mirror of https://github.com/Wilfred/difftastic/
307 lines
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307 lines
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<main>
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<h1 id="internals-diffing"><a class="header" href="#internals-diffing">Internals: Diffing</a></h1>
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<p>Difftastic treats diff calculations as a route finding problem on a
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directed acyclic graph.</p>
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<h2 id="graph-representation"><a class="header" href="#graph-representation">Graph Representation</a></h2>
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<p>A vertex in the graph represents a position in two syntax trees.</p>
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<p>The start vertex has both positions pointing to the first syntax node
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in both trees. The end vertex has both positions just
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after the last syntax node in both trees.</p>
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<p>Consider comparing <code>A</code> with <code>X A</code>.</p>
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<pre><code>START
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+---------------------+
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| Left: A Right: X A |
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| ^ ^ |
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+---------------------+
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END
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+---------------------+
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| Left: A Right: X A |
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| ^ ^|
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+---------------------+
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</code></pre>
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<p>From the start vertex, we have two options:</p>
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<ul>
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<li>we can mark the first syntax node on the left as novel, and advance
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to the next syntax node on the left (vertex 1 above), or</li>
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<li>we can mark the first syntax node on the right as novel, and advance
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to the next syntax node on the right (vertex 2 above).</li>
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</ul>
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<pre><code> START
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+---------------------+
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| Left: A Right: X A |
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| ^ ^ |
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+---------------------+
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/ \
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Novel atom L / \ Novel atom R
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1 v 2 v
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+---------------------+ +---------------------+
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| Left: A Right: X A | | Left: A Right: X A |
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| ^ ^ | | ^ ^ |
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+---------------------+ +---------------------+
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</code></pre>
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<p>Choosing "novel atom R" to vertex 2 will turn out to be the best
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choice. From vertex 2, we can see three routes to the end vertex.</p>
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<pre><code> 2
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+---------------------+
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| Left: A Right: X A |
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| ^ ^ |
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+---------------------+
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/ | \
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Novel atom L / | \ Novel atom R
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v | v
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+---------------------+ | +---------------------+
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| Left: A Right: X A | | | Left: A Right: X A |
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| ^ ^ | | | ^ ^|
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+---------------------+ | +---------------------+
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| | |
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| Novel atom R | Nodes match | Novel atom L
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| | |
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| END v |
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| +---------------------+ |
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+-------->| Left: A Right: X A |<---------+
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| ^ ^|
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+---------------------+
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</code></pre>
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<h2 id="comparing-routes"><a class="header" href="#comparing-routes">Comparing Routes</a></h2>
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<p>We assign a cost to each edge. Marking a syntax node as novel is worse
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than finding a matching syntax node, so the "novel atom" edge has a
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higher cost than the "syntax nodes match" edge.</p>
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<p>The best route is the lowest cost route from the start vertex to the
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end vertex.</p>
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<h2 id="finding-the-best-route"><a class="header" href="#finding-the-best-route">Finding The Best Route</a></h2>
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<p>Difftastic uses Dijkstra's algorithm to find the best (i.e. lowest cost)
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route.</p>
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<p>One big advantage of this algorithm is that we don't need to construct
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the graph in advance. Constructing the whole graph would require
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exponential memory relative to the number of syntax nodes. Instead,
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vertex neighbours are constructed as the graph is explored.</p>
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<p>There are lots of resources explaining Dijkstra's algorithm online,
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but I particularly recommend the <a href="https://www.redblobgames.com/pathfinding/a-star/introduction.html#dijkstra">graph search section of Red Blob
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Games</a>.</p>
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