Unable to Get Bitwise XOR to work in GameMaker 2 - hash

Question

I'm trying to implement Zobrist hashing for a chess game and thought I was doing things correctly. However, when I try to apply XOR to a Hash value, it simply changes between 0 (the starting value for the hash) and -2147483648, in spite of conducting a bitwise XOR against a random number between 0 and 18446744073709551616.
Here's some code. I thought it was relatively simple with a struct for each entry in the Grid, a constructor function to make an entry, a function to create the Hash Table, and finally one for the Hash itself.
//Hash Table struct. This can be expanded for each new piece type
HashTableEntry = {
QueenHash:-1,
BishopHash:-1,
KnightHash:-1,
RookHash:-1,
PawnHash:-1,
KingHash:-1
};
//A function for creating a Hash Table Entry
function toHashTableEntryStruct() constructor {
var HashUpperBound = 18446744073709551616;
QueenHash = random(HashUpperBound);
BishopHash = random(HashUpperBound);
KnightHash = random(HashUpperBound);
RookHash = random(HashUpperBound);
PawnHash = random(HashUpperBound);
KingHash = random(HashUpperBound);
}
//A function to create a ds_grid Hash Table of the size passed to it, which it then returns
function CreateHashTable(_width, _height){
//Copy the local variables
var width = _width;
var height = _height;
//Create the grid for the Hash Table
var NewHashTable = ds_grid_create(width, height);
//Copy the original seed and set the game's seed to create the same table each time
var OriginalSeed = random_get_seed;
random_set_seed(280804);
//Loop through the Grid and fill each value with a HashTableEntry struct
for(var i = 0; i < width; i++;){
for(var j = 0; j < height; j++;){
var NewHash = new toHashTableEntryStruct();
ds_grid_set(NewHashTable, i, j, NewHash);
}
}
//Reset the seed
random_set_seed(OriginalSeed);
//Return the shiny new Hash Table
return NewHashTable;
}
//A function for creating the original Hash of a board. This should only be called on
initialising a Board as it sets the Original Hash
function CreateBoardHash(_board){
var Board = _board;
var width = ds_grid_width(Board);
var height = ds_grid_height(Board);
HashTable = CreateHashTable(width, height);
var FinalHash = 0;
for(var i = 0; i < width; i++;){
for(var j = 0; j < height; j++;){
var PieceToCheck = ds_grid_get(Board, i, j);
if(PieceToCheck == Pieces.BLANK){ continue; }
var HashEntry = ds_grid_get(HashTable, i, j);
var XorValue = MatchPieceToHashTableEntry(PieceToCheck, HashEntry);
FinalHash ^= XorValue;
}
}
if(FinalHash == 0){
//Error here
return;
}
//Set the instance variable for this original Hash
OriginalHash = FinalHash;
//Add it to the Current Hash stack
CurrentHash = FinalHash;
//Return said hash for the calling function
return FinalHash;
}
The problem comes in the FinalHash ^= XorValue; line, where XorValue each time is a random number, but FinalHash only ever comes out as 0 or -2147483648.
Can someone help with why this is the case and what I may be doing wrong with my XOR function?

Answer 1

Thanks to Reddit, I have fixed this. GameMaker must be doing something weird with the ^= operator, as the solution was as follows:
FinalHash = FinalHash ^ XorValue;