Robert Prest hasn't bought No Man's Sky yet, but in a way, he's already played it. For the past three weeks, the man has used his spare time to build a version of Hello Games's ambitious—and controversial, depending on who you talk to—space exploration game in id Software's Doom, released back in 1993.

Yes, that Doom.

"The main reason for working on it was just to see if I could do it," Prest said recently.

He hasn't settled on a name for his mod yet, wavering between No Guy's Sky and Doom Guy's Sky. (My vote's for the latter.) The mod is more than an aesthetic makeover; Doom Guy's Sky tries to replicate the core features of No Man's Sky, right down to applying random generation algorithms to the creatures and planets. You can even mine resources to provide additional fuel for your ship, allowing you to leave a planet. Alien languages are randomized, too.

How No Man's Sky algorithm works is a bit of a mystery, but Prest did his best to explain his approach.

"The algorithm is incredibly basic," he said. "Every [space] jump you do, it randomly raises and lowers multiple areas. Then [it] has a base water level that rises to a random level. Any holes that go lower than this will have water in them. The game then randomizes the tree type and bush type for the level, then spawns it at random locations based on tones of predefined possible locations. The grass and rocks are spawned in the same way. It then changes the color, floor texture, sky texture and fade, changes and resets the aliens, and then by the time it finishes doing this, your spaceship has traveled to the world and it comes down to land."

The result of that algorithm is a planet looking like this:

Or this:

As Prest hasn't played No Man's Sky, he based his entire mod on the game's trailers. When he'd initially show early versions of the mod to friends and colleagues, they'd have to correct him on how features actually ended up working in the game. He even found time to needle the original developers of No Man's Sky over the backlash the game endured in the weeks after its release.

"Don't worry, the patches will make it fun," reads one message as you quit the mod.

Doom Guy's Sky started as a three-day project, but much like the feature creep that seemed to impact the actual No Man's Sky over its several years of development, Prest worked on the mod for another three weeks. The mod built upon his previous work applying randomization algorithms to Doom, having worked on transforming it into one of the most popular survival games of the last five years, DayZ. Prest is still actively developing DoomZ today.

I hope they [Hello Games] don't sue me, and I hope they get a laugh from it as I have. And don't sue me. —Robert Prest

Making video games isn't Prest's day job; he works network operations for a broadband ISP. As a kid, he would make Doom levels in his spare time, but walked away from the series for ten years before being drawn back by the ridiculousness of trying to bring DayZ into Doom.

"This way, I'm free to work on whatever I like whether anyone else is interested, and that keeps it fun to do," he said. "Several of my friends went into game development, and I follow their work really proudly—must be great to have your name on a full game. It would be great to work in games development, but I'd feel uncomfortable ever taking money for it, as it might take some of the fun away from it."

The reason Prest chose Doom as his medium of expression, rather than a modern engine like Unity or Unreal Engine 4, is precisely because the technology limitations require getting creative to solve problems. It helps that Doom mod tools have continued to evolve since the game's release decades ago, meaning amateur coders can tackle ambitious ideas.

Ultimately, Doom Guy's Sky, like DoomZ, is for fun, and Prest just wants it to stay that way.

"I hope they [Hello Games] don't sue me," he said. "And I hope they get a laugh from it as I have. And don't sue me."

You can download Doom Guy's Sky on Prest's website right now.