replacing if-then-else strcmp ladders with switch cases

replacing if-then-else strcmp ladders with switch cases

[jim.cromie]

considering  lib/dynamic_debug.c
we have

...
} else if (!strcmp(words[i], "module")) {
    rc = check_set(&query->module, words[i+1], "module");
} else if (!strcmp(words[i], "format")) {
...

are there any built-in hash functions which would allow this ?

switch (keyword) {
case Hash("module"):
    ..... break;
case Hash("format"):
    .... break;
default: return -EINVAL
}

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Re: replacing if-then-else strcmp ladders with switch cases

[Valdis Klētnieks]

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On Fri, 12 Jun 2020 12:05:55 -0600, jim.cromie@gmail.com said:
> considering  lib/dynamic_debug.c
> we have
>
> ...
> } else if (!strcmp(words[i], "module")) {
>     rc = check_set(&query->module, words[i+1], "module");
> } else if (!strcmp(words[i], "format")) {
> ...
>
> are there any built-in hash functions which would allow this ?
>
> switch (keyword) {
> case Hash("module"):
>     ..... break;
> case Hash("format"):
>     .... break;
> default: return -EINVAL
> }

There's hash functions.  But they're all cryptographic hashes that return
things that are far too many bits to use as the index of a switch.

Also, you have the problem that the cases of a switch have to be something
that can be evaluated at compile time....

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Re: replacing if-then-else strcmp ladders with switch cases

[jim.cromie]

On Fri, Jun 12, 2020 at 11:02 PM Valdis Klētnieks
<valdis.kletnieks@vt.edu> wrote:
>
> On Fri, 12 Jun 2020 12:05:55 -0600, jim.cromie@gmail.com said:
> > considering  lib/dynamic_debug.c
> > we have
> >
> > ...
> > } else if (!strcmp(words[i], "module")) {
> >     rc = check_set(&query->module, words[i+1], "module");
> > } else if (!strcmp(words[i], "format")) {
> > ...
> >
> > are there any built-in hash functions which would allow this ?
> >
> > switch (keyword) {
> > case Hash("module"):
> >     ..... break;

that should be:

/* 2 compatible implementations */
#define chash(keywd)  simple_hash_computed_by_compiler ( keywd )
#define rhash(keywd)  simple_hash_computed_at_runtime ( keywd )

  switch ( rhash (keyword) ) {
  case chash("module"):
      ... break;

> There's hash functions.  But they're all cryptographic hashes that return
> things that are far too many bits to use as the index of a switch.
>
> Also, you have the problem that the cases of a switch have to be something
> that can be evaluated at compile time....

ok, so they dont exist.
I suspect its doable, but needs big -fu, more than I got

that said, I could imagine a compile-time check to insure that
the simple-hash on a fixed dictionary yields no collision

DEFINE_DICTIONARY( "dyndbg.control.adverbs", "module", "file", "line", "func" )

but thats not essential, since if theyre in case:s,
the compiler would notice the collision,
and so chash() doesnt need to do collision detection either.

this would be slick, and I think usable more than once.

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