Read Write Format

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Lists of Integers

This function fails for negative singed values i.e. -86000 !

-- 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 "xV41)a",

--     a 7 bytes string whose characters are in hex: 78 56 34 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

More Integers

This functions fail for negative singed values i.e. -86000 !

Another way to read and write integers, written by Tom N Harris.

-- Read an integer in LSB order.

function stringtonumber(str)

  local function _b2n(exp, num, digit, ...)

    if not digit then return num end

    return _b2n(exp*256, num + digit*exp, ...)

  end

  return _b2n(256, string.byte(str, 1, -1))

end



-- Read an integer in MSB order.

function stringtonumber(str)

  local function _b2n(num, digit, ...)

    if not digit then return num end

    return _b2n(num*256 + digit, ...)

  end

  return _b2n(0, string.byte(str, 1, -1))

end



-- Write an integer in LSB order using width bytes.

function numbertobytes(num, width)

  local function _n2b(width, num, rem)

    rem = rem * 256

    if width == 0 then return rem end

    return rem, _n2b(width-1, math.modf(num/256))

  end

  return string.char(_n2b(width-1, math.modf(num/256)))

end



-- Write an integer in MSB order using width bytes.

function numbertobytes(num, width)

  local function _n2b(t, width, num, rem)

    if width == 0 then return table.concat(t) end

    table.insert(t, 1, string.char(rem * 256))

    return _n2b(t, width-1, math.modf(num/256))

  end

  return _n2b({}, width, math.modf(num/256))

end

Bitfields

function unpackbits(bits, width)

  local fl = {}

  local num,rem = flags

  for i = 1,width do

    num,rem = math.modf(num/2)

    fl[#fl+1] = rem>=0.5

  end

  return unpack(fl)

end

Floating-Point

function convertfloat(str)

  -- Change to b4,b3,b2,b1 to unpack an LSB float

  local b1,b2,b3,b4 = string.byte(str, 1, 4)

  local exponent = (b1 % 128) * 2 + math.floor(b2 / 128)

  if exponent == 0 then return 0 end

  local sign = (b1 > 127) and -1 or 1

  local mantissa = ((b2 % 128) * 256 + b3) * 256 + b4

  mantissa = (math.ldexp(mantissa, -23) + 1) * sign

  return math.ldexp(mantissa, exponent - 127)

end

Libraries

Original citation, Sirmabus: September 13, 2009