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+/*
+ * Copyright 2018 Google
+ *
+ * Licensed 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.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_FILTER_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_FILTER_H_
+
+#include <memory>
+#include <string>
+
+#include "Firestore/core/src/firebase/firestore/model/document.h"
+#include "Firestore/core/src/firebase/firestore/model/field_path.h"
+#include "Firestore/core/src/firebase/firestore/model/field_value.h"
+
+namespace firebase {
+namespace firestore {
+namespace core {
+
+/** Interface used for all query filters. All filters are immutable. */
+class Filter {
+ public:
+  enum class Operator {
+    LessThan,
+    LessThanOrEqual,
+    Equal,
+    GreaterThan,
+    GreaterThanOrEqual,
+  };
+
+  /**
+   * Creates a Filter instance for the provided path, operator, and value.
+   *
+   * Note that if the relational operator is Equal and the value is NullValue or
+   * NaN, then this will return the appropriate NullFilter or NaNFilter class
+   * instead of a RelationFilter.
+   */
+  static std::shared_ptr<Filter> Create(model::FieldPath path,
+                                        Operator op,
+                                        model::FieldValue value);
+
+  virtual ~Filter() {
+  }
+
+  /** Returns the field the Filter operates over. */
+  virtual const model::FieldPath& field() const = 0;
+
+  /** Returns true if a document matches the filter. */
+  virtual bool Matches(const model::Document& doc) const = 0;
+
+  /** A unique ID identifying the filter; used when serializing queries. */
+  virtual std::string CanonicalId() const = 0;
+};
+
+}  // namespace core
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_CORE_FILTER_H_