util/ByteOrder: add class PackedBE64

src/util/ByteOrder.hxx

@@ -351,6 +351,58 @@ public:
 static_assert(sizeof(PackedBE32) == sizeof(uint32_t), "Wrong size");
 static_assert(alignof(PackedBE32) == 1, "Wrong alignment");
 
+/**
+ * A packed big-endian 64 bit integer.
+ */
+class PackedBE64 {
+	uint8_t a, b, c, d, e, f, g, h;
+
+public:
+	PackedBE64() = default;
+
+	constexpr PackedBE64(uint64_t src) noexcept
+		:a(uint8_t(src >> 56)),
+		 b(uint8_t(src >> 48)),
+		 c(uint8_t(src >> 40)),
+		 d(uint8_t(src >> 32)),
+		 e(uint8_t(src >> 24)),
+		 f(uint8_t(src >> 16)),
+		 g(uint8_t(src >> 8)),
+		 h(uint8_t(src)) {}
+
+	/**
+	 * Construct an instance from an integer which is already
+	 * big-endian.
+	 */
+	static constexpr auto FromBE(uint64_t src) noexcept {
+		union {
+			uint64_t in;
+			PackedBE64 out;
+		} u{src};
+		return u.out;
+	}
+
+	constexpr operator uint64_t() const noexcept {
+		return (uint64_t(a) << 56) | (uint64_t(b) << 48) |
+			(uint64_t(c) << 40) | (uint64_t(d) << 32) |
+			(uint64_t(e) << 24) | (uint64_t(f) << 16) |
+			(uint64_t(g) << 8) | uint64_t(h);
+	}
+
+	/**
+	 * Reads the raw, big-endian value.
+	 */
+	constexpr uint64_t raw() const noexcept {
+		uint64_t x = *this;
+		if (IsLittleEndian())
+			x = ByteSwap64(x);
+		return x;
+	}
+};
+
+static_assert(sizeof(PackedBE64) == sizeof(uint64_t), "Wrong size");
+static_assert(alignof(PackedBE64) == 1, "Wrong alignment");
+
 /**
  * A packed little-endian 16 bit integer.
  */