Reimplement prefix_map::find_sup() to iterate over key, not entries

1 files changed, 41 insertions, 1 deletions

diff --git a/libbutl/prefix-map.txx b/libbutl/prefix-map.txx
index fb50570..fc94409 100644
--- a/libbutl/prefix-map.txx
+++ b/libbutl/prefix-map.txx
@@ -55,12 +55,15 @@ LIBBUTL_MODEXPORT namespace butl //@@ MOD Clang needs this for some reason.
     //
     // The drawback of the first approach is that we have to create a new key
     // which will most likely involve a memory allocation (we can probably
-    // limit it to a single allocation by reusing the key).
+    // limit it to a single allocation by reusing the key instance).
     //
     // The drawback of the second approch is that we may have a lot of entries
     // between the lower bound and the prefix (in contrast, keys normally only
     // have a handful of components).
     //
+    // Tests have shown that (2) can be significantly faster than (1).
+    //
+#if 0
     const auto& c (this->key_comp ());
 
     for (auto i (this->upper_bound (k)), b (this->begin ()); i != b; )
@@ -71,12 +74,32 @@ LIBBUTL_MODEXPORT namespace butl //@@ MOD Clang needs this for some reason.
     }
 
     return this->end ();
+#else
+    // First look for the exact match before making any copies.
+    //
+    auto i (this->find (k)), e (this->end ());
+
+    if (i == e)
+    {
+      const auto& c (this->key_comp ());
+
+      for (key_type p (k); c.prefix (p); )
+      {
+        i = this->find (p);
+        if (i != e)
+          break;
+      }
+    }
+
+    return i;
+#endif
   }
 
   template <typename M>
   auto prefix_map_common<M>::
   find_sup (const key_type& k) const -> const_iterator
   {
+#if 0
     const auto& c (this->key_comp ());
 
     for (auto i (this->upper_bound (k)), b (this->begin ()); i != b; )
@@ -87,5 +110,22 @@ LIBBUTL_MODEXPORT namespace butl //@@ MOD Clang needs this for some reason.
     }
 
     return this->end ();
+#else
+    auto i (this->find (k)), e (this->end ());
+
+    if (i == e)
+    {
+      const auto& c (this->key_comp ());
+
+      for (key_type p (k); c.prefix (p); )
+      {
+        i = this->find (p);
+        if (i != e)
+          break;
+      }
+    }
+
+    return i;
+#endif
   }
 }