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InfluxDB
Power Real-Time Data Analytics at Scale. Get real-time insights from all types of time series data with InfluxDB. Ingest, query, and analyze billions of data points in real-time with unbounded cardinality.
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ja2
The source code for Jagged Alliance 2. I didn't write this; see the Strategy First license agreement for details. Supplementary material for the Jagged Alliance 2 Boss Fight Book.
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freeciv
Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization. Upstream repository for the standalone Freeciv client and server. Report bugs and submit patches at https://osdn.net/projects/freeciv/ticket/
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game-hacking
Tutorials, tools, and more as related to reverse engineering video games. (by hdbreaker)
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cassette-elite-beebasm
Fully documented and annotated source code for the cassette version of Elite on the BBC Micro
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SaaSHub
SaaSHub - Software Alternatives and Reviews. SaaSHub helps you find the best software and product alternatives
Doom might be the best one, they released their code i don't remember when. It's in github if you want to check it out: link
Not most, but quite a few legendary ones were released in a "source available engine, paid maps" kind of way. Check out Id Software's github for a couple versions of DOOM, Quake and Wolfenstein. EA has released Command & Conquer's source code (here).
Some games, yeah. Here's the source code for Jagged Alliance 2.
A team or something reverse engineered super mario. I think that's the repo and it's mostly in C
To add to all the answers, you would also do something like cheat engine to understand the variables and memory use while running the game. You can find an example in this video and here you can find a github repo with a lot of good resources for decompiling and reverse engineering.