Add utils::Substr which can use different allocator

Reviewers: mtomic, msantl, mferencevic

Reviewed By: msantl

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D2103

src/utils/string.hpp

@@ -430,4 +430,17 @@ inline std::string Escape(const std::string_view &src) {
   return res;
 }
 
+/**
+ * Return a view into substring [pos, pos+count).
+ * If the bounds extend past the length of the string then both sides are
+ * clamped to a valid interval. Therefore, this function never throws
+ * std::out_of_range, unlike std::basic_string::substr.
+ */
+inline std::string_view Substr(const std::string_view &string, size_t pos = 0,
+                               size_t count = std::string::npos) {
+  if (pos >= string.size()) return std::string_view(string.data(), 0);
+  auto len = std::min(string.size() - pos, count);
+  return std::string_view(string.data() + pos, len);
+}
+
 }  // namespace utils

tests/unit/utils_string.cpp

@@ -145,3 +145,22 @@ TEST(String, RandomString) {
 
   EXPECT_EQ(string_set.size(), 20);
 }
+
+// NOLINTNEXTLINE(hicpp-special-member-functions)
+TEST(String, Substr) {
+  const std::string string("memgraph");
+  EXPECT_EQ(Substr(string), string.substr());
+  EXPECT_EQ(Substr(string, string.size()), string.substr(string.size()));
+  EXPECT_THROW(string.substr(string.size() + 1), std::out_of_range);
+  EXPECT_TRUE(Substr(string, string.size() + 1).empty());
+  EXPECT_EQ(Substr(string, 1, string.size()), string.substr(1, string.size()));
+  EXPECT_EQ(Substr(string, 0, string.size()), string.substr(0, string.size()));
+  EXPECT_EQ(Substr(string, 0, string.size() + 1),
+            string.substr(0, string.size() + 1));
+  EXPECT_EQ(Substr(string, 0, string.size() - 1),
+            string.substr(0, string.size() - 1));
+  EXPECT_EQ(Substr(string, string.size() - 1, 1),
+            string.substr(string.size() - 1, 1));
+  EXPECT_EQ(Substr(string, string.size() - 1, 2),
+            string.substr(string.size() - 1, 2));
+}