Improve mappings merge

Buffer collection on stack that acts like usage map of indexes also serves as orderer.
More memory efficient solution and faster approach for at least portion of queries

Author: alex-kulakov

Orm/Xtensive.Orm/Orm/Rse/Transformation/ColumnMappingInspector.cs

@@ -436,6 +436,10 @@ protected override void OnRecursionEntrance(Provider provider)
 
     private static List<int> Merge(IEnumerable<int> left, IEnumerable<int> right)
     {
+      if (TryMergeFast(left, right, out var result))
+        return result;
+
+      // slow
       var hs = new HashSet<int>(left);
       foreach (var r in right) {
         _ = hs.Add(r);
@@ -445,18 +449,50 @@ private static List<int> Merge(IEnumerable<int> left, IEnumerable<int> right)
       return resultList;
     }
 
-    private static List<int> Merge(List<int> leftMap, IEnumerable<int> rightMap)
+    private static bool TryMergeFast(IEnumerable<int> leftMap, IEnumerable<int> rightMap, out List<int> result)
     {
-      var preReturn = leftMap.Union(rightMap).ToList(leftMap.Count * 2);
-      preReturn.Sort();
-      return preReturn;
-    }
+      Span<bool> usageMap = stackalloc bool[512];
+      usageMap.Fill(false);
+      var uniqueCount = 0;
+      var biggestIndex = 0;
 
-    private static List<int> Merge(List<int> leftMap, IList<int> rightMap)
-    {
-      var preReturn = leftMap.Union(rightMap).ToList(leftMap.Count + rightMap.Count);
-      preReturn.Sort();
-      return preReturn;
+      // leftMap.Concat(rightMap) is slower!
+      foreach (var idx in leftMap) {
+        if (idx >= usageMap.Length) {
+          result = null;
+          return false;
+        }
+        if (!usageMap[idx]) {
+          uniqueCount++;
+          usageMap[idx] = true;
+        }
+        if (biggestIndex < idx)
+          biggestIndex = idx;
+      }
+      foreach (var idx in rightMap) {
+        if (idx >= usageMap.Length) {
+          result = null;
+          return false;
+        }
+        if (!usageMap[idx]) {
+          uniqueCount++;
+          usageMap[idx] = true;
+        }
+        if (biggestIndex < idx)
+          biggestIndex = idx;
+      }
+      var resultList = new List<int>(uniqueCount);
+      for (int i = 0; i < biggestIndex + 1; i++) {
+        if (usageMap[i])
+          resultList.Add(i);
+      }
+      if (resultList.Count != uniqueCount) {
+        //
+        result = null;
+        return false;
+      }
+      result = resultList;
+      return true;
     }
 
     private static List<int> MergeMappings(Provider originalLeft, List<int> leftMap, List<int> rightMap)