[GitHub] [kafka] ableegoldman commented on a change in pull request #10052: KAFKA-12289: Adding test cases for prefix scan in InMemoryKeyValueStore

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ableegoldman commented on a change in pull request #10052:
URL: https://github.com/apache/kafka/pull/10052#discussion_r577140223



##########
File path: streams/src/test/java/org/apache/kafka/streams/state/internals/InMemoryKeyValueStoreTest.java
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@@ -60,4 +67,22 @@ public void shouldRemoveKeysWithNullValues() {
 
         assertThat(store.get(0), nullValue());
     }
+
+
+    @Test
+    public void shouldReturnKeysWithGivenPrefix(){
+        store = createKeyValueStore(driver.context());
+        final String value = "value";
+        final List<KeyValue<Integer, String>> entries = new ArrayList<>();
+        entries.add(new KeyValue<>(1, value));
+        entries.add(new KeyValue<>(2, value));
+        entries.add(new KeyValue<>(11, value));
+        entries.add(new KeyValue<>(13, value));
+
+        store.putAll(entries);
+        final KeyValueIterator<Integer, String> keysWithPrefix = store.prefixScan(1, new IntegerSerializer());

Review comment:
       Yeah, the underlying store compares the serializer bytes lexicographically, it doesn't have any concept of "Integer" or any other type. And the really tricky thing is that it scans lexicographically, which means from left to right, whereas when we serialize things we usually do so from right to left. eg `2` in binary is `10` whereas 11 in binary is `1011` and 13 is `1101`. 
   The problem here is that the serialized version of 2 is a different number of bytes than the serialized form of 11/13, so the lexicographical comparator is effectively comparing digits of a different magnitude. 




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