That's a pretty standard way to comment out a huge chunk of code, because /* */ will get screwed up if there's another comment using the same convention inside, and putting // in front of every single line looks bad/makes copying code around painful.

#if 0 is also a convenient way to flip bits of test/debug code on and off.

  #if 0 <...> #else <...> #endif

Is a pretty standard way to switch between new/old code while refactoring to check if behavior/output matches. Just change 0 to 1 to switch between "versions".