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-<span id="Path-Finding"></span><div class="header">
-<p>
-Previous: <a href="Grids.html" accesskey="p" rel="prev">Grids</a>, Up: <a href="Data-Structures.html" accesskey="u" rel="up">Data Structures</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html" title="Index" rel="index">Index</a>]</p>
-</div>
-<hr />
-<span id="Path-Finding-1"></span><h4 class="subsection">5.6.6 Path Finding</h4>
-
-<p>Most game worlds have maps. Often, these games have a need to move
-non-player characters around in an unscripted fashion. For example,
-in a real-time strategy game, the player may command one of their
-units to attack something in the enemy base. To do so, the unit must
-calculate the shortest route to get there. It wouldn&rsquo;t be a very fun
-game if units didn&rsquo;t know how to transport themselves efficiently.
-This is where path finding algorithms come in handy. The
-<code>(chickadee data path-finding)</code> module provides a generic
-implementation of the popular A* path finding algorithm. Just add a
-map implementation!
-</p>
-<p>The example below defines a very simple town map and finds the
-quickest way to get from the town common to the school.
-</p>
-<div class="lisp">
-<pre class="lisp"><span class="syntax-open">(</span><span class="syntax-special">define</span> <span class="syntax-symbol">world-map</span>
- <span class="syntax-symbol">'</span><span class="syntax-open">(</span><span class="syntax-open">(</span><span class="syntax-symbol">town-common</span> <span class="syntax-symbol">.</span> <span class="syntax-open">(</span><span class="syntax-symbol">town-hall</span> <span class="syntax-special">library</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
- <span class="syntax-open">(</span><span class="syntax-symbol">town-hall</span> <span class="syntax-symbol">.</span> <span class="syntax-open">(</span><span class="syntax-symbol">town-common</span> <span class="syntax-symbol">school</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
- <span class="syntax-open">(</span><span class="syntax-special">library</span> <span class="syntax-symbol">.</span> <span class="syntax-open">(</span><span class="syntax-symbol">town-common</span> <span class="syntax-symbol">cafe</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
- <span class="syntax-open">(</span><span class="syntax-symbol">school</span> <span class="syntax-symbol">.</span> <span class="syntax-open">(</span><span class="syntax-symbol">town-hall</span> <span class="syntax-symbol">cafe</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
- <span class="syntax-open">(</span><span class="syntax-symbol">cafe</span> <span class="syntax-symbol">.</span> <span class="syntax-open">(</span><span class="syntax-special">library</span> <span class="syntax-symbol">school</span><span class="syntax-close">)</span><span class="syntax-close">)</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
-<span class="syntax-open">(</span><span class="syntax-special">define</span> <span class="syntax-open">(</span><span class="syntax-symbol">neighbors</span> <span class="syntax-symbol">building</span><span class="syntax-close">)</span>
- <span class="syntax-open">(</span><span class="syntax-symbol">assq-ref</span> <span class="syntax-symbol">town-map</span> <span class="syntax-symbol">building</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
-<span class="syntax-open">(</span><span class="syntax-special">define</span> <span class="syntax-open">(</span><span class="syntax-symbol">cost</span> <span class="syntax-symbol">a</span> <span class="syntax-symbol">b</span><span class="syntax-close">)</span> <span class="syntax-symbol">1</span><span class="syntax-close">)</span>
-<span class="syntax-open">(</span><span class="syntax-special">define</span> <span class="syntax-open">(</span><span class="syntax-symbol">distance</span> <span class="syntax-symbol">a</span> <span class="syntax-symbol">b</span><span class="syntax-close">)</span> <span class="syntax-symbol">1</span><span class="syntax-close">)</span>
-<span class="syntax-open">(</span><span class="syntax-special">define</span> <span class="syntax-symbol">pf</span> <span class="syntax-open">(</span><span class="syntax-symbol">make-path-finder</span><span class="syntax-close">)</span><span class="syntax-close">)</span>
-<span class="syntax-open">(</span><span class="syntax-symbol">a*</span> <span class="syntax-symbol">pf</span> <span class="syntax-symbol">'town-common</span> <span class="syntax-symbol">'school</span> <span class="syntax-symbol">neighbors</span> <span class="syntax-symbol">cost</span> <span class="syntax-symbol">distance</span><span class="syntax-close">)</span>
-</pre></div>
-
-<p>In this case, the <code>a*</code> procedure will return the list
-<code>(town-common town-hall school)</code>, which is indeed the shortest
-route. (The other possible route is <code>(town-common library cafe
-school)</code>.)
-</p>
-<p>The <code>a*</code> procedure does not know anything about about any kind of
-map and therefore must be told how to look up neighboring nodes, which
-is what the <code>neighbors</code> procedure in the example does. To
-simulate different types of terrain, a cost procedure is used. In
-this example, it is just as easy to move between any two nodes because
-<code>cost</code> always returns 1. In a real game, perhaps moving from
-from a field to a rocky hill would cost a lot more than moving from
-one field to another. Finally, a heuristic is used to calculate an
-approximate distance between two nodes on the map. In this simple
-association list based graph it is tough to calculate a distance
-between nodes, so the <code>distance</code> procedure isn&rsquo;t helpful and
-always returns 1. In a real game with a tile-based map, for example,
-the heuristic could be a quick Manhattan distance calculation based on
-the coordinates of the two map tiles. Choose an appropriate heuristic
-for optimal path finding!
-</p>
-<dl>
-<dt id="index-make_002dpath_002dfinder">Procedure: <strong>make-path-finder</strong></dt>
-<dd><p>Return a new path finder object.
-</p></dd></dl>
-
-<dl>
-<dt id="index-path_002dfinder_003f">Procedure: <strong>path-finder?</strong> <em>obj</em></dt>
-<dd><p>Return <code>#t</code> if <var>obj</var> is a path finder.
-</p></dd></dl>
-
-<dl>
-<dt id="index-a_002a">Procedure: <strong>a*</strong> <em>path-finder start goal neighbors cost distance</em></dt>
-<dd>
-<p>Return a list of nodes forming a path from <var>start</var> to <var>goal</var>
-using <var>path-finder</var> to hold state. <var>neighbors</var> is a procedure
-that accepts a node and returns a list of nodes that neighbor it.
-<var>cost</var> is a procedure that accepts two neighboring nodes and
-returns the cost of moving from the first to the second as a real
-number. <var>distance</var> is a procedure that accepts two nodes and
-returns an approximate distance between them.
-</p></dd></dl>
-
-<hr />
-<div class="header">
-<p>
-Previous: <a href="Grids.html" accesskey="p" rel="prev">Grids</a>, Up: <a href="Data-Structures.html" accesskey="u" rel="up">Data Structures</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index.html" title="Index" rel="index">Index</a>]</p>
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