-- NBS file format documentation: http://www.stuffbydavid.com/nbs
-- Only parts 1 and 2 are used here.

instrumentMap = {[0]=0, [1]=4, [2]=1, [3]=2, [4]=3}

-- Parse arguments
args = {...}
if #args < 1 then
  print("Usage: convertnbs filename")
  print("Converts a Note Block Studio (.nbs) file to a .sng file for playback with playsng.")
  return
end

-- Get file
filename = table.concat(args, " ")
file = fs.open(filename, "rb")
if not file then
  print("File not found: "..filename)
  return
end

-- Helper functions for reading values from the file (in little endian!)
function readShort(f)
  return f.read() + f.read() * 256
end

function readInt(f)
  return readShort(f) + f.read() * 65536 + f.read() * 16777216
end

function readString(f) -- int length + char payload
  count = readInt(f)
  ret = ""
  for i = 1, count do
    char = string.char(f.read())
    if char == "\r" then char = "\n" end -- for some weird reason, NBS uses CR as line break
    ret = ret..char
  end
  return ret
end

-- Initialize song
song = {}

-- Part 1: metadata
song["length"] = readShort(file)
song["height"] = readShort(file)
song["title"] = readString(file)
song["author"] = readString(file)
song["originalAuthor"] = readString(file)
song["description"] = readString(file)
song["tempo"] = readShort(file) / 100
song["interval"] = 1 / song["tempo"] -- sleep between ticks, saves computation
song["autoSave"] = file.read()
song["autoSaveDuration"] = file.read()
song["timeSignature"] = file.read()
song["minutesSpent"] = readInt(file)
song["leftClicks"] = readInt(file)
song["rightClicks"] = readInt(file)
song["blocksAdded"] = readInt(file)
song["blocksRemoved"] = readInt(file)
song["originalFile"] = readString(file)

print(song["title"])
print(song["author"])
print(song["originalAuthor"])
print(song["description"])
print("")
term.write("0 / "..song["length"].." t")

-- Part 2: the actual song - based on David's own pseudocode
function printTick(tick)
  if tick % 5 == 0 then
    x, y = term.getCursorPos()
    term.setCursorPos(1, y)
    term.write(tick.." / "..song["length"].." t")
  end
end

song["song"] = {}
tick = -1
jumps = 0
while true do
  jumps = readShort(file)
  if jumps == 0 then break -- end of song
  elseif jumps > 1 then -- if I'm leaving a gap while jumping, fill it in
    for i = 1, jumps - 1 do
      t = tick + i
      song["song"][t] = {} -- fill
      printTick(t)
    end
  end
  tick = tick + jumps -- jump
  layer = -1
  song["song"][tick] = {} -- initialize the tick
  while true do
    jumps = readShort(file)
    if jumps == 0 then break end -- end of tick
    instrument = instrumentMap[file.read()] -- NBS uses a different instrument value than the game
    if instrument == nil then instrument = 0 end -- custom instrument? fall back to piano
    table.insert(song["song"][tick], instrument)
    note = file.read()
    if note < 33 then note = 33         -- prevent notes from going out
    elseif note > 57 then note = 57 end -- of the game's octave limit
    note = note - 33
    table.insert(song["song"][tick], note)
  end
  
  printTick(tick)
end

printTick(song["length"])

print("")

-- Save it
outFile = fs.open(filename..".sng", "w")
outFile.write(textutils.serialize(song))
outFile.close()