Reduces string allocations in GUI and maths.

2025-10-11 19:08:02 +02:00
parent 4bf107352f
commit bedb6129f2
16 changed files with 315 additions and 51 deletions
@@ -0,0 +1,105 @@
+/* Copyright (C) 2025 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. 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, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. 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 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef INCLUDED_PS_STRINGBUILDER
+#define INCLUDED_PS_STRINGBUILDER
+
+#include <charconv>
+#include <cstdint>
+#include <span>
+#include <string>
+#include <string_view>
+#include <type_traits>
+#include <version>
+
+#if defined(__cpp_lib_to_chars)
+#include <charconv>
+#else
+#include <fmt/core.h>
+#endif
+
+namespace PS
+{
+
+/**
+ * A helper class to build string in-place without additional cost of
+ * allocations, locales and format parsing. Prefer to use the class with stack
+ * allocated buffers over standard tools in high-load cases.
+ * TODO: in case of overflow it might use a heap allocated storage.
+ */
+class StringBuilder
+{
+public:
+	StringBuilder(std::span<char> buffer) noexcept
+		: m_Buffer{buffer}, m_BufferBegin{buffer.data()}
+	{
+		ENSURE(!m_Buffer.empty());
+	}
+
+	template<typename T>
+	typename std::enable_if<std::is_arithmetic_v<T>>::type Append(const T value) noexcept
+	{
+		ENSURE(m_Buffer.size() > 0);
+#if defined(__cpp_lib_to_chars)
+		const std::to_chars_result result{std::to_chars(m_Buffer.data(), m_Buffer.data() + m_Buffer.size() - 1, value)};
+		ENSURE(result.ec == std::errc());
+		m_Buffer = m_Buffer.subspan(result.ptr - m_Buffer.data());
+		// to_chars doesn't write terminating null.
+		m_Buffer.front() = 0;
+#else
+		// TODO: switch to std::to_chars after minimal libcxx supports it.
+		const fmt::format_to_n_result result{
+			fmt::format_to_n(m_Buffer.data(), m_Buffer.size() - 1, "{}", value)};
+		ENSURE(m_Buffer.data() != result.out);
+		m_Buffer = m_Buffer.subspan(result.size);
+		// format_to_n doesn't write terminating null.
+		m_Buffer.front() = 0;
+#endif
+	}
+
+	void Append(const char ch)
+	{
+		ENSURE(m_Buffer.size() > 1);
+		m_Buffer.front() = ch;
+		m_Buffer = m_Buffer.subspan(1);
+		m_Buffer.front() = 0;
+	}
+
+	void Append(const std::string_view str) noexcept
+	{
+		ENSURE(m_Buffer.size() >= static_cast<size_t>(str.size()) + 1);
+		std::copy(str.begin(), str.end(), m_Buffer.begin());
+		m_Buffer = m_Buffer.subspan(str.size());
+		m_Buffer.front() = 0;
+	}
+
+	/**
+	 * Returns a string_view to a null-terminated string.
+	 */
+	std::string_view Str() noexcept
+	{
+		return {m_BufferBegin, m_Buffer.data()};
+	}
+
+private:
+	std::span<char> m_Buffer;
+	char* m_BufferBegin;
+};
+
+} // namespace PS
+
+#endif // INCLUDED_PS_STRINGBUILDER
@@ -0,0 +1,66 @@
+/* Copyright (C) 2025 Wildfire Games.
+ * This file is part of 0 A.D.
+ *
+ * 0 A.D. 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, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 0 A.D. 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 0 A.D.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "lib/self_test.h"
+
+#include "ps/strings/StringBuilder.h"
+
+#include <array>
+#include <limits>
+
+class TestStringBuilder : public CxxTest::TestSuite
+{
+public:
+	template<typename T>
+	void Check(const T value, const std::string_view str)
+	{
+		char buffer[64] = {};
+
+		PS::StringBuilder builder{{std::begin(buffer), std::end(buffer)}};
+		builder.Append(value);
+		TS_ASSERT_EQUALS(builder.Str(), str);
+	}
+
+	void test_append()
+	{
+		Check<int>(0, "0");
+		Check<int>(std::numeric_limits<int>::min(), "-2147483648");
+		Check<int>(std::numeric_limits<int>::max(), "2147483647");
+
+		Check<uint64_t>(std::numeric_limits<uint64_t>::min(), "0");
+		Check<uint64_t>(std::numeric_limits<uint64_t>::max(), "18446744073709551615");
+
+		Check<float>(0.0f, "0");
+		Check<float>(1.0f, "1");
+		Check<float>(-1.0f, "-1");
+		Check<float>(0.5f, "0.5");
+		Check<float>(1e-3f, "0.001");
+	}
+
+	void test_overflow()
+	{
+		char buffer[8] = {};
+		buffer[3] = 1;
+		buffer[4] = 2;
+		PS::StringBuilder builder{{std::begin(buffer), std::begin(buffer) + 4}};
+		builder.Append("abc");
+		TS_ASSERT_EQUALS(buffer[0], 'a');
+		TS_ASSERT_EQUALS(buffer[3], 0);
+		TS_ASSERT_EQUALS(buffer[4], 2);
+		TS_ASSERT_EQUALS(builder.Str(), "abc");
+	}
+};