Change entry representation to reduce number of entries

VersionedMap.cpp

@@ -50,16 +50,21 @@ namespace weaselab {
 
 struct Entry {
   int64_t insertVersion;
-  int param1Len;
-  int param2Len;
+  int keyLen;
+  // Negative if this key is cleared
+  int valLen;
   mutable int refCount;
   uint32_t priority;
-  VersionedMap::MutationType mutationType;
+  // True if mutations in (pred, this) are cleared. If false, (pred, this)
+  // should be read through to the underlying data structure.
+  bool clearTo;
 
-  const uint8_t *getParam1() const { return (const uint8_t *)(this + 1); }
+  // There's an extra zero byte past the end of getKey, used for
+  // reconstructing logical mutations without copies.
+  const uint8_t *getKey() const { return (const uint8_t *)(this + 1); }
 
-  const uint8_t *getParam2() const {
-    return (const uint8_t *)(this + 1) + param1Len;
+  const uint8_t *getVal() const {
+    return (const uint8_t *)(this + 1) + 1 + keyLen;
   }
 
   void addref() const { ++refCount; }
@@ -70,18 +75,18 @@ struct Entry {
     }
   }
 
-  static Entry *make(int64_t insertVersion, const uint8_t *param1,
-                     int param1Len, const uint8_t *param2, int param2Len,
-                     VersionedMap::MutationType mutationType) {
-    auto e = (Entry *)malloc(sizeof(Entry) + param1Len + param2Len);
+  static Entry *make(int64_t insertVersion, const uint8_t *key, int keyLen,
+                     const uint8_t *val, int valLen, bool clearTo) {
+    auto e = (Entry *)malloc(sizeof(Entry) + keyLen + 1 + std::max(valLen, 0));
     e->insertVersion = insertVersion;
-    e->param1Len = param1Len;
-    e->param2Len = param2Len;
+    e->keyLen = keyLen;
+    e->valLen = valLen;
     e->refCount = 1;
     e->priority = rand(); // TODO
-    e->mutationType = mutationType;
-    memcpy((uint8_t *)e->getParam1(), param1, param1Len);
-    memcpy((uint8_t *)e->getParam2(), param2, param2Len);
+    e->clearTo = clearTo;
+    memcpy((uint8_t *)e->getKey(), key, keyLen);
+    ((uint8_t *)e->getKey())[keyLen] = 0;
+    memcpy((uint8_t *)e->getVal(), val, std::max(valLen, 0));
     return e;
   }
 };

include/VersionedMap.h

@@ -141,7 +141,7 @@ struct VersionedMap {
 
   /** Map starts with no mutations, with `getOldestVersion()` == `getVersion()`
    * == `version`. */
-  VersionedMap(int64_t version);
+  explicit VersionedMap(int64_t version);
 
   ~VersionedMap();