sketch shrinking

Author: Gillgamesh

src/sketch_columns.cpp

@@ -391,6 +391,15 @@ void ResizeableSketchColumn::merge(ResizeableSketchColumn const& other) {
   for (size_t i = 0; i < other.capacity; ++i) {
     buckets[i] ^= other.buckets[i];
   }
+
+  // Shrink after merge when the resulting column is shallower.
+  // Keep only a small safety margin above observed depth.
+  constexpr size_t merge_slack = 1;
+  size_t depth = get_depth();
+  size_t target_capacity = std::max<size_t>(1, depth + merge_slack);
+  if (target_capacity < capacity) {
+    reallocate(static_cast<uint8_t>(target_capacity));
+  }
 }
 
 uint8_t ResizeableSketchColumn::get_depth() const {

test/sketch_test.cpp

@@ -575,7 +575,10 @@ TEST(SketchColumnTestSuite, TestMergeResizing) {
   }
   ASSERT_EQ(column1.capacity, 10);
   column1.merge(column2);
-  ASSERT_EQ(column1.capacity, column2.capacity);
+  ASSERT_LE(column1.capacity, column2.capacity);
+  ASSERT_GE(column1.capacity, 1);
+  auto sample = column1.sample();
+  ASSERT_EQ(sample.result, GOOD);
 }
 
 // as we insert more unique items, maximum nonzero bucket depth should never decrease  
@@ -606,7 +609,7 @@ TEST(SketchColumnTestSuite, TestClear) {
   auto sample = column.sample();
   ASSERT_EQ(sample.result, GOOD);
   column.clear();
-  for (size_t i = 0; i < capacity; i++) {
+  for (size_t i = 0; i < column.capacity; i++) {
     bool good = Bucket_Boruvka::is_empty(column.buckets[i]);
     ASSERT_EQ(good, true);
   }
@@ -624,7 +627,7 @@ TEST(SketchColumnTestSuite, TestClearMerge) {
   }
   column1.merge(column2);
   column2.clear();
-  for (size_t i = 0; i < capacity; i++) {
+  for (size_t i = 0; i < column2.capacity; i++) {
     bool good = Bucket_Boruvka::is_empty(column2.buckets[i]);
     ASSERT_EQ(good, true);
   }
@@ -640,7 +643,7 @@ TEST(SketchColumnTestSuite, TestClearMerge) {
   }
   column3.merge(column4);
   column3.clear();
-  for (size_t i = 0; i < capacity; i++) {
+  for (size_t i = 0; i < column3.capacity; i++) {
     bool empty = Bucket_Boruvka::is_empty(column3.buckets[i]);
     ASSERT_EQ(empty, true);
   }