The Universe of Disco

More about auto-generated switch-cases

Yesterday I described what I thought was a cool hack I had seen in rsync, to try several possible methods and then remember which one worked so as to skip the others on future attempts. This was abetted by a different hack, for automatically generating the case labels for the switch, which I thought was less cool.

Simon Tatham wrote to me with a technique for compile-time generation of case labels that I liked better. Recall that the context is:

        int set_the_mtime(...) {
          static int switch_step = 0;
          switch (switch_step) {

        #ifdef METHOD_1_MIGHT_WORK
            case ???:
              if (method_1_works(...))
                break;
              switch_step++;
              /* FALLTHROUGH */
        #endif

        #ifdef METHOD_2_MIGHT_WORK
            case ???:
              if (method_2_works(...))
                break;
              switch_step++;
              /* FALLTHROUGH */
        #endif

        ... etc. ...
          }
          return 1;
        }

M. Tatham suggested this:

        #define NEXT_CASE   switch_step = __LINE__; case __LINE__

You use it like this:

        int set_the_mtime(...) {
          static int switch_step = 0;
          switch (switch_step) {

            default:
        #ifdef METHOD_1_MIGHT_WORK
            NEXT_CASE:
              if (method_1_works(...))
                break;
              /* FALLTHROUGH */
        #endif

        #ifdef METHOD_2_MIGHT_WORK
            NEXT_CASE:
              if (method_2_works(...))
                break;
              /* FALLTHROUGH */
        #endif

        ... etc. ...
          }
          return 1;
        }

The case labels are no longer consecutive, but that doesn't matter; all that is needed is for them to be distinct. Nobody is ever going to see them except the compiler. M. Tatham called this “the case __LINE__ trick”, which suggested to me that it was generally known. But it was new to me.

One possible drawback of this method is that if the file contains more than 255 lines, the case labels will not fit in a single byte. The ultimate effect of this depends on how the compiler handles switch. It might be compiled into a jump table with !!2^{16}!! entries, which would only be a problem if you had to run your program in 1986. Or it might be compiled to an if-else tree, or something else we don't want. Still, it seems like a reasonable bet.

You could use case 0: at the beginning instead of default:, but that's not as much fun. M. Tatham observes that it's one of very few situations in which it makes sense not to put default: last. He says this is the only other one he knows:

        switch (month) {
            case SEPTEMBER:
            case APRIL:
            case JUNE:
            case NOVEMBER:
                days = 30;
                break;
            default:
                days = 31;
                break;
            case FEBRUARY:
                days = 28;
                if (leap_year)
                    days = 29;
                break;
        }

Addendum 20181029: Several people have asked for an explanation of why the default is in the middle of the last switch. It follows the pattern of a very well-known mnemonic poem that goes

Thirty days has September,
  April, June and November.
All the rest have thirty-one
  Except February, it's a different one:
It has 28 days clear,
  and 29 each leap year.

Wikipedia says:

[The poem has] been called “one of the most popular and oft-repeated verses in the English language” and “probably the only sixteenth-century poem most ordinary citizens know by heart”.

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