Add case-insensitive string/wcstring search

src/common.h

@@ -345,6 +345,38 @@ bool string_suffixes_string_case_insensitive(const wcstring &proposed_suffix,
 bool string_prefixes_string_case_insensitive(const wcstring &proposed_prefix,
                                              const wcstring &value);
 
+/// Helper struct for ifind, templated to allow using it with both std::string and std::wstring
+/// Supports locale-specific search for characters that transform into a different character
+/// when upper/lower-cased, such as those in Turkish and German.
+template<typename T>
+struct string_iequal_t {
+private:
+    const std::locale &_locale;
+public:
+    string_iequal_t(const std::locale &locale)
+        : _locale(locale) {}
+    bool operator() (T char1, T char2) {
+        return std::toupper(char1, _locale) == std::toupper(char2, _locale);
+    }
+};
+
+/// Case-insensitive string search, templated for use with both std::string and std::wstring.
+/// Modeled after std::string::find().
+/// \return the offset of the first case-insensitive matching instance of `needle` within
+/// `haystack`, or `string::npos()` if no results were found.
+template<typename T>
+size_t ifind(const T &haystack, const T &needle,
+        const std::locale &locale = std::locale()) {
+    auto result = std::search(haystack.begin(), haystack.end(), needle.begin(), needle.end(),
+            string_iequal_t<typename T::value_type>(locale));
+
+    if (result != haystack.end()) {
+        return result - haystack.begin();
+    }
+
+    return T::npos;
+}
+
 /// Split a string by a separator character.
 wcstring_list_t split_string(const wcstring &val, wchar_t sep);