[base] Helper functions that remove ("eat") prefixes and suffixes of strings.

base/base_tests/string_utils_test.cpp

@@ -825,6 +825,48 @@ UNIT_TEST(EndsWith)
   }
 }
 
+UNIT_TEST(EatPrefix_EatSuffix)
+{
+  // <original string, prefix/suffix to cut, success, string after cutting>
+  std::vector<std::tuple<std::string, std::string, bool, std::string>> kPrefixTestcases = {
+    {"abc", "a", true, "bc"},
+    {"abc", "b", false, "abc"},
+    {"abc", "", true, "abc"},
+    {"abc", "abc", true, ""},
+    {"abc", "abc00", false, "abc"},
+    {"", "", true, ""},
+    {"абв", "а", true, "бв"},
+    {"абв", "б", false, "абв"},
+    {"字符串", "字", true, "符串"},
+  };
+
+  std::vector<std::tuple<std::string, std::string, bool, std::string>> kSuffixTestcases = {
+    {"abc", "c", true, "ab"},
+    {"abc", "b", false, "abc"},
+    {"abc", "", true, "abc"},
+    {"abc", "abc", true, ""},
+    {"abc", "00abc", false, "abc"},
+    {"", "", true, ""},
+    {"абв", "в", true, "аб"},
+    {"абв", "б", false, "абв"},
+    {"字符串", "串", true, "字符"},
+  };
+
+  for (auto const & [original, toCut, success, afterCutting] : kPrefixTestcases)
+  {
+    auto s = original;
+    TEST_EQUAL(strings::EatPrefix(s, toCut), success, (original, toCut));
+    TEST_EQUAL(s, afterCutting, ());
+  }
+
+  for (auto const & [original, toCut, success, afterCutting] : kSuffixTestcases)
+  {
+    auto s = original;
+    TEST_EQUAL(strings::EatSuffix(s, toCut), success, (original, toCut));
+    TEST_EQUAL(s, afterCutting, ());
+  }
+}
+
 UNIT_TEST(UniString_LessAndEqualsAndNotEquals)
 {
   std::vector<strings::UniString> v;

base/string_utils.cpp

@@ -325,6 +325,26 @@ bool EndsWith(std::string const & s1, std::string const & s2)
   return s1.size() >= s2.size() && s1.compare(s1.size() - s2.size(), s2.size(), s2) == 0;
 }
 
+bool EatPrefix(std::string & s, std::string const & prefix)
+{
+  if (!StartsWith(s, prefix))
+    return false;
+
+  CHECK_LESS_OR_EQUAL(prefix.size(), s.size(), ());
+  s = s.substr(prefix.size());
+  return true;
+}
+
+bool EatSuffix(std::string & s, std::string const & suffix)
+{
+  if (!EndsWith(s, suffix))
+    return false;
+
+  CHECK_LESS_OR_EQUAL(suffix.size(), s.size(), ());
+  s = s.substr(0, s.size() - suffix.size());
+  return true;
+}
+
 std::string to_string_dac(double d, int dac)
 {
   dac = std::min(std::numeric_limits<double>::digits10, dac);

base/string_utils.hpp

@@ -634,6 +634,13 @@ bool EndsWith(std::string const & s1, char const * s2);
 
 bool EndsWith(std::string const & s1, std::string const & s2);
 
+// If |s| starts with |prefix|, deletes it from |s| and returns true.
+// Otherwise, leaves |s| unmodified and returns false.
+bool EatPrefix(std::string & s, std::string const & prefix);
+// If |s| ends with |suffix|, deletes it from |s| and returns true.
+// Otherwise, leaves |s| unmodified and returns false.
+bool EatSuffix(std::string & s, std::string const & suffix);
+
 /// Try to guess if it's HTML or not. No guarantee.
 bool IsHTML(std::string const & utf8);