text: /* -*- Mode: C; tab-width: 4; c-basic-offset: 4; indent-tabs-mode: nil -*- */
  /*
   * Hash table
   *
   * The hash function used here is by Bob Jenkins, 1996:
   *    <http://burtleburtle.net/bob/hash/doobs.html>
   *       "By Bob Jenkins, 1996.  bob_jenkins@burtleburtle.net.
   *       You may use this code any way you wish, private, educational,
   *       or commercial.  It's free."
   *
   * The rest of the file is licensed under the BSD license.  See LICENSE.
   */
  
  #include "memcached.h"
  #include <sys/stat.h>
  #include <errno.h>
  #include <stdlib.h>
  #include <stdio.h>
  #include <string.h>
  #include <platform/cbassert.h>
  #include "log.h"
  
  static cb_cond_t maintenance_cond;
  
  
  typedef  unsigned long  int  ub4;   /* unsigned 4-byte quantities */
  typedef  unsigned       char ub1;   /* unsigned 1-byte quantities */
  
  /* how many powers of 2's worth of buckets we use */
  static unsigned int hashpower = 16;
  
  #define hashsize(n) ((ub4)1<<(n))
  #define hashmask(n) (hashsize(n)-1)
  
  /* Main hash table. This is where we look except during expansion. */
  static item** primary_hashtable = 0;
  
  /*
   * Previous hash table. During expansion, we look here for keys that haven't
   * been moved over to the primary yet.
   */
  static item** old_hashtable = 0;
  
  /* Number of items in the hash table. */
  static unsigned int hash_items = 0;
  
  /* Flag: Are we in the middle of expanding now? */
  static bool expanding = false;
  
  /*
   * During expansion we migrate values with bucket granularity; this is how
   * far we've gotten so far. Ranges from 0 .. hashsize(hashpower - 1) - 1.
   */
  static unsigned int expand_bucket = 0;
  
  void assoc_init(void) {
      cb_cond_initialize(&maintenance_cond);
      primary_hashtable = calloc(hashsize(hashpower), sizeof(void *));
      if (! primary_hashtable) {
          moxi_log_write("Failed to init hashtable.\n");
          exit(EXIT_FAILURE);
      }
  }
  
  item *assoc_find(const char *key, const size_t nkey) {
      uint32_t hv = hash(key, nkey, 0);
      item *it;
      unsigned int oldbucket;
  
      if (expanding &&
          (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)
      {
          it = old_hashtable[oldbucket];
      } else {
          it = primary_hashtable[hv & hashmask(hashpower)];
      }
  
      item *ret = NULL;
      int depth = 0;
      while (it) {
          if ((nkey == it->nkey) && (memcmp(key, ITEM_key(it), nkey) == 0)) {
              ret = it;
              break;
          }
          it = it->h_next;
          ++depth;
      }
      MEMCACHED_ASSOC_FIND(key, nkey, depth);
      return ret;
  }
  
  /* returns the address of the item pointer before the key.  if *item == 0,
     the item wasn't found */
  
  static item** _hashitem_before (const char *key, const size_t nkey) {
      uint32_t hv = hash(key, nkey, 0);
      item **pos;
      unsigned int oldbucket;
  
      if (expanding &&
          (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)
      {
          pos = &old_hashtable[oldbucket];
      } else {
          pos = &primary_hashtable[hv & hashmask(hashpower)];
      }
  
      while (*pos && ((nkey != (*pos)->nkey) || memcmp(key, ITEM_key(*pos), nkey))) {
          pos = &(*pos)->h_next;
      }
      return pos;
  }
  
  /* grows the hashtable to the next power of 2. */
  static void assoc_expand(void) {
      old_hashtable = primary_hashtable;
  
      primary_hashtable = calloc(hashsize(hashpower + 1), sizeof(void *));
      if (primary_hashtable) {
          if (settings.verbose > 1)
              moxi_log_write("Hash table expansion starting\n");
          hashpower++;
          expanding = true;
          expand_bucket = 0;
          cb_cond_signal(&maintenance_cond);
      } else {
          primary_hashtable = old_hashtable;
          /* Bad news, but we can keep running. */
      }
  }
  
  /* Note: this isn't an assoc_update.  The key must not already exist to call this */
  int assoc_insert(item *it) {
      uint32_t hv;
      unsigned int oldbucket;
  
      cb_assert(assoc_find(ITEM_key(it), it->nkey) == 0);  /* shouldn't have duplicately named things defined */
  
      hv = hash(ITEM_key(it), it->nkey, 0);
      if (expanding &&
          (oldbucket = (hv & hashmask(hashpower - 1))) >= expand_bucket)
      {
          it->h_next = old_hashtable[oldbucket];
          old_hashtable[oldbucket] = it;
      } else {
          it->h_next = primary_hashtable[hv & hashmask(hashpower)];
          primary_hashtable[hv & hashmask(hashpower)] = it;
      }
  
      hash_items++;
      if (! expanding && hash_items > (hashsize(hashpower) * 3) / 2) {
          assoc_expand();
      }
  
      MEMCACHED_ASSOC_INSERT(ITEM_key(it), it->nkey, hash_items);
      return 1;
  }
  
  void assoc_delete(const char *key, const size_t nkey) {
      item **before = _hashitem_before(key, nkey);
  
      if (*before) {
          item *nxt;
          hash_items--;
          /* The DTrace probe cannot be triggered as the last instruction
           * due to possible tail-optimization by the compiler
           */
          MEMCACHED_ASSOC_DELETE(key, nkey, hash_items);
          nxt = (*before)->h_next;
          (*before)->h_next = 0;   /* probably pointless, but whatever. */
          *before = nxt;
          return;
      }
      /* Note:  we never actually get here.  the callers don't delete things
         they can't find. */
      cb_assert(*before != 0);
  }
  
  
  static volatile int do_run_maintenance_thread = 1;
  
  #define DEFAULT_HASH_BULK_MOVE 1
  int hash_bulk_move = DEFAULT_HASH_BULK_MOVE;
  
  static void assoc_maintenance_thread(void *arg) {
      (void)arg;
  
      while (do_run_maintenance_thread) {
          int ii = 0;
  
          /* Lock the cache, and bulk move multiple buckets to the new
           * hash table. */
          cb_mutex_enter(&cache_lock);
  
          for (ii = 0; ii < hash_bulk_move && expanding; ++ii) {
              item *it, *next;
              int bucket;
  
              for (it = old_hashtable[expand_bucket]; NULL != it; it = next) {
                  next = it->h_next;
  
                  bucket = hash(ITEM_key(it), it->nkey, 0) & hashmask(hashpower);
                  it->h_next = primary_hashtable[bucket];
                  primary_hashtable[bucket] = it;
              }
  
              old_hashtable[expand_bucket] = NULL;
  
              expand_bucket++;
              if (expand_bucket == hashsize(hashpower - 1)) {
                  expanding = false;
                  free(old_hashtable);
                  if (settings.verbose > 1)
                      moxi_log_write("Hash table expansion done\n");
              }
          }
  
          if (!expanding) {
              /* We are done expanding.. just wait for next invocation */
              cb_cond_wait(&maintenance_cond, &cache_lock);
          }
  
          cb_mutex_exit(&cache_lock);
      }
  }
  
  static cb_thread_t maintenance_tid;
  
  int start_assoc_maintenance_thread() {
      int ret;
      char *env = getenv("MEMCACHED_HASH_BULK_MOVE");
      if (env != NULL) {
          hash_bulk_move = atoi(env);
          if (hash_bulk_move == 0) {
              hash_bulk_move = DEFAULT_HASH_BULK_MOVE;
          }
      }
      if ((ret = cb_create_thread(&maintenance_tid,
                                   assoc_maintenance_thread, NULL, 0)) != 0) {
          moxi_log_write("Can't create thread: %s\n", strerror(ret));
          return -1;
      }
      return 0;
  }
  
  void stop_assoc_maintenance_thread() {
      cb_mutex_enter(&cache_lock);
      do_run_maintenance_thread = 0;
      cb_cond_signal(&maintenance_cond);
      cb_mutex_exit(&cache_lock);
  
      /* Wait for the maintenance thread to stop */
      cb_join_thread(maintenance_tid);
  }
  
  
  