Acpi

    pub const Acpi = union(Subtype) {
        acpi: *const BaseAcpiDevicePath,
        expanded_acpi: *const ExpandedAcpiDevicePath,
        adr: *const AdrDevicePath,

        pub const Subtype = enum(u8) {
            acpi = 1,
            expanded_acpi = 2,
            adr = 3,
            _,
        };

        pub const BaseAcpiDevicePath = extern struct {
            type: DevicePath.Type,
            subtype: Subtype,
            length: u16 align(1),
            hid: u32 align(1),
            uid: u32 align(1),
        };

        comptime {
            assert(12 == @sizeOf(BaseAcpiDevicePath));
            assert(1 == @alignOf(BaseAcpiDevicePath));

            assert(0 == @offsetOf(BaseAcpiDevicePath, "type"));
            assert(1 == @offsetOf(BaseAcpiDevicePath, "subtype"));
            assert(2 == @offsetOf(BaseAcpiDevicePath, "length"));
            assert(4 == @offsetOf(BaseAcpiDevicePath, "hid"));
            assert(8 == @offsetOf(BaseAcpiDevicePath, "uid"));
        }

        pub const ExpandedAcpiDevicePath = extern struct {
            type: DevicePath.Type,
            subtype: Subtype,
            length: u16 align(1),
            hid: u32 align(1),
            uid: u32 align(1),
            cid: u32 align(1),
            // variable length u16[*:0] strings
            // hid_str, uid_str, cid_str
        };

        comptime {
            assert(16 == @sizeOf(ExpandedAcpiDevicePath));
            assert(1 == @alignOf(ExpandedAcpiDevicePath));

            assert(0 == @offsetOf(ExpandedAcpiDevicePath, "type"));
            assert(1 == @offsetOf(ExpandedAcpiDevicePath, "subtype"));
            assert(2 == @offsetOf(ExpandedAcpiDevicePath, "length"));
            assert(4 == @offsetOf(ExpandedAcpiDevicePath, "hid"));
            assert(8 == @offsetOf(ExpandedAcpiDevicePath, "uid"));
            assert(12 == @offsetOf(ExpandedAcpiDevicePath, "cid"));
        }

        pub const AdrDevicePath = extern struct {
            type: DevicePath.Type,
            subtype: Subtype,
            length: u16 align(1),
            adr: u32 align(1),

            // multiple adr entries can optionally follow
            pub fn adrs(self: *const AdrDevicePath) []align(1) const u32 {
                // self.length is a minimum of 8 with one adr which is size 4.
                const entries = (self.length - 4) / @sizeOf(u32);
                return @as([*]align(1) const u32, @ptrCast(&self.adr))[0..entries];
            }
        };

        comptime {
            assert(8 == @sizeOf(AdrDevicePath));
            assert(1 == @alignOf(AdrDevicePath));

            assert(0 == @offsetOf(AdrDevicePath, "type"));
            assert(1 == @offsetOf(AdrDevicePath, "subtype"));
            assert(2 == @offsetOf(AdrDevicePath, "length"));
            assert(4 == @offsetOf(AdrDevicePath, "adr"));
        }
    }