diff --git a/include/dwarfs/inode.h b/include/dwarfs/inode.h
index 45266929..0e9edd74 100644
--- a/include/dwarfs/inode.h
+++ b/include/dwarfs/inode.h
@@ -27,6 +27,7 @@
 
 #include <folly/small_vector.h>
 
+#include "dwarfs/nilsimsa.h"
 #include "dwarfs/object.h"
 
 namespace dwarfs {
@@ -50,7 +51,7 @@ class inode : public object {
   virtual void set_num(uint32_t num) = 0;
   virtual uint32_t num() const = 0;
   virtual uint32_t similarity_hash() const = 0;
-  virtual std::vector<uint64_t> const& nilsimsa_similarity_hash() const = 0;
+  virtual nilsimsa::hash_type const& nilsimsa_similarity_hash() const = 0;
   virtual size_t size() const = 0;
   virtual file const* any() const = 0;
   virtual files_vector const& files() const = 0;
diff --git a/include/dwarfs/nilsimsa.h b/include/dwarfs/nilsimsa.h
index 3059b900..13d63713 100644
--- a/include/dwarfs/nilsimsa.h
+++ b/include/dwarfs/nilsimsa.h
@@ -21,10 +21,10 @@
 
 #pragma once
 
+#include <array>
 #include <cstdint>
 #include <memory>
 #include <type_traits>
-#include <vector>
 
 #include "dwarfs/compiler.h"
 
@@ -45,11 +45,13 @@ namespace dwarfs {
 
 class nilsimsa {
  public:
+  using hash_type = std::array<uint64_t, 4>;
+
   nilsimsa();
   ~nilsimsa();
 
   void update(uint8_t const* data, size_t size);
-  std::vector<uint64_t> finalize() const;
+  void finalize(hash_type& hash) const;
 
 #ifdef DWARFS_MULTIVERSIONING
   __attribute__((target("popcnt"))) static int
diff --git a/src/dwarfs/inode_manager.cpp b/src/dwarfs/inode_manager.cpp
index 0eb3f946..ffe24ecd 100644
--- a/src/dwarfs/inode_manager.cpp
+++ b/src/dwarfs/inode_manager.cpp
@@ -93,6 +93,10 @@ class inode_ : public inode {
  public:
   using chunk_type = thrift::metadata::chunk;
 
+  inode_() {
+    std::fill(nilsimsa_similarity_hash_.begin(), nilsimsa_similarity_hash_.end(), 0);
+  }
+
   void set_num(uint32_t num) override {
     DWARFS_CHECK(!num_, "attempt to set inode number multiple times");
     num_ = num;
@@ -107,7 +111,7 @@ class inode_ : public inode {
     return similarity_hash_;
   }
 
-  std::vector<uint64_t> const& nilsimsa_similarity_hash() const override {
+  nilsimsa::hash_type const& nilsimsa_similarity_hash() const override {
     if (files_.empty()) {
       DWARFS_THROW(runtime_error, "inode has no file");
     }
@@ -156,7 +160,7 @@ class inode_ : public inode {
       }
 
       if (opts.with_nilsimsa) {
-        nilsimsa_similarity_hash_ = nc.finalize();
+        nc.finalize(nilsimsa_similarity_hash_);
       }
     }
   }
@@ -189,7 +193,7 @@ class inode_ : public inode {
   uint32_t similarity_hash_{0};
   files_vector files_;
   std::vector<chunk_type> chunks_;
-  std::vector<uint64_t> nilsimsa_similarity_hash_;
+  nilsimsa::hash_type nilsimsa_similarity_hash_;
 };
 
 } // namespace
diff --git a/src/dwarfs/nilsimsa.cpp b/src/dwarfs/nilsimsa.cpp
index cf88ea50..f19fa4be 100644
--- a/src/dwarfs/nilsimsa.cpp
+++ b/src/dwarfs/nilsimsa.cpp
@@ -19,8 +19,6 @@
  * along with dwarfs.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-#include <array>
-
 #include "dwarfs/compiler.h"
 #include "dwarfs/nilsimsa.h"
 
@@ -76,7 +74,7 @@ class nilsimsa::impl {
     update_fast(data, size);
   }
 
-  std::vector<uint64_t> finalize() const {
+  void finalize(hash_type& hash) const {
     size_t total = 0;
 
     if (size_ == 3) {
@@ -89,16 +87,13 @@ class nilsimsa::impl {
 
     size_t threshold = total / acc_.size();
 
-    std::vector<uint64_t> hash;
-    hash.resize(4);
+    std::fill(hash.begin(), hash.end(), 0);
 
     for (size_t i = 0; i < acc_.size(); i++) {
       if (acc_[i] > threshold) {
         hash[i >> 6] |= UINT64_C(1) << (i & 0x3F);
       }
     }
-
-    return hash;
   }
 
  private:
@@ -195,7 +190,7 @@ void nilsimsa::update(uint8_t const* data, size_t size) {
   impl_->update(data, size);
 }
 
-std::vector<uint64_t> nilsimsa::finalize() const { return impl_->finalize(); }
+void nilsimsa::finalize(hash_type& hash) const { impl_->finalize(hash); }
 
 #ifdef DWARFS_MULTIVERSIONING
 __attribute__((target("popcnt"))) int