Refactoring FilterPath.parse by using an iterative approach instead of recursion. (#14200)

* Refactor FilterPath parse function (#12067)
Signed-off-by: Robin Friedmann <robinfriedmann.rf@gmail.com>

* Implement unit tests for FilterPathTests (#12067)
Signed-off-by: Robin Friedmann <robinfriedmann.rf@gmail.com>

* Write warn log if Filter is empty; Add comments (#12067)
Signed-off-by: Robin Friedmann <robinfriedmann.rf@gmail.com>

* Add changelog

Signed-off-by: Siddhant Deshmukh <deshsid@amazon.com>

* Remove unnecessary log statement

Signed-off-by: Siddhant Deshmukh <deshsid@amazon.com>

* Remove unused logger

Signed-off-by: Siddhant Deshmukh <deshsid@amazon.com>

* Spotless apply

Signed-off-by: Siddhant Deshmukh <deshsid@amazon.com>

* Remove incorrect changelog

Signed-off-by: Siddhant Deshmukh <deshsid@amazon.com>

---------

Signed-off-by: Siddhant Deshmukh <deshsid@amazon.com>
Co-authored-by: Robin Friedmann <robinfriedmann.rf@gmail.com>

Author: deshsidd

CHANGELOG.md

@@ -54,6 +54,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix FuzzyQuery in keyword field will use IndexOrDocValuesQuery when both of index and doc_value are true ([#14378](https://github.com/opensearch-project/OpenSearch/pull/14378))
 - Fix file cache initialization ([#14004](https://github.com/opensearch-project/OpenSearch/pull/14004))
 - Handle NPE in GetResult if "found" field is missing ([#14552](https://github.com/opensearch-project/OpenSearch/pull/14552))
+- Refactoring FilterPath.parse by using an iterative approach ([#14200](https://github.com/opensearch-project/OpenSearch/pull/14200))
 
 ### Security
 

libs/core/src/main/java/org/opensearch/core/xcontent/filtering/FilterPath.java

@@ -46,7 +46,6 @@
 public class FilterPath {
 
     static final FilterPath EMPTY = new FilterPath();
-
     private final String filter;
     private final String segment;
     private final FilterPath next;
@@ -99,32 +98,29 @@ public static FilterPath[] compile(Set<String> filters) {
 
         List<FilterPath> paths = new ArrayList<>();
         for (String filter : filters) {
-            if (filter != null) {
+            if (filter != null && !filter.isEmpty()) {
                 filter = filter.trim();
                 if (filter.length() > 0) {
-                    paths.add(parse(filter, filter));
+                    paths.add(parse(filter));
                 }
             }
         }
         return paths.toArray(new FilterPath[0]);
     }
 
-    private static FilterPath parse(final String filter, final String segment) {
-        int end = segment.length();
-
-        for (int i = 0; i < end;) {
-            char c = segment.charAt(i);
+    private static FilterPath parse(final String filter) {
+        // Split the filter into segments using a regex
+        // that avoids splitting escaped dots.
+        String[] segments = filter.split("(?<!\\\\)\\.");
+        FilterPath next = EMPTY;
 
-            if (c == '.') {
-                String current = segment.substring(0, i).replaceAll("\\\\.", ".");
-                return new FilterPath(filter, current, parse(filter, segment.substring(i + 1)));
-            }
-            ++i;
-            if ((c == '\\') && (i < end) && (segment.charAt(i) == '.')) {
-                ++i;
-            }
+        for (int i = segments.length - 1; i >= 0; i--) {
+            // Replace escaped dots with actual dots in the current segment.
+            String segment = segments[i].replaceAll("\\\\.", ".");
+            next = new FilterPath(filter, segment, next);
         }
-        return new FilterPath(filter, segment.replaceAll("\\\\.", "."), EMPTY);
+
+        return next;
     }
 
     @Override

libs/core/src/test/java/org/opensearch/core/xcontent/filtering/FilterPathTests.java

@@ -35,6 +35,7 @@
 import org.opensearch.common.util.set.Sets;
 import org.opensearch.test.OpenSearchTestCase;
 
+import java.util.HashSet;
 import java.util.Set;
 
 import static java.util.Collections.singleton;
@@ -369,4 +370,20 @@ public void testMultipleFilterPaths() {
         assertThat(filterPath.getSegment(), is(emptyString()));
         assertSame(filterPath, FilterPath.EMPTY);
     }
+
+    public void testCompileWithEmptyString() {
+        Set<String> filters = new HashSet<>();
+        filters.add("");
+        FilterPath[] filterPaths = FilterPath.compile(filters);
+        assertNotNull(filterPaths);
+        assertEquals(0, filterPaths.length);
+    }
+
+    public void testCompileWithNull() {
+        Set<String> filters = new HashSet<>();
+        filters.add(null);
+        FilterPath[] filterPaths = FilterPath.compile(filters);
+        assertNotNull(filterPaths);
+        assertEquals(0, filterPaths.length);
+    }
 }