Fix long-jump table growth after compaction

utils/src/main/java/datadog/instrument/utils/ClassNameTrie.java

@@ -899,8 +899,8 @@ private char newJump(int jump) {
       if (longJumps == null) {
         longJumps = new int[16]; // create table on first use
       } else if (longJumpCount == longJumps.length) {
-        // expand the table by 50% to fit additional entries
-        longJumps = Arrays.copyOf(longJumps, longJumpCount + (longJumpCount >> 1));
+        // expand the table by 50%, ensuring progress even after compaction to one entry
+        longJumps = Arrays.copyOf(longJumps, longJumpCount + Math.max(1, longJumpCount >> 1));
       }
       longJumps[longJumpCount] = jump;
       return (char) (longJumpCount++ | LONG_JUMP_MARKER);

utils/src/test/java/datadog/instrument/utils/ClassNameTrieTest.java

@@ -42,6 +42,7 @@
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.Arguments;
 import org.junit.jupiter.params.provider.MethodSource;
+import org.junit.jupiter.params.provider.ValueSource;
 
 @SuppressFBWarnings("NP_NONNULL_PARAM_VIOLATION")
 class ClassNameTrieTest {
@@ -262,6 +263,30 @@ void tooManyLongJumps() {
         IllegalArgumentException.class, () -> builder.put("" + ((char) 67) + ((char) 2), 1));
   }
 
+  @ParameterizedTest
+  @ValueSource(booleans = {false, true})
+  void growsLongJumpTableAfterCompaction(boolean newBuilder) {
+    ClassNameTrie.Builder builder = new ClassNameTrie.Builder();
+    String padding = padding(7700);
+    for (char branch = 'a'; branch <= 'h'; branch++) {
+      builder.put("aa" + branch + padding, 1);
+    }
+    builder.put("ab", 2); // first long jump skips the padded sub-trie
+    ClassNameTrie compacted = builder.buildTrie(); // compacts the long-jump table to one entry
+    if (newBuilder) {
+      builder = new ClassNameTrie.Builder(compacted);
+    }
+
+    builder.put("ac", 3); // needs a second long jump
+
+    ClassNameTrie trie = builder.buildTrie();
+    for (char branch = 'a'; branch <= 'h'; branch++) {
+      assertEquals(1, trie.apply("aa" + branch + padding));
+    }
+    assertEquals(2, trie.apply("ab"));
+    assertEquals(3, trie.apply("ac"));
+  }
+
   @Test
   void failedInsertionPreservesExistingMappings() {
     String padding = padding(7700);