dsmiley commented on code in PR #13570:
URL: https://github.com/apache/lucene/pull/13570#discussion_r1690256783


##########
lucene/core/src/java/org/apache/lucene/store/MMapDirectory.java:
##########
@@ -83,6 +94,41 @@ public class MMapDirectory extends FSDirectory {
    */
   public static final BiPredicate<String, IOContext> NO_FILES = (filename, 
context) -> false;
 
+  /**
+   * This sysprop allows to control the total maximum number of mmapped files 
that can be associated
+   * with a single shared {@link java.lang.foreign.Arena foreign Arena}. For 
example, to set the max
+   * number of permits to 256, pass the following on the command line pass 
{@code
+   * -Dorg.apache.lucene.store.MMapDirectory.sharedArenaMaxPermits=256}. 
Setting a value of 1
+   * associates one file to one shared arena.
+   *
+   * @lucene.internal
+   */
+  public static final String SHARED_ARENA_MAX_PERMITS_SYSPROP =
+      "org.apache.lucene.store.MMapDirectory.sharedArenaMaxPermits";
+
+  /** Argument for {@link #setGroupingFunction(Function)} that configures no 
grouping. */
+  public static final Function<String, Optional<String>> NO_GROUPING = 
filename -> Optional.empty();
+
+  /** Argument for {@link #setGroupingFunction(Function)} that configures 
grouping by segment. */
+  public static final Function<String, Optional<String>> GROUP_BY_SEGMENT =
+      filename -> {
+        if (!CODEC_FILE_PATTERN.matcher(filename).matches()) {
+          return Optional.empty();
+        }
+        String groupKey = 
IndexFileNames.parseSegmentName(filename).substring(1);
+        try {
+          // keep the original generation (=0) in base group, later 
generations in extra group
+          if (IndexFileNames.parseGeneration(filename) > 0) {
+            groupKey += "-g";
+          }
+        } catch (
+            @SuppressWarnings("unused")
+            NumberFormatException unused) {
+          // does not confirm to the generation syntax, or trash

Review Comment:
   I would use an assert here



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