Well, it is every version of RAR. Documenting the quirks of rar 1.4, 1.5, 2.0, 2.9, 3.0, 4.0, 5.0 and 7.0, multiple compression strategies, PPMd, RARVM, compression levels, encryption, multi volume support, a huge test corpus, round trips for compatibility... The spec docs are linked.

5 week is a decompressor for 1 version. If this supports multiple versions of RAR, then writing decompressors alone for all of them is probably a year effort of work.