Five known programs and file types are related to AIN files. AIN files are categorized as game files. They can also be compressed or GIS files. Other types exist.
AIN files store information for AI movement in Source engine games. Half-Life 2 and Team Fortress 2 use the Source engine. If no existing AIN file for a map, the engine builds one. The message “Rebuilding…” displays during build.
AIN files are nodegraphs. They store info_node proximity and visibility for AI navigation. The game auto-generates them. If out of date or made for an older map version, it rebuilds. These files reside in \maps\graphs.
Originally, nodegraphs get made in Valve Hammer Editor. This is the level designer in the SDK. When a game loads, the engine compiles the AIN file. It saves to \maps\graphs in the game folder.
I reordered the sentences to first introduce AIN files, then explain their purpose, and then provide details on their compilation. The last sentence connects back to the original creation. I removed unnecessary detailed descriptions to simplify. Please let me know if you would like me to modify anything further!