Find all hex starting with and has fixed length

[akshaybabloo]

Improving on top of #1996 (comment).

I want to find all packets starting from 55 AA and continuing find till a packet_length is reached. Here is what I have done so far

import type.base;
import std.mem;

u32 PACKET_SIZE = 74;

struct Packet {
    $ += 2;

    u16 length;

    type::Hex<u8> data[while((length > PACKET_SIZE) && ($[$]!=0x55 || $[$+1] != 0xAA) && !std::mem::eof())];
};

std::mem::MagicSearch<"\x55\xAA", Packet> find@0;

This doesn't seem to work, because the loop doesn't wait for the length for the packet, here is a screenshot

You can see that the size differs between each 55 AA. I am not sue what I am doing wrong.

[paxcut]

length is being read after the 0x55AA is found and wont change for the entire struct instance so i an not sure what the purpose of this (length > PACKET_SIZE) inside the conditional is for since it will always be either true or false.

The instructions of what you want to accomplish seem contradictory. First you imply that you want to read from the magic sequence for a fixed length but then you state that the sizes between magic numbers is not the same. Depending on which of the two is the one you are after the code will have to be different.

[akshaybabloo]

Yeah that's the thing, the packet size is 74 and 0x55 0xAA can come anytime between them, so on the first find of 0x55 0xAA it should loop till 74 and get the next packet and ignore if nothing else is found.

Yeah, I contradicted it, sorry.

[paxcut]

I am still not clear on what it is supposed to do. For example, you define a variable for the packet size but inside the pattern you also define a variable called length with no explanation as to what it is for. Also you don't state if 74 includes the magic number or not. Another missing piece of information is about the smallest size a packet may have.

I'm going to take a guess and write code based on the guess. My guess is that you look for the sequence 0x55 0xAA and from there you either take 73 bytes after the sequence or as many bytes as you can find before another 0x55 0xAA is found but never more that 73 bytes. (I chose 73 because that's in the middle of either including the magic sequence in the packet size or not). At a minimum the packet contains 2 bytes labeled as length (if there are two consecutive magic sequences then the second will be treated as length).

This code should do that. If that is not correct please state what part is incorrect so the code can be revised:

import type.base;
import std.mem;

u32 PACKET_SIZE = 74;

struct Packet {
    u64 savedAddress = $;
    $ += 2;

    u16 length; // what is this for? I am not using it for now

    type::Hex<u8> data[while((($-savedAddress) < PACKET_SIZE) && ($[$]!=0x55 && $[$+1] != 0xAA) && !std::mem::eof())];
};

std::mem::MagicSearch<"\x55\xAA", Packet> find@0;

[akshaybabloo]

Ok, this kinda works, when a 0x55 0xAA is found, take 72 bytes after that, so in that case, here is an example data

24 00 55 96 1A E8 00 00  00 00 00 [55 AA 47 04 06
00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
10 75 0F AF 00 00 00 00  01 00 00 00 00 00 00 00
00 00 00 00 CB 19 00 00  00 00 7E 00 00 00 00 01
00 17 47 01 05 1E 00 8C  00 02 03 00 00 00 00 00
00 00 24 00 55] E6 9D 06  01 00 00 00 00 [55 AA 47
04 06 00 00 00 00 00 00  00 00 00 00 00 00 00 00
00 00 10 75 0F AF 00 00  00 00 01 00 00 00 00 00
00 00 00 00 00 00 DF 19  00 00 00 00 7E 00 00 00
00 01 00 17 47 01 05 1E  00 8C 00 02 03 00 00 00
00 00 00 00 24 00 55 26  5F 25 01 00 00 00 00 55]
AA 47 04 06 00 00 00 00  00 00 00 00 00 00 00 00

When I ran your code, I get (denoted by [<hex>] from above:

                                                      55 AA 47 04 06
00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00
10 75 0F AF 00 00 00 00  01 00 00 00 00 00 00 00
00 00 00 00 CB 19 00 00  00 00 7E 00 00 00 00 01
00 17 47 01 05 1E 00 8C  00 02 03 00 00 00 00 00
00 00 24 00

and

                                                                 55 AA 47
04 06 00 00 00 00 00 00  00 00 00 00 00 00 00 00
00 00 10 75 0F AF 00 00  00 00 01 00 00 00 00 00
00 00 00 00 00 00 DF 19  00 00 00 00 7E 00 00 00
00 01 00 17 47 01 05 1E  00 8C 00 02 03 00 00 00
00 00 00 00 24 00 55 26  5F 25 01 00 00 00 00

It doesn't add the 55 at the end, I tried increasing the PACKET_SIZE to 78 and it stops right before 55.

[paxcut]

There is a mistake. if you wanted to find the magic sequence you would write

bool found = false;
if ($[$]==0x55 && $[$+1] == 0xAA) 
    found = true;

the while inside the array wants to avoid finding them which is the negative of finding them so it should be

!($[$]==0x55 && $[$+1] == 0xAA) =>  ($[$]!=0x55 || $[$+1] != 0xAA) 

So in the code change ($[$]!=0x55 && $[$+1] != 0xAA) to ($[$]!=0x55 || $[$+1] != 0xAA) and then it should work for you. The reason it didnt read the 0x55 is because the && makes it avoid either 0x55 or 0xAA so when it finds 0x55 it stops.

It actually was correct in the code you posted.

[akshaybabloo]

Thanks for the help!

[akshaybabloo]

What is $ here?

[paxcut]

The symbol $ stands for the current position of the cursor, in other words, the current offset of the input file that the pattern language is evaluating the pattern. the symbol $[$] stands for the byte value of the input signal at that offset. The added code savesthe offset of the starting of the struct and then check to see if enough bytes have been read in order to stop.