compute tree (#298)

* compute merkle tree

cpp/src/barretenberg/stdlib/merkle_tree/hash.hpp

@@ -58,6 +58,30 @@ inline barretenberg::fr compute_tree_root_native(std::vector<barretenberg::fr> c
     return layer[0];
 }
 
+// TODO write test
+inline std::vector<barretenberg::fr> compute_tree_native(std::vector<barretenberg::fr> const& input)
+{
+    // Check if the input vector size is a power of 2.
+    ASSERT(input.size() > 0);
+    ASSERT(numeric::is_power_of_two(input.size()));
+    auto layer = input;
+    std::vector<barretenberg::fr> tree(input);
+    while (layer.size() > 1) {
+        std::vector<barretenberg::fr> next_layer(layer.size() / 2);
+        for (size_t i = 0; i < next_layer.size(); ++i) {
+            if (plonk::SYSTEM_COMPOSER == plonk::PLOOKUP) {
+                next_layer[i] = crypto::pedersen_hash::lookup::hash_multiple({ layer[i * 2], layer[i * 2 + 1] });
+            } else {
+                next_layer[i] = crypto::pedersen_hash::hash_multiple({ layer[i * 2], layer[i * 2 + 1] });
+            }
+            tree.push_back(next_layer[i]);
+        }
+        layer = std::move(next_layer);
+    }
+
+    return tree;
+}
+
 } // namespace merkle_tree
 } // namespace stdlib
 } // namespace proof_system::plonk