mirror of
https://github.com/mfocko/blog.git
synced 2024-11-22 21:13:47 +01:00
104 lines
71 KiB
HTML
104 lines
71 KiB
HTML
|
<!doctype html>
|
|||
|
|
<html lang="en" dir="ltr" class="docs-wrapper plugin-docs plugin-id-algorithms docs-version-current docs-doc-page docs-doc-id-paths/2024-01-01-bf-to-astar/astar" data-has-hydrated="false">
|
|||
|
|
<head>
|
|||
|
|
<meta charset="UTF-8">
|
|||
|
|
<meta name="generator" content="Docusaurus v3.0.1">
|
|||
|
|
<title data-rh="true">A* algorithm | mf</title><meta data-rh="true" name="viewport" content="width=device-width,initial-scale=1"><meta data-rh="true" name="twitter:card" content="summary_large_image"><meta data-rh="true" property="og:url" content="https://blog.mfocko.xyz/algorithms/paths/bf-to-astar/astar/"><meta data-rh="true" property="og:locale" content="en"><meta data-rh="true" name="docusaurus_locale" content="en"><meta data-rh="true" name="docsearch:language" content="en"><meta data-rh="true" name="docusaurus_version" content="current"><meta data-rh="true" name="docusaurus_tag" content="docs-algorithms-current"><meta data-rh="true" name="docsearch:version" content="current"><meta data-rh="true" name="docsearch:docusaurus_tag" content="docs-algorithms-current"><meta data-rh="true" property="og:title" content="A* algorithm | mf"><meta data-rh="true" name="description" content="Moving from Dijkstra's algorithm into the A* algorithm.
|
|||
|
|
"><meta data-rh="true" property="og:description" content="Moving from Dijkstra's algorithm into the A* algorithm.
|
|||
|
|
"><link data-rh="true" rel="icon" href="/img/favicon.ico"><link data-rh="true" rel="canonical" href="https://blog.mfocko.xyz/algorithms/paths/bf-to-astar/astar/"><link data-rh="true" rel="alternate" href="https://blog.mfocko.xyz/algorithms/paths/bf-to-astar/astar/" hreflang="en"><link data-rh="true" rel="alternate" href="https://blog.mfocko.xyz/algorithms/paths/bf-to-astar/astar/" hreflang="x-default"><link data-rh="true" rel="preconnect" href="https://0VXRFPR4QF-dsn.algolia.net" crossorigin="anonymous"><link rel="search" type="application/opensearchdescription+xml" title="mf" href="/opensearch.xml">
|
|||
|
|
|
|||
|
|
|
|||
|
|
|
|||
|
|
<link rel="alternate" type="application/rss+xml" href="/blog/rss.xml" title="mf RSS Feed">
|
|||
|
|
<link rel="alternate" type="application/atom+xml" href="/blog/atom.xml" title="mf Atom Feed">
|
|||
|
|
<link rel="alternate" type="application/json" href="/blog/feed.json" title="mf JSON Feed">
|
|||
|
|
|
|||
|
|
|
|||
|
|
|
|||
|
|
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/katex@0.13.24/dist/katex.min.css" integrity="sha384-odtC+0UGzzFL/6PNoE8rX/SPcQDXBJ+uRepguP4QkPCm2LBxH3FA3y+fKSiJ+AmM" crossorigin="anonymous"><link rel="stylesheet" href="/assets/css/styles.e1ac7597.css">
|
|||
|
|
<script src="/assets/js/runtime~main.65095f6f.js" defer="defer"></script>
|
|||
|
|
<script src="/assets/js/main.2a166133.js" defer="defer"></script>
|
|||
|
|
</head>
|
|||
|
|
<body class="navigation-with-keyboard">
|
|||
|
|
<script>!function(){function t(t){document.documentElement.setAttribute("data-theme",t)}var e=function(){try{return new URLSearchParams(window.location.search).get("docusaurus-theme")}catch(t){}}()||function(){try{return localStorage.getItem("theme")}catch(t){}}();t(null!==e?e:"light")}(),function(){try{const c=new URLSearchParams(window.location.search).entries();for(var[t,e]of c)if(t.startsWith("docusaurus-data-")){var a=t.replace("docusaurus-data-","data-");document.documentElement.setAttribute(a,e)}}catch(t){}}()</script><div id="__docusaurus"><div role="region" aria-label="Skip to main content"><a class="skipToContent_fXgn" href="#__docusaurus_skipToContent_fallback">Skip to main content</a></div><nav aria-label="Main" class="navbar navbar--fixed-top"><div class="navbar__inner"><div class="navbar__items"><button aria-label="Toggle navigation bar" aria-expanded="false" class="navbar__toggle clean-btn" type="button"><svg width="30" height="30" viewBox="0 0 30 30" aria-hidden="true"><path stroke="currentColor" stroke-linecap="round" stroke-miterlimit="10" stroke-width="2" d="M4 7h22M4 15h22M4 23h22"></path></svg></button><a class="navbar__brand" href="/"><b class="navbar__title text--truncate">mf</b></a><div class="navbar__item dropdown dropdown--hoverable"><a href="#" aria-haspopup="true" aria-expanded="false" role="button" class="navbar__link">Additional FI MU materials</a><ul class="dropdown__menu"><li><a aria-current="page" class="dropdown__link dropdown__link--active" href="/algorithms/">Algorithms</a></li><li><a class="dropdown__link" href="/c/">C</a></li><li><a class="dropdown__link" href="/cpp/">C++</a></li></ul></div><a class="navbar__item navbar__link" href="/contributions/">Contributions</a><a class="navbar__item navbar__link" href="/talks/">Talks</a></div><div class="navbar__items navbar__items--right"><a class="navbar__item navbar__link" href="/blog/">Blog</a><div class="toggle_vylO colorModeToggle_DEke"><button class="clean-btn toggleButton_gllP toggleButtonDisabled_aARS" type="button" disabled="" title="Switch between dark and light mode (currently light mode)" aria-label="Switch between dark and light mode (currently light mode)" aria-live="polite"><svg viewBox="0 0 24 24" width="24" height="24" class="lightToggleIcon_pyhR"><path fill="currentColor" d="M12,9c1.65,0,3,1.35,3,3s-1.35,3-3,3s-3-1.35-3-3S10.35,9,12,9 M12,7c-2.76,0-5,2.24-5,5s2.24,5,5,5s5-2.24,5-5 S14.76,7,12,7L12,7z M2,13l2,0c0.55,0,1-0.45,1-1s-0.45-1-1-1l-2,0c-0.55,0-1,0.45-1,1S1.45,13,2,13z M20,13l2,0c0.55,0,1-0.45,1-1 s-0.45-1-1-1l-2,0c-0.55,0-1,0.45-1,1S19.45,13,20,13z M11,2v2c0,0.55,0.45,1,1,1s1-0.45,1-1V2c0-0.55-0.45-1-1-1S11,1.45,11,2z M11,20v2c0,0.55,0.45,1,1,1s1-0.45,1-1v-2c0-0.55-0.45-1-1-1C11.45,19,11,19.45,11,20z M5.99,4.58c-0.39-0.39-1.03-0.39-1.41,0 c-0.39,0.39-0.39,1.03,0,1.41l1.06,1.06c0.39,0.39,1.03,0.39,1.41,0s0.39-1.03,0-1.41L5.99,4.58z M18.36,16.95 c-0.39-0.39-1.03-0.39-1.41,0c-0.39,0.39-0.39,1.03,0,1.41l1.06,1.06c0.39,0.39,1.03,0.39,1.41,0c0.39-0.39,0.39-1.03,0-1.41 L18.36,16.95z M19.42,5.99c0.39-0.39,0.39-1.03,0-1.41c-0.39-0.39-1.03-0.39-1.41,0l-1.06,1.06c-0.39,0.39-0.39,1.03,0,1.41 s1.03,0.39,1.41,0L19.42,5.99z M7.05,18.36c0.39-0.39,0.39-1.03,0-1.41c-0.39-0.39-1.03-0.39-1.41,0l-1.06,1.06 c-0.39,0.39-0.39,1.03,0,1.41s1.03,0.39,1.41,0L7.05,18.36z"></path></svg><svg viewBox="0 0 24 24" width="24" height="24" class="darkToggleIcon_wfgR"><path fill="currentColor" d="M9.37,5.51C9.19,6.15,9.1,6.82,9.1,7.5c0,4.08,3.32,7.4,7.4,7.4c0.68,0,1.35-0.09,1.99-0.27C17.45,17.19,14.93,19,12,19 c-3.86,0-7-3.14-7-7C5,9.07,6.81,6.55,9.37,5.51z M12,3c-4.97,0-9,4.03-9,9s4.03,9,9,9s9-4.03,9-9c0-0.46-0.04-0.92-0.1-1.36 c-0.98,1.37-2.58,2.26-4.4,2.26c-2.98,0-5.4-2.42-5.4-5.4c0-1.81,0.89-3.42,2.26-4.4C12.92,3.04,12.46,3,12,3L12,3z"></path></svg></button></div><div class="navbarSearchContainer_Bca1"><button type="button" class="DocSearch DocSearch-Button" aria-label="Search"><span class="DocSearch-Button-Container"><svg width="20" height="20" class="DocSearch-Search-Icon" viewBox="0 0 20 20"><path d="M14.386 14.386l4.0877 4.0877-4.0877-4.0877c-2.
|
|||
|
|
<p>Let's start by the recap of what we've achieved so far:</p>
|
|||
|
|
<ol>
|
|||
|
|
<li>We have implemented a naïve brute-force algorithm that tries to relax paths
|
|||
|
|
as long as there are any paths to be relaxed.</li>
|
|||
|
|
<li>Then we have fixed an issue caused by negative loops that can result in
|
|||
|
|
a non-terminating run of our brute-force method. At this moment we have made
|
|||
|
|
some small arguments why are bounding is enough and doesn't prevent any
|
|||
|
|
shortest path to <em>not be</em> discovered.</li>
|
|||
|
|
<li>Finally we have converted our bounded brute-force algorithm into the
|
|||
|
|
Bellman-Ford algorithm.</li>
|
|||
|
|
<li>We have mentioned the worst-case time complexity of our bounded naïve
|
|||
|
|
approach and also the Bellman-Ford algorithm. Our worst-case depended on the
|
|||
|
|
fact that we assumed the worst possible ordering of the relaxations. However
|
|||
|
|
we could also try to relax in the most ideal ordering which could result in a
|
|||
|
|
faster algorithm and that's how we got to the Dijkstra's algorithm.</li>
|
|||
|
|
</ol>
|
|||
|
|
<p>Now the question is, could we improve the Dijkstra's algorithm to get even
|
|||
|
|
better results? And the answer is <em>maybe</em>!</p>
|
|||
|
|
<p>Dijkstra's algorithm chooses the next cheapest vertex for relaxing. This is good
|
|||
|
|
as long as there is no additional information. However, imagine a roadmap of
|
|||
|
|
some country. If you're in the middle of the map and you want to go south, it
|
|||
|
|
doesn't make much sense for you to go to the north (in the opposite direction),
|
|||
|
|
but a little bit might make sense, so that you can switch to highway and go much
|
|||
|
|
faster.</p>
|
|||
|
|
<p>The important question here is how to <em>influence</em> the algorithm, so that it does
|
|||
|
|
choose the path that <em>makes more sense</em> rather than the one that costs the
|
|||
|
|
least.</p>
|
|||
|
|
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="a-description">A* description<a href="#a-description" class="hash-link" aria-label="Direct link to A* description" title="Direct link to A* description"></a></h2>
|
|||
|
|
<p>The <em>A* algorithm</em> can be considered a modification of Dijkstra's algorithm. The
|
|||
|
|
cost is still the same, we cannot change it, right? However when we pick the
|
|||
|
|
vertices from the heap, we can influence the order by some <em>heuristic</em>. In this
|
|||
|
|
case, we introduce a function that can suggest how feasible the vertex is.</p>
|
|||
|
|
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="roadmap-heuristic">Roadmap heuristic<a href="#roadmap-heuristic" class="hash-link" aria-label="Direct link to Roadmap heuristic" title="Direct link to Roadmap heuristic"></a></h2>
|
|||
|
|
<p>Let's have a look at the heuristic we could use for the roadmap example. There
|
|||
|
|
are roads (the edges) and towns (the vertices). Cost could be an average time to
|
|||
|
|
travel the road. What heuristic could we use to influence our algorithm to
|
|||
|
|
choose a better ordering of the vertices when relaxing?</p>
|
|||
|
|
<p>In the former example we've said that it doesn't make much sense to go in the
|
|||
|
|
opposite direction than our goal is… We could choose the distance from our goal
|
|||
|
|
as the heuristic, e.g. right now we're 100 km away from our goal, using this
|
|||
|
|
road makes us 50 km away and using the other road we will be 200 km away.</p>
|
|||
|
|
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="heuristic-for-our-map">Heuristic for our map<a href="#heuristic-for-our-map" class="hash-link" aria-label="Direct link to Heuristic for our map" title="Direct link to Heuristic for our map"></a></h2>
|
|||
|
|
<p>Our map is a bit simpler, but we can use a very similar principle. We will use
|
|||
|
|
the <em>Manhattan distance</em>, which is defined in a following way:</p>
|
|||
|
|
<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mi mathvariant="normal">∣</mi><msub><mi>x</mi><mi>a</mi></msub><mo>−</mo><msub><mi>x</mi><mi>b</mi></msub><mi mathvariant="normal">∣</mi><mo>+</mo><mi mathvariant="normal">∣</mi><msub><mi>y</mi><mi>a</mi></msub><mo>−</mo><msub><mi>y</mi><mi>b</mi></msub><mi mathvariant="normal">∣</mi></mrow><annotation encoding="application/x-tex">\vert x_a - x_b \vert + \vert y_a - y_b \vert</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em"></span><span class="mord">∣</span><span class="mord"><span class="mord mathnormal">x</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.1514em"><span style="top:-2.55em;margin-left:0em;margin-right:0.05em"><span class="pstrut" style="height:2.7em"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">a</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.15em"><span></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2222em"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222em"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em"></span><span class="mord"><span class="mord mathnormal">x</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.3361em"><span style="top:-2.55em;margin-left:0em;margin-right:0.05em"><span class="pstrut" style="height:2.7em"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">b</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.15em"><span></span></span></span></span></span></span><span class="mord">∣</span><span class="mspace" style="margin-right:0.2222em"></span><span class="mbin">+</span><span class="mspace" style="margin-right:0.2222em"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em"></span><span class="mord">∣</span><span class="mord"><span class="mord mathnormal" style="margin-right:0.03588em">y</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.1514em"><span style="top:-2.55em;margin-left:-0.0359em;margin-right:0.05em"><span class="pstrut" style="height:2.7em"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">a</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.15em"><span></span></span></span></span></span></span><span class="mspace" style="margin-right:0.2222em"></span><span class="mbin">−</span><span class="mspace" style="margin-right:0.2222em"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em"></span><span class="mord"><span class="mord mathnormal" style="margin-right:0.03588em">y</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.3361em"><span style="top:-2.55em;margin-left:-0.0359em;margin-right:0.05em"><span class="pstrut" style="height:2.7em"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight">b</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.15em"><span></span></span></span></span></span></span><span class="mord">∣</span></span></span></span></span>
|
|||
|
|
<p>Since we cannot move in diagonals, it makes sense to maintain the distance in
|
|||
|
|
the actual steps from the goal.</p>
|
|||
|
|
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="passing-the-heuristic">Passing the heuristic<a href="#passing-the-heuristic" class="hash-link" aria-label="Direct link to Passing the heuristic" title="Direct link to Passing the heuristic"></a></h2>
|
|||
|
|
<p>In our case, when we're using C++, we can just template the function that will
|
|||
|
|
calculate the shortest path and pass the heuristic as a parameter.</p>
|
|||
|
|
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="implementation">Implementation<a href="#implementation" class="hash-link" aria-label="Direct link to Implementation" title="Direct link to Implementation"></a></h2>
|
|||
|
|
<p>Actual implementation is very easy once we have the Dijkstra's algorithm:</p>
|
|||
|
|
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token keyword" style="color:#00009f">auto</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">astar</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> graph</span><span class="token operator" style="color:#393A34">&</span><span class="token plain"> g</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> vertex_t</span><span class="token operator" style="color:#393A34">&</span><span class="token plain"> source</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">auto</span><span class="token operator" style="color:#393A34">&</span><span class="token plain"> h</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token operator" style="color:#393A34">-></span><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:#393A34">::</span><span class="token plain">vector</span><span class="token operator" style="color:#393A34"><</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:#393A34">::</span><span class="token plain">vector</span><span class="token operator" style="color:#393A34"><</span><span class="token keyword" style="color:#00009f">int</span><span class="token operator" style="color:#393A34">>></span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// make sure that ‹source› exists</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token function" style="color:#d73a49">assert</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">g</span><span class="token punctuation" style="color:#393A34">.</span><span class="token function" style="color:#d73a49">has</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">source</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain" style="display:inline-block"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token comment" style="color:#999988;font-style:italic">// initialize the distances</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:#393A34">::</span><span class="token plain">vector</span><span class="token operator" style="color:#393A34"><</span><span class="token plain">std</span><span class="token double-colon punctuation" style="color:#393A34">::</span><span class="token plain">vector</span><span cl
|
|||
|
|
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="running-on-our-map">Running on our map<a href="#running-on-our-map" class="hash-link" aria-label="Direct link to Running on our map" title="Direct link to Running on our map"></a></h2>
|
|||
|
|
<p>For this algorithm I will also show the example of a call:</p>
|
|||
|
|
<div class="language-cpp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-cpp codeBlock_bY9V thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">distances </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> </span><span class="token function" style="color:#d73a49">astar</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">g</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:#393A34">::</span><span class="token function" style="color:#d73a49">make_pair</span><span class="token punctuation" style="color:#393A34">(</span><span class="token number" style="color:#36acaa">1</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">9</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token punctuation" style="color:#393A34">]</span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">const</span><span class="token plain"> </span><span class="token keyword" style="color:#00009f">auto</span><span class="token operator" style="color:#393A34">&</span><span class="token plain"> u</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">{</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">auto</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">[</span><span class="token plain">x</span><span class="token punctuation" style="color:#393A34">,</span><span class="token plain"> y</span><span class="token punctuation" style="color:#393A34">]</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">=</span><span class="token plain"> u</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token keyword" style="color:#00009f">return</span><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:#393A34">::</span><span class="token function" style="color:#d73a49">abs</span><span class="token punctuation" style="color:#393A34">(</span><span class="token number" style="color:#36acaa">1</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">-</span><span class="token plain"> x</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">+</span><span class="token plain"> std</span><span class="token double-colon punctuation" style="color:#393A34">::</span><span class="token function" style="color:#d73a49">abs</span><span class="token punctuation" style="color:#393A34">(</span><span class="token number" style="color:#36acaa">7</span><span class="token plain"> </span><span class="token operator" style="color:#393A34">-</span><span class="token plain"> y</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">;</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">}<
|
|||
|
|
<p>First argument to the function is the graph itself. Second argument is the
|
|||
|
|
source vertex where we start. And finally the lambda returns
|
|||
|
|
<em>Manhattan distance</em> to the goal.</p>
|
|||
|
|
<p>And we get the following result:</p>
|
|||
|
|
<div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">Normal cost: 1</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Vortex cost: 5</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Graph:</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#############</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#..#..*.*.**#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">##***.....**#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#..########.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#...###...#.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#..#...##.#.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#..#.*.#..#.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#D...#....#.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">########*.*.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#S..........#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#############</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[Finite BF] Cost: 22</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[Bellman-Ford] Cost: 22</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[Dijkstra] Cost: 22</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[A*] Cost: 22</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
|
|||
|
|
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="comparison">Comparison<a href="#comparison" class="hash-link" aria-label="Direct link to Comparison" title="Direct link to Comparison"></a></h2>
|
|||
|
|
<p>Now you may wonder how does it compare to the previous algorithms. Supposedly it
|
|||
|
|
should be faster. Let's add counters and debugging output when we update
|
|||
|
|
distance to our goal. And now if we run our code, we get the following output:</p>
|
|||
|
|
<div class="codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-text codeBlock_bY9V thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain">Normal cost: 1</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Vortex cost: 5</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Graph:</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#############</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#..#..*.*.**#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">##***.....**#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#..########.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#...###...#.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#..#...##.#.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#..#.*.#..#.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#D...#....#.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">########*.*.#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#S..........#</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#############</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Relaxing path to goal in 40. relaxation</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Relaxing path to goal in 68. relaxation</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Relaxing path to goal in 89. relaxation</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[Finite BF] Cost: 22</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Relaxing path to goal in 40. relaxation</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Relaxing path to goal in 68. relaxation</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Relaxing path to goal in 89. relaxation</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[Bellman-Ford] Cost: 22</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Relaxing path to goal in 41. iteration</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[Dijkstra] Cost: 22</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">Relaxing path to goal in 31. iteration</span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">[A*] Cost: 22</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
|
|||
|
|
<p>From the output we can easily deduce that for both brute-force and Bellman-Ford,
|
|||
|
|
which are in our case identical, we actually relax three times and for the last
|
|||
|
|
time in the 89th iteration.</p>
|
|||
|
|
<p>Dijkstra's algorithm manages to find the shortest path to our goal already in
|
|||
|
|
the 41st iteration.</p>
|
|||
|
|
<p>And finally after introducing some heuristic, we could find the shortest path
|
|||
|
|
in the 31st iteration.</p>
|
|||
|
|
<div class="theme-admonition theme-admonition-danger admonition_xJq3 alert alert--danger"><div class="admonitionHeading_Gvgb"><span class="admonitionIcon_Rf37"><svg viewBox="0 0 12 16"><path fill-rule="evenodd" d="M5.05.31c.81 2.17.41 3.38-.52 4.31C3.55 5.67 1.98 6.45.9 7.98c-1.45 2.05-1.7 6.53 3.53 7.7-2.2-1.16-2.67-4.52-.3-6.61-.61 2.03.53 3.33 1.94 2.86 1.39-.47 2.3.53 2.27 1.67-.02.78-.31 1.44-1.13 1.81 3.42-.59 4.78-3.42 4.78-5.56 0-2.84-2.53-3.22-1.25-5.61-1.52.13-2.03 1.13-1.89 2.75.09 1.08-1.02 1.8-1.86 1.33-.67-.41-.66-1.19-.06-1.78C8.18 5.31 8.68 2.45 5.05.32L5.03.3l.02.01z"></path></svg></span>danger</div><div class="admonitionContent_BuS1"><p>Please keep in mind that choosing bad heuristic can actually lead to worse
|
|||
|
|
results than using no heuristic at all.</p></div></div>
|
|||
|
|
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="summary">Summary<a href="#summary" class="hash-link" aria-label="Direct link to Summary" title="Direct link to Summary"></a></h2>
|
|||
|
|
<p>And there we have it. We have made our way from the brute-force algorithm all
|
|||
|
|
the way to more optimal ones. Hopefully we could notice how the small
|
|||
|
|
improvements of the already existing algorithms made them much better.</p></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-tags-row row margin-bottom--sm"><div class="col"><b>Tags:</b><ul class="tags_jXut padding--none margin-left--sm"><li class="tag_QGVx"><a class="tag_zVej tagRegular_sFm0" href="/algorithms/tags/cpp/">cpp</a></li><li class="tag_QGVx"><a class="tag_zVej tagRegular_sFm0" href="/algorithms/tags/dynamic-programming/">dynamic programming</a></li><li class="tag_QGVx"><a class="tag_zVej tagRegular_sFm0" href="/algorithms/tags/astar/">astar</a></li></ul></div></div><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/mfocko/blog/tree/main/algorithms/11-paths/2024-01-01-bf-to-astar/03-astar.md" target="_blank" rel="noopener noreferrer" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_vwxv"><span class="theme-last-updated">Last updated<!-- --> on <b><time datetime="2024-01-03T00:00:00.000Z">Jan 3, 2024</time></b></span></div></div></footer></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="/algorithms/paths/bf-to-astar/dijkstra/"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">Dijkstra's algorithm</div></a><a class="pagination-nav__link pagination-nav__link--next" href="/algorithms/category/hash-tables/"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">Hash Tables</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="#intro" class="table-of-contents__link toc-highlight">Intro</a></li><li><a href="#a-description" class="table-of-contents__link toc-highlight">A* description</a></li><li><a href="#roadmap-heuristic" class="table-of-contents__link toc-highlight">Roadmap heuristic</a></li><li><a href="#heuristic-for-our-map" class="table-of-contents__link toc-highlight">Heuristic for our map</a></li><li><a href="#passing-the-heuristic" class="table-of-contents__link toc-highlight">Passing the heuristic</a></li><li><a href="#implementation" class="table-of-contents__link toc-highlight">Implementation</a></li><li><a href="#running-on-our-map" class="table-of-contents__link toc-highlight">Running on our map</a></li><li><a href="#comparison" class="table-of-contents__link toc-highlight">Comparison</a></li><li><a href="#summary" class="table-of-contents__link toc-highlight">Summary</a></li></ul></div></div></div></div></main></div></div></div><footer class="footer footer--dark"><div class="container container-fluid"><div class="row footer__links"><div class="col footer__col"><div class="footer__title">Git</div><ul class="footer__items clean-list"><li class="footer__item"><a href="https://github.com/mfocko" target="_blank" rel="noopener noreferrer" class="footer__link-item">GitHub<svg width="13.5" height="13.5" aria-hidden="true" viewBox="0 0 24 24" class="iconExternalLink_nPIU"><path fill="currentColor" d="M21 13v10h-21v-19h12v2h-10v15h17v-8h2zm3-12h-10.988l4.035 4-6.977 7.07 2.828 2.828 6.977-7.07 4.125 4.172v-11z"></path></svg></a></li><li class="footer__item"><a href="https://gitlab.com/mfocko" target="_blank" rel="noopener noreferrer" class="footer__link-item">GitLab<svg width="13.5" height="13.5" aria-hidden="true" viewBox="0 0 24 24" class="iconExternalLink_nPIU"><path fill="currentColor" d="M21 13v10h-21v-19h12v2h-10v15h17v-8h2zm3-12h-10.988l4.035 4-6.977 7.07 2.828 2.828 6.977-7.07 4.125 4.172v-11z"></path></svg></a></li><li class="footer__item"><a href="https://git.mfocko.xyz/mfocko" target="_blank" rel="noopener noreferrer"
|
|||
|
|
</body>
|
|||
|
|
</html>
|