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