Question

In a computer contest, I was given a problem where I had to manipulate input data. The input has been split() into an array where data[0] is the number of repetitions. There can be up to 10^18 repetitions. My program returned Exception in thread "main" java.lang.OutOfMemoryError: Java heap space and I failed the contest.

Here’s a piece of my code that’s eating up memory and CPU:

long product[][]=new long[data[0]][2];
product[0][0]=data[1];
product[0][1]=data[2];
for(int a=1;a<data[0];a++){
  product[a][0]=((data[5]*product[a-1][0] + data[6]) % data[3]) + 1; // Pi = ((A*Pi-1 + B) mod M) + 1 (for all i = 2..N)
  product[a][1]=((data[7]*product[a-1][1] + data[8]) % data[4]) + 1; // Wi = ((C*Wi-1 + D) mod K) + 1 (for all i = 2..N)
}

Here’s some of the input data:

980046644627629799 9 123456 18 10000000 831918484 451864686 840000324 650000765
972766173386786486 123 1 10000000 10000000 590000001 680000000 610000001 970000002
299896237124947938 681206 164538 2280874 981991 416793690 904023823 813682336 774801135

My program can only work up to about 7 or 8 digits, then it takes minutes to run. With 18 digits, it crashed almost as soon as I clicked “Run” in Eclipse.

I’m curious as to how is it possible to manipulate that much data on a normal computer. Please let me know if my question is unclear or you need more information. Thanks!

Answer 1

You can’t have, and don’t need, an array of such a huge length. You just need to track the most recent 2values. E.g., just have product1 and product2.

Also, consider testing if either number is a NaN after each iteration. If so, throw an Exception and give the iteration number.
Because once you get a NaN they will all be NaN. Except you are using long, so scratch that. “Nevermind”. 🙂