Add: Utf8View and iterator.

src/core/utf8.hpp

@@ -0,0 +1,121 @@
+/*
+ * This file is part of OpenTTD.
+ * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
+ * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/**
+ * @file utf8.hpp Handling of UTF-8 encoded data.
+ */
+
+#ifndef UTF8_HPP
+#define UTF8_HPP
+
+#include <iterator>
+#include "../string_func.h"
+
+[[nodiscard]] std::pair<char[4], size_t> EncodeUtf8(char32_t c);
+[[nodiscard]] std::pair<size_t, char32_t> DecodeUtf8(std::string_view buf);
+
+/**
+ * Constant span of UTF-8 encoded data.
+ */
+class Utf8View {
+	std::string_view src;
+public:
+	Utf8View() = default;
+	Utf8View(std::string_view src) : src(src) {}
+
+	/**
+	 * Bidirectional input iterator over codepoints.
+	 *
+	 * If invalid encodings are present:
+	 * - the iterator will skip overlong encodings, and
+	 * - dereferencing returns a placeholder char '?'.
+	 */
+	class iterator {
+		std::string_view src;
+		size_t position = 0;
+	public:
+		using value_type = char32_t;
+		using difference_type = std::ptrdiff_t;
+		using iterator_category = std::bidirectional_iterator_tag;
+		using pointer = void;
+		using reference = void;
+
+		iterator() = default;
+		iterator(std::string_view src, size_t position) : src(src), position(position) {}
+
+		size_t GetByteOffset() const
+		{
+			return this->position;
+		}
+
+		bool operator==(const iterator &rhs) const
+		{
+			assert(this->src.data() == rhs.src.data());
+			return this->position == rhs.position;
+		}
+
+		std::strong_ordering operator<=>(const iterator &rhs) const
+		{
+			assert(this->src.data() == rhs.src.data());
+			return this->position <=> rhs.position;
+		}
+
+		char32_t operator*() const
+		{
+			assert(this->position < this->src.size());
+			auto [len, c] = DecodeUtf8(this->src.substr(this->position));
+			return len > 0 ? c : '?';
+		}
+
+		iterator& operator++()
+		{
+			auto size = this->src.size();
+			assert(this->position < size);
+			do {
+				++this->position;
+			} while (this->position < size && IsUtf8Part(this->src[this->position]));
+			return *this;
+		}
+
+		iterator operator++(int)
+		{
+			iterator result = *this;
+			++*this;
+			return result;
+		}
+
+		iterator& operator--()
+		{
+			assert(this->position > 0);
+			do {
+				--this->position;
+			} while (this->position > 0 && IsUtf8Part(this->src[this->position]));
+			return *this;
+		}
+
+		iterator operator--(int)
+		{
+			iterator result = *this;
+			--*this;
+			return result;
+		}
+	};
+
+	iterator begin() const
+	{
+		return iterator(this->src, 0);
+	}
+
+	iterator end() const
+	{
+		return iterator(this->src, this->src.size());
+	}
+
+	iterator GetIterAtByte(size_t offset) const;
+};
+
+#endif /* UTF8_HPP */