A peek at the experiments Spelunky players use to manipulate the game's code

Players of Mossmouth's Spelunky HD are working to understand the game's inner workings, a process that has become integral to snagging a spot on the game's leaderboards.
"We took the precaution of making sure our tests were consistent and replicable, even if we didn't understand the dupe yet."

- A Spelunky player by the name of ContraMuffin details the process of testing potential score exploits.

The leaderboards of Mossmouth's Spelunky HD are ruled by scores from players that understand how to use the game’s seemingly basic mechanics in a complex way, sometimes in ways that the game’s original developer never anticipated.

Waypoint’s Patrick Klepek has shared his exploration of one such technique, offering developers a look at the methods used by a very dedicated community of Spelunky fans to understand (and later manipulate) how the game itself handles something as basic as item scoring.

Spelunky players typically compete for world records in either lowest completion time or highest score. The latter depends on picking up as much gold and gems as possible, each type of which has a varying monetary value that becomes a player’s leaderboard score when the level completes.

There are various things intentionally built into the game to let the player pad their score as the game goes on, like trips to the hidden City of Gold or kiting the invincible ghost enemy to float over gems and convert those into more lucrative diamonds. But some players noticed that piling up a stack of gems sometimes led to the game count some of them more than in their running score total. Klepek’s story dives into the exact how and why this happens, but in short players began to work out that when an item is ‘picked up’ in Spelunky, the game quickly gives players a set amount of money, marks the item to be deleted from the in-game environment, then deletes the item itself.

The duplication exploit, as the Spelunky community worked out, happens when a player picks up a bunch of items simultaneously and the game gets confused about that reward, mark, delete order, causing it to forget to delete an item or two. That bug allows some items to essentially to be picked up and scored twice.

There are some caveats to this method, all worked out by those Spelunky players through no small amount of testing, that involve how other items can interfere with that duplication glitch. The full story on Waypoint has more to say on those complications and explores how players experimented and came up with unconventional methods of manipulating the game’s code through in-game actions to best exploit that glitch. 

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