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338 lines
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<h1 id="parsing-and-rendering"><a class="header" href="#parsing-and-rendering">Parsing and rendering</a></h1>
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<p>This document goes through the internals of how we take a markdown file and finish rendering it into the terminal
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screen.</p>
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<h2 id="parsing"><a class="header" href="#parsing">Parsing</a></h2>
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<p>Markdown file parsing is done via the <a href="https://github.com/kivikakk/comrak">comrak</a> crate. This crate parses the markdown
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file and gives you back an AST that contains the contents and structure of the input file.</p>
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<p>ASTs are a logical way of representing the markdown file but this structure makes it a bit hard to process. Given our
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ultimate goal is to render this input, we want it to be represented in a way that facilitates that. Because of this we
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first do a pass on this AST and construct a list of <code>MarkdownElement</code>s. This enum represents each of the markdown
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elements in a flattened, non-recursive, way:</p>
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<ul>
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<li>Inline text is flattened so that instead of having a recursive structure you have chunks of text, each with their own
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style. So for example the text "<strong>hello <em>my name is <del>bob</del></em></strong>" which would look like a 3 level tree (I think?) in the
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AST, gets transformed to something like <code>[Bold(hello), ItalicsBold(my name is), ItalicsBoldStrikethrough(bob)]</code> (names
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are completely not what they are in the code, this is just to illustrate flattening). This makes it much easier to
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render text because we don't need to walk the tree and keep the state between levels.</li>
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<li>Lists are flattened into a single <code>MarkdownElement::List</code> element that contains a list of items that contain their
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text, prefix ("*" for bullet lists), and nesting depth. This also simplifies processing as list elements can also
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contain formatted text so we would otherwise have the same problem as above.</li>
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</ul>
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<p>This first step then produces a list of elements that can easily be processed.</p>
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<h2 id="building-the-presentation"><a class="header" href="#building-the-presentation">Building the presentation</a></h2>
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<p>The format above is <em>nicer</em> than an AST but it's still not great to be used as the input to the code that renders the
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presentation for various reasons:</p>
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<ul>
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<li>The presentation needs to be styled, which means we need to apply a theme on top of it to transform it. Putting this
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responsibility in the render code creates too much coupling: now the render needs to understand markdown <em>and</em> how
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themes work.</li>
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<li>The render code tends to be a bit annoying: we need to jump around in the screen, print text, change colors, etc. If
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we add the responsibility of transforming the markdown into visible text to the render code itself, we end up having a
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mess of UI code mixed with the markdown element processing.</li>
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<li>Some elements can't be printed as-is. For example, a list item has text and a prefix, so we don't want the render code
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to be in charge of understanding and executing those transformations.</li>
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</ul>
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<p>Because of this, we introduce a step in between parsing and rendering where we build a presentation. A presentation is
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made up of a list of slides and each slide is made up of render operations. Render operations are the primitives that
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the render code understands to print text on the screen. These can be the following, among others:</p>
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<ul>
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<li>Render text.</li>
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<li>Clear the screen.</li>
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<li>Set the default colors to be used.</li>
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<li>Render a line break.</li>
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<li>Jump to the middle of the screen.</li>
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</ul>
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<p>This allows us to have a simple model where the logic that takes markdown elements and a theme and chooses <em>how</em> it will
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be rendered is in one place, and the logic that takes those instructions and executes them is elsewhere. So for example,
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this step will take a bullet point and concatenate is suffix ("*" for bullet points for example), turn that into a
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single string and generate a "render text" operation.</p>
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<p>This has the nice added bonus that the rendering code doesn't have to be fiddling around with string concatenation or
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other operations that could take up CPU cycles: it just takes these render operations and executes them. Not that
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performance matters here but it's nice to get better performance for free.</p>
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<h2 id="render-a-slide"><a class="header" href="#render-a-slide">Render a slide</a></h2>
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<p>The rendering code is straightforward and simply takes the current slide, iterates all of its rendering operations, and
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executes those one by one. This is done via the <a href="https://github.com/crossterm-rs/crossterm">crossterm</a> crate.</p>
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<p>The only really complicated part is fitting text into the screen. Because we apply our own margins, we perform word
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splitting and wrapping around manually, so there's some logic that takes the text to be printed and the width of the
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terminal and splits it accordingly.</p>
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<p>Note that this piece of code is the only one aware of the current screen size. This lets us forget in previous steps
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about how large the screen is and simply delegate that responsibility to this piece.</p>
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<h2 id="entire-flow"><a class="header" href="#entire-flow">Entire flow</a></h2>
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<p><img src="../assets/parse-flow.png" alt="" /></p>
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