src/base/hash/cl_hash1weak.h

   1 // Weak hash tables with 1 key and a value
   2
   3 #ifndef _CL_HASH1WEAK_H
   4 #define _CL_HASH1WEAK_H
   5
   6 #include "cl_hash1.h"
   7
   8 // This is a hash table in which an entry can be removed when a user-defined
   9 // condition is fulfilled (e.g. the value is not referenced any more).
  10 // We don't remove unused entries immediately, only when the hash table
  11 // wants to grow. This way the hash table also serves as a cache.
  12
  13 // Requirements:
  14 // - same as for hash1,
  15 // - key1_type must be a subclass of cl_gc[object|pointer],
  16 // - value_type must be a subclass of cl_[gc|rc][object|pointer],
  17 // - function  maygc_htentry(const cl_htentry1<key1_type,value_type>&);
  18 //   must be filled in at runtime.
  19
  20 template <class key1_type, class value_type>
  21 struct cl_heap_weak_hashtable_1 : public cl_heap_hashtable_1 <key1_type,value_type> {
  22         // Allocation.
  23         void* operator new (size_t size) { return cl_malloc_hook(size); }
  24         // Deallocation.
  25         void operator delete (void* ptr) { cl_free_hook(ptr); }
  26 public:
  27         // Function which tells when an unused entry may be garbage collected.
  28         cl_boolean (* const _maygc_htentry) (const cl_htentry1<key1_type,value_type>&);
  29         // Constructor.
  30         cl_heap_weak_hashtable_1 (cl_boolean (*maygc_htentry) (const cl_htentry1<key1_type,value_type>&))
  31                 : cl_heap_hashtable_1 <key1_type,value_type> (),
  32                   _maygc_htentry (maygc_htentry)
  33         {
  34                 _garcol_fun = garcol;
  35         }
  36 private:
  37         // Garbage collection.
  38         // Before growing the table, we check whether we can remove unused
  39         // entries.
  40         static cl_boolean garcol (cl_heap* _ht)
  41         {
  42                 var cl_heap_weak_hashtable_1* ht = (cl_heap_weak_hashtable_1*)_ht;
  43                 // Now ht->_garcol_fun = garcol.
  44                 // It is not worth doing a garbage collection if the table
  45                 // is small, say, has fewer than 100 entries.
  46                 if (ht->_count < 100)
  47                         return cl_false;
  48                 // Do a garbage collection.
  49                 var long removed = 0;
  50                 for (long i = 0; i < ht->_size; i++)
  51                     if (ht->_entries[i].next >= 0) {
  52                         var cl_htentry1<key1_type,value_type>& entry = ht->_entries[i].entry;
  53                         if (ht->_maygc_htentry(entry)) {
  54                                 // This is hairy. We remove the entry and
  55                                 // free the value after its refcount has
  56                                 // dropped to zero. But in order to protect
  57                                 // against too early destruction
  58                                 // we have to temporarily increase the refcount.
  59                                 if (entry.val.pointer_p())
  60                                         entry.val.inc_pointer_refcount();
  61                                 ht->remove(entry.key);
  62                                 if (entry.val.pointer_p()) {
  63                                         var cl_heap* p = entry.val.heappointer;
  64                                         if (!(--p->refcount == 0)) cl_abort();
  65                                         cl_free_heap_object(p);
  66                                 }
  67                                 removed++;
  68                         }
  69                     }
  70                 if (removed == 0)
  71                         // Unsuccessful. Let the table grow immediately.
  72                         return cl_false;
  73                 else if (2*removed < ht->_count) {
  74                         // Table shrank by less than a factor of 1/1.5.
  75                         // Don't expand the table now, but expand it next time.
  76                         ht->_garcol_fun = garcol_nexttime;
  77                         return cl_true;
  78                 } else {
  79                         // Table shrank much. Don't expand the table now,
  80                         // and try a GC next time.
  81                         return cl_true;
  82                 }
  83         }
  84         static cl_boolean garcol_nexttime (cl_heap* _ht)
  85         {
  86                 var cl_heap_weak_hashtable_1* ht = (cl_heap_weak_hashtable_1*)_ht;
  87                 // Now ht->_garcol_fun = garcol_nexttime.
  88                 ht->_garcol_fun = garcol;
  89                 return cl_false;
  90         }
  91 };
  92
  93 #endif /* _CL_HASH1WEAK_H */