struct.lua

local unpack = table.unpack or _G.unpack

local struct = {}

function struct.pack(format, ...)
  local stream = {}
  local vars = {...}
  local endianness = true

  for i = 1, format:len() do
    local opt = format:sub(i, i)

    if opt == '<' then
      endianness = true
    elseif opt == '>' then
      endianness = false
    elseif opt:find('[bBhHiIlL]') then
      local n = opt:find('[hH]') and 2 or opt:find('[iI]') and 4 or opt:find('[lL]') and 8 or 1
      local val = tonumber(table.remove(vars, 1))

      local bytes = {}
      for j = 1, n do
        table.insert(bytes, string.char(val % (2 ^ 8)))
        val = math.floor(val / (2 ^ 8))
      end

      if not endianness then
        table.insert(stream, string.reverse(table.concat(bytes)))
      else
        table.insert(stream, table.concat(bytes))
      end
    elseif opt:find('[fd]') then
      local val = tonumber(table.remove(vars, 1))
      local sign = 0

      if val < 0 then
        sign = 1
        val = -val
      end

      local mantissa, exponent = math.frexp(val)
      if val == 0 then
        mantissa = 0
        exponent = 0
      else
        mantissa = (mantissa * 2 - 1) * math.ldexp(0.5, (opt == 'd') and 53 or 24)
        exponent = exponent + ((opt == 'd') and 1022 or 126)
      end

      local bytes = {}
      if opt == 'd' then
        val = mantissa
        for i = 1, 6 do
          table.insert(bytes, string.char(math.floor(val) % (2 ^ 8)))
          val = math.floor(val / (2 ^ 8))
        end
      else
        table.insert(bytes, string.char(math.floor(mantissa) % (2 ^ 8)))
        val = math.floor(mantissa / (2 ^ 8))
        table.insert(bytes, string.char(math.floor(val) % (2 ^ 8)))
        val = math.floor(val / (2 ^ 8))
      end

      table.insert(bytes, string.char(math.floor(exponent * ((opt == 'd') and 16 or 128) + val) % (2 ^ 8)))
      val = math.floor((exponent * ((opt == 'd') and 16 or 128) + val) / (2 ^ 8))
      table.insert(bytes, string.char(math.floor(sign * 128 + val) % (2 ^ 8)))
      val = math.floor((sign * 128 + val) / (2 ^ 8))

      if not endianness then
        table.insert(stream, string.reverse(table.concat(bytes)))
      else
        table.insert(stream, table.concat(bytes))
      end
    elseif opt == 's' then
      table.insert(stream, tostring(table.remove(vars, 1)))
      table.insert(stream, string.char(0))
    elseif opt == 'c' then
      local n = format:sub(i + 1):match('%d+')
      local str = tostring(table.remove(vars, 1))
      local len = tonumber(n)
      if len <= 0 then
        len = str:len()
      end
      if len - str:len() > 0 then
        str = str .. string.rep(' ', len - str:len())
      end
      table.insert(stream, str:sub(1, len))
      i = i + n:len()
    end
  end

  return table.concat(stream)
end

function struct.unpack(format, stream, pos)
  local vars = {}
  local iterator = pos or 1
  local endianness = true

  for i = 1, format:len() do
    local opt = format:sub(i, i)

    if opt == '<' then
      endianness = true
    elseif opt == '>' then
      endianness = false
    elseif opt:find('[bBhHiIlL]') then
      local n = opt:find('[hH]') and 2 or opt:find('[iI]') and 4 or opt:find('[lL]') and 8 or 1
      local signed = opt:lower() == opt

      local val = 0
      for j = 1, n do
        local byte = string.byte(stream:sub(iterator, iterator))
        if endianness then
          val = val + byte * (2 ^ ((j - 1) * 8))
        else
          val = val + byte * (2 ^ ((n - j) * 8))
        end
        iterator = iterator + 1
      end

      if signed and val >= 2 ^ (n * 8 - 1) then
        val = val - 2 ^ (n * 8)
      end

      table.insert(vars, math.floor(val))
    elseif opt:find('[fd]') then
      local n = (opt == 'd') and 8 or 4
      local x = stream:sub(iterator, iterator + n - 1)
      iterator = iterator + n

      if not endianness then
        x = string.reverse(x)
      end

      local sign = 1
      local mantissa = string.byte(x, (opt == 'd') and 7 or 3) % ((opt == 'd') and 16 or 128)
      for i = n - 2, 1, -1 do
        mantissa = mantissa * (2 ^ 8) + string.byte(x, i)
      end

      if string.byte(x, n) > 127 then
        sign = -1
      end

      local exponent = (string.byte(x, n) % 128) * ((opt == 'd') and 16 or 2) + math.floor(string.byte(x, n - 1) / ((opt == 'd') and 16 or 128))
      if exponent == 0 then
        table.insert(vars, 0.0)
      else
        mantissa = (math.ldexp(mantissa, (opt == 'd') and -52 or -23) + 1) * sign
        table.insert(vars, math.ldexp(mantissa, exponent - ((opt == 'd') and 1023 or 127)))
      end
    elseif opt == 's' then
      local bytes = {}
      for j = iterator, stream:len() do
        if stream:sub(j) == '' then
          break
        end

        table.insert(bytes, stream:sub(j, j))
      end

      local str = table.concat(bytes)
      iterator = iterator + str:len() + 1
      table.insert(vars, str)
    elseif opt == 'c' then
      local n = format:sub(i + 1):match('%d+')
      local len = tonumber(n)
      if len <= 0 then
        len = table.remove(vars)
      end

      table.insert(vars, stream:sub(iterator, iterator + len - 1))
      iterator = iterator + len
      i = i + n:len()
    end
  end

  return unpack(vars)
end

return struct