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6.3.1. &nbsp; 哈希算法的目标
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<a href="https://github.com/krahets/hello-algo/tree/main/docs/chapter_hashing/hash_algorithm.md" title="编辑此页" class="md-content__button md-icon">
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<h1 id="63">6.3. &nbsp; 哈希算法<a class="headerlink" href="#63" title="Permanent link">&para;</a></h1>
<p>在上两节中,我们了解了哈希表的工作原理,以及哈希冲突的处理方法。然而,无论是开放寻址还是链地址法,<strong>它们只能保证哈希表可以在发生冲突时正常工作,但无法减少哈希冲突的发生</strong></p>
<p>如果哈希冲突过于频繁,哈希表的性能则会急剧劣化。例如对于链地址哈希表,理想情况下键值对平均分布在各个桶中,达到最好的查询效率;最差情况下全部键值都被存储到同一个桶中,时间复杂度退化至 <span class="arithmatex">\(O(n)\)</span></p>
<p><img alt="哈希冲突的最佳与最差情况" src="../hash_algorithm.assets/hash_collision_best_worst_condition.png" /></p>
<p align="center"> Fig. 哈希冲突的最佳与最差情况 </p>
<p><strong>键值对的分布情况是由哈希函数决定的</strong>。回忆哈希函数的计算步骤,先计算哈希值,再对数组长度取模:</p>
<div class="highlight"><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="nv">index</span><span class="w"> </span><span class="o">=</span><span class="w"> </span>hash<span class="o">(</span>key<span class="o">)</span><span class="w"> </span>%<span class="w"> </span>capacity
</code></pre></div>
<p>观察以上公式,当哈希表容量 <code>capacity</code> 固定时,<strong>哈希算法 <code>hash()</code> 决定了输出值</strong>,进而决定了键值对在哈希表中的分布。因此,为了减小哈希冲突的发生概率,我们需要将注意力集中在哈希算法 <code>hash()</code> 的设计上。</p>
<h2 id="631">6.3.1. &nbsp; 哈希算法的目标<a class="headerlink" href="#631" title="Permanent link">&para;</a></h2>
<p>为了在编程语言中实现“既快又稳”的哈希表数据结构,哈希算法应包含以下特点:</p>
<ul>
<li><strong>确定性</strong>:对于相同的输入,哈希算法应始终产生相同的输出。这样才能确保哈希表是可靠的。</li>
<li><strong>效率高</strong>:计算哈希值的过程应该足够快。计算开销越小,哈希表的实用性越高。</li>
<li><strong>均匀分布</strong>:哈希算法应使得键值对平均分布在哈希表中。分布越平均,哈希冲突的概率就越低。</li>
</ul>
<p>实际上,哈希算法除了可以用于实现哈希表,还广泛应用于其他领域中,包括:</p>
<ul>
<li><strong>密码存储</strong>:为了保护用户密码的安全,系统通常不会直接存储用户的明文密码,而是存储密码的哈希值。当用户输入密码时,系统会对输入的密码计算哈希值,然后与存储的哈希值进行比较。如果两者匹配,那么密码就被视为正确。</li>
<li><strong>数据完整性检查</strong>:数据发送方可以计算数据的哈希值并将其一同发送;接收方可以重新计算接收到的数据的哈希值,并与接收到的哈希值进行比较。如果两者匹配,那么数据就被视为完整的。</li>
</ul>
<p>对于密码学的相关应用,哈希算法需要满足更高的安全标准,以防止从哈希值推导出原始密码等逆向工程,包括:</p>
<ul>
<li><strong>抗碰撞性</strong>:应当极其困难找到两个不同的输入,使得它们的哈希值相同。</li>
<li><strong>雪崩效应</strong>:输入的微小变化应当导致输出的显著且不可预测的变化。</li>
</ul>
<p>注意,<strong>“均匀分布”与“抗碰撞性”是两个独立的概念</strong>,满足均匀分布不一定满足抗碰撞性。例如,在随机输入 <code>key</code> 下,哈希函数 <code>key % 100</code> 可以产生均匀分布的输出。然而,该哈希算法过于简单,所有后两位相等的 <code>key</code> 的输出都相同,因此我们可以很容易地从哈希值反推出可用的 <code>key</code> ,从而破解密码。</p>
<h2 id="632">6.3.2. &nbsp; 哈希算法的设计<a class="headerlink" href="#632" title="Permanent link">&para;</a></h2>
<p>哈希算法的设计是一个复杂且需要考虑许多因素的问题。然而,对于一些简单场景,我们也能设计一些简单的哈希算法,以字符串哈希为例:</p>
<ul>
<li><strong>加法哈希</strong>:对输入的每个字符的 ASCII 码进行相加,将得到的总和作为哈希值。</li>
<li><strong>乘法哈希</strong>:利用了乘法的不相关性,每轮乘以一个常数,将各个字符的 ASCII 码累积到哈希值中。</li>
<li><strong>异或哈希</strong>:将输入数据的每个元素通过异或操作累积到一个哈希值中。</li>
<li><strong>旋转哈希</strong>:将每个字符的 ASCII 码累积到一个哈希值中,每次累积之前都会对哈希值进行旋转操作。</li>
</ul>
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<div class="highlight"><span class="filename">simple_hash.java</span><pre><span></span><code><a id="__codelineno-1-1" name="__codelineno-1-1" href="#__codelineno-1-1"></a><span class="o">[</span><span class="kd">class</span><span class="err">]{</span><span class="nc">simple_hash</span><span class="p">}</span><span class="o">-[</span><span class="n">func</span><span class="o">]</span><span class="p">{</span><span class="n">addHash</span><span class="p">}</span>
<a id="__codelineno-1-2" name="__codelineno-1-2" href="#__codelineno-1-2"></a>
<a id="__codelineno-1-3" name="__codelineno-1-3" href="#__codelineno-1-3"></a><span class="o">[</span><span class="kd">class</span><span class="err">]{</span><span class="nc">simple_hash</span><span class="p">}</span><span class="o">-[</span><span class="n">func</span><span class="o">]</span><span class="p">{</span><span class="n">mulHash</span><span class="p">}</span>
<a id="__codelineno-1-4" name="__codelineno-1-4" href="#__codelineno-1-4"></a>
<a id="__codelineno-1-5" name="__codelineno-1-5" href="#__codelineno-1-5"></a><span class="o">[</span><span class="kd">class</span><span class="err">]{</span><span class="nc">simple_hash</span><span class="p">}</span><span class="o">-[</span><span class="n">func</span><span class="o">]</span><span class="p">{</span><span class="n">xorHash</span><span class="p">}</span>
<a id="__codelineno-1-6" name="__codelineno-1-6" href="#__codelineno-1-6"></a>
<a id="__codelineno-1-7" name="__codelineno-1-7" href="#__codelineno-1-7"></a><span class="o">[</span><span class="kd">class</span><span class="err">]{</span><span class="nc">simple_hash</span><span class="p">}</span><span class="o">-[</span><span class="n">func</span><span class="o">]</span><span class="p">{</span><span class="n">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.cpp</span><pre><span></span><code><a id="__codelineno-2-1" name="__codelineno-2-1" href="#__codelineno-2-1"></a><span class="p">[</span><span class="k">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">addHash</span><span class="p">}</span>
<a id="__codelineno-2-2" name="__codelineno-2-2" href="#__codelineno-2-2"></a>
<a id="__codelineno-2-3" name="__codelineno-2-3" href="#__codelineno-2-3"></a><span class="p">[</span><span class="k">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">mulHash</span><span class="p">}</span>
<a id="__codelineno-2-4" name="__codelineno-2-4" href="#__codelineno-2-4"></a>
<a id="__codelineno-2-5" name="__codelineno-2-5" href="#__codelineno-2-5"></a><span class="p">[</span><span class="k">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">xorHash</span><span class="p">}</span>
<a id="__codelineno-2-6" name="__codelineno-2-6" href="#__codelineno-2-6"></a>
<a id="__codelineno-2-7" name="__codelineno-2-7" href="#__codelineno-2-7"></a><span class="p">[</span><span class="k">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.py</span><pre><span></span><code><a id="__codelineno-3-1" name="__codelineno-3-1" href="#__codelineno-3-1"></a><span class="k">def</span> <span class="nf">add_hash</span><span class="p">(</span><span class="n">key</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<a id="__codelineno-3-2" name="__codelineno-3-2" href="#__codelineno-3-2"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;加法哈希&quot;&quot;&quot;</span>
<a id="__codelineno-3-3" name="__codelineno-3-3" href="#__codelineno-3-3"></a> <span class="nb">hash</span> <span class="o">=</span> <span class="mi">0</span>
<a id="__codelineno-3-4" name="__codelineno-3-4" href="#__codelineno-3-4"></a> <span class="n">modulus</span> <span class="o">=</span> <span class="mi">1000000007</span>
<a id="__codelineno-3-5" name="__codelineno-3-5" href="#__codelineno-3-5"></a> <span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">key</span><span class="p">:</span>
<a id="__codelineno-3-6" name="__codelineno-3-6" href="#__codelineno-3-6"></a> <span class="nb">hash</span> <span class="o">+=</span> <span class="nb">ord</span><span class="p">(</span><span class="n">c</span><span class="p">)</span>
<a id="__codelineno-3-7" name="__codelineno-3-7" href="#__codelineno-3-7"></a> <span class="k">return</span> <span class="nb">hash</span> <span class="o">%</span> <span class="n">modulus</span>
<a id="__codelineno-3-8" name="__codelineno-3-8" href="#__codelineno-3-8"></a>
<a id="__codelineno-3-9" name="__codelineno-3-9" href="#__codelineno-3-9"></a><span class="k">def</span> <span class="nf">mul_hash</span><span class="p">(</span><span class="n">key</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<a id="__codelineno-3-10" name="__codelineno-3-10" href="#__codelineno-3-10"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;乘法哈希&quot;&quot;&quot;</span>
<a id="__codelineno-3-11" name="__codelineno-3-11" href="#__codelineno-3-11"></a> <span class="nb">hash</span> <span class="o">=</span> <span class="mi">0</span>
<a id="__codelineno-3-12" name="__codelineno-3-12" href="#__codelineno-3-12"></a> <span class="n">modulus</span> <span class="o">=</span> <span class="mi">1000000007</span>
<a id="__codelineno-3-13" name="__codelineno-3-13" href="#__codelineno-3-13"></a> <span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">key</span><span class="p">:</span>
<a id="__codelineno-3-14" name="__codelineno-3-14" href="#__codelineno-3-14"></a> <span class="nb">hash</span> <span class="o">=</span> <span class="mi">31</span> <span class="o">*</span> <span class="nb">hash</span> <span class="o">+</span> <span class="nb">ord</span><span class="p">(</span><span class="n">c</span><span class="p">)</span>
<a id="__codelineno-3-15" name="__codelineno-3-15" href="#__codelineno-3-15"></a> <span class="k">return</span> <span class="nb">hash</span> <span class="o">%</span> <span class="n">modulus</span>
<a id="__codelineno-3-16" name="__codelineno-3-16" href="#__codelineno-3-16"></a>
<a id="__codelineno-3-17" name="__codelineno-3-17" href="#__codelineno-3-17"></a><span class="k">def</span> <span class="nf">xor_hash</span><span class="p">(</span><span class="n">key</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<a id="__codelineno-3-18" name="__codelineno-3-18" href="#__codelineno-3-18"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;异或哈希&quot;&quot;&quot;</span>
<a id="__codelineno-3-19" name="__codelineno-3-19" href="#__codelineno-3-19"></a> <span class="nb">hash</span> <span class="o">=</span> <span class="mi">0</span>
<a id="__codelineno-3-20" name="__codelineno-3-20" href="#__codelineno-3-20"></a> <span class="n">modulus</span> <span class="o">=</span> <span class="mi">1000000007</span>
<a id="__codelineno-3-21" name="__codelineno-3-21" href="#__codelineno-3-21"></a> <span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">key</span><span class="p">:</span>
<a id="__codelineno-3-22" name="__codelineno-3-22" href="#__codelineno-3-22"></a> <span class="nb">hash</span> <span class="o">^=</span> <span class="nb">ord</span><span class="p">(</span><span class="n">c</span><span class="p">)</span>
<a id="__codelineno-3-23" name="__codelineno-3-23" href="#__codelineno-3-23"></a> <span class="k">return</span> <span class="nb">hash</span> <span class="o">%</span> <span class="n">modulus</span>
<a id="__codelineno-3-24" name="__codelineno-3-24" href="#__codelineno-3-24"></a>
<a id="__codelineno-3-25" name="__codelineno-3-25" href="#__codelineno-3-25"></a><span class="k">def</span> <span class="nf">rot_hash</span><span class="p">(</span><span class="n">key</span><span class="p">:</span> <span class="nb">str</span><span class="p">)</span> <span class="o">-&gt;</span> <span class="nb">int</span><span class="p">:</span>
<a id="__codelineno-3-26" name="__codelineno-3-26" href="#__codelineno-3-26"></a><span class="w"> </span><span class="sd">&quot;&quot;&quot;旋转哈希&quot;&quot;&quot;</span>
<a id="__codelineno-3-27" name="__codelineno-3-27" href="#__codelineno-3-27"></a> <span class="nb">hash</span> <span class="o">=</span> <span class="mi">0</span>
<a id="__codelineno-3-28" name="__codelineno-3-28" href="#__codelineno-3-28"></a> <span class="n">modulus</span> <span class="o">=</span> <span class="mi">1000000007</span>
<a id="__codelineno-3-29" name="__codelineno-3-29" href="#__codelineno-3-29"></a> <span class="k">for</span> <span class="n">c</span> <span class="ow">in</span> <span class="n">key</span><span class="p">:</span>
<a id="__codelineno-3-30" name="__codelineno-3-30" href="#__codelineno-3-30"></a> <span class="nb">hash</span> <span class="o">=</span> <span class="p">(</span><span class="nb">hash</span> <span class="o">&lt;&lt;</span> <span class="mi">4</span><span class="p">)</span> <span class="o">^</span> <span class="p">(</span><span class="nb">hash</span> <span class="o">&gt;&gt;</span> <span class="mi">28</span><span class="p">)</span> <span class="o">^</span> <span class="nb">ord</span><span class="p">(</span><span class="n">c</span><span class="p">)</span>
<a id="__codelineno-3-31" name="__codelineno-3-31" href="#__codelineno-3-31"></a> <span class="k">return</span> <span class="nb">hash</span> <span class="o">%</span> <span class="n">modulus</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.go</span><pre><span></span><code><a id="__codelineno-4-1" name="__codelineno-4-1" href="#__codelineno-4-1"></a><span class="p">[</span><span class="nx">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="kd">func</span><span class="p">]{</span><span class="nx">addHash</span><span class="p">}</span>
<a id="__codelineno-4-2" name="__codelineno-4-2" href="#__codelineno-4-2"></a>
<a id="__codelineno-4-3" name="__codelineno-4-3" href="#__codelineno-4-3"></a><span class="p">[</span><span class="nx">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="kd">func</span><span class="p">]{</span><span class="nx">mulHash</span><span class="p">}</span>
<a id="__codelineno-4-4" name="__codelineno-4-4" href="#__codelineno-4-4"></a>
<a id="__codelineno-4-5" name="__codelineno-4-5" href="#__codelineno-4-5"></a><span class="p">[</span><span class="nx">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="kd">func</span><span class="p">]{</span><span class="nx">xorHash</span><span class="p">}</span>
<a id="__codelineno-4-6" name="__codelineno-4-6" href="#__codelineno-4-6"></a>
<a id="__codelineno-4-7" name="__codelineno-4-7" href="#__codelineno-4-7"></a><span class="p">[</span><span class="nx">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="kd">func</span><span class="p">]{</span><span class="nx">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.js</span><pre><span></span><code><a id="__codelineno-5-1" name="__codelineno-5-1" href="#__codelineno-5-1"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="nx">func</span><span class="p">]{</span><span class="nx">addHash</span><span class="p">}</span>
<a id="__codelineno-5-2" name="__codelineno-5-2" href="#__codelineno-5-2"></a>
<a id="__codelineno-5-3" name="__codelineno-5-3" href="#__codelineno-5-3"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="nx">func</span><span class="p">]{</span><span class="nx">mulHash</span><span class="p">}</span>
<a id="__codelineno-5-4" name="__codelineno-5-4" href="#__codelineno-5-4"></a>
<a id="__codelineno-5-5" name="__codelineno-5-5" href="#__codelineno-5-5"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="nx">func</span><span class="p">]{</span><span class="nx">xorHash</span><span class="p">}</span>
<a id="__codelineno-5-6" name="__codelineno-5-6" href="#__codelineno-5-6"></a>
<a id="__codelineno-5-7" name="__codelineno-5-7" href="#__codelineno-5-7"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="nx">func</span><span class="p">]{</span><span class="nx">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.ts</span><pre><span></span><code><a id="__codelineno-6-1" name="__codelineno-6-1" href="#__codelineno-6-1"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="nx">func</span><span class="p">]{</span><span class="nx">addHash</span><span class="p">}</span>
<a id="__codelineno-6-2" name="__codelineno-6-2" href="#__codelineno-6-2"></a>
<a id="__codelineno-6-3" name="__codelineno-6-3" href="#__codelineno-6-3"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="nx">func</span><span class="p">]{</span><span class="nx">mulHash</span><span class="p">}</span>
<a id="__codelineno-6-4" name="__codelineno-6-4" href="#__codelineno-6-4"></a>
<a id="__codelineno-6-5" name="__codelineno-6-5" href="#__codelineno-6-5"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="nx">func</span><span class="p">]{</span><span class="nx">xorHash</span><span class="p">}</span>
<a id="__codelineno-6-6" name="__codelineno-6-6" href="#__codelineno-6-6"></a>
<a id="__codelineno-6-7" name="__codelineno-6-7" href="#__codelineno-6-7"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="nx">func</span><span class="p">]{</span><span class="nx">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.c</span><pre><span></span><code><a id="__codelineno-7-1" name="__codelineno-7-1" href="#__codelineno-7-1"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">addHash</span><span class="p">}</span>
<a id="__codelineno-7-2" name="__codelineno-7-2" href="#__codelineno-7-2"></a>
<a id="__codelineno-7-3" name="__codelineno-7-3" href="#__codelineno-7-3"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">mulHash</span><span class="p">}</span>
<a id="__codelineno-7-4" name="__codelineno-7-4" href="#__codelineno-7-4"></a>
<a id="__codelineno-7-5" name="__codelineno-7-5" href="#__codelineno-7-5"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">xorHash</span><span class="p">}</span>
<a id="__codelineno-7-6" name="__codelineno-7-6" href="#__codelineno-7-6"></a>
<a id="__codelineno-7-7" name="__codelineno-7-7" href="#__codelineno-7-7"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.cs</span><pre><span></span><code><a id="__codelineno-8-1" name="__codelineno-8-1" href="#__codelineno-8-1"></a><span class="na">[class]</span><span class="p">{</span><span class="n">simple_hash</span><span class="p">}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">addHash</span><span class="p">}</span>
<a id="__codelineno-8-2" name="__codelineno-8-2" href="#__codelineno-8-2"></a>
<a id="__codelineno-8-3" name="__codelineno-8-3" href="#__codelineno-8-3"></a><span class="na">[class]</span><span class="p">{</span><span class="n">simple_hash</span><span class="p">}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">mulHash</span><span class="p">}</span>
<a id="__codelineno-8-4" name="__codelineno-8-4" href="#__codelineno-8-4"></a>
<a id="__codelineno-8-5" name="__codelineno-8-5" href="#__codelineno-8-5"></a><span class="na">[class]</span><span class="p">{</span><span class="n">simple_hash</span><span class="p">}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">xorHash</span><span class="p">}</span>
<a id="__codelineno-8-6" name="__codelineno-8-6" href="#__codelineno-8-6"></a>
<a id="__codelineno-8-7" name="__codelineno-8-7" href="#__codelineno-8-7"></a><span class="na">[class]</span><span class="p">{</span><span class="n">simple_hash</span><span class="p">}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.swift</span><pre><span></span><code><a id="__codelineno-9-1" name="__codelineno-9-1" href="#__codelineno-9-1"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="kd">func</span><span class="p">]{</span><span class="n">addHash</span><span class="p">}</span>
<a id="__codelineno-9-2" name="__codelineno-9-2" href="#__codelineno-9-2"></a>
<a id="__codelineno-9-3" name="__codelineno-9-3" href="#__codelineno-9-3"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="kd">func</span><span class="p">]{</span><span class="n">mulHash</span><span class="p">}</span>
<a id="__codelineno-9-4" name="__codelineno-9-4" href="#__codelineno-9-4"></a>
<a id="__codelineno-9-5" name="__codelineno-9-5" href="#__codelineno-9-5"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="kd">func</span><span class="p">]{</span><span class="n">xorHash</span><span class="p">}</span>
<a id="__codelineno-9-6" name="__codelineno-9-6" href="#__codelineno-9-6"></a>
<a id="__codelineno-9-7" name="__codelineno-9-7" href="#__codelineno-9-7"></a><span class="p">[</span><span class="kd">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="kd">func</span><span class="p">]{</span><span class="n">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.zig</span><pre><span></span><code><a id="__codelineno-10-1" name="__codelineno-10-1" href="#__codelineno-10-1"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">addHash</span><span class="p">}</span>
<a id="__codelineno-10-2" name="__codelineno-10-2" href="#__codelineno-10-2"></a>
<a id="__codelineno-10-3" name="__codelineno-10-3" href="#__codelineno-10-3"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">mulHash</span><span class="p">}</span>
<a id="__codelineno-10-4" name="__codelineno-10-4" href="#__codelineno-10-4"></a>
<a id="__codelineno-10-5" name="__codelineno-10-5" href="#__codelineno-10-5"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">xorHash</span><span class="p">}</span>
<a id="__codelineno-10-6" name="__codelineno-10-6" href="#__codelineno-10-6"></a>
<a id="__codelineno-10-7" name="__codelineno-10-7" href="#__codelineno-10-7"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">rotHash</span><span class="p">}</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">simple_hash.dart</span><pre><span></span><code><a id="__codelineno-11-1" name="__codelineno-11-1" href="#__codelineno-11-1"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">add_hash</span><span class="p">}</span>
<a id="__codelineno-11-2" name="__codelineno-11-2" href="#__codelineno-11-2"></a>
<a id="__codelineno-11-3" name="__codelineno-11-3" href="#__codelineno-11-3"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">mul_hash</span><span class="p">}</span>
<a id="__codelineno-11-4" name="__codelineno-11-4" href="#__codelineno-11-4"></a>
<a id="__codelineno-11-5" name="__codelineno-11-5" href="#__codelineno-11-5"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">xor_hash</span><span class="p">}</span>
<a id="__codelineno-11-6" name="__codelineno-11-6" href="#__codelineno-11-6"></a>
<a id="__codelineno-11-7" name="__codelineno-11-7" href="#__codelineno-11-7"></a><span class="p">[</span><span class="n">class</span><span class="p">]{}</span><span class="o">-</span><span class="p">[</span><span class="n">func</span><span class="p">]{</span><span class="n">rot_hash</span><span class="p">}</span>
</code></pre></div>
</div>
</div>
</div>
<p>观察发现,每种哈希算法的最后一步都是对大质数 <span class="arithmatex">\(1000000007\)</span> 取模,以确保哈希值在合适的范围内。值得思考的是,为什么要强调对质数取模,对合数取模的弊端是什么?这是一个有趣的问题。</p>
<p>先抛出结论:<strong>当我们使用大质数作为模数时,可以最大化地保证哈希值的均匀分布</strong>。因为质数不会与其他数字存在公约数,可以减少因取模操作而产生的周期性模式,从而避免哈希冲突。</p>
<p>举个例子,假设我们选择合数 <span class="arithmatex">\(9\)</span> 作为模数,它可以被 <span class="arithmatex">\(3\)</span> 整除。那么所有可以被 <span class="arithmatex">\(3\)</span> 整除的 <code>key</code> 都会被映射到 <span class="arithmatex">\(0\)</span> , <span class="arithmatex">\(3\)</span> , <span class="arithmatex">\(6\)</span> 这三个哈希值。</p>
<div class="arithmatex">\[
\begin{aligned}
\text{modulus} &amp; = 9 \newline
\text{key} &amp; = \{ 0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, \cdots \} \newline
\text{hash} &amp; = \{ 0, 3, 6, 0, 3, 6, 0, 3, 6, 0, 3, 6,\cdots \}
\end{aligned}
\]</div>
<p>如果输入 <code>key</code> 恰好满足这种等差数列的数据分布,那么哈希值就会出现聚堆,从而加重哈希冲突。现在,假设将 <code>modulus</code> 替换为质数 <span class="arithmatex">\(13\)</span> ,由于 <code>key</code><code>modulus</code> 之间不存在公约数,输出的哈希值的均匀性会明显提升。</p>
<div class="arithmatex">\[
\begin{aligned}
\text{modulus} &amp; = 13 \newline
\text{key} &amp; = \{ 0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, \cdots \} \newline
\text{hash} &amp; = \{ 0, 3, 6, 9, 12, 2, 5, 8, 11, 1, 4, 7, \cdots \}
\end{aligned}
\]</div>
<p>值得强调的是,如果能够保证 <code>key</code> 是随机均匀分布的,那么选择质数或者合数作为模数都是可以的,它们都能输出均匀分布的哈希值。而当 <code>key</code> 的分布存在某种周期性时,对合数取模更容易出现聚集现象。</p>
<p>总而言之,我们通常选取质数作为模数,并且这个质数最好大一些,以提升哈希算法的稳健性。</p>
<h2 id="633">6.3.3. &nbsp; 常见哈希算法<a class="headerlink" href="#633" title="Permanent link">&para;</a></h2>
<p>不难发现,以上介绍的简单哈希算法都比较“脆弱”,远远没有达到哈希算法的设计目标。例如,由于加法和异或满足交换律,因此加法哈希和异或哈希无法区分内容相同但顺序不同的字符串,这可能会加剧哈希冲突,并引起一些安全问题。</p>
<p>在实际中,我们通常会用一些标准哈希算法,例如 MD5, SHA-1, SHA-2, SHA3 等。它们可以将任意长度的输入数据映射到恒定长度的哈希值。近一个世纪以来,哈希算法处在不断升级与优化的过程中。一部分研究人员努力提升哈希算法的性能,另一部分研究人员和黑客则致力于寻找哈希算法的安全性问题。</p>
<p>直至目前MD5 和 SHA-1 已多次被成功攻击因此它们被各类安全应用弃用。SHA-2 系列中的 SHA-256 是最安全的哈希算法之一仍未出现成功的攻击案例因此常被用在各类安全应用与协议中。SHA-3 相较 SHA-2 的实现开销更低、计算效率更高,但目前使用覆盖度不如 SHA-2 系列。</p>
<table>
<thead>
<tr>
<th></th>
<th>MD5</th>
<th>SHA-1</th>
<th>SHA-2</th>
<th>SHA-3</th>
</tr>
</thead>
<tbody>
<tr>
<td>推出时间</td>
<td>1992</td>
<td>1995</td>
<td>2002</td>
<td>2008</td>
</tr>
<tr>
<td>输出长度</td>
<td>128 bits</td>
<td>160 bits</td>
<td>256 / 512 bits</td>
<td>224/256/384/512 bits</td>
</tr>
<tr>
<td>哈希冲突</td>
<td>较多</td>
<td>较多</td>
<td>很少</td>
<td>很少</td>
</tr>
<tr>
<td>安全等级</td>
<td>低,已被成功攻击</td>
<td>低,已被成功攻击</td>
<td></td>
<td></td>
</tr>
<tr>
<td>应用</td>
<td>已被弃用,仍用于数据完整性检查</td>
<td>已被弃用</td>
<td>加密货币交易验证、数字签名等</td>
<td>可用于替代 SHA-2</td>
</tr>
</tbody>
</table>
<h2 id="634">6.3.4. &nbsp; 数据结构的哈希值<a class="headerlink" href="#634" title="Permanent link">&para;</a></h2>
<p>我们知道,哈希表的 <code>key</code> 可以是整数、小数或字符串等数据类型。编程语言通常会为这些数据类型提供内置的哈希算法,用于计算哈希表中的桶索引。以 Python 为例,我们可以调用 <code>hash()</code> 函数来计算各种数据类型的哈希值,包括:</p>
<ul>
<li>整数和布尔量的哈希值就是其本身。</li>
<li>浮点数和字符串的哈希值计算较为复杂,有兴趣的同学请自行学习。</li>
<li>元组的哈希值是对其中每一个元素进行哈希,然后将这些哈希值组合起来,得到单一的哈希值。</li>
<li>对象的哈希值基于其内存地址生成。通过重写对象的哈希方法,可实现基于内容生成哈希值。</li>
</ul>
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<div class="highlight"><span class="filename">built_in_hash.java</span><pre><span></span><code><a id="__codelineno-12-1" name="__codelineno-12-1" href="#__codelineno-12-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.cpp</span><pre><span></span><code><a id="__codelineno-13-1" name="__codelineno-13-1" href="#__codelineno-13-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.py</span><pre><span></span><code><a id="__codelineno-14-1" name="__codelineno-14-1" href="#__codelineno-14-1"></a><span class="n">num</span> <span class="o">=</span> <span class="mi">3</span>
<a id="__codelineno-14-2" name="__codelineno-14-2" href="#__codelineno-14-2"></a><span class="n">hash_num</span> <span class="o">=</span> <span class="nb">hash</span><span class="p">(</span><span class="n">num</span><span class="p">)</span>
<a id="__codelineno-14-3" name="__codelineno-14-3" href="#__codelineno-14-3"></a><span class="c1"># 整数 3 的哈希值为 3</span>
<a id="__codelineno-14-4" name="__codelineno-14-4" href="#__codelineno-14-4"></a>
<a id="__codelineno-14-5" name="__codelineno-14-5" href="#__codelineno-14-5"></a><span class="n">bol</span> <span class="o">=</span> <span class="kc">True</span>
<a id="__codelineno-14-6" name="__codelineno-14-6" href="#__codelineno-14-6"></a><span class="n">hash_bol</span> <span class="o">=</span> <span class="nb">hash</span><span class="p">(</span><span class="n">bol</span><span class="p">)</span>
<a id="__codelineno-14-7" name="__codelineno-14-7" href="#__codelineno-14-7"></a><span class="c1"># 布尔量 True 的哈希值为 1</span>
<a id="__codelineno-14-8" name="__codelineno-14-8" href="#__codelineno-14-8"></a>
<a id="__codelineno-14-9" name="__codelineno-14-9" href="#__codelineno-14-9"></a><span class="n">dec</span> <span class="o">=</span> <span class="mf">3.14159</span>
<a id="__codelineno-14-10" name="__codelineno-14-10" href="#__codelineno-14-10"></a><span class="n">hash_dec</span> <span class="o">=</span> <span class="nb">hash</span><span class="p">(</span><span class="n">dec</span><span class="p">)</span>
<a id="__codelineno-14-11" name="__codelineno-14-11" href="#__codelineno-14-11"></a><span class="c1"># 小数 3.14159 的哈希值为 326484311674566659</span>
<a id="__codelineno-14-12" name="__codelineno-14-12" href="#__codelineno-14-12"></a>
<a id="__codelineno-14-13" name="__codelineno-14-13" href="#__codelineno-14-13"></a><span class="nb">str</span> <span class="o">=</span> <span class="s2">&quot;Hello 算法&quot;</span>
<a id="__codelineno-14-14" name="__codelineno-14-14" href="#__codelineno-14-14"></a><span class="n">hash_str</span> <span class="o">=</span> <span class="nb">hash</span><span class="p">(</span><span class="nb">str</span><span class="p">)</span>
<a id="__codelineno-14-15" name="__codelineno-14-15" href="#__codelineno-14-15"></a><span class="c1"># 字符串 Hello 算法 的哈希值为 4617003410720528961</span>
<a id="__codelineno-14-16" name="__codelineno-14-16" href="#__codelineno-14-16"></a>
<a id="__codelineno-14-17" name="__codelineno-14-17" href="#__codelineno-14-17"></a><span class="n">tup</span> <span class="o">=</span> <span class="p">(</span><span class="mi">12836</span><span class="p">,</span> <span class="s2">&quot;小哈&quot;</span><span class="p">)</span>
<a id="__codelineno-14-18" name="__codelineno-14-18" href="#__codelineno-14-18"></a><span class="n">hash_tup</span> <span class="o">=</span> <span class="nb">hash</span><span class="p">(</span><span class="n">tup</span><span class="p">)</span>
<a id="__codelineno-14-19" name="__codelineno-14-19" href="#__codelineno-14-19"></a><span class="c1"># 元组 (12836, &#39;小哈&#39;) 的哈希值为 1029005403108185979</span>
<a id="__codelineno-14-20" name="__codelineno-14-20" href="#__codelineno-14-20"></a>
<a id="__codelineno-14-21" name="__codelineno-14-21" href="#__codelineno-14-21"></a><span class="n">obj</span> <span class="o">=</span> <span class="n">ListNode</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<a id="__codelineno-14-22" name="__codelineno-14-22" href="#__codelineno-14-22"></a><span class="n">hash_obj</span> <span class="o">=</span> <span class="nb">hash</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span>
<a id="__codelineno-14-23" name="__codelineno-14-23" href="#__codelineno-14-23"></a><span class="c1"># 节点对象 &lt;ListNode object at 0x1058fd810&gt; 的哈希值为 274267521</span>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.go</span><pre><span></span><code><a id="__codelineno-15-1" name="__codelineno-15-1" href="#__codelineno-15-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.js</span><pre><span></span><code><a id="__codelineno-16-1" name="__codelineno-16-1" href="#__codelineno-16-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.ts</span><pre><span></span><code><a id="__codelineno-17-1" name="__codelineno-17-1" href="#__codelineno-17-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.c</span><pre><span></span><code><a id="__codelineno-18-1" name="__codelineno-18-1" href="#__codelineno-18-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.cs</span><pre><span></span><code><a id="__codelineno-19-1" name="__codelineno-19-1" href="#__codelineno-19-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.swift</span><pre><span></span><code><a id="__codelineno-20-1" name="__codelineno-20-1" href="#__codelineno-20-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.zig</span><pre><span></span><code><a id="__codelineno-21-1" name="__codelineno-21-1" href="#__codelineno-21-1"></a>
</code></pre></div>
</div>
<div class="tabbed-block">
<div class="highlight"><span class="filename">built_in_hash.dart</span><pre><span></span><code><a id="__codelineno-22-1" name="__codelineno-22-1" href="#__codelineno-22-1"></a>
</code></pre></div>
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</div>
<p>在大多数编程语言中,<strong>只有不可变对象才可作为哈希表的 <code>key</code></strong> 。假如我们将列表(动态数组)作为 <code>key</code> ,当列表的内容发生变化时,它的哈希值也随之改变,我们就无法在哈希表中查询到原先的 <code>value</code> 了。</p>
<p>虽然自定义对象(例如链表节点)的成员变量是可变的,但它是可哈希的,这是因为对象的哈希值默认基于内存地址生成。即使对象的内容发生了变化,但它的内存地址不变,哈希值仍然是不变的。</p>
<div class="admonition tip">
<p class="admonition-title">向哈希函数加盐</p>
<p>Python 解释器在每次启动时都会为字符串哈希函数加入一个随机的盐Salt值。因此在不同的 Python 运行实例中,同一字符串的哈希值通常是不同的。此做法可以有效防止 HashDoS 攻击,提升哈希算法的安全性。</p>
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