Don't scan+apply every setOldestVersion call

Bench.cpp

@@ -3,11 +3,11 @@
 #include <nanobench.h>
 
 void monotonicallyIncreasing() {
-  constexpr int kWindow = 100;
+  constexpr int kWindow = 1000;
   ankerl::nanobench::Bench bench;
   Facade facade{0};
 
-  bench.minEpochIterations(1000);
+  bench.minEpochIterations(kWindow * 10);
 
   bench.warmup(kWindow).run("monotonically increasing", [&] {
     const int64_t remove = __builtin_bswap64(facade.getVersion() - kWindow);
@@ -29,18 +29,6 @@ void monotonicallyIncreasing() {
   const auto end = facade.versioned.end(v);
   auto iter = begin;
 
-  bench.run("scan", [&] {
-    for (iter = begin; iter != end; ++iter) {
-    }
-  });
-
-  bench.run("reverse scan", [&] {
-    iter = end;
-    do {
-      --iter;
-    } while (iter != begin);
-  });
-
   bench.run("deref", [&] { bench.doNotOptimizeAway(*iter); });
 
   iter = begin;

Facade.h

@@ -12,7 +12,8 @@ using String = std::basic_string<uint8_t>;
 struct Facade {
 
   explicit Facade(int64_t version)
-      : unversionedVersion(version), versioned{version} {}
+      : oldestVersion(version), nextPurgeVersion(version),
+        unversionedVersion(version), versioned(version) {}
 
   struct View {
 
@@ -131,6 +132,15 @@ struct Facade {
   }
 
   void setOldestVersion(int64_t version) {
+    // Don't scan and apply mutations every time setOldestVersion is called.
+
+    oldestVersion = version;
+    if (version >= nextPurgeVersion) {
+      nextPurgeVersion = versioned.getVersion();
+    } else {
+      return;
+    }
+
     for (auto iter = versioned.begin(version), end = versioned.end(version);
          iter != end; ++iter) {
       auto m = *iter;
@@ -162,10 +172,14 @@ struct Facade {
     versioned.setOldestVersion(version);
   }
 
-  View viewAt(int64_t version) const { return View{this, version}; }
+  View viewAt(int64_t version) const {
+    assert(version >= oldestVersion);
+    assert(version <= versioned.getVersion());
+    return View{this, version};
+  }
 
   int64_t getVersion() const { return versioned.getVersion(); }
-  int64_t getOldestVersion() const { return versioned.getOldestVersion(); }
+  int64_t getOldestVersion() const { return oldestVersion; }
 
   void unversionedRead(const String &begin, const String &end, int &limit,
                        bool reverse,
@@ -197,6 +211,8 @@ struct Facade {
     }
   }
 
+  int64_t oldestVersion;
+  int64_t nextPurgeVersion;
   int64_t unversionedVersion;
   std::map<String, String> unversioned;
   weaselab::VersionedMap versioned;