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+/*
+ * Copyright 2017 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.
+ */
+
+// Useful string functions and so forth.  This is a grab-bag file.
+//
+// These functions work fine for UTF-8 strings as long as you can
+// consider them to be just byte strings.  For example, due to the
+// design of UTF-8 you do not need to worry about accidental matches,
+// as long as all your inputs are valid UTF-8 (use \uHHHH, not \xHH or \oOOO).
+
+#ifndef IPHONE_FIRESTORE_PORT_STRING_UTIL_H_
+#define IPHONE_FIRESTORE_PORT_STRING_UTIL_H_
+
+#include <string>
+
+namespace leveldb {
+class Slice;
+}
+
+namespace Firestore {
+
+// Returns the smallest lexicographically larger string of equal or smaller
+// length. Returns an empty string if there is no such successor (if the input
+// is empty or consists entirely of 0xff bytes).
+// Useful for calculating the smallest lexicographically larger string
+// that will not be prefixed by the input string.
+//
+// Examples:
+// "a" -> "b", "aaa" -> "aab", "aa\xff" -> "ab", "\xff" -> "", "" -> ""
+std::string PrefixSuccessor(leveldb::Slice prefix);
+
+// Returns the immediate lexicographically-following string. This is useful to
+// turn an inclusive range into something that can be used with Bigtable's
+// SetLimitRow():
+//
+//     // Inclusive range [min_element, max_element].
+//     string min_element = ...;
+//     string max_element = ...;
+//
+//     // Equivalent range [range_start, range_end).
+//     string range_start = min_element;
+//     string range_end = ImmediateSuccessor(max_element);
+//
+// WARNING: Returns the input string with a '\0' appended; if you call c_str()
+// on the result, it will compare equal to s.
+//
+// WARNING: Transforms "" -> "\0"; this doesn't account for Bigtable's special
+// treatment of "" as infinity.
+std::string ImmediateSuccessor(leveldb::Slice s);
+
+}  // namespace Firestore
+
+#endif  // IPHONE_FIRESTORE_PORT_STRING_UTIL_H_