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Telegram/lib_base/base/qt/qt_string_view.h

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+// This file is part of Desktop App Toolkit,
+// a set of libraries for developing nice desktop applications.
+//
+// For license and copyright information please follow this link:
+// https://github.com/desktop-app/legal/blob/master/LEGAL
+//
+#pragma once
+
+#include <QtCore/QString>
+
+namespace base {
+namespace details {
+
+struct ContainerImplHelper {
+	enum CutResult { Null, Empty, Full, Subset };
+	static constexpr CutResult mid(
+			qsizetype originalLength,
+			qsizetype *_position,
+			qsizetype *_length) {
+		qsizetype &position = *_position;
+		qsizetype &length = *_length;
+		if (position > originalLength) {
+			position = 0;
+			length = 0;
+			return Null;
+		}
+
+		if (position < 0) {
+			if (length < 0 || length + position >= originalLength) {
+				position = 0;
+				length = originalLength;
+				return Full;
+			}
+			if (length + position <= 0) {
+				position = length = 0;
+				return Null;
+			}
+			length += position;
+			position = 0;
+		} else if (size_t(length) > size_t(originalLength - position)) {
+			length = originalLength - position;
+		}
+
+		if (position == 0 && length == originalLength)
+			return Full;
+
+		return length > 0 ? Subset : Empty;
+	}
+};
+
+} // namespace details
+
+[[nodiscard]] inline QStringView StringViewMid(
+		QStringView view,
+		qsizetype pos,
+		qsizetype n = -1) {
+	const auto result = details::ContainerImplHelper::mid(
+		view.size(),
+		&pos,
+		&n);
+	return (result == details::ContainerImplHelper::Null)
+		? QStringView()
+		: view.mid(pos, n);
+}
+
+} // namespace base