Let digesters support Reset() when this is more efficient than assi…

…gning a

newly constructed instance.

PiperOrigin-RevId: 647650005

riegeli/digests/highwayhash/highwayhash_digester.h

@@ -43,6 +43,14 @@ class HighwayHashDigester {
  public:
   explicit HighwayHashDigester(const HighwayHashKey& key) : cat_(key) {}
 
+  HighwayHashDigester(const HighwayHashDigester& that) = default;
+  HighwayHashDigester& operator=(const HighwayHashDigester& that) = default;
+
+  void Reset(const HighwayHashKey& key) {
+    cat_.Reset(key);
+    is_open_ = true;
+  }
+
   void Write(absl::string_view chunk) {
     cat_.Append(chunk.data(), chunk.size());
   }

riegeli/digests/openssl_digester.h

@@ -35,6 +35,11 @@ class OpenSslDigester {
   OpenSslDigester(const OpenSslDigester& that) = default;
   OpenSslDigester& operator=(const OpenSslDigester& that) = default;
 
+  void Reset() {
+    init(&ctx_);
+    is_open_ = true;
+  }
+
   void Write(absl::string_view src) { update(&ctx_, src.data(), src.size()); }
 
   void Close() {

riegeli/digests/wrapping_digester.h

@@ -83,7 +83,7 @@ class WrappingDigester {
               absl::negation<std::is_same<std::tuple<std::decay_t<Args>...>,
                                           std::tuple<WrappingDigester>>>,
               std::is_constructible<BaseDigester, Args&&...>>::value,
-          int> = true>
+          int> = 0>
   explicit WrappingDigester(Args&&... args)
       : base_(riegeli::Maker(std::forward<Args>(args)...)) {}
 
@@ -93,6 +93,13 @@ class WrappingDigester {
   WrappingDigester(WrappingDigester&& that) = default;
   WrappingDigester& operator=(WrappingDigester&& that) = default;
 
+  template <typename... Args,
+            std::enable_if_t<
+                std::is_constructible<BaseDigester, Args&&...>::value, int> = 0>
+  void Reset(Args&&... args) {
+    base_.Reset(riegeli::Maker(std::forward<Args>(args)...));
+  }
+
   bool Write(absl::string_view src) { return base_.get().Write(src); }
   bool Write(const Chain& src) { return base_.get().Write(src); }
   bool Write(const absl::Cord& src) { return base_.get().Write(src); }