Interview With a Slot Programmer


The casino industry is a restrained one, game developers in particular. It doesn’t necessarily have to do with unethical business practices as many seem to believe seeing how poorly the industry tends to score on various trust surveys. All companies need to protect their intellectual properties, and game developers are no different.

However, operating in the casino industry involves strict rules and heavy regulation. With the money in circulation, game developers also need to minimise the risk of exposing themselves and their games to vulnerabilities that could potentially be taken advantage of. Therefore, it’s hard to find people from the inside willing to talk about the structure of their games and the methods in use. We were lucky enough to have a sitdown with a slot programmer with many years of experience working on both land-based, as well as server-based gaming systems.

programming casino

We’re a bit curious about jackpot machines. Are they truly random or are they set to be triggered at some point?

There are in general two different types of jackpot machines – the progressive multi-million Euro jackpots, and the machine jackpots which usually only pay out a few thousand at most. In comparison, they are different beasts.

When it comes to machine jackpots, the random number generator is the engine that spits out the results. When it hits the right combo, which of course happens randomly given that its an RNG, the jackpot is triggered. By the nature of it, it’s never more or less likely to be triggered on any given spin. The progressive jackpot slots are also governed by random number generators but will usually have a threshold. This doesn’t mean that a player is more or less likely to trigger the jackpot at any given time, just that the probability of it happening becomes more likely the closer to the threshold they get.

But how can something be random while at the same time have an increase in probability?

That’s a good point. Random means that you tell the random number generator you need a number between 1 and 100. It will then guarantee that you hit a random value within the specified range which is how slots usually work. Translated to progressives, let’s assume a progressive must be triggered by €10k. Every time a player presses the max bet, the slot asks the RNG if “1 in 1000” is true. If it’s a no, then no jackpot win. However, as it comes closer and closer to the €10k threshold, the question narrows down further to become “1/100”, and eventually “1 in 10”. This means all players have an equal chance of triggering the jackpot while at the same time it becomes increasingly likely to happen.

Are the theoretical payout percentages of slot machines audited? If so, how are they validated?

Indeed they are. There are stringent regulations in this industry, and the gaming commission audits everything! Any slot can be set to whatever payout percentages as specified in the paytable of the game which for example means slot X could be set to 101%, 99.1%, 97.5%, 85.1% or 72.5%. In order for a slot to become certified and allowed to go live, millions of simulations are run on the game to verify that the RNG adds up with the RTP (read more about certification of online slots here). The provider has to submit these results to the gaming commission and must also be able to be repeated and proven whenever asked for.

The operator (casino) is allowed to choose whichever payout percentage it wants. The regulatory body does not actually oversee which RTP a slot is currently set to, and it is not its job to do so. The results are normally not followed up in practice, but the threat and fear of losing a license is enough to keep casinos and developers away from pulling any stunts.

Is there any way to find out the payout percentage?

If the payout percentage must be displayed or not depends on the jurisdiction. In countries such as Australia and in the UK, the RTP must be displayed at all times and is normally found in the paytable. Other jurisdictions, such as Costa Rica, Cyprus or Curacao does not have such requirements. This essentially means you will have no idea whatsoever which odds you’re up against. Could be 98% or it could be 85%.

How does a random number generator work exactly? Is it similar to the one found on

In general, no. It is much more advanced. Most providers hire mathematicians to do this job, and I would say these people are the true artists behind a slot. A substantial amount of work goes into the RNG to make sure it is 100% random as well as making sure the game is balanced enough to make it enjoyable. If the result turned out to be a random number generator that only achieved pseudorandomness, the consequences would potentially be catastrophic.

Many players believe programmers can write in secret code that would enable them to win on their own slots. Is this possible?

I can’t speak for all jurisdictions, but where I’m active, all programmers need a clean criminal record to get hired. This at least eliminates some unscrupulous people. I myself have never ever considered such a thing. The industry doesn’t attract such people, and besides, there are more than one of us working on each individual game which would make it hard, if not impossible, to pull such thing off. Moreover, game developers these days are paranoid enough to make sure more than one person understands every piece of code that goes into any slot. The only known person to have tried it back in the 90s, Ron Harris, went to prison and served serious time.

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