src/base/hash/cl_hashuniqweak.h

   1 // Weak hash tables for making objects unique
   2
   3 #ifndef _CL_HASHUNIQWEAK_H
   4 #define _CL_HASHUNIQWEAK_H
   5
   6 #include "cl_hashuniq.h"
   7
   8 namespace cln {
   9
  10 // This is a hashuniq table in which an entry can be removed when the
  11 // value is not referenced any more.
  12 // Best example: string -> symbol uniquification. When a symbol is not
  13 // referenced any more (except from the hash table itself), the (string,symbol)
  14 // pair may be removed from the hash table.
  15 // We don't remove unused entries immediately, only when the hash table
  16 // wants to grow. This way the hash table also serves as a cache.
  17
  18 // Requirements:
  19 // - same as for hashuniq,
  20 // - value_type must be a subclass of cl_[gc|rc][object|pointer].
  21 // Note that since the reference counts are compared against 1, it doesn't
  22 // make sense to have more than one weak hash table for the same value_type.
  23
  24 template <class key1_type, class value_type>
  25 struct cl_heap_weak_hashtable_uniq : public cl_heap_hashtable_uniq <key1_type,value_type> {
  26         // Allocation.
  27         void* operator new (size_t size) { return malloc_hook(size); }
  28         // Deallocation.
  29         void operator delete (void* ptr) { free_hook(ptr); }
  30 public:
  31         // Constructor.
  32         cl_heap_weak_hashtable_uniq ()
  33                 : cl_heap_hashtable_uniq <key1_type,value_type> ()
  34         {
  35                 this->_garcol_fun = garcol;
  36         }
  37 private:
  38         // Garbage collection.
  39         // Before growing the table, we check whether we can remove unused
  40         // entries.
  41         static cl_boolean garcol (cl_heap* _ht)
  42         {
  43                 var cl_heap_weak_hashtable_uniq* ht = (cl_heap_weak_hashtable_uniq*)_ht;
  44                 // Now ht->_garcol_fun = garcol.
  45                 // It is not worth doing a garbage collection if the table
  46                 // is small, say, has fewer than 100 entries.
  47                 if (ht->_count < 100)
  48                         return cl_false;
  49                 // Do a garbage collection.
  50                 var long removed = 0;
  51                 for (long i = 0; i < ht->_size; i++)
  52                     if (ht->_entries[i].next >= 0) {
  53                         var value_type& v = ht->_entries[i].entry.val;
  54                         if (!v.pointer_p() || (v.heappointer->refcount == 1)) {
  55                                 // This is hairy. We remove the entry and
  56                                 // free the value after its refcount has
  57                                 // dropped to zero. But in order to protect
  58                                 // against too early destruction (depending on
  59                                 // how the C++ compiler optimizes hashkey())
  60                                 // we have to temporarily increase the refcount.
  61                                 if (v.pointer_p())
  62                                         v.inc_pointer_refcount();
  63                                 ht->remove(hashkey(v));
  64                                 if (v.pointer_p()) {
  65                                         var cl_heap* p = v.heappointer;
  66                                         if (!(--p->refcount == 0)) cl_abort();
  67                                         cl_free_heap_object(p);
  68                                 }
  69                                 removed++;
  70                         }
  71                     }
  72                 if (removed == 0)
  73                         // Unsuccessful. Let the table grow immediately.
  74                         return cl_false;
  75                 else if (2*removed < ht->_count) {
  76                         // Table shrank by less than a factor of 1/1.5.
  77                         // Don't expand the table now, but expand it next time.
  78                         ht->_garcol_fun = garcol_nexttime;
  79                         return cl_true;
  80                 } else {
  81                         // Table shrank much. Don't expand the table now,
  82                         // and try a GC next time.
  83                         return cl_true;
  84                 }
  85         }
  86         static cl_boolean garcol_nexttime (cl_heap* _ht)
  87         {
  88                 var cl_heap_weak_hashtable_uniq* ht = (cl_heap_weak_hashtable_uniq*)_ht;
  89                 // Now ht->_garcol_fun = garcol_nexttime.
  90                 ht->_garcol_fun = garcol;
  91                 return cl_false;
  92         }
  93 };
  94
  95 }  // namespace cln
  96
  97 #endif /* _CL_HASHUNIQWEAK_H */