Legacy GM Hashing a datastructure

Ronchon

Member
Hi,
I would like to generate a "hash" out of datastructures such as ds_grids that would allow to quickly compare 2 different grids to know if they hold the same values, for example.
Comparing each tile's values is too slow, and too problematic if the values are of different types.
How would you approach this problem? How would you design a simple algorithm cheap and reliable enough to limit the odds of generating the same hash for 2 different grids ?
 

Dmi7ry

Member
Simple algorithm: store grids using ds_grid_write and compare results.
Reliable algorithm: compare each cell.
Cheap algorithm: I dont know. It depends on your data structures (size, how often you need compare it, etc).
 

Ronchon

Member
Thanks!
I have grids that can be quite large, like 200x200 holding strings or floats.
So i'm looking for a system as cheap as possible that i can use to "version" grids and compare the client / server ones in the context of a multiplayer game.
These functions would be used regularly on the server, every-time an update is made to the data.

I was thinking about ds_grid_write + md5_string but i'm worried it would be too slow. I'll make some tests...
Comparing the ds_grid_write strings directly is not possible.
Comparing each cell is what i currently do but its taking too much processing power, which is why i'm looking for alternatives.
 

chamaeleon

Member
Depending on what you are storing, perhaps some variant of Zobrist Hashing will help, if you have some fixed/limited number of things you store, or if you can reliably and cheaply generate a seemingly random number for a given value in a grid cell and the value that is going to replace it.
 

Ronchon

Member
Thanks, but I need a technique that is flexible and works with any type of grids and content! However today i learned about "xor" which i didnt know before :p

I have experimented a bit. Using the simple existing functions the fastest one was this :
Code:
key = md5_string_utf8(ds_grid_write(grid))
I also experimented a homemade method. On 100x100 grids, its nearly 10x faster than the previous technique on my machine, but it only works for grids holding numbers and no strings:
Code:
    str = string(w)+"*"+string(h)
    str += string(ds_grid_get_sum(grid,0,0,w,h))
 
    n=0
    repeat(w/2)
    { str += string(ds_grid_get_sum(grid,n,0,n,h));n+=2 }
    n=1
    repeat(h/2)
    { str += string(ds_grid_get_sum(grid,0,n,w,n));n+=2 }

    key = md5_string_utf8(str)
I can't find a way to detect changes in strings yet... :(
 
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chamaeleon

Member
Thanks, but I need a technique that is flexible and works with any type of grids and content! However today i learned about "xor" which i didnt know before :p
The principle holds for any data, as long as you have a consistent method of converting what you have store or will store in a grid element into a number that you can use xor with. Without actually attempting to do any coding and benchmarking, my gut tells me it would be faster convert a string value (or a number converted to a string) to a sha1 value, convert the first couple of characters in the sha1 value into a number from the hexadecimal representation for the old value in the grid and and new value that will replace it, respectively, and use xor with these, than performing a string conversion of data for 10,000 cells and then one sha1 on that whole string.
 

Ronchon

Member
I'm not sure i get it... But from what i understand that would mean inspecting the whole grid and testing each cell to see if its a string, to then be able to apply the operations you describe to each one.
But after testing, the simple operation of inspecting each cell and testing if its a string is already much longer than the ds_grid_write method, without doing anything else to the strings. Just the "for" loops + is_string is already too long.
 

chamaeleon

Member
I should mention that this work is really only relevant if the grid comparison is performed frequently. If grid updates happen all the time and comparisons between grids are rare, then it may not be worthwhile.

Anyway.. Starting with a grid hash of zero, when you set a value (ideally using a script to keep it clean when reading the code) in the grid you update the grid hash using xor by first performing an xor with the old cell hash value (an empty/uninitialized cell should have a value of zero), then xor with the new cell hash value. Done. There is no iteration over the whole grid to do anything. After this, the grid hash is available for use without any further iterations over any other cells.

Again, this may or may not be relevant for your use case. It is very useful for hash tables for board games where the state of the board changes continuously.
 

Dmi7ry

Member
Why your situation happens? Can't you just send info about changed cells after they were changed?
 

The-any-Key

Member
Is the strings you store fixed? Give an example of the string data. Maybe you can index it instead.
Ex:
1="this string"
2="another string"
This will make the grid only hold numbers.
 

Homunculus

Member
The MD5 of ds_grid_write sounds like a good idea to me, I use the same when comparing ds_maps. It's true that a 200x200 grid holds a lot of data, but I doubt you need to do this frequently, so why not?

Other than that, you may find other solutions depending on how you handle the data and that kind of data you are storing. If the grid is updated infrequently for example you can generate a new hash whenever something changes, instead of doing so on the fly when comparing.
 
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