The engine for the first Halflife was a customized and modified version of Id’s engine, so originally they shared not only some principles but also code. “Gold

The engine for the first Halflife was a customized and modified version of Id’s engine, so originally they shared not only some principles but also code. “GoldSrc is a game engine developed by Valve Corporation, first showcased in the 1998 first-person shooter game Half-Life. Elements of GoldSrc are based on a heavily modified version of id Software's Quake engine. “ https://en.wikipedia.org/wiki/GoldSrc https://en.wikipedia.org/wiki/GoldSrc