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Apply proofreading changes to web. Also:
- Ackknowledgements - Add chapter navigation to top of pages - Some other minor style tweaks
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‎asset/style.scss‎

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}
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}
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nav.top {
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margin-top: 0;
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}
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.page {
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background: hsl(40, 30%, 98%);
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max-width: 640px;
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}
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}
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aside + aside {
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margin-top: 10px;
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}
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// The front page gets a bit of special styling.
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.index {
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h1 {
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nav {
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margin: 20px -18px 0 -18px;
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padding: 10px 18px;
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padding: 6px 18px;
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font-size: 13px;
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}
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‎asset/template.html‎

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<body id="top">
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<div class="page sidebar">
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<div class="content">
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<nav class="top">
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{{prev}}
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{{next}}
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<span class="toc">&equiv; <a href="/">The Book</a></span>
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</nav>
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<h1>{{header}}</h1>
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<h1 class="book"><a href="/">Game Programming Patterns</a>{{section_header}}</h1>
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{{body}}
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<nav>
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{{prev}}
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{{next}}
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<span class="toc">&equiv; <a href="/">Table of Contents</a></span>
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<span class="toc">&equiv; <a href="/">The Book</a></span>
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</nav>
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</div>
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</div>

‎book/acknowledgements.markdown‎

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^title Acknowledge&shy;ments
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I've heard only other authors know what's involved in writing a book, but there
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is another tribe who know the precise weight of that burden -- those with the
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misfortune of being in a relationship with a writer. I wrote this in a space of
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time painstakingly carved from the dense rock of life for me by my wife Megan.
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Washing dishes and giving the kids baths may not be "writing", but without her
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doing those, this book wouldn't be here.
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I started this project while a programmer at Electronic Arts. I don't think the
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company knew quite what to make of it, and I'm grateful to Michael Malone,
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Olivier Nallet, and Richard Wifall for supporting it and providing detailed,
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insightful feedback on the first few chapters.
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<span name="editor">About</span> halfway through writing, I decided to forgo a traditional publisher. I
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knew that meant losing the guidance an editor brings, but I had email from
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dozens of readers telling me where they wanted the book to go. I'd lose
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proofreaders, but I had over <span name="colm">250 bug reports</span> to help improve the prose. I'd give
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up the incentive of a writing schedule, but with readers patting my back when I
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finished each chapter, I had more than enough motivation.
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<aside name="editor">
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What I didn't lose was a copy editor. Lauren Briese showed up just when I
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needed her and did a wonderful job.
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</aside>
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They call this "self publishing", but "crowd publishing" is closer to the mark.
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Writing can be lonely work, but I was never alone. Even when I put the book on a
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shelf for two years, the encouragement continued. Without the dozens of people
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who didn't let me forget that they were waiting for more chapters, I never would
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have picked it back up and finished.
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<aside name="colm">
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Special thanks go to Colm Sloan who pored over every single chapter in the book
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*twice* and gave me mountains of fantastic feedback, all out of the goodness of
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his own heart. I owe you a beer or twenty.
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</aside>
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To everyone who emailed or commented, upvoted or favorited, tweeted or
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retweeted, anyone who reached out to me, or told a friend about the book, or
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sent me a bug report: my heart is filled with gratitude for you. Completing this
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book was one of my biggest goals in life, and you made it happen.
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Thank you!

‎book/architecture-performance-and-games.markdown‎

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</aside>
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We tend to gloss over this step, but it's often the most time-consuming part of
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programming. If you think paging some data from RAM into disk is slow, try
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programming. If you think paging some data from disk into RAM is slow, try
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paging it into a simian cerebrum over a pair of optical nerves.
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Once you've got all the right context into your wetware, you think for a bit and
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I won't get on a soapbox here, but I'll ask you to consider doing more automated
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testing if you aren't already. Don't you have better things to do than manually
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test stuff over and over again?
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validate stuff over and over again?
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</aside>
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You can define "decoupling" a bunch of ways, but I think if two pieces of code
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are coupled, it means you can't understand one without understanding the other.
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If you *de*couple them, you can reason about either side independently. That's
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If you *de*-couple them, you can reason about either side independently. That's
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great because if only one of those pieces is relevant to your problem, you just
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need to load it into your monkey brain and not the other half too.
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need to load *it* into your monkey brain and not the other half too.
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To me, this is a key goal of software architecture: **minimize the amount of
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knowledge you need to have in-cranium before you can make progress.**
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code that does something. When you need to make a change, sure, there's probably
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an interface there to help, but good luck finding it. In theory, all of this
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decoupling means you have less code to understand before you can extend it, but
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the layers of abstraction themselves end up filling up your mental scratch disk.
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the layers of abstraction themselves end up filling your mental scratch disk.
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Codebases like this are what turn people *against* software architecture, and
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design patterns in particular. It's easy to get so wrapped up in the code itself

‎book/bytecode.markdown‎

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A literal number expression simply evaluates to its value. Addition and
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multiplication are a bit more complex because they contain subexpressions.
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Before they can evaluate themselves, they need to recursively evaluate the
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Before they can evaluate themselves, they need to recursively evaluate their
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subexpressions. Like so:
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<span name="addition"></span>
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</aside>
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It's a <span name="ruby">beautiful</span>, simple pattern, but is has some
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It's a <span name="ruby">beautiful</span>, simple pattern, but it has some
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problems. Look up at the illustration. What do you see? Lots of little boxes,
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and lots of arrows between them. Code is represented as a sprawling fractal tree
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of tiny objects. That has some unpleasant consequences:
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## The Pattern
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An **instruction set** defines the low-level operations that can be performed.
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These are encoded as a **sequence of bytes**. A **virtual machine** executes
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A series of instructions is encoded as a **sequence of bytes**. A **virtual machine** executes
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these instructions one at a time, using a **stack for intermediate values**. By
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combining instructions, complex high-level behavior can be defined.
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Programming is hard. We know what we want the machine to do, but we don't always
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communicate that correctly -- we write bugs. To help find and fix those, we've
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amassed a pile of tools to understand what our code is doing wrong, and how to
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right it.
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We have debuggers, static analyzers, decompilers, etc. All of those tools are
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right it. We have debuggers, static analyzers, decompilers, etc. All of those tools are
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designed to work with some existing language: either machine code or something
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higher level.
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To encode a spell in data, we store an array of `enum` values. We've only got
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a few different primitives, so the range of `enum` values easily fits into a byte.
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This means a spell is just a list of <span name="byte">bytes</span> -- ergo
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This means the code for a spell is just a list of <span name="byte">bytes</span> -- ergo
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"bytecode".
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<img src="images/bytecode-code.png" alt="A sequence of bytecode instructions: 0x00 HEALTH, 0x03 SOUND, 0x004 PARTICLES, ..." />
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### How are values represented?
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Our sample VM only works with one kind of value, integers. That makes it easy --
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Our sample VM only works with one kind of value, integers. That makes answering this easy --
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the stack is just a stack of `int`s. A more full-featured VM will support
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different data types: strings, objects, lists, etc. You'll have to decide how
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those are stored internally.
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ways.
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Most non-programmers don't think of plaintext like that. To them, text
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files feel like filling in tax forms for a robotic auditor that yells at
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files feel like filling in tax forms for an angry robotic auditor that yells at
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them if they forget a single semicolon.
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* **If you define a graphical authoring tool:**
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<aside name="lines">
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Except for line endings. *Sigh...*
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Except for line endings. And encodings.
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</aside>
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‎book/command.markdown‎

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I think we can all agree that that's a terrible sentence. First of all, it
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mangles whatever metaphor it's trying to establish. Outside of the weird world
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of software where words can mean anything, a "client" is a *person* -- someone
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you do business with. Last I checked, people can't be "parameterized".
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you do business with. Last I checked, human beings can't be "parameterized".
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Then, the rest of that sentence is just a list of stuff you could maybe possibly
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## Undo and Redo
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The last example is the most well-known use of this pattern. If a command object
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The final example is the most well-known use of this pattern. If a command object
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can *do* things, it's a small step for it to be able to *undo* them. Undo is
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used in some strategy games where you can roll back moves that you didn't like.
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It's *de rigueur* in tools that people use to *create* games. The <span
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and then execute that command. If they choose a new command after undoing some,
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everything in the list after the current command is discarded.
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The first time I implemented this in a level editor, I felt like a wizard. I was
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The first time I implemented this in a level editor, I felt like a genius. I was
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astonished at how straightforward it was and how well it worked. It takes
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discipline to make sure every data modification goes through a command, but once
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you do that, the rest is easy.

‎book/component.markdown‎

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isolate each pattern. Many design patterns exist to contain code that itself
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isn't part of the pattern. In order to distill the pattern down to its essence,
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I try to cut as much of that out as possible, but at some point it becomes a bit
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like explaining how an engine works without mentioning fuel or oil.
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like explaining how to organize a closet without showing any clothes.
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The Component pattern is a particularly hard one. You can't get a real feel for
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it without seeing some code for each of the domains that it decouples, so I'll

‎book/data-locality.markdown‎

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precise timer. To catch poor cache usage, you'll want something a little more
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sophisticated. You really want to see how many cache misses are occurring and
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Fortunately, there are <span name="cachegrind">profilers</span> out there that
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report cache misses. It's worth spending the time to get one of these working and make
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report this. It's worth spending the time to get one of these working and make
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### Contiguous arrays
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Let's start with a <a href="game-loop.html" class="pattern">Game Loop</a> that
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Let's start with a <a href="game-loop.html" class="pattern">game loop</a> that
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processes a bunch of game entities. Those entities are decomposed into different
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</aside>
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You can see how this starts to get fuzzy, though. In my example here, it's
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pretty obvious which data should be hot and cold, but it's rarely so clear-cut.
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pretty obvious which data should be hot and cold, but in a real game it's
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rarely so clear-cut.
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What if you have fields that are used when an entity is in a certain mode but
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certain parts of the level?

‎book/dirty-flag.markdown‎

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## Motivation
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"Flag" and "bit" are synonymous in programming -- they both mean a single micron
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of data that can be in one of two states. We call those "true" and "false", or
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sometimes "set" and "cleared". I'll use all of these interchangeably. "Dirty
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bit" is an equally <span name="specific">common</span> name for this pattern,
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but I figured I'd stick with the name that didn't seem as prurient.
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<aside name="specific">
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Wikipedia's editors don't have my level of self-control and went with [dirty
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bit](http://en.wikipedia.org/wiki/Dirty_bit).
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</aside>
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### Locating a ship at sea
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Many games have something called a *scene graph*. This is a big data structure
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that contains all of the objects in the world. The rendering engine uses this to
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However, most scene graphs are <span name="hierarchical">*hierarchical*</span>.
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An object in the graph may have a parent object that it is anchored to. In that
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case, its transform is relative to the *parent's* position and isn't an
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For example, imagine our game world has a pirate ship at sea. Atop the ship's
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mast is a crow's nest. Hunched in that crow's nest is a pirate. Clutching the
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pirate's shoulder is a parrot. The ship's local transform will position it in
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the sea. The crow's nest's transform positions it on the ship, and so on.
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pirate's shoulder is a parrot. The ship's local transform positions the ship in
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the sea. The crow's nest's transform positions the nest on the ship, and so on.
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<span name="pirate"></span>
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We only moved four objects, but we did *ten* world transform calculations.
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by the renderer. We calculated the parrot's world transform *four* times, but it
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only got rendered once.
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is only rendered once.
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</aside>
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To do this, we add a flag to each object in the graph. When the local transform
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synonymous in programming -- they both mean a single micron of data that can be
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in one of two states. We call those "true" and "false", or sometimes "set" and
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"cleared". I'll use all of these interchangeably.
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When the local transform
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changes, we set it. When we need the object's world transform, we check the
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flag. If it's set, we calculate the world transform and then clear the flag. The
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flag represents, "Is the world transform out of date?" For reasons that aren't
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entirely clear, the traditional name for this "out-of-date-ness" is "dirty".
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Hence: *a dirty flag*. "Dirty bit" is an equally
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<span name="specific">common</span> name for this pattern, but I figured I'd
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stick with the name that didn't seem as prurient.
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<aside name="specific">
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Wikipedia's editors don't have my level of self-control and went with [dirty
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bit](http://en.wikipedia.org/wiki/Dirty_bit).
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</aside>
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If we apply this pattern and then move all of the objects in our previous
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that's often a better choice than using this pattern and calculating the
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All of this makes it sound like dirty flags are never appropriate, but you'll
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This makes it sound like dirty flags are rarely appropriate, but you'll
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find a place here or there where they help. <span name="hacks">Searching</span>
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your average game codebase for the word "dirty" will often turn up uses of this
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we store the result in a field instead of a local variable. When the node is
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clean, we skip `combine()` completely and use the old but still correct `world_`
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clean, we skip `combine()` completely and use the old-but-still-correct `world_`
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<aside name="hysteresis">
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The term in human-computer interaction for in intentional delay between
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The term in human-computer interaction for an intentional delay between
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when a program receives user input and when it responds is [*hysteresis*](http://en.wikipedia.org/wiki/Hysteresis).
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</aside>

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