local elf = require("elf")
local ec = require("elf.constants")

local function die(fmt, ...)
    io.stderr:write("mkelf: ", string.format(fmt, ...), "\n")
    os.exit(1, true)
end

local function usage()
    die("[-o output] input")
end

---@param fn string|nil
---@return {header: Header, sections: Section[], segments: Segment[]}
local function loadconfig(fn)
    local spec = {}
    local env = {
        -- Keep in sync with types/elf.d.lua
        Class = elf.Class,
        Endian = elf.Endian,
        Osabi = elf.Osabi,
        Type = elf.Type,
        Machine = elf.Machine,
        SegmentType = elf.SegmentType,
        SegmentFlags = elf.SegmentFlags,
        ELFVERSION = elf.VERSION,

        header = function(t) spec.header = t end,
        sections = function(t) spec.sections = t end,
        segments = function(t) spec.segments = t end,
    }
    -- TODO error check for multiple headers
    local chunk, err = loadfile(fn, "t", env)
    if not chunk then die("%s", err) end
    local ok, perr = pcall(chunk)
    if not ok then die("%s", perr) end
    return spec
end

local function main()
    local outfn, infn = "a.out", nil
    local i = 1
    while arg[i] do
        if arg[i] == "-o" then
            i = i + 1
            outfn = arg[i] or die("missing filename after -o")
        elseif infn then
            usage()
        else
            infn = arg[i]
        end
        i = i + 1
    end
    local c = loadconfig(infn)
    c.header.phnum = #c.segments
    c.header.phentsize = (c.header.class == ec.Class.bit64) and 0x38 or 0x20
    -- TODO write to a temp file and atomic rename to outfn.
    local out <close> = assert(io.open(outfn, "wb"))
    out:write(elf.header.pack(c.header))
    out:write(elf.segment.pack(c.segments, c.header.data, c.header.class))
    -- TODO finish section.pack
    -- out:write(elf.section.pack(c.sections, c.header.data))
end

main()