zig/doc/langref/test_packed_structs.zig

const std = @import("std");
const native_endian = @import("builtin").target.cpu.arch.endian();
const expect = std.testing.expect;

const Full = packed struct {
    number: u16,
};
const Divided = packed struct {
    half1: u8,
    quarter3: u4,
    quarter4: u4,
};

test "@bitCast between packed structs" {
    try doTheTest();
    try comptime doTheTest();
}

fn doTheTest() !void {
    try expect(@sizeOf(Full) == 2);
    try expect(@sizeOf(Divided) == 2);
    const full = Full{ .number = 0x1234 };
    const divided: Divided = @bitCast(full);
    try expect(divided.half1 == 0x34);
    try expect(divided.quarter3 == 0x2);
    try expect(divided.quarter4 == 0x1);

    const ordered: [2]u8 = @bitCast(full);
    switch (native_endian) {
        .big => {
            try expect(ordered[0] == 0x12);
            try expect(ordered[1] == 0x34);
        },
        .little => {
            try expect(ordered[0] == 0x34);
            try expect(ordered[1] == 0x12);
        },
    }
}

// test