Storing and returning pointer values?

[ArcaneEngineer]

I'm replacing my own map implementation with khash in a given project. My old map could return pointers or values. This pattern still makes sense for my project. However, khash is built to return values for performance reasons, I believe, so returning pointers reliably isn't really supported, FWICS (?).

I'd like to use khash for both performance-critical and less demanding work where I just need string-key/pointer-value pairs, so am here to ask, what would you do to store/return a pointer in a khash map?

I can see 3 ways:

• Use khash to store array indices rather than pointers... but here we'd have to also ref into that array
• Hack it using int64_t... this should theoretically work on systems with smaller word sizes as well, but it seems a bit dodgy... my project needs to work for Arm (32-bit) & Intel (64-bit)
• The better solution seems to be to use intptr_t as the value type, but am worried this will have some problems.

Any advice greatly appreciated.

[attractivechaos]

You can have something like this (you would like to use a better hash function as pointers are not random):

#if UINTPTR_MAX == UINT64_MAX
static inline uint32_t hash_intptr(uintptr_t key) // by Thomas Wang
{
  key = (~key) + (key << 18);
  key = key ^ (key >> 31);
  key = (key + (key << 2)) + (key << 4);
  key = key ^ (key >> 11);
  key = key + (key << 6);
  key = key ^ (key >> 22);
  return (uint32_t)key;
}
#else
static inline uint32_t hash_intptr(uintptr_t key)
{
  key = ~key + (key << 15);
  key = key ^ (key >> 12);
  key = key + (key << 2);
  key = key ^ (key >> 4);
  key = (key + (key << 3)) + (key << 11);
  key = key ^ (key >> 16);
  return key;
}
#endif

#define my_hash_eq(a,b) ((a)==(b))
KHASH_INIT(name, uintptr_t, value_type, 1, hash_intptr, my_hash_eq)

Probably there are better ways to test the size of uintptr_t.

[ArcaneEngineer]

Thanks. Actually my original question didn't make sense (I was thinking of applying hash and equals to values... eh!) It was relatively simple in the end.

In my header:
static const int StrPtr = 36;
KHASH_DECLARE(StrPtr, kh_cstr_t, uintptr_t)

In my source (note, I have removed static from khash's SCOPE to get things working; YMMV):

__KHASH_IMPL(StrPtr, kh_inline klib_unused, kh_cstr_t, uintptr_t, 1, kh_str_hash_func, kh_str_hash_equal)

And of course the khash instance is set up as:

khash_t(StrPtr) * devicesByName = kh_init(StrPtr);

And access is as all other examples. For now, this seems to be working. This issue can be closed.