Yojouhan Shinwa Taikei - 1x10 - 四畳半主義者- Followmy.tv

Disillusioned with any form of campus life, the protagonist joins no clubs as a freshman in college, choosing instead to spend all his time within his 4½ tatami room. He awakens after 2 years to find that he is surrounded by an infinite number of seemingly identical rooms beyond every door, window, and wall. He sustains himself on castellas he finds in the rooms, and explores, collecting bits of money. It becomes clear that the rooms are not in fact identical, but bear slight variations corresponding to the choices he could have (and has) made throughout the rest of the series.