util/RoundPowerOfTwo: new library

src/util/RoundPowerOfTwo.hxx

@@ -0,0 +1,50 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// Copyright CM4all GmbH
+// author: Max Kellermann <mk@cm4all.com>
+
+#pragma once
+
+#include <concepts>
+
+template<std::unsigned_integral T>
+constexpr T
+RoundUpToPowerOfTwo(T value) noexcept
+{
+	if (value <= 0)
+		return 1;
+
+	--value;
+
+	for (unsigned bits = 1; bits < sizeof(T) * 8; bits <<= 1)
+		value |= value >> bits;
+
+	++value;
+
+	return value;
+}
+
+static_assert(RoundUpToPowerOfTwo(0U) == 1U);
+static_assert(RoundUpToPowerOfTwo(2U) == 2U);
+static_assert(RoundUpToPowerOfTwo(3U) == 4U);
+static_assert(RoundUpToPowerOfTwo(4U) == 4U);
+static_assert(RoundUpToPowerOfTwo(5U) == 8U);
+static_assert(RoundUpToPowerOfTwo(0x7fffU) == 0x8000U);
+static_assert(RoundUpToPowerOfTwo(0x7ffffU) == 0x80000U);
+static_assert(RoundUpToPowerOfTwo(0x1000000000000000ULL) == 0x1000000000000000ULL);
+static_assert(RoundUpToPowerOfTwo(0x1fffffffffffffffULL) == 0x2000000000000000ULL);
+static_assert(RoundUpToPowerOfTwo(0x7fffffffffffffffULL) == 0x8000000000000000ULL);
+static_assert(RoundUpToPowerOfTwo(0x8000000000000000ULL) == 0x8000000000000000ULL);
+
+template<std::unsigned_integral T>
+constexpr T
+RoundUpToPowerOfTwo(T value, T power_of_two) noexcept
+{
+	return ((value - 1) | (power_of_two - 1)) + 1;
+}
+
+template<std::unsigned_integral T>
+constexpr T
+RoundDownToPowerOfTwo(T value, T power_of_two) noexcept
+{
+	return value & ~(power_of_two - 1);
+}