util/ByteOrder: add class PackedLE64

src/util/ByteOrder.hxx

@@ -1,8 +1,7 @@
 // SPDX-License-Identifier: BSD-2-Clause
 // author: Max Kellermann <max.kellermann@gmail.com>
 
-#ifndef BYTE_ORDER_HXX
-#define BYTE_ORDER_HXX
+#pragma once
 
 #include <cstdint>
 
@@ -470,4 +469,54 @@ public:
 static_assert(sizeof(PackedLE32) == sizeof(uint32_t), "Wrong size");
 static_assert(alignof(PackedLE32) == 1, "Wrong alignment");
 
-#endif
+/**
+ * A packed little-endian 64 bit integer.
+ */
+class PackedLE64 {
+	uint8_t a, b, c, d, e, f, g, h;
+
+public:
+	PackedLE64() = default;
+
+	constexpr PackedLE64(uint64_t src) noexcept
+		:a(uint8_t(src)),
+		 b(uint8_t(src >> 8)),
+		 c(uint8_t(src >> 16)),
+		 d(uint8_t(src >> 24)),
+		 e(uint8_t(src >> 32)),
+		 f(uint8_t(src >> 40)),
+		 g(uint8_t(src >> 48)),
+		 h(uint8_t(src >> 56)) {}
+
+	/**
+	 * Construct an instance from an integer which is already
+	 * little-endian.
+	 */
+	static constexpr auto FromLE(uint64_t src) noexcept {
+		union {
+			uint64_t in;
+			PackedLE64 out;
+		} u{src};
+		return u.out;
+	}
+
+	constexpr operator uint64_t() const noexcept {
+		return uint64_t(a) | (uint64_t(b) << 8) |
+			(uint64_t(c) << 16) | (uint64_t(d) << 24) |
+			(uint64_t(e) << 32) | (uint64_t(f) << 40) |
+			(uint64_t(g) << 48) | (uint64_t(h) << 56);
+	}
+
+	/**
+	 * Reads the raw, big-endian value.
+	 */
+	constexpr uint64_t raw() const noexcept {
+		uint64_t x = *this;
+		if (IsBigEndian())
+			x = ByteSwap64(x);
+		return x;
+	}
+};
+
+static_assert(sizeof(PackedLE64) == sizeof(uint64_t), "Wrong size");
+static_assert(alignof(PackedLE64) == 1, "Wrong alignment");