pallet-mmr: get hash for block that added node

frame/merkle-mountain-range/src/lib.rs

@@ -271,7 +271,7 @@ impl<T: Config<I>, I: 'static> Pallet<T, I> {
 	/// all the leaves to be present.
 	/// It may return an error or panic if used incorrectly.
 	pub fn generate_batch_proof(
-		leaf_indices: Vec<NodeIndex>,
+		leaf_indices: Vec<LeafIndex>,
 	) -> Result<
 		(Vec<LeafOf<T, I>>, primitives::BatchProof<<T as Config<I>>::Hash>),
 		primitives::Error,

frame/merkle-mountain-range/src/mmr/storage.rs

@@ -59,11 +59,18 @@ impl<StorageType, T, I, L> Default for Storage<StorageType, T, I, L> {
 }
 
 fn parent_hash_of_ancestor_that_added_node<T: frame_system::Config>(
-	_pos: NodeIndex,
+	pos: NodeIndex,
 ) -> <T as frame_system::Config>::Hash {
-	// TODO: implement
-	let block_num_from_pos: <T as frame_system::Config>::BlockNumber = 42_u32.into();
-	<frame_system::Pallet<T>>::block_hash(block_num_from_pos)
+	// TODO: turn this in a pallet storage item recording block-num when pallet was activated.
+	let block_offset = 0;
+
+	let leaf_idx = NodesUtils::leaf_index_that_added_node(pos);
+	let block_num: <T as frame_system::Config>::BlockNumber =
+		u32::try_from(leaf_idx + block_offset)
+			.expect("leaf-idx < block-num; qed")
+			.into();
+	// TODO: I think this only holds recent history, so old block hashes might not be here.
+	<frame_system::Pallet<T>>::block_hash(block_num)
 }
 
 impl<T, I, L> mmr_lib::MMRStore<NodeOf<T, I, L>> for Storage<OffchainStorage, T, I, L>

frame/merkle-mountain-range/src/mmr/utils.rs

@@ -53,12 +53,60 @@ impl NodesUtils {
 
 		64 - self.no_of_leaves.next_power_of_two().leading_zeros()
 	}
+
+	/// Calculate `LeafIndex` for the leaf that added `node_index` to the MMR.
+	pub fn leaf_index_that_added_node(node_index: NodeIndex) -> LeafIndex {
+		let rightmost_leaf_pos = Self::rightmost_leaf_node_index_from_pos(node_index);
+		Self::leaf_node_index_to_leaf_index(rightmost_leaf_pos)
+	}
+
+	// Translate a _leaf_ `NodeIndex` to its `LeafIndex`.
+	fn leaf_node_index_to_leaf_index(pos: NodeIndex) -> LeafIndex {
+		if pos == 0 {
+			return 0
+		}
+		let (leaf_count, _) =
+			mmr_lib::helper::get_peaks(pos)
+				.iter()
+				.fold((0, 0), |(mut acc, last_peak), peak| {
+					let leaves = (peak - last_peak) >> 1;
+					acc += leaves + 1;
+					// last_peak, leaves, acc);
+					(acc, peak.clone())
+				});
+		leaf_count
+	}
+
+	// Starting from any node position get position of rightmost leaf; this is the leaf
+	// responsible for the addition of node `pos`.
+	fn rightmost_leaf_node_index_from_pos(mut pos: NodeIndex) -> NodeIndex {
+		use mmr_lib::helper::pos_height_in_tree;
+		if pos > 0 {
+			let mut current_height = pos_height_in_tree(pos);
+			let mut right_child_height = pos_height_in_tree(pos - 1);
+			while right_child_height < current_height {
+				pos = pos - 1;
+				current_height = right_child_height;
+				right_child_height = pos_height_in_tree(pos - 1);
+			}
+		}
+		pos
+	}
 }
 
 #[cfg(test)]
 mod tests {
 	use super::*;
 
+	#[test]
+	fn test_leaf_node_index_to_leaf_index() {
+		use mmr_lib::helper::leaf_index_to_pos;
+		for index in 0..100000 {
+			let pos = leaf_index_to_pos(index);
+			assert_eq!(NodesUtils::leaf_node_index_to_leaf_index(pos), index);
+		}
+	}
+
 	#[test]
 	fn should_calculate_number_of_leaves_correctly() {
 		assert_eq!(