javascript关联数组使用什么样的散列函数/算法？

Question

我们了解到有许多不同的散列算法/函数,我很好奇javascript使用了哪一个(v8,如果实现很重要).

Answer 1

由于V8是开源的,所以你去了源码:

这是GetHash():https://github.com/v8/v8/blob/master/src/objects.cc#L903

并且,以下是一些针对不同类型的哈希函数:https://github.com/v8/v8-git-mirror/blob/bda7fb22465fc36d99b4053f0ef60cfaa8441209/src/utils.h#L347

而且,这看起来像字符串的核心哈希计算:https://code.google.com/p/v8/source/browse/trunk/src/objects.cc？specpec = svn6&r = 6#3574

uint32_t String::ComputeHashCode(unibrow::CharacterStream* buffer,
                                 int length) {
  // Large string (please note large strings cannot be an array index).
  if (length > kMaxMediumStringSize) return HashField(length, false);

  // Note: the Jenkins one-at-a-time hash function
  uint32_t hash = 0;
  while (buffer->has_more()) {
    uc32 r = buffer->GetNext();
    hash += r;
    hash += (hash << 10);
    hash ^= (hash >> 6);
  }
  hash += (hash << 3);
  hash ^= (hash >> 11);
  hash += (hash << 15);

  // Short string.
  if (length <= kMaxShortStringSize) {
    // Make hash value consistent with value returned from String::Hash.
    buffer->Rewind();
    uint32_t index;
    hash = HashField(hash, ComputeArrayIndex(buffer, &index, length));
    hash = (hash & 0x00FFFFFF) | (length << kShortLengthShift);
    return hash;
  }

  // Medium string (please note medium strings cannot be an array index).
  ASSERT(length <= kMaxMediumStringSize);
  // Make hash value consistent with value returned from String::Hash.
  hash = HashField(hash, false);
  hash = (hash & 0x0000FFFF) | (length << kMediumLengthShift);
  return hash;
}

值得一提的是,V8尽可能避免在可能的情况下使用哈希表作为对象属性,而不是出于性能原因将已知属性引用编译为直接索引引用而不是运行时哈希查找(尽管有时只能这样做 - 取决于代码).