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<h1 id="example-find-the-best-lowestping-peer-in-a-dht-table">Example: Find the best (lowestping) peer in a DHT table</h1>
<p>This example shows how to model a "made up" DHT routing table and use Poindexter's <code>KDTree</code> to quickly find:</p>
<ul>
<li>the single best peer by ping (nearest neighbor)</li>
<li>the top N best peers by ping (knearest neighbors)</li>
<li>all peers under a ping threshold (radius search)</li>
</ul>
<p>We keep it simple by mapping each peer to a 1dimensional coordinate: its ping in milliseconds. Using 1D means the KDTree's distance is just the absolute difference between pings.</p>
<blockquote>
<p>Tip: In a real system, you might expand to multiple dimensions (e.g., <code>[ping_ms, hop_count, geo_distance, score]</code>) and choose a metric (<code>L1</code>, <code>L2</code>, or <code>L∞</code>) that best matches your routing heuristic. See how to build normalized, weighted multidimensional points with the public helpers <code>poindexter.Build2D/3D/4D</code> here: <a href="../kdtree-multidimensional/">Multi-Dimensional KDTree (DHT)</a>.</p>
</blockquote>
<hr />
<h2 id="full-example">Full example</h2>
<div class="highlight"><pre><span></span><code><a id="__codelineno-0-1" name="__codelineno-0-1" href="#__codelineno-0-1"></a><span class="kn">package</span><span class="w"> </span><span class="nx">main</span>
<a id="__codelineno-0-2" name="__codelineno-0-2" href="#__codelineno-0-2"></a>
<a id="__codelineno-0-3" name="__codelineno-0-3" href="#__codelineno-0-3"></a><span class="kn">import</span><span class="w"> </span><span class="p">(</span>
<a id="__codelineno-0-4" name="__codelineno-0-4" href="#__codelineno-0-4"></a><span class="w"> </span><span class="s">&quot;fmt&quot;</span>
<a id="__codelineno-0-5" name="__codelineno-0-5" href="#__codelineno-0-5"></a><span class="w"> </span><span class="nx">poindexter</span><span class="w"> </span><span class="s">&quot;github.com/Snider/Poindexter&quot;</span>
<a id="__codelineno-0-6" name="__codelineno-0-6" href="#__codelineno-0-6"></a><span class="p">)</span>
<a id="__codelineno-0-7" name="__codelineno-0-7" href="#__codelineno-0-7"></a>
<a id="__codelineno-0-8" name="__codelineno-0-8" href="#__codelineno-0-8"></a><span class="c1">// Peer is our DHT peer entry (made up for this example).</span>
<a id="__codelineno-0-9" name="__codelineno-0-9" href="#__codelineno-0-9"></a><span class="kd">type</span><span class="w"> </span><span class="nx">Peer</span><span class="w"> </span><span class="kd">struct</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-0-10" name="__codelineno-0-10" href="#__codelineno-0-10"></a><span class="w"> </span><span class="nx">Addr</span><span class="w"> </span><span class="kt">string</span><span class="w"> </span><span class="c1">// multiaddr or host:port</span>
<a id="__codelineno-0-11" name="__codelineno-0-11" href="#__codelineno-0-11"></a><span class="w"> </span><span class="nx">Ping</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="c1">// measured ping in milliseconds</span>
<a id="__codelineno-0-12" name="__codelineno-0-12" href="#__codelineno-0-12"></a><span class="p">}</span>
<a id="__codelineno-0-13" name="__codelineno-0-13" href="#__codelineno-0-13"></a>
<a id="__codelineno-0-14" name="__codelineno-0-14" href="#__codelineno-0-14"></a><span class="kd">func</span><span class="w"> </span><span class="nx">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-0-15" name="__codelineno-0-15" href="#__codelineno-0-15"></a><span class="w"> </span><span class="c1">// A toy DHT routing table with made-up ping values</span>
<a id="__codelineno-0-16" name="__codelineno-0-16" href="#__codelineno-0-16"></a><span class="w"> </span><span class="nx">table</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="p">[]</span><span class="nx">Peer</span><span class="p">{</span>
<a id="__codelineno-0-17" name="__codelineno-0-17" href="#__codelineno-0-17"></a><span class="w"> </span><span class="p">{</span><span class="nx">Addr</span><span class="p">:</span><span class="w"> </span><span class="s">&quot;peer1.example:4001&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">Ping</span><span class="p">:</span><span class="w"> </span><span class="mi">74</span><span class="p">},</span>
<a id="__codelineno-0-18" name="__codelineno-0-18" href="#__codelineno-0-18"></a><span class="w"> </span><span class="p">{</span><span class="nx">Addr</span><span class="p">:</span><span class="w"> </span><span class="s">&quot;peer2.example:4001&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">Ping</span><span class="p">:</span><span class="w"> </span><span class="mi">52</span><span class="p">},</span>
<a id="__codelineno-0-19" name="__codelineno-0-19" href="#__codelineno-0-19"></a><span class="w"> </span><span class="p">{</span><span class="nx">Addr</span><span class="p">:</span><span class="w"> </span><span class="s">&quot;peer3.example:4001&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">Ping</span><span class="p">:</span><span class="w"> </span><span class="mi">110</span><span class="p">},</span>
<a id="__codelineno-0-20" name="__codelineno-0-20" href="#__codelineno-0-20"></a><span class="w"> </span><span class="p">{</span><span class="nx">Addr</span><span class="p">:</span><span class="w"> </span><span class="s">&quot;peer4.example:4001&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">Ping</span><span class="p">:</span><span class="w"> </span><span class="mi">35</span><span class="p">},</span>
<a id="__codelineno-0-21" name="__codelineno-0-21" href="#__codelineno-0-21"></a><span class="w"> </span><span class="p">{</span><span class="nx">Addr</span><span class="p">:</span><span class="w"> </span><span class="s">&quot;peer5.example:4001&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">Ping</span><span class="p">:</span><span class="w"> </span><span class="mi">60</span><span class="p">},</span>
<a id="__codelineno-0-22" name="__codelineno-0-22" href="#__codelineno-0-22"></a><span class="w"> </span><span class="p">{</span><span class="nx">Addr</span><span class="p">:</span><span class="w"> </span><span class="s">&quot;peer6.example:4001&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">Ping</span><span class="p">:</span><span class="w"> </span><span class="mi">44</span><span class="p">},</span>
<a id="__codelineno-0-23" name="__codelineno-0-23" href="#__codelineno-0-23"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-0-24" name="__codelineno-0-24" href="#__codelineno-0-24"></a>
<a id="__codelineno-0-25" name="__codelineno-0-25" href="#__codelineno-0-25"></a><span class="w"> </span><span class="c1">// Map peers to KD points in 1D where coordinate = ping (ms).</span>
<a id="__codelineno-0-26" name="__codelineno-0-26" href="#__codelineno-0-26"></a><span class="w"> </span><span class="c1">// Use stable string IDs so we can delete/update later.</span>
<a id="__codelineno-0-27" name="__codelineno-0-27" href="#__codelineno-0-27"></a><span class="w"> </span><span class="nx">pts</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="nb">make</span><span class="p">([]</span><span class="nx">poindexter</span><span class="p">.</span><span class="nx">KDPoint</span><span class="p">[</span><span class="nx">Peer</span><span class="p">],</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="nb">len</span><span class="p">(</span><span class="nx">table</span><span class="p">))</span>
<a id="__codelineno-0-28" name="__codelineno-0-28" href="#__codelineno-0-28"></a><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="nx">i</span><span class="p">,</span><span class="w"> </span><span class="nx">p</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="k">range</span><span class="w"> </span><span class="nx">table</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-0-29" name="__codelineno-0-29" href="#__codelineno-0-29"></a><span class="w"> </span><span class="nx">pts</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="nb">append</span><span class="p">(</span><span class="nx">pts</span><span class="p">,</span><span class="w"> </span><span class="nx">poindexter</span><span class="p">.</span><span class="nx">KDPoint</span><span class="p">[</span><span class="nx">Peer</span><span class="p">]{</span>
<a id="__codelineno-0-30" name="__codelineno-0-30" href="#__codelineno-0-30"></a><span class="w"> </span><span class="nx">ID</span><span class="p">:</span><span class="w"> </span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Sprintf</span><span class="p">(</span><span class="s">&quot;peer-%d&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">i</span><span class="o">+</span><span class="mi">1</span><span class="p">),</span>
<a id="__codelineno-0-31" name="__codelineno-0-31" href="#__codelineno-0-31"></a><span class="w"> </span><span class="nx">Coords</span><span class="p">:</span><span class="w"> </span><span class="p">[]</span><span class="kt">float64</span><span class="p">{</span><span class="nb">float64</span><span class="p">(</span><span class="nx">p</span><span class="p">.</span><span class="nx">Ping</span><span class="p">)},</span>
<a id="__codelineno-0-32" name="__codelineno-0-32" href="#__codelineno-0-32"></a><span class="w"> </span><span class="nx">Value</span><span class="p">:</span><span class="w"> </span><span class="nx">p</span><span class="p">,</span>
<a id="__codelineno-0-33" name="__codelineno-0-33" href="#__codelineno-0-33"></a><span class="w"> </span><span class="p">})</span>
<a id="__codelineno-0-34" name="__codelineno-0-34" href="#__codelineno-0-34"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-0-35" name="__codelineno-0-35" href="#__codelineno-0-35"></a>
<a id="__codelineno-0-36" name="__codelineno-0-36" href="#__codelineno-0-36"></a><span class="w"> </span><span class="c1">// Build a KDTree. Euclidean metric is fine for 1D ping comparisons.</span>
<a id="__codelineno-0-37" name="__codelineno-0-37" href="#__codelineno-0-37"></a><span class="w"> </span><span class="nx">kdt</span><span class="p">,</span><span class="w"> </span><span class="nx">err</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="nx">poindexter</span><span class="p">.</span><span class="nx">NewKDTree</span><span class="p">(</span><span class="nx">pts</span><span class="p">,</span><span class="w"> </span><span class="nx">poindexter</span><span class="p">.</span><span class="nx">WithMetric</span><span class="p">(</span><span class="nx">poindexter</span><span class="p">.</span><span class="nx">EuclideanDistance</span><span class="p">{}))</span>
<a id="__codelineno-0-38" name="__codelineno-0-38" href="#__codelineno-0-38"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="nx">err</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="kc">nil</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-0-39" name="__codelineno-0-39" href="#__codelineno-0-39"></a><span class="w"> </span><span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<a id="__codelineno-0-40" name="__codelineno-0-40" href="#__codelineno-0-40"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-0-41" name="__codelineno-0-41" href="#__codelineno-0-41"></a>
<a id="__codelineno-0-42" name="__codelineno-0-42" href="#__codelineno-0-42"></a><span class="w"> </span><span class="c1">// 1) Find the best (lowest-ping) peer.</span>
<a id="__codelineno-0-43" name="__codelineno-0-43" href="#__codelineno-0-43"></a><span class="w"> </span><span class="c1">// Query is a 1D point representing desired ping target. Using 0 finds the min.</span>
<a id="__codelineno-0-44" name="__codelineno-0-44" href="#__codelineno-0-44"></a><span class="w"> </span><span class="nx">best</span><span class="p">,</span><span class="w"> </span><span class="nx">d</span><span class="p">,</span><span class="w"> </span><span class="nx">ok</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="nx">kdt</span><span class="p">.</span><span class="nx">Nearest</span><span class="p">([]</span><span class="kt">float64</span><span class="p">{</span><span class="mi">0</span><span class="p">})</span>
<a id="__codelineno-0-45" name="__codelineno-0-45" href="#__codelineno-0-45"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">!</span><span class="nx">ok</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-0-46" name="__codelineno-0-46" href="#__codelineno-0-46"></a><span class="w"> </span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;no peers found&quot;</span><span class="p">)</span>
<a id="__codelineno-0-47" name="__codelineno-0-47" href="#__codelineno-0-47"></a><span class="w"> </span><span class="k">return</span>
<a id="__codelineno-0-48" name="__codelineno-0-48" href="#__codelineno-0-48"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-0-49" name="__codelineno-0-49" href="#__codelineno-0-49"></a><span class="w"> </span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;Best peer: %s (ping=%d ms), distance=%.0f\n&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">best</span><span class="p">.</span><span class="nx">Value</span><span class="p">.</span><span class="nx">Addr</span><span class="p">,</span><span class="w"> </span><span class="nx">best</span><span class="p">.</span><span class="nx">Value</span><span class="p">.</span><span class="nx">Ping</span><span class="p">,</span><span class="w"> </span><span class="nx">d</span><span class="p">)</span>
<a id="__codelineno-0-50" name="__codelineno-0-50" href="#__codelineno-0-50"></a><span class="w"> </span><span class="c1">// Example output: Best peer: peer4.example:4001 (ping=35 ms), distance=35</span>
<a id="__codelineno-0-51" name="__codelineno-0-51" href="#__codelineno-0-51"></a>
<a id="__codelineno-0-52" name="__codelineno-0-52" href="#__codelineno-0-52"></a><span class="w"> </span><span class="c1">// 2) Top-N best peers by ping.</span>
<a id="__codelineno-0-53" name="__codelineno-0-53" href="#__codelineno-0-53"></a><span class="w"> </span><span class="nx">top</span><span class="p">,</span><span class="w"> </span><span class="nx">dists</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="nx">kdt</span><span class="p">.</span><span class="nx">KNearest</span><span class="p">([]</span><span class="kt">float64</span><span class="p">{</span><span class="mi">0</span><span class="p">},</span><span class="w"> </span><span class="mi">3</span><span class="p">)</span>
<a id="__codelineno-0-54" name="__codelineno-0-54" href="#__codelineno-0-54"></a><span class="w"> </span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;Top 3 peers by ping:&quot;</span><span class="p">)</span>
<a id="__codelineno-0-55" name="__codelineno-0-55" href="#__codelineno-0-55"></a><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="nx">i</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="k">range</span><span class="w"> </span><span class="nx">top</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-0-56" name="__codelineno-0-56" href="#__codelineno-0-56"></a><span class="w"> </span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot; #%d %s (ping=%d ms), distance=%.0f\n&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">i</span><span class="o">+</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="nx">top</span><span class="p">[</span><span class="nx">i</span><span class="p">].</span><span class="nx">Value</span><span class="p">.</span><span class="nx">Addr</span><span class="p">,</span><span class="w"> </span><span class="nx">top</span><span class="p">[</span><span class="nx">i</span><span class="p">].</span><span class="nx">Value</span><span class="p">.</span><span class="nx">Ping</span><span class="p">,</span><span class="w"> </span><span class="nx">dists</span><span class="p">[</span><span class="nx">i</span><span class="p">])</span>
<a id="__codelineno-0-57" name="__codelineno-0-57" href="#__codelineno-0-57"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-0-58" name="__codelineno-0-58" href="#__codelineno-0-58"></a>
<a id="__codelineno-0-59" name="__codelineno-0-59" href="#__codelineno-0-59"></a><span class="w"> </span><span class="c1">// 3) All peers under a threshold (e.g., &lt;= 50 ms): radius search.</span>
<a id="__codelineno-0-60" name="__codelineno-0-60" href="#__codelineno-0-60"></a><span class="w"> </span><span class="nx">within</span><span class="p">,</span><span class="w"> </span><span class="nx">wd</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="nx">kdt</span><span class="p">.</span><span class="nx">Radius</span><span class="p">([]</span><span class="kt">float64</span><span class="p">{</span><span class="mi">0</span><span class="p">},</span><span class="w"> </span><span class="mi">50</span><span class="p">)</span>
<a id="__codelineno-0-61" name="__codelineno-0-61" href="#__codelineno-0-61"></a><span class="w"> </span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;Peers with ping &lt;= 50 ms:&quot;</span><span class="p">)</span>
<a id="__codelineno-0-62" name="__codelineno-0-62" href="#__codelineno-0-62"></a><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="nx">i</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="k">range</span><span class="w"> </span><span class="nx">within</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-0-63" name="__codelineno-0-63" href="#__codelineno-0-63"></a><span class="w"> </span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot; %s (ping=%d ms), distance=%.0f\n&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">within</span><span class="p">[</span><span class="nx">i</span><span class="p">].</span><span class="nx">Value</span><span class="p">.</span><span class="nx">Addr</span><span class="p">,</span><span class="w"> </span><span class="nx">within</span><span class="p">[</span><span class="nx">i</span><span class="p">].</span><span class="nx">Value</span><span class="p">.</span><span class="nx">Ping</span><span class="p">,</span><span class="w"> </span><span class="nx">wd</span><span class="p">[</span><span class="nx">i</span><span class="p">])</span>
<a id="__codelineno-0-64" name="__codelineno-0-64" href="#__codelineno-0-64"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-0-65" name="__codelineno-0-65" href="#__codelineno-0-65"></a>
<a id="__codelineno-0-66" name="__codelineno-0-66" href="#__codelineno-0-66"></a><span class="w"> </span><span class="c1">// 4) Dynamic updates: if a peer improves ping, we can delete &amp; re-insert with a new ID</span>
<a id="__codelineno-0-67" name="__codelineno-0-67" href="#__codelineno-0-67"></a><span class="w"> </span><span class="c1">// (or keep the same ID and just update the point if your application tracks indices).</span>
<a id="__codelineno-0-68" name="__codelineno-0-68" href="#__codelineno-0-68"></a><span class="w"> </span><span class="c1">// Here we simulate peer5 dropping from 60 ms to 30 ms.</span>
<a id="__codelineno-0-69" name="__codelineno-0-69" href="#__codelineno-0-69"></a><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="nx">kdt</span><span class="p">.</span><span class="nx">DeleteByID</span><span class="p">(</span><span class="s">&quot;peer-5&quot;</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<a id="__codelineno-0-70" name="__codelineno-0-70" href="#__codelineno-0-70"></a><span class="w"> </span><span class="nx">improved</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="nx">poindexter</span><span class="p">.</span><span class="nx">KDPoint</span><span class="p">[</span><span class="nx">Peer</span><span class="p">]{</span>
<a id="__codelineno-0-71" name="__codelineno-0-71" href="#__codelineno-0-71"></a><span class="w"> </span><span class="nx">ID</span><span class="p">:</span><span class="w"> </span><span class="s">&quot;peer-5&quot;</span><span class="p">,</span><span class="w"> </span><span class="c1">// keep the same ID for simplicity</span>
<a id="__codelineno-0-72" name="__codelineno-0-72" href="#__codelineno-0-72"></a><span class="w"> </span><span class="nx">Coords</span><span class="p">:</span><span class="w"> </span><span class="p">[]</span><span class="kt">float64</span><span class="p">{</span><span class="mi">30</span><span class="p">},</span>
<a id="__codelineno-0-73" name="__codelineno-0-73" href="#__codelineno-0-73"></a><span class="w"> </span><span class="nx">Value</span><span class="p">:</span><span class="w"> </span><span class="nx">Peer</span><span class="p">{</span><span class="nx">Addr</span><span class="p">:</span><span class="w"> </span><span class="s">&quot;peer5.example:4001&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">Ping</span><span class="p">:</span><span class="w"> </span><span class="mi">30</span><span class="p">},</span>
<a id="__codelineno-0-74" name="__codelineno-0-74" href="#__codelineno-0-74"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-0-75" name="__codelineno-0-75" href="#__codelineno-0-75"></a><span class="w"> </span><span class="nx">_</span><span class="w"> </span><span class="p">=</span><span class="w"> </span><span class="nx">kdt</span><span class="p">.</span><span class="nx">Insert</span><span class="p">(</span><span class="nx">improved</span><span class="p">)</span>
<a id="__codelineno-0-76" name="__codelineno-0-76" href="#__codelineno-0-76"></a><span class="w"> </span><span class="p">}</span>
<a id="__codelineno-0-77" name="__codelineno-0-77" href="#__codelineno-0-77"></a>
<a id="__codelineno-0-78" name="__codelineno-0-78" href="#__codelineno-0-78"></a><span class="w"> </span><span class="c1">// Recompute the best after update</span>
<a id="__codelineno-0-79" name="__codelineno-0-79" href="#__codelineno-0-79"></a><span class="w"> </span><span class="nx">best2</span><span class="p">,</span><span class="w"> </span><span class="nx">d2</span><span class="p">,</span><span class="w"> </span><span class="nx">_</span><span class="w"> </span><span class="o">:=</span><span class="w"> </span><span class="nx">kdt</span><span class="p">.</span><span class="nx">Nearest</span><span class="p">([]</span><span class="kt">float64</span><span class="p">{</span><span class="mi">0</span><span class="p">})</span>
<a id="__codelineno-0-80" name="__codelineno-0-80" href="#__codelineno-0-80"></a><span class="w"> </span><span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;After update, best peer: %s (ping=%d ms), distance=%.0f\n&quot;</span><span class="p">,</span><span class="w"> </span><span class="nx">best2</span><span class="p">.</span><span class="nx">Value</span><span class="p">.</span><span class="nx">Addr</span><span class="p">,</span><span class="w"> </span><span class="nx">best2</span><span class="p">.</span><span class="nx">Value</span><span class="p">.</span><span class="nx">Ping</span><span class="p">,</span><span class="w"> </span><span class="nx">d2</span><span class="p">)</span>
<a id="__codelineno-0-81" name="__codelineno-0-81" href="#__codelineno-0-81"></a><span class="p">}</span>
</code></pre></div>
<h3 id="why-does-querying-with-0-work">Why does querying with <code>[0]</code> work?</h3>
<p>We use Euclidean distance in 1D, so <code>distance = |ping - target|</code>. With target <code>0</code>, minimizing the distance is equivalent to minimizing the ping itself.</p>
<h3 id="extending-the-metricspace">Extending the metric/space</h3>
<ul>
<li>Multi-objective: encode more routing features (lower is better) as extra dimensions, e.g. <code>[ping_ms, hops, queue_delay_ms]</code>.</li>
<li>Metric choice:</li>
<li><code>EuclideanDistance</code> (L2): balances outliers smoothly.</li>
<li><code>ManhattanDistance</code> (L1): linear penalty; robust for sparsity.</li>
<li><code>ChebyshevDistance</code> (L∞): cares about the worst dimension.</li>
<li>Normalization: when mixing units (ms, hops, km), normalize or weight dimensions so the metric reflects your priority.</li>
</ul>
<h3 id="notes">Notes</h3>
<ul>
<li>This KDTree currently uses an internal linear scan for queries. The API is stable and designed so it can be swapped to use <code>gonum.org/v1/gonum/spatial/kdtree</code> under the hood later for sub-linear queries on large datasets.</li>
<li>IDs are optional but recommended for O(1)-style deletes; keep them unique per tree.</li>
</ul>
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