protobuf/upb_table.c

/*
 * upb - a minimalist implementation of protocol buffers.
 *
 * Copyright (c) 2009 Joshua Haberman.  See LICENSE for details.
 */

#include "upb_table.h"

#include <assert.h>
#include <stdlib.h>
#include <string.h>

static int compare_entries(const void *f1, const void *f2)
{
  return ((struct upb_inttable_entry*)f1)->key -
         ((struct upb_inttable_entry*)f2)->key;
}

static uint32_t max(uint32_t a, uint32_t b) { return a > b ? a : b; }

static uint32_t round_up_to_pow2(uint32_t v)
{
  /* cf. Bit Twiddling Hacks:
   * http://www-graphics.stanford.edu/~seander/bithacks.html#RoundUpPowerOf2 */
  v--;
  v |= v >> 1; v |= v >> 2; v |= v >> 4; v |= v >> 8; v |= v >> 16;
  v++;
  return v;
}

static struct upb_inttable_entry *find_empty_slot(struct upb_inttable *table)
{
  /* TODO: does it matter that this is biased towards the front of the table? */
  for(uint32_t i = 0; i < table->size; i++) {
    struct upb_inttable_entry *e =
        upb_inttable_entry_get(table->entries, i, table->entry_size);
    if(e->key == UPB_EMPTY_ENTRY) return e;
  }
  assert(false);
  return NULL;
}

void upb_inttable_init(struct upb_inttable *table, void *entries,
                       int num_entries, int entry_size)
{
  qsort(entries, num_entries, entry_size, compare_entries);

  /* Find the largest n for which at least half the keys <n are used.  We
   * make sure our table size is at least n.  This allows all keys <n to be
   * in their main position (as if it were an array) and only numbers >n might
   * possibly have collisions.  Start at 8 to avoid noise of small numbers. */
  upb_inttable_key_t n = 0, maybe_n;
  bool all_in_array = true;
  for(int i = 0; i < num_entries; i++) {
    struct upb_inttable_entry *e =
        upb_inttable_entry_get(entries, i, entry_size);
    maybe_n = e->key;
    if(maybe_n > 8 && maybe_n/(i+1) >= 2) {
      all_in_array = false;
      break;
    }
    n = maybe_n;
  }

  /* TODO: measure, tweak, optimize this choice of table size.  Possibly test
   * (at runtime) maximum chain length for each proposed size. */
  uint32_t min_size_by_load = all_in_array ? n : (double)num_entries / 0.85;
  uint32_t min_size = max(n, min_size_by_load);
  table->size = round_up_to_pow2(min_size);
  table->entry_size = entry_size;
  table->entries = malloc(table->size * entry_size);

  /* Initialize to empty. */
  for(size_t i = 0; i < table->size; i++) {
    struct upb_inttable_entry *e =
        upb_inttable_entry_get(table->entries, i, entry_size);
    e->key = UPB_EMPTY_ENTRY;
    e->next = NULL;
  }

  /* Insert the elements. */
  for(int i = 0; i < num_entries; i++) {
    struct upb_inttable_entry *e, *table_e;
    e = upb_inttable_entry_get(entries, i, entry_size);
    table_e = upb_inttable_mainpos(table, e->key);
    if(table_e->key != UPB_EMPTY_ENTRY) {  /* Collision. */
      if(table_e == upb_inttable_mainpos(table, table_e->key)) {
        /* Existing element is in its main posisiton.  Find an empty slot to
         * place our new element and append it to this key's chain. */
        struct upb_inttable_entry *empty = find_empty_slot(table);
        while (table_e->next) table_e = table_e->next;
        table_e->next = empty;
        table_e = empty;
      } else {
        /* Existing element is not in its main position.  Move it to an empty
         * slot and put our element in its main position. */
        struct upb_inttable_entry *empty, *colliding_key_mainpos;
        empty = find_empty_slot(table);
        colliding_key_mainpos = upb_inttable_mainpos(table, table_e->key);
        assert(colliding_key_mainpos->key != UPB_EMPTY_ENTRY);
        assert(colliding_key_mainpos->next);
        memcpy(empty, table_e, entry_size);  /* next is copied also. */
        while(1) {
          assert(colliding_key_mainpos->next);
          if(colliding_key_mainpos->next == table_e) {
            colliding_key_mainpos->next = empty;
            break;
          }
        }
        /* table_e remains set to our mainpos. */
      }
    }
    memcpy(table_e, e, entry_size);
    table_e->next = NULL;
  }
}

void upb_inttable_free(struct upb_inttable *table)
{
  free(table->entries);
}

/* Emit definition for inline functions. */