From e422e5824f763592f8d0be83088c0d1204413275 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eduardo=20Leegwater=20Sim=C3=B5es?= <eduardo@dusk.network>
Date: Mon, 9 Sep 2024 14:45:50 +0200
Subject: [PATCH] dusk-merkle: add bounds check at `Tree::insert`

A panic on an out of bounds insertion is expected in the merkle tree,
however, the panic message is a bit cryptic. This commit adds a bounds
check (`index < capacity`) and improves the error message such that it
is clear to whomever calls it.

Resolves: #91
---
 dusk-merkle/CHANGELOG.md |  5 +++++
 dusk-merkle/src/tree.rs  | 20 +++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/dusk-merkle/CHANGELOG.md b/dusk-merkle/CHANGELOG.md
index b661377..823008b 100644
--- a/dusk-merkle/CHANGELOG.md
+++ b/dusk-merkle/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## Added
+
+- Add bounds check on `Tree::insert`, improving panic message [#91]
+
 ## [0.5.2] - 2023-10-27
 
 ### Fixed
@@ -102,6 +106,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix `CheckBytes` derivation in `Node` [#15]
 
 <!-- ISSUES -->
+[#91]: https://github.com/dusk-network/merkle/issues/91
 [#73]: https://github.com/dusk-network/merkle/issues/73
 [#62]: https://github.com/dusk-network/merkle/issues/62
 [#58]: https://github.com/dusk-network/merkle/issues/58
diff --git a/dusk-merkle/src/tree.rs b/dusk-merkle/src/tree.rs
index 662cc86..eeaf7d9 100644
--- a/dusk-merkle/src/tree.rs
+++ b/dusk-merkle/src/tree.rs
@@ -37,10 +37,18 @@ where
     /// Insert an `item` at the given `position` in the tree.
     ///
     /// # Panics
-    /// If `position >= capacity`.
-    pub fn insert(&mut self, position: u64, item: impl Into<T>) {
-        self.root.insert(0, position, item);
-        self.positions.insert(position);
+    /// If `index >= capacity`.
+    pub fn insert(&mut self, index: u64, item: impl Into<T>) {
+        let capacity = self.capacity();
+
+        assert!(
+            index < capacity,
+            "index out of bounds: \
+             the capacity is {capacity} but the index is {index}"
+        );
+
+        self.root.insert(0, index, item);
+        self.positions.insert(index);
     }
 
     /// Remove and return the item at the given `position` in the tree if it
@@ -200,7 +208,9 @@ mod tests {
     }
 
     #[test]
-    #[should_panic]
+    #[should_panic(
+        expected = "index out of bounds: the capacity is 8 but the index is 8"
+    )]
     fn tree_insertion_out_of_bounds() {
         let mut tree = SumTree::new();
         tree.insert(tree.capacity(), 42);