normalize string_util (#708)

* refactoring string_util
* port string_util to iOS

1 files changed, 72 insertions, 0 deletions

diff --git a/Firestore/core/src/firebase/firestore/util/string_util.h b/Firestore/core/src/firebase/firestore/util/string_util.h
new file mode 100644
index 0000000..3de177d
--- /dev/null
+++ b/Firestore/core/src/firebase/firestore/util/string_util.h
@@ -0,0 +1,72 @@
+/*
+ * 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 FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_STRING_UTIL_H_
+#define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_STRING_UTIL_H_
+
+#include <string>
+
+#include "absl/strings/string_view.h"
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+/*
+ * 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(absl::string_view 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(absl::string_view s);
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_UTIL_STRING_UTIL_H_