local version <const> = 1

---@enum Osabi
local Osabi <const> = {
    sysv = 0x00,
    hpux = 0x01,
    netbsd = 0x02,
    linux = 0x03,
    hurd = 0x04,
    solaris = 0x06,
    aix = 0x07,
    irix = 0x08,
    freebsd = 0x09,
    tru64 = 0x0a,
    modesto = 0x0b,
    openbsd = 0x0c,
    openvms = 0x0d,
    nsk = 0x0e,
    aros = 0x0f,
    fenixos = 0x10,
    cloudabi = 0x11,
    opevos = 0x12,
}

---@enum Class
local Class <const> = {
    bit32 = 0x1,
    bit64 = 0x2,
}

---@enum Endian
local Endian <const> = {
    little = 0x1,
    big = 0x2,
}

---@enum Type
local Type <const> = {
    none = 0x00,
    rel = 0x01,
    exec = 0x02,
    dyn = 0x03,
    core = 0x04,
    loos = 0xfe00,
    hios = 0xfeff,
    loproc = 0xfe00,
    hiproc = 0xfeff,
}

---@enum Machine
local Machine <const> = {
    none = 0x00,
    att = 0x01,
    sparc = 0x02,
    x86 = 0x03,
    m68k = 0x04,
    m88k = 0x05,
    -- todo
}

---@alias ElfMagic [integer, integer, integer, integer]
---@class ElfHeader
---@field magic ElfMagic
---@field class Class
---@field data Endian
---@field version integer
---@field osabi Osabi
---@field abiversion integer
---@field type Type
---@field machine Machine

---@type ElfHeader
local header <const> = {
    magic = { 0x7f, 0x45, 0x4c, 0x46 },
    class = Class.bit32,
    data = Endian.big,
    version = version,
    osabi = Osabi.netbsd,
    abiversion = 0,
    type = Type.exec,
    machine = Machine.x86,

    pack = function(self)
        local ident = string.pack("BBBBBBBBBxxxxxxx",
            self.magic[1], self.magic[2], self.magic[3], self.magic[4],
            self.class,
            self.data,
            self.version,
            self.osabi,
            self.abiversion)
        local e = (self.data == Endian.big) and ">" or "<"
        local sz = (self.class == Class.bit64) and "I8" or "I4"
        local hdr = string.pack(e .. "I2I2I4" ..
            sz .. sz .. sz ..
            "I4I2I2I2I2I2I2",
            self.type,
            self.machine,
            self.version,
            0,
            0x34,
            0xff,
            0,
            52,
            0x20,
            0,
            0x28,
            0,
            0)
        return ident .. hdr
    end
}

local function main()
    local output = "a.out"
    for i in ipairs(arg) do
        if arg[i] ~= "-o" then goto continue end
        if not arg[i + 1] then
            error("mkelf: missing filename after -o")
        end
        output = arg[i + 1]
        ::continue::
    end
    local f = assert(io.open(output, "w"))
    f:write(header:pack())
    f:close()
end

main()