Need help in converting the python command into lua

I need help in converting the below python commands which are basically extracting the CRC of a file(*.LUH or *.bin) file into Lua commands.

filename_expected = os.path.abspath(header_path)

Read and extract data from Header.LUH

f = open(filename_expected, 'rb')
header_data = f.read()
f.close()

Create a list of Python integers representing header_bytes in file

header_bytes = map(ord, header_data)
partnum_len = header_bytes\[27\]

Convert ordinals to a list of string'ized two-digit hex numbers e.g. ['0a', '02', '34', 'ff', 'e7', ...]

bytes_in_hex_representation = map(lambda n: "%.2x" % n, header_bytes)

Get hex representation of desired header_bytes

header_crc_number = ''.join(bytes_in_hex_representation[-4:])

I have started to write that into lua, came only this till reading the data.

header_file = 'header.LUH'
header_path = ".\\table\\" .. header_file

\-- Read and extract data from Header.LUH
file = io.open(header_path, 'r')
t = file:read("\*all")
print(t)
file:close()

Does Lua has any commands like 'map'or 'ord' or lambda ??

Lua does not need lambda since functions in Lua are all anonymous (that is, they are values).

Lua is minimalistic; it does not have map, but you can implement it yourself. The equivalent to ord is string.byte.

Anyways, let's break it down:

Setting the path:

local header_path = [[.\table\header.LUH]]

I've used a local variable and a "long string literal" here. Your code is fine too.

Opening the file:

local file = io.open(header_path, "rb")

Note that you need rb in Lua for binary files (just like in Python or any other sane language); you may not use just r - otherwise Windows systems will perform line feed normalization, making you read the CRLF bytes instead of LF (the PNG header for instance contains LF to check against this).

For reading the entire contents:

local content = file:read"*a"

newer Lua versions will also accept just a. You may not escape the asterisk - doing so should throw a syntax error ("invalid escape sequence").

Closing the file using file:close() is fine - however, if an error happens in-between, you may end up not closing the file. In Lua 5.4, you could mark file as a to-be-closed variable using <close> to have it be closed automatically as it goes out of scope (successfully or through error): local file <close> = io.open(header_path, "rb").

In Python, you're now converting the bytes to a list of integers. In Lua, you wouldn't take this extra step. You'd directly treat the string as a bytestring and convert bytes to hex pairs using string.gsub:

local content_hex = content:gsub(".", function(char) return ("%02x"):format(char:byte()) end)

This uses char:byte() to convert the character to its numeric representation and string.format to format this number as a two-digit hex string.

You can then substring the desired hex digits (the last 4 digits, equivalent to the last 2 bytes) using string.sub:

local crc_hex = content_hex:sub(-4)

and you're done! However, this code does plenty of extra work; it reads the entire file into memory and converts it into hex just for the CRC checksum at the end. You should instead seek 2 bytes before the end, read the two bytes, then convert them to hex as described above:

file:seek("end", -2)
local crc = file:read(2)
local crc_hex = content:gsub(".", function(char) return ("%02x"):format(char:byte()) end)

This will be vastly more efficient (much, much less disk I/O), will require much less memory (only 2 bytes are read into memory) and much less CPU time (only 2 bytes are converted to hex). Overall, you're looking at going from linear to constant time!