在升压幻数:: hash_combine [英] Magic number in boost::hash_combine

问题描述

在的boost :: hash_combine 模板函数需要引用一个哈希（称为籽）和对象 v 。根据<一href="http://www.boost.org/doc/libs/release/doc/html/hash/reference.html#boost.hash_combine">docs,它结合了籽与散v 按

The boost::hash_combine template function takes a reference to a hash (called seed) and an object v. According to the docs, it combines seed with the hash of v by

seed ^= hash_value(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);

我可以看到，这是确定的。我明白为什么一个XOR被使用。

I can see that this is deterministic. I see why a XOR is used.

我敢打赌，除了有助于映射相似的价值观广开这么探测哈希表不会破裂，但有人可以解释的神奇不变的是什么？

I bet the addition helps in mapping similar values widely apart so probing hash tables won't break down, but can someone explain what the magic constant is?

推荐答案

的幻数应该是32个随机比特，其中每个也同样可能是0或1，并与位之间没有简单的相关性。找到这样的比特串的常用方法是使用一个无理数的二进制扩展;在这种情况下，这个数字是黄金比例的倒数：

The magic number is supposed to be 32 random bits, where each is equally likely to be 0 or 1, and with no simple correlation between the bits. A common way to find a string of such bits is to use the binary expansion of an irrational number; in this case, that number is the reciprocal of the golden ratio:

phi = (1 + sqrt(5)) / 2
2^32 / phi = 0x9e3779b9

所以，包括本数随机改变种子的每一位;就像你说的，这意味着连续的值将是相距甚远。包括老种子的转移版本可以确保，即使 HASH_VALUE（）有一个相当小的范围值，差异将很快为s $ P $所有垫位。

So including this number "randomly" changes each bit of the seed; as you say, this means that consecutive values will be far apart. Including the shifted versions of the old seed makes sure that, even if hash_value() has a fairly small range of values, differences will soon be spread across all the bits.

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