local ELFVERSION <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,
    m32 = 0x01,       -- AT&T WE 32100
    sparc = 0x02,     -- SPARC
    i386 = 0x03,      -- Intel 80386
    m68k = 0x04,      -- Motorola 68000 (M68k)
    m88k = 0x05,      -- Motorola 88000 (M88k)
    mcu = 0x06,       -- Intel MCU
    i860 = 0x07,      -- Intel 80860
    mips = 0x08,      -- MIPS
    s370 = 0x09,      -- IBM System/370
    mipsrs3le = 0x0a, -- MIPS RS3000 Little Endian
    parisc = 0x0f,    -- Hewlett-Packard PA-RISC
    vpp500 = 0x11,    -- Fujitsu VPP500
    sparc32 = 0x12,   -- Enhanced instruction set SPARC
    i960 = 0x13,      -- Intel 80960
    ppc = 0x14,       -- PowerPC
    ppc64 = 0x15,     -- PowerPC (64-bit)
    s390 = 0x16,      -- IBM System/390
    spu = 0x17,       -- IBM SPC/SPU
    v800 = 0x24,      -- NEC V800
    fr20 = 0x25,      -- Fujitsu FR20
    -- todo
}

---@alias ElfMagic [integer, integer, integer, integer]
---@class ElfHeader
---@field magic ElfMagic
---@field class Class
---@field data Endian
---@field ident_version integer
---@field osabi Osabi
---@field abiversion integer
---@field type Type
---@field machine Machine
---@field version integer
---@field entry integer
---@field phoff integer
---@field shoff integer
---@field flags integer
---@field ehsize integer
---@field phentsize integer
---@field phnum integer
---@field shentsize integer
---@field shnum integer
---@field shstrndx integer

---@param endian Endian
---@nodiscard
local function efmt(endian)
    return (endian == Endian.big) and ">" or "<"
end

---@param class Class
---@nodiscard
local function szfmt(class)
    return (class == Class.bit64) and "I8" or "I4"
end

---@param hdr ElfHeader
---@nodiscard
local function ElfHeader_pack(hdr)
    return string.pack("BBBBBBBBBxxxxxxx" ..
        efmt(hdr.data) ..
        "I2I2I4" ..
        string.rep(szfmt(hdr.class), 3) ..
        "I4I2I2I2I2I2I2",
        hdr.magic[1], hdr.magic[2], hdr.magic[3], hdr.magic[4],
        hdr.class,
        hdr.data,
        hdr.ident_version,
        hdr.osabi,
        hdr.abiversion,
        hdr.type,
        hdr.machine,
        hdr.version,
        hdr.entry,
        hdr.phoff,
        hdr.shoff,
        hdr.flags,
        hdr.ehsize,
        hdr.phentsize,
        hdr.phnum,
        hdr.shentsize,
        hdr.shnum,
        hdr.shstrndx)
end

---@class ProgramHeader
---@field type integer
---@field flags integer
---@field offset integer
---@field vaddr integer
---@field paddr integer
---@field filesz integer
---@field memsz integer
---@field align integer

---@enum SegmentType
local SegmentType <const> = {
    null = 0x00000000,
    load = 0x00000001,
    dynamic = 0x00000002,
    interp = 0x00000003,
    note = 0x00000004,
    shlib = 0x00000005,
    phdr = 0x00000006,
    tls = 0x00000007,
    loos = 0x60000000,
    hios = 0x6FFFFFFF,
    loproc = 0x70000000,
    hiproc = 0x7FFFFFFF,
}

---@enum SegmentFlags
local SegmentFlags <const> = {
    x = 0x1,
    w = 0x2,
    r = 0x3,
}

---@param hdr ProgramHeader
---@param endian Endian
---@param class Class
---@nodiscard
local function ElfPHeader_pack(hdr, endian, class)
    if class == Class.bit32 then
        return string.pack(efmt(endian) .. string.rep(szfmt(class), 8),
            hdr.type,
            hdr.offset,
            hdr.vaddr,
            hdr.paddr,
            hdr.filesz,
            hdr.memsz,
            hdr.flags,
            hdr.align)
    elseif class == Class.bit64 then
        return string.pack(efmt(endian) .. "I4I4" .. string.rep(szfmt(class), 6),
            hdr.type,
            hdr.flags,
            hdr.offset,
            hdr.vaddr,
            hdr.paddr,
            hdr.filesz,
            hdr.memsz,
            hdr.align)
    end
end

local function main()
    ---@type ElfHeader
    local header <const> = {
        magic = { 0x7f, 0x45, 0x4c, 0x46 },
        class = Class.bit32,
        data = Endian.big,
        ident_version = ELFVERSION,
        osabi = Osabi.netbsd,
        abiversion = 0,
        type = Type.exec,
        machine = Machine.i386,
        version = ELFVERSION,
        entry = 0,
        phoff = 0x34,
        shoff = 0xff,
        flags = 0,
        ehsize = 52,
        phentsize = 0x20,
        phnum = 0,
        shentsize = 0x28,
        shnum = 0,
        shstrndx = 0,
    }
    ---@type ProgramHeader
    local pheader <const> = {
        type = SegmentType.dynamic,
        flags = SegmentFlags.x,
        offset = 0xf,
        vaddr = 0xff,
        paddr = 0xfff,
        filesz = 0xffff,
        memsz = 0xfffff,
        align = 1,
    }
    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(ElfHeader_pack(header))
    f:write(ElfPHeader_pack(pheader, header.data, header.class))
    f:close()
end

main()