optimization for reading peak hashes from backend file (#3575)

core/src/core/pmmr/backend.rs

@@ -46,6 +46,11 @@ pub trait Backend<T: PMMRable> {
 	/// (ignoring the remove log).
 	fn get_from_file(&self, position: u64) -> Option<Hash>;
 
+	/// Get hash for peak pos.
+	/// Optimized for reading peak hashes rather than arbitrary pos hashes.
+	/// Peaks can be assumed to not be compacted.
+	fn get_peak_from_file(&self, position: u64) -> Option<Hash>;
+
 	/// Get a Data Element by original insertion position
 	/// (ignoring the remove log).
 	fn get_data_from_file(&self, position: u64) -> Option<T::E>;

core/src/core/pmmr/pmmr.rs

@@ -39,6 +39,11 @@ pub trait ReadablePMMR {
 	/// Get the hash from the underlying MMR file (ignores the remove log).
 	fn get_from_file(&self, pos: u64) -> Option<Hash>;
 
+	/// Get the hash for the provided peak pos.
+	/// Optimized for reading peak hashes rather than arbitrary pos hashes.
+	/// Peaks can be assumed to not be compacted.
+	fn get_peak_from_file(&self, pos: u64) -> Option<Hash>;
+
 	/// Get the data element at provided position in the MMR (ignores the remove log).
 	fn get_data_from_file(&self, pos: u64) -> Option<Self::Item>;
 
@@ -84,11 +89,7 @@ pub trait ReadablePMMR {
 	fn peaks(&self) -> Vec<Hash> {
 		peaks(self.unpruned_size())
 			.into_iter()
-			.filter_map(move |pi| {
-				// here we want to get from underlying hash file
-				// as the pos *may* have been "removed"
-				self.get_from_file(pi)
-			})
+			.filter_map(move |pi| self.get_peak_from_file(pi))
 			.collect()
 	}
 
@@ -98,7 +99,7 @@ pub trait ReadablePMMR {
 		let mut res = peaks(self.unpruned_size())
 			.into_iter()
 			.filter(|&x| x < peak_pos)
-			.filter_map(|x| self.get_from_file(x))
+			.filter_map(|x| self.get_peak_from_file(x))
 			.collect::<Vec<_>>();
 		if let Some(rhs) = rhs {
 			res.push(rhs);
@@ -227,7 +228,7 @@ where
 			let left_sibling = pos + 1 - 2 * peak;
 			let left_hash = self
 				.backend
-				.get_from_file(left_sibling)
+				.get_peak_from_file(left_sibling)
 				.ok_or("missing left sibling in tree, should not have been pruned")?;
 			peak *= 2;
 			pos += 1;
@@ -414,6 +415,14 @@ where
 		}
 	}
 
+	fn get_peak_from_file(&self, pos: u64) -> Option<Hash> {
+		if pos > self.last_pos {
+			None
+		} else {
+			self.backend.get_peak_from_file(pos)
+		}
+	}
+
 	fn get_data_from_file(&self, pos: u64) -> Option<Self::Item> {
 		if pos > self.last_pos {
 			None

core/src/core/pmmr/readonly_pmmr.rs

@@ -148,6 +148,14 @@ where
 		}
 	}
 
+	fn get_peak_from_file(&self, pos: u64) -> Option<Hash> {
+		if pos > self.last_pos {
+			None
+		} else {
+			self.backend.get_peak_from_file(pos)
+		}
+	}
+
 	fn get_data_from_file(&self, pos: u64) -> Option<Self::Item> {
 		if pos > self.last_pos {
 			None

core/src/core/pmmr/vec_backend.rs

@@ -64,6 +64,10 @@ impl<T: PMMRable> Backend<T> for VecBackend<T> {
 		self.hashes.get(idx).cloned()
 	}
 
+	fn get_peak_from_file(&self, position: u64) -> Option<Hash> {
+		self.get_from_file(position)
+	}
+
 	fn get_data_from_file(&self, position: u64) -> Option<T::E> {
 		if let Some(data) = &self.data {
 			let idx = usize::try_from(pmmr::n_leaves(position).saturating_sub(1))

store/src/pmmr.rs

@@ -94,6 +94,11 @@ impl<T: PMMRable> Backend<T> for PMMRBackend<T> {
 		self.hash_file.read(position - shift)
 	}
 
+	fn get_peak_from_file(&self, position: u64) -> Option<Hash> {
+		let shift = self.prune_list.get_shift(position);
+		self.hash_file.read(position - shift)
+	}
+
 	fn get_data_from_file(&self, position: u64) -> Option<T::E> {
 		if !pmmr::is_leaf(position) {
 			return None;
@@ -284,11 +289,15 @@ impl<T: PMMRable> PMMRBackend<T> {
 		self.prune_list.is_pruned_root(pos)
 	}
 
+	// Check if pos is pruned but not a pruned root itself.
+	// Checking for pruned root is faster so we do this check first.
+	// We can do a fast initial check as well -
+	// if its in the current leaf_set then we know it is not compacted.
 	fn is_compacted(&self, pos: u64) -> bool {
 		if self.leaf_set.includes(pos) {
 			return false;
 		}
-		self.is_pruned(pos) && !self.is_pruned_root(pos)
+		!self.is_pruned_root(pos) && self.is_pruned(pos)
 	}
 
 	/// Number of hashes in the PMMR stored by this backend. Only produces the