += 1;
-        }
-
-        // Build the authentication path
-        let mut authentication_path: Vec<Digest> = vec![];
-        let mut index = node_index;
-        let (mut right_ancestor_count, mut index_height): (u32, u32) =
-            shared_advanced::right_lineage_length_and_own_height(index);
-        while index_height < top_height as u32 {
-            if right_ancestor_count != 0 {
-                // index is right child
-                let left_sibling_index = shared_advanced::left_sibling(index, index_height);
-                authentication_path.push(self.digests.get(left_sibling_index).await);
-
-                // parent of right child is index + 1
-                index += 1;
-            } else {
-                // index is left child
-                let right_sibling_index = shared_advanced::right_sibling(index, index_height);
-                authentication_path.push(self.digests.get(right_sibling_index).await);
-
-                // parent of left child:
-                index += 1 << (index_height + 1);
-            }
-            let next_index_info = shared_advanced::right_lineage_length_and_own_height(index);
-            right_ancestor_count = next_index_info.0;
-            index_height = next_index_info.1;
-        }
+        let (_, own_index_into_peaks_list) =
+            leaf_index_to_mt_index_and_peak_index(leaf_index, num_leafs);
+        let (_, peak_indices) = get_peak_heights_and_peak_node_indices(num_leafs);
+        let num_nodes = self.digests.len().await;
+        let sibling_indices = get_authentication_path_node_indices(
+            node_index,
+            peak_indices[own_index_into_peaks_list as usize],
+            num_nodes,
+        )
+        .unwrap();
+
+        let authentication_path = self.digests.get_many(&sibling_indices).await;
 
         MmrMembershipProof::new(leaf_index, authentication_path)
     }