turbonss/lib/header.zig

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

const max_shells = @import("shell.zig").max_shells;
const magic = [4]u8{ 0xf0, 0x9f, 0xa4, 0xb7 };
const version = 0;

const Endian = enum(u8) {
    big,
    little,

    fn native() Endian {
        return switch (native_endian) {
            .Little => Endian.little,
            .Big => Endian.big,
        };
    }
};

pub const section_length_bits = 6;
pub const section_length = 1 << section_length_bits;

pub const Invalid = error{
    InvalidMagic,
    InvalidVersion,
    InvalidEndianess,
};

pub const Header = packed struct {
    magic: [4]u8 = magic,
    version: u8 = version,
    endian: Endian = Endian.native(),
    nblocks_shell_blob: u8,
    num_shells: u8,
    num_groups: u32,
    num_users: u32,
    nblocks_bdz_gid: u32,
    nblocks_bdz_groupname: u32,
    nblocks_bdz_uid: u32,
    nblocks_bdz_username: u32,
    nblocks_groups: u64,
    nblocks_users: u64,
    nblocks_groupmembers: u64,
    nblocks_additional_gids: u64,

    pub fn fromBytes(blob: *const [@sizeOf(Header)]u8) Invalid!*const Header {
        const self = mem.bytesAsValue(Header, blob);

        if (!mem.eql(u8, magic[0..4], blob[0..4]))
            return error.InvalidMagic;

        if (self.version != 0)
            return error.InvalidVersion;

        if (self.endian != Endian.native())
            return error.InvalidEndianess;

        return self;
    }
};

const testing = std.testing;

test "Section length is a power of two" {
    try testing.expect(std.math.isPowerOfTwo(section_length));
}

test "Header fits into a section" {
    try testing.expect(@sizeOf(Header) <= section_length);
}

test "bit header size is equal to @sizeOf(Header)" {
    try testing.expectEqual(@sizeOf(Header) * 8, @bitSizeOf(Header));
}

test "header pack and unpack" {
    const header1 = Header{
        .nblocks_shell_blob = 0,
        .num_shells = 0,
        .num_groups = 0,
        .num_users = 0,
        .nblocks_bdz_gid = 0,
        .nblocks_bdz_groupname = 0,
        .nblocks_bdz_uid = 0,
        .nblocks_bdz_username = 0,
        .nblocks_groups = 0,
        .nblocks_users = 0,
        .nblocks_groupmembers = 0,
        .nblocks_additional_gids = 1,
    };

    const bytes = mem.asBytes(&header1);
    const header = try Header.fromBytes(bytes);
    try testing.expectEqual(header1, header.*);
}