danthev commented on code in PR #22175:
URL: https://github.com/apache/beam/pull/22175#discussion_r919438611


##########
sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/firestore/QueryUtils.java:
##########
@@ -0,0 +1,387 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.beam.sdk.io.gcp.firestore;
+
+import com.google.firestore.v1.Document;
+import com.google.firestore.v1.StructuredQuery;
+import com.google.firestore.v1.StructuredQuery.Direction;
+import com.google.firestore.v1.StructuredQuery.FieldFilter;
+import com.google.firestore.v1.StructuredQuery.FieldFilter.Operator;
+import com.google.firestore.v1.StructuredQuery.FieldReference;
+import com.google.firestore.v1.StructuredQuery.Filter;
+import com.google.firestore.v1.StructuredQuery.Order;
+import com.google.firestore.v1.StructuredQuery.UnaryFilter;
+import com.google.firestore.v1.Value;
+import com.google.firestore.v1.Value.ValueTypeCase;
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+import org.apache.beam.vendor.guava.v26_0_jre.com.google.common.base.Ascii;
+import 
org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.ImmutableList;
+import 
org.apache.beam.vendor.guava.v26_0_jre.com.google.common.collect.ImmutableSet;
+import org.checkerframework.checker.nullness.qual.Nullable;
+
+/**
+ * Contains several internal utility functions for Firestore query handling, 
such as filling
+ * implicit ordering or escaping field references.
+ */
+class QueryUtils {
+
+  private static final ImmutableSet<Operator> INEQUALITY_FIELD_FILTER_OPS =
+      ImmutableSet.of(
+          FieldFilter.Operator.LESS_THAN,
+          FieldFilter.Operator.LESS_THAN_OR_EQUAL,
+          FieldFilter.Operator.GREATER_THAN,
+          FieldFilter.Operator.GREATER_THAN_OR_EQUAL,
+          FieldFilter.Operator.NOT_EQUAL,
+          FieldFilter.Operator.NOT_IN);
+  private static final ImmutableSet<UnaryFilter.Operator> 
INEQUALITY_UNARY_FILTER_OPS =
+      ImmutableSet.of(UnaryFilter.Operator.IS_NOT_NAN, 
UnaryFilter.Operator.IS_NOT_NULL);
+
+  private static final String UNQUOTED_NAME_REGEX_STRING = 
"([a-zA-Z_][a-zA-Z_0-9]*)";
+  private static final String QUOTED_NAME_REGEX_STRING = 
"(`(?:[^`\\\\]|(?:\\\\.))+`)";
+  // After each segment follows a dot and more characters, or the end of the 
string.
+  private static final Pattern FIELD_PATH_SEGMENT_REGEX =
+      Pattern.compile(
+          String.format("(?:%s|%s)(\\..+|$)", UNQUOTED_NAME_REGEX_STRING, 
QUOTED_NAME_REGEX_STRING),
+          Pattern.DOTALL);
+
+  /**
+   * Populates implicit orderBy of a query in accordance with our 
documentation. * Required
+   * inequality fields are appended in field name order. * __name__ is 
appended if not specified.
+   * See <a
+   * 
href=https://github.com/googleapis/googleapis/tree/master/google/firestore/v1/query.proto#L254>here</a>
+   * for more details.
+   *
+   * @param query The StructuredQuery of the original request.
+   * @return A list of additional orderBy fields, excluding the explicit ones.
+   */
+  static List<Order> getImplicitOrderBy(StructuredQuery query) {
+    List<OrderByFieldPath> expectedImplicitOrders = new ArrayList<>();
+    if (query.hasWhere()) {
+      fillInequalityFields(query.getWhere(), expectedImplicitOrders);
+    }
+    Collections.sort(expectedImplicitOrders);
+    if 
(expectedImplicitOrders.stream().noneMatch(OrderByFieldPath::isDocumentName)) {
+      expectedImplicitOrders.add(OrderByFieldPath.fromString("__name__"));
+    }
+    for (Order order : query.getOrderByList()) {
+      OrderByFieldPath orderField = 
OrderByFieldPath.fromString(order.getField().getFieldPath());
+      expectedImplicitOrders.remove(orderField);
+    }
+
+    List<Order> additionalOrders = new ArrayList<>();
+    if (!expectedImplicitOrders.isEmpty()) {
+      Direction lastDirection =
+          query.getOrderByCount() == 0
+              ? Direction.ASCENDING
+              : query.getOrderByList().get(query.getOrderByCount() - 
1).getDirection();
+
+      for (OrderByFieldPath field : expectedImplicitOrders) {
+        additionalOrders.add(
+            Order.newBuilder()
+                .setDirection(lastDirection)
+                
.setField(FieldReference.newBuilder().setFieldPath(field.canonicalString()).build())
+                .build());
+      }
+    }
+
+    return additionalOrders;
+  }
+
+  private static void fillInequalityFields(Filter filter, 
List<OrderByFieldPath> result) {
+    switch (filter.getFilterTypeCase()) {
+      case FIELD_FILTER:
+        if 
(INEQUALITY_FIELD_FILTER_OPS.contains(filter.getFieldFilter().getOp())) {
+          OrderByFieldPath fieldPath =
+              
OrderByFieldPath.fromString(filter.getFieldFilter().getField().getFieldPath());
+          if (!result.contains(fieldPath)) {
+            result.add(fieldPath);
+          }
+        }
+        break;
+      case COMPOSITE_FILTER:
+        filter.getCompositeFilter().getFiltersList().forEach(f -> 
fillInequalityFields(f, result));
+        break;
+      case UNARY_FILTER:
+        if 
(INEQUALITY_UNARY_FILTER_OPS.contains(filter.getUnaryFilter().getOp())) {
+          OrderByFieldPath fieldPath =
+              
OrderByFieldPath.fromString(filter.getUnaryFilter().getField().getFieldPath());
+          if (!result.contains(fieldPath)) {
+            result.add(fieldPath);
+          }
+        }
+        break;
+      default:
+        break;
+    }
+  }
+
+  static @Nullable Value lookupDocumentValue(Document document, String 
fieldPath) {
+    OrderByFieldPath resolvedPath = OrderByFieldPath.fromString(fieldPath);
+    // __name__ is a special field and doesn't exist in (top-level) valueMap
+    if (resolvedPath.isDocumentName()) {
+      return Value.newBuilder().setReferenceValue(document.getName()).build();
+    }
+    return findMapValue(new ArrayList<>(resolvedPath.getSegments()), 
document.getFieldsMap());
+  }
+
+  private static @Nullable Value findMapValue(List<String> segments, 
Map<String, Value> valueMap) {
+    if (segments.isEmpty()) {
+      return null;
+    }
+    String field = segments.remove(0);
+    Value value = valueMap.get(field);
+    if (segments.isEmpty()) {
+      return value;
+    }
+    // Field path traversal is not done, recurse into map values.
+    if (value == null || 
!value.getValueTypeCase().equals(ValueTypeCase.MAP_VALUE)) {
+      return null;
+    }
+    return findMapValue(segments, value.getMapValue().getFieldsMap());
+  }
+
+  private static class OrderByFieldPath implements 
Comparable<OrderByFieldPath> {
+
+    public static OrderByFieldPath fromString(String fieldPath) {
+      if (fieldPath.isEmpty()) {
+        throw new IllegalArgumentException("Could not resolve empty field 
path");
+      }
+      List<String> segments = new ArrayList<>();
+      while (!fieldPath.isEmpty()) {
+        Matcher segmentMatcher = FIELD_PATH_SEGMENT_REGEX.matcher(fieldPath);
+        boolean foundMatch = segmentMatcher.lookingAt();
+        if (!foundMatch) {
+          throw new IllegalArgumentException("OrderBy field path was 
malformed");
+        }
+        String fieldName;
+        if ((fieldName = segmentMatcher.group(1)) != null) {
+          segments.add(fieldName);
+        } else if ((fieldName = segmentMatcher.group(2)) != null) {
+          String unescaped = unescapeFieldName(fieldName.substring(1, 
fieldName.length() - 1));
+          segments.add(unescaped);
+        } else {
+          throw new IllegalArgumentException("OrderBy field path was 
malformed");
+        }
+        fieldPath = fieldPath.substring(fieldName.length());
+        // Due to the regex, any non-empty fieldPath will have a dot before 
the next nested field.
+        if (fieldPath.startsWith(".")) {
+          fieldPath = fieldPath.substring(1);
+        }
+      }
+      return new OrderByFieldPath(ImmutableList.copyOf(segments));
+    }
+
+    private final ImmutableList<String> segments;
+
+    private OrderByFieldPath(ImmutableList<String> segments) {
+      this.segments = segments;
+    }
+
+    public String canonicalString() {
+      return String.join(".", segments);

Review Comment:
   Ah yeah, added a test case that shows it. I've switched it to returning the 
original string.



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