> They used the same routines they wrote for their day jobs at Softdisk in the Keen code. [...] Most of the IDLIB.C code must have come directly from the PC version of Dangerous Dave. [...] there is some extremely strong evidence showing that the id founders used Softdisk's code in their own game. Sure, it's not the code responsible for the smooth scrolling, but it is code they probably didn't have the rights to use.

Huh, this is interesting. Is someone able to provide more detail?

The pace at which Id produced games has always been an inspiration for me. Large amounts of code reuse seems like an important clue as to how they were able to do that.[1] But how were they able to reuse code effectively to such a degree?

[1]: The other clues I have so far are Romero's legendary tool-making abilities, and Carmack's tendency to produce code that gets computers to do things they couldn't before.

Id Software very much skirted the edge of legality by making Commander Keen outside of office hours while still employed by SoftDisk and using SoftDisk computers, and SoftDisk could have easily sued them if they wanted to. They managed to avoid that by striking a deal where the Id guys would continue to make games for SoftDisk while working on Keen and later Wolfenstein 3D.

There was a lot of code reuse between games. John Carmack is on record somewhere that the enemy navigation code from Doom and Quake still has its origins in some of the earliest 8-bit games he wrote in the 1980's.

In Masters of Doom they are depicted as taking work computers out of the office to go work on their side project. I doubt IP would be the thing they’d stand on.

To some degree this is amusing. For a decade or so, we people would talk about the “borrowed” PCs. Now hacker forums talk about who owns the IP. In my childhood I never would have guessed this culture shift towards IP maximalism but I imagine the lesson that copyleft licenses only work in a copyright enforced environment finally took!