From 940cc04670924511777d9d3ef627a5ce3f528792 Mon Sep 17 00:00:00 2001
From: Andrey Khmuro <andrey@polygon.technology>
Date: Fri, 18 Oct 2024 00:27:50 +0300
Subject: [PATCH] feat: add `Smt::is_empty` (#337)

---
 CHANGELOG.md                   |  1 +
 src/merkle/smt/full/mod.rs     |  7 +++++++
 src/merkle/smt/full/tests.rs   | 10 ++++++++++
 src/merkle/smt/mod.rs          |  3 +++
 src/merkle/smt/simple/mod.rs   |  7 +++++++
 src/merkle/smt/simple/tests.rs | 17 +++++++++++++++++
 6 files changed, 45 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index bd17037..acc6a6b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 - Added `Smt::compute_mutations()` and `Smt::apply_mutations()` for validation-checked insertions (#327).
 - [BREAKING] Changed return value of the `Mmr::verify()` and `MerklePath::verify()` from `bool` to
   `Result<>` (#).
+- Added `is_empty()` functions to the `SimpleSmt` and `Smt` structures. Added `EMPTY_ROOT` constant to the `SparseMerkleTree` trait (#337).
 
 ## 0.10.1 (2024-09-13)
 
diff --git a/src/merkle/smt/full/mod.rs b/src/merkle/smt/full/mod.rs
index 9c64002..6fe8534 100644
--- a/src/merkle/smt/full/mod.rs
+++ b/src/merkle/smt/full/mod.rs
@@ -130,6 +130,12 @@ impl Smt {
         <Self as SparseMerkleTree<SMT_DEPTH>>::open(self, key)
     }
 
+    /// Returns a boolean value indicating whether the SMT is empty.
+    pub fn is_empty(&self) -> bool {
+        debug_assert_eq!(self.leaves.is_empty(), self.root == Self::EMPTY_ROOT);
+        self.root == Self::EMPTY_ROOT
+    }
+
     // ITERATORS
     // --------------------------------------------------------------------------------------------
 
@@ -252,6 +258,7 @@ impl SparseMerkleTree<SMT_DEPTH> for Smt {
     type Opening = SmtProof;
 
     const EMPTY_VALUE: Self::Value = EMPTY_WORD;
+    const EMPTY_ROOT: RpoDigest = *EmptySubtreeRoots::entry(SMT_DEPTH, 0);
 
     fn root(&self) -> RpoDigest {
         self.root
diff --git a/src/merkle/smt/full/tests.rs b/src/merkle/smt/full/tests.rs
index 1613c8f..03eb382 100644
--- a/src/merkle/smt/full/tests.rs
+++ b/src/merkle/smt/full/tests.rs
@@ -516,6 +516,16 @@ fn test_smt_entries() {
     assert!(entries.next().is_none());
 }
 
+/// Tests that `EMPTY_ROOT` constant generated in the `Smt` equals to the root of the empty tree of
+/// depth 64
+#[test]
+fn test_smt_check_empty_root_constant() {
+    // get the root of the empty tree of depth 64
+    let empty_root_64_depth = EmptySubtreeRoots::empty_hashes(64)[0];
+
+    assert_eq!(empty_root_64_depth, Smt::EMPTY_ROOT);
+}
+
 // SMT LEAF
 // --------------------------------------------------------------------------------------------
 
diff --git a/src/merkle/smt/mod.rs b/src/merkle/smt/mod.rs
index 0b7ceb9..056c221 100644
--- a/src/merkle/smt/mod.rs
+++ b/src/merkle/smt/mod.rs
@@ -56,6 +56,9 @@ pub(crate) trait SparseMerkleTree<const DEPTH: u8> {
     /// The default value used to compute the hash of empty leaves
     const EMPTY_VALUE: Self::Value;
 
+    /// The root of the empty tree with provided DEPTH
+    const EMPTY_ROOT: RpoDigest;
+
     // PROVIDED METHODS
     // ---------------------------------------------------------------------------------------------
 
diff --git a/src/merkle/smt/simple/mod.rs b/src/merkle/smt/simple/mod.rs
index 1744430..819b6f7 100644
--- a/src/merkle/smt/simple/mod.rs
+++ b/src/merkle/smt/simple/mod.rs
@@ -158,6 +158,12 @@ impl<const DEPTH: u8> SimpleSmt<DEPTH> {
         <Self as SparseMerkleTree<DEPTH>>::open(self, key)
     }
 
+    /// Returns a boolean value indicating whether the SMT is empty.
+    pub fn is_empty(&self) -> bool {
+        debug_assert_eq!(self.leaves.is_empty(), self.root == Self::EMPTY_ROOT);
+        self.root == Self::EMPTY_ROOT
+    }
+
     // ITERATORS
     // --------------------------------------------------------------------------------------------
 
@@ -298,6 +304,7 @@ impl<const DEPTH: u8> SparseMerkleTree<DEPTH> for SimpleSmt<DEPTH> {
     type Opening = ValuePath;
 
     const EMPTY_VALUE: Self::Value = EMPTY_WORD;
+    const EMPTY_ROOT: RpoDigest = *EmptySubtreeRoots::entry(DEPTH, 0);
 
     fn root(&self) -> RpoDigest {
         self.root
diff --git a/src/merkle/smt/simple/tests.rs b/src/merkle/smt/simple/tests.rs
index 2f59ae6..b1dd28d 100644
--- a/src/merkle/smt/simple/tests.rs
+++ b/src/merkle/smt/simple/tests.rs
@@ -444,6 +444,23 @@ fn test_simplesmt_set_subtree_entire_tree() {
     assert_eq!(tree.root(), *EmptySubtreeRoots::entry(DEPTH, 0));
 }
 
+/// Tests that `EMPTY_ROOT` constant generated in the `SimpleSmt` equals to the root of the empty
+/// tree of depth 64
+#[test]
+fn test_simplesmt_check_empty_root_constant() {
+    // get the root of the empty tree of depth 64
+    let empty_root_64_depth = EmptySubtreeRoots::empty_hashes(64)[0];
+    assert_eq!(empty_root_64_depth, SimpleSmt::<64>::EMPTY_ROOT);
+
+    // get the root of the empty tree of depth 32
+    let empty_root_32_depth = EmptySubtreeRoots::empty_hashes(32)[0];
+    assert_eq!(empty_root_32_depth, SimpleSmt::<32>::EMPTY_ROOT);
+
+    // get the root of the empty tree of depth 0
+    let empty_root_1_depth = EmptySubtreeRoots::empty_hashes(1)[0];
+    assert_eq!(empty_root_1_depth, SimpleSmt::<1>::EMPTY_ROOT);
+}
+
 // HELPER FUNCTIONS
 // --------------------------------------------------------------------------------------------