local fs = require "filesystem"
local args = {...}

local dev = args[1] or error("Must specify a file")

local device = fs.open(dev, "wb")

local sectors = math.floor(device:seek("end", 0)/512)
print(sectors .. " sectors")
device:seek("set", 0)

local fat12 = sectors/4 < 4085
local fatSize = math.ceil(sectors*(fat12 and 1.5 or 2)/2048)

local startseq = "\xEB\x3C\x90ocdosfs\0"
local bytesPerSector = "\0\2"
local sectorsPerCluster = "\4"
local reservedSectors = "\1\0"
local fatnum = "\2"
local dirent = "\0\2"
local sectors = string.char(sectors%256) .. string.char(math.floor(sectors/256))
local descriptor = "\xF8"
local sectorsPerFat = string.char(fatSize%256) .. string.char(math.floor(fatSize/256))
local sectorsPerTrack = "\x20\0"
local heads = "\x40\0"
local hiddenSect = "\0\0\0\0"
local largeSectorAmount = "\0\0\0\0"

local drivenum = "\x80"
local ntflags = "\0"
local signature = "\x29"
local id = "\0\0\0\0"
local label = "NO NAME    "
local sysid = fat12 and "FAT12   " or "FAT16   "
local bootcode = ("\0"):rep(448)
local bootsig = "\x55\xAA"

local bootRecord = startseq .. bytesPerSector .. sectorsPerCluster .. reservedSectors .. fatnum .. dirent .. sectors .. descriptor
    .. sectorsPerFat .. sectorsPerTrack .. heads .. hiddenSect .. largeSectorAmount
    .. drivenum .. ntflags .. signature .. id .. label .. sysid .. bootcode .. bootsig

print("Boot record size: " .. #bootRecord)

device:write(bootRecord)

print("Cleaning FAT tables")
os.sleep(0)

for i = 1, fatSize * 2 do --related to fat sizes/count
    device:write(("\0"):rep(512))
end

print("Cleaning Root Directory")
os.sleep(0)

for i = 1, 512 do
    device:write(("\0"):rep(32))
end