Question

I’m trying to convert some python code to Lua. What is the Lua equivalent to:

value2 = ''
key = 'cmpg'
value1 = '\x00\x00\x00\x00\x00\x00\x00\x01'
Value2 += '%s%s%s' % (key, struct.pack('>i', len(value1)), value1)

Answer 1

You said in a comment:

I may be able to get by with knowing the string generated by each of
the following: struct.pack(‘>i’,4),struct.pack(‘>i’,8), and
struct.pack(‘>i’,10)

The effector ‘>i’ means bigendian signed 32-bit integer. For non-negative input x, the simple Python equivalent would be

chr((x >> 24) & 255) + chr((x >> 16) & 255) + chr((x >> 8) & 255) + chr(x & 255)

You should be able to express that in Lua without much difficulty.

You said in yet another comment:

I … don’t understand an answer (@john machin)

chr(x) is easy to find in the docs. Lua should have such a function, perhaps even with the same name.

i >> n shifts i right by n bits. If i is unsigned, this is equivalent to i // ( 2 ** n) where // is Python’s integer floor division.

i & 255 is a bitwise-and which is equivalent to i % 256.

Lua should have both of those.

The + is in this case string concatenation.

Have a look at this:

>>> import binascii
>>> def pack_be_I(x):
...     return (
...         chr((x >> 24) & 255) +
...         chr((x >> 16) & 255) +
...         chr((x >>  8) & 255) +
...         chr(x         & 255)
...         )
...
>>> for anint in (4, 8, 10, 0x01020304, 0x04030201):
...     packed = pack_be_I(anint)
...     hexbytes = binascii.hexlify(packed)
...     print anint, repr(packed), hexbytes
...
4 '\x00\x00\x00\x04' 00000004
8 '\x00\x00\x00\x08' 00000008
10 '\x00\x00\x00\n' 0000000a
16909060 '\x01\x02\x03\x04' 01020304
67305985 '\x04\x03\x02\x01' 04030201
>>>

You’ll notice that the required output for 10 is '\x00\x00\x00\n' … note that '\x0a' aka '\n' aka chr(10) needs care. If you are writing this stuff to a file on Windows, you must open the file in binary mode ('wb', not 'w') otherwise the run-time library will insert a carriage-return byte to conform with Windows,MS-DOS,CP/M conventions for text files.