util/LocaleString: add SplitAtCharMB()

src/BasicMarquee.cxx

@@ -14,8 +14,7 @@ using std::string_view_literals::operator""sv;
 std::string_view
 BasicMarquee::ScrollString() const noexcept
 {
-	const char *p = AtCharMB(buffer, offset);
-	return TruncateAtWidthMB(p, width);
+	return TruncateAtWidthMB(SplitAtCharMB(buffer, offset).second, width);
 }
 
 bool

src/util/LocaleString.hxx

@@ -3,6 +3,8 @@
 
 #pragma once
 
+#include "util/StringSplit.hxx"
+
 #include <cstddef>
 #include <string_view>
 
@@ -48,6 +50,13 @@ PrevCharMB(const char *start, const char *reference) noexcept;
 const char *
 AtCharMB(std::string_view s, size_t i) noexcept;
 
+inline std::pair<std::string_view, std::string_view>
+SplitAtCharMB(std::string_view s, std::size_t i) noexcept
+{
+	const char *p = AtCharMB(s, i);
+	return Partition(s, p - s.data());
+}
+
 /**
  * Returns the number of terminal cells occupied by this multi-byte
  * string.

src/util/StringSplit.hxx

@@ -0,0 +1,93 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// author: Max Kellermann <[email protected]>
+
+#pragma once
+
+#include <algorithm>
+#include <string_view>
+#include <type_traits>
+
+template<typename T>
+constexpr std::pair<std::basic_string_view<T>, std::basic_string_view<T>>
+Partition(const std::basic_string_view<T> haystack,
+	  const typename std::basic_string_view<T>::size_type position) noexcept
+{
+	return {
+		haystack.substr(0, position),
+		haystack.substr(position),
+	};
+}
+
+template<typename T>
+constexpr std::pair<std::basic_string_view<T>, std::basic_string_view<T>>
+Partition(const std::basic_string_view<T> haystack,
+	  const typename std::basic_string_view<T>::const_pointer position) noexcept
+{
+	return Partition(haystack, position - haystack.data());
+}
+
+template<typename T>
+requires(!std::is_same_v<typename std::basic_string_view<T>::const_pointer,
+	 typename std::basic_string_view<T>::const_iterator>)
+constexpr std::pair<std::basic_string_view<T>, std::basic_string_view<T>>
+Partition(const std::basic_string_view<T> haystack,
+	  const typename std::basic_string_view<T>::const_iterator i) noexcept
+{
+	return Partition(haystack, i - haystack.begin());
+}
+
+template<typename T>
+constexpr std::pair<std::basic_string_view<T>, std::basic_string_view<T>>
+PartitionWithout(const std::basic_string_view<T> haystack,
+		 const typename std::basic_string_view<T>::size_type separator) noexcept
+{
+	return {
+		haystack.substr(0, separator),
+		haystack.substr(separator + 1),
+	};
+}
+
+/**
+ * Split the string at the first occurrence of the given character.
+ * If the character is not found, then the first value is the whole
+ * string and the second value is nullptr.
+ */
+template<typename T>
+constexpr std::pair<std::basic_string_view<T>, std::basic_string_view<T>>
+Split(const std::basic_string_view<T> haystack, const T ch) noexcept
+{
+	const auto i = haystack.find(ch);
+	if (i == haystack.npos)
+		return {haystack, {}};
+
+	return PartitionWithout(haystack, i);
+}
+
+/**
+ * Split the string at the last occurrence of the given
+ * character.  If the character is not found, then the first
+ * value is the whole string and the second value is nullptr.
+ */
+template<typename T>
+constexpr std::pair<std::basic_string_view<T>, std::basic_string_view<T>>
+SplitLast(const std::basic_string_view<T> haystack, const T ch) noexcept
+{
+	const auto i = haystack.rfind(ch);
+	if (i == haystack.npos)
+		return {haystack, {}};
+
+	return PartitionWithout(haystack, i);
+}
+
+/**
+ * Find the first character that does not match the given predicate
+ * and split at this boundary.
+ */
+template<typename T, typename P>
+constexpr std::pair<std::basic_string_view<T>, std::basic_string_view<T>>
+SplitWhile(const std::basic_string_view<T> haystack, P &&predicate) noexcept
+{
+	const auto i = std::find_if_not(haystack.begin(), haystack.end(),
+					std::forward<P>(predicate));
+	return Partition(haystack, i);
+}