Fixed used size calculation logic

References: #32

Author: ilyalukyanov

src/LightningDB.Tests/EnvironmentTests.cs

@@ -152,6 +152,8 @@ public void CanGetUsedSize()
             _env = new LightningEnvironment(_path, EnvironmentOpenFlags.None);
             _env.Open();
 
+            var initialUsedSize = _env.UsedSize;
+
             using (var txn = _env.BeginTransaction())
             using (var db = txn.OpenDatabase(null, DatabaseOpenFlags.None))
             {
@@ -162,10 +164,10 @@ public void CanGetUsedSize()
             }
 
             //act
-            var size = _env.UsedSize;
+            var sizeDelta = _env.UsedSize - initialUsedSize;
 
             //act-assert;
-            Assert.AreEqual(_env.PageSize, size);
+            Assert.AreEqual(_env.PageSize, sizeDelta);
         }
 
     }

src/LightningDB/LightningEnvironment.cs

@@ -117,13 +117,10 @@ public long UsedSize
         {
             get
             {
-                var stat = GetStat();
-                var totalPages = 
-                    stat.ms_branch_pages.ToInt64() + 
-                    stat.ms_leaf_pages.ToInt64() + 
-                    stat.ms_overflow_pages.ToInt64();
+                var envInfo = new MDBEnvInfo();
+                NativeMethods.Execute(lib => lib.mdb_env_info(_handle, out envInfo));
 
-                return stat.ms_psize * totalPages;
+                return envInfo.me_last_pgno.ToInt64() * PageSize;
             }
         }