Read Write Format

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Description

-- This format function packs a list of integers into a binary string.
-- The sizes of the integers can be specified, both little and big endian
-- ordering are supported. The format parameter is a string composed of 
-- ASCII digit numbers, the size in bytes of the corresponding value.
-- Example:
--   write_format(true, "421", 0x12345678, 0x432931, 0x61) returns "xV4.1)a",
--     a 7 bytes string whose characters are in hex: 78 56 45 12 31 29 61
function write_format(little_endian, format, ...)
  local res = ''
  local values = {...}
  for i=1,#format do
    local size = tonumber(format:sub(i,i))
    local value = values[i]
    local str = ""
    for j=1,size do
      str = str .. string.char(value % 256)
      value = math.floor(value / 256)
    end
    if not little_endian then
      str = string.reverse(str)
    end
    res = res .. str
  end
  return res
end

-- This format function does the inverse of write_format. It unpacks a binary
-- string into a list of integers of specified size, supporting big and little 
-- endian ordering. Example:
--   read_format(true, "421", "xV4.1)a") returns 0x12345678, 0x2931 and 0x61.
function read_format(little_endian, format, str)
  local idx = 0
  local res = {}
  for i=1,#format do
    local size = tonumber(format:sub(i,i))
    local val = str:sub(idx+1,idx+size)
    local value = 0
    idx = idx + size
    if little_endian then
      val = string.reverse(val)
    end
    for j=1,size do
      value = value * 256 + val:byte(j)
    end
    res[i] = value
  end
  return unpack(res)
end