diff --git a/example/web/build-tdlib.sh b/example/web/build-tdlib.sh
index 548c77e3..1dbc85bb 100755
--- a/example/web/build-tdlib.sh
+++ b/example/web/build-tdlib.sh
@@ -37,8 +37,8 @@ eval emconfigure cmake -DCMAKE_BUILD_TYPE=Release $OPENSSL_OPTIONS -DASMJS=1 $TD
 cd ../..
 
 echo "Generating TDLib autogenerated source files..."
-cmake --build build/generate --target prepare_cross_compiling || exit 1
+cmake --build build/generate -j --target prepare_cross_compiling || exit 1
 echo "Building TDLib to WebAssembly..."
-cmake --build build/wasm --target td_wasm || exit 1
+cmake --build build/wasm -j --target td_wasm || exit 1
 echo "Building TDLib to asm.js..."
-cmake --build build/asmjs --target td_asmjs || exit 1
+cmake --build build/asmjs -j --target td_asmjs || exit 1
diff --git a/td/telegram/files/FileBitmask.cpp b/td/telegram/files/FileBitmask.cpp
index 1ff6cb28..4e413d83 100644
--- a/td/telegram/files/FileBitmask.cpp
+++ b/td/telegram/files/FileBitmask.cpp
@@ -20,12 +20,27 @@ Bitmask::Bitmask(Ones, int64 count) : data_(narrow_cast<size_t>((count + 7) / 8)
   }
 }
 
-std::string Bitmask::encode(int32 prefix_count) const {
+std::string Bitmask::encode(int32 prefix_count) {
   // remove zeroes in the end to make encoding deterministic
   td::Slice data(data_);
+
+  int save_i = -1;
+  char save_c;
   if (prefix_count != -1) {
-    data.truncate(prefix_count);
+    auto truncated_size = (prefix_count + 7) / 8;
+    data.truncate(truncated_size);
+    if (prefix_count % 8 != 0) {
+      save_i = truncated_size - 1;
+      save_c = data_[save_i];
+      uint8 mask = 0xff >> (8 - prefix_count % 8);
+      data_[save_i] &= mask;
+    }
   }
+  SCOPE_EXIT {
+    if (save_i != -1) {
+      data_[save_i] = save_c;
+    }
+  };
   while (!data.empty() && data.back() == '\0') {
     data.remove_suffix(1);
   }
diff --git a/td/telegram/files/FileBitmask.h b/td/telegram/files/FileBitmask.h
index ef17c0a4..390eeacd 100644
--- a/td/telegram/files/FileBitmask.h
+++ b/td/telegram/files/FileBitmask.h
@@ -19,7 +19,7 @@ class Bitmask {
   Bitmask() = default;
   Bitmask(Decode, Slice data);
   Bitmask(Ones, int64 count);
-  std::string encode(int32 prefix_count = -1) const;
+  std::string encode(int32 prefix_count = -1);
   int64 get_ready_prefix_size(int64 offset, int64 part_size, int64 file_size) const;
   int64 get_total_size(int64 part_size, int64 file_size) const;
   bool get(int64 offset_part) const;
diff --git a/td/telegram/files/PartsManager.cpp b/td/telegram/files/PartsManager.cpp
index 5799c8fe..aaed84ea 100644
--- a/td/telegram/files/PartsManager.cpp
+++ b/td/telegram/files/PartsManager.cpp
@@ -169,7 +169,7 @@ int32 PartsManager::get_ready_prefix_count() {
   }
   return res;
 }
-string PartsManager::get_bitmask() const {
+string PartsManager::get_bitmask() {
   int32 prefix_count = -1;
   if (need_check_) {
     prefix_count = narrow_cast<int32>(checked_prefix_size_ / part_size_);
diff --git a/td/telegram/files/PartsManager.h b/td/telegram/files/PartsManager.h
index e0e56fab..c86b0afe 100644
--- a/td/telegram/files/PartsManager.h
+++ b/td/telegram/files/PartsManager.h
@@ -47,7 +47,7 @@ class PartsManager {
   int32 get_part_count() const;
   int32 get_unchecked_ready_prefix_count();
   int32 get_ready_prefix_count();
-  string get_bitmask() const;
+  string get_bitmask();
 
  private:
   static constexpr int MAX_PART_COUNT = 3000;