turbonss/src/header.zig

const std = @import("std");
const shell = @import("shell.zig");

const HeaderSize = @divExact(@bitSizeOf(Header), 8);
const Magic = [4]u8{ 0xf0, 0x9f, 0xa4, 0xb7 };
const Version = 0;
const Bom = 0x1234;

pub const SectionLength = 64;

const InvalidHeader = error{
    InvalidMagic,
    InvalidVersion,
    InvalidBom,
    TooManyShells,
    InvalidOffset,
};

const Header = packed struct {
    magic: [4]u8,
    version: u8,
    bom: u16,
    num_shells: u8,
    num_users: u32,
    num_groups: u32,
    offset_bdz_uid2user: u32,
    offset_bdz_groupname2group: u32,
    offset_bdz_username2user: u32,
    offset_idx: u32,
    offset_groups: u32,
    offset_users: u32,
    offset_groupmembers: u32,
    offset_additional_gids: u32,

    pub fn init(blob: [HeaderSize]u8) InvalidHeader!Header {
        const self = @bitCast(Header, blob);
        for (Magic) |item, index| {
            if (self.magic[index] != item) return error.InvalidMagic;
        }
        if (self.version != 0) {
            return error.InvalidVersion;
        }
        if (self.bom != Bom) {
            return error.InvalidBom;
        }
        if (self.num_shells > shell.max_shells) {
            return error.TooManyShells;
        }

        const offsets = [_]u32{
            self.offset_bdz_uid2user,
            self.offset_bdz_groupname2group,
            self.offset_bdz_username2user,
            self.offset_idx,
            self.offset_groups,
            self.offset_users,
            self.offset_groupmembers,
            self.offset_additional_gids,
        };
        for (offsets) |offset| {
            if (offset & (SectionLength - 1) != 0) {
                return error.InvalidOffset;
            }
        }

        return self;
    }

    pub fn asArray(self: *const Header) [HeaderSize]u8 {
        return @bitCast([HeaderSize]u8, self.*);
    }
};

const testing = std.testing;

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

test "header pack, unpack and validation" {
    const goodHeader = Header{
        .magic = Magic,
        .version = Version,
        .bom = Bom,
        .num_shells = 0,
        .num_users = 0,
        .num_groups = 0,
        .offset_bdz_uid2user = 0,
        .offset_bdz_groupname2group = 0,
        .offset_bdz_username2user = 0,
        .offset_idx = 0,
        .offset_groups = 0,
        .offset_users = 0,
        .offset_groupmembers = 0,
        .offset_additional_gids = 0,
    };

    const gotHeader = try Header.init(goodHeader.asArray());
    try testing.expectEqual(goodHeader, gotHeader);

    {
        var header = goodHeader;
        header.magic[0] = 0;
        try testing.expectError(error.InvalidMagic, Header.init(header.asArray()));
    }

    {
        var header = goodHeader;
        header.bom = 0x3412;
        try testing.expectError(error.InvalidBom, Header.init(header.asArray()));
    }

    {
        var header = goodHeader;
        header.offset_bdz_uid2user = 65;
        try testing.expectError(error.InvalidOffset, Header.init(header.asArray()));
    }
}