<h1>Implementing a game rule system<aclass="headerlink"href="#implementing-a-game-rule-system"title="Permalink to this headline">¶</a></h1>
<p>The simplest way to create an online roleplaying game (at least from a code perspective) is to
simply grab a paperback RPG rule book, get a staff of game masters together and start to run scenes
with whomever logs in. Game masters can roll their dice in front of their computers and tell the
players the results. This is only one step away from a traditional tabletop game and puts heavy
demands on the staff - it is unlikely staff will be able to keep up around the clock even if they
are very dedicated.</p>
<p>Many games, even the most roleplay-dedicated, thus tend to allow for players to mediate themselves
to some extent. A common way to do this is to introduce <em>coded systems</em> - that is, to let the
computer do some of the heavy lifting. A basic thing is to add an online dice-roller so everyone can
make rolls and make sure noone is cheating. Somewhere at this level you find the most bare-bones
roleplaying MUSHes.</p>
<p>The advantage of a coded system is that as long as the rules are fair the computer is too - it makes
no judgement calls and holds no personal grudges (and cannot be accused of holding any). Also, the
computer doesn’t need to sleep and can always be online regardless of when a player logs on. The
drawback is that a coded system is not flexible and won’t adapt to the unprogrammed actions human
players may come up with in role play. For this reason many roleplay-heavy MUDs do a hybrid
variation - they use coded systems for things like combat and skill progression but leave role play
to be mostly freeform, overseen by staff game masters.</p>
<p>Finally, on the other end of the scale are less- or no-roleplay games, where game mechanics (and
thus player fairness) is the most important aspect. In such games the only events with in-game value
are those resulting from code. Such games are very common and include everything from hack-and-slash
MUDs to various tactical simulations.</p>
<p>So your first decision needs to be just what type of system you are aiming for. This page will try
to give some ideas for how to organize the “coded” part of your system, however big that may be.</p>
<sectionid="overall-system-infrastructure">
<h2>Overall system infrastructure<aclass="headerlink"href="#overall-system-infrastructure"title="Permalink to this headline">¶</a></h2>
<p>We strongly recommend that you code your rule system as stand-alone as possible. That is, don’t
spread your skill check code, race bonus calculation, die modifiers or what have you all over your
game.</p>
<ul>
<li><p>Put everything you would need to look up in a rule book into a module in <codeclass="docutils literal notranslate"><spanclass="pre">mygame/world</span></code>. Hide away
as much as you can. Think of it as a black box (or maybe the code representation of an all-knowing
game master). The rest of your game will ask this black box questions and get answers back. Exactly
how it arrives at those results should not need to be known outside the box. Doing it this way
makes it easier to change and update things in one place later.</p></li>
<li><p>Store only the minimum stuff you need with each game object. That is, if your Characters need
values for Health, a list of skills etc, store those things on the Character - don’t store how to
roll or change them.</p></li>
<li><p>Next is to determine just how you want to store things on your Objects and Characters. You can
choose to either store things as individual <aclass="reference internal"href="../Components/Attributes.html"><spanclass="doc std std-doc">Attributes</span></a>, like <codeclass="docutils literal notranslate"><spanclass="pre">character.db.STR=34</span></code> and
<codeclass="docutils literal notranslate"><spanclass="pre">character.db.Hunting_skill=20</span></code>. But you could also use some custom storage method, like a
dictionary <codeclass="docutils literal notranslate"><spanclass="pre">character.db.skills</span><spanclass="pre">=</span><spanclass="pre">{"Hunting":34,</span><spanclass="pre">"Fishing":20,</span><spanclass="pre">...}</span></code>. A much more fancy solution is
to look at the Ainneve <aclass="reference external"href="https://github.com/evennia/ainneve/blob/master/world/traits.py">Trait
handler</a>. Finally you could even go
with a <aclass="reference internal"href="../Concepts/New-Models.html"><spanclass="doc std std-doc">custom django model</span></a>. Which is the better depends on your game and the
complexity of your system.</p></li>
<li><p>Make a clear <aclass="reference external"href="https://en.wikipedia.org/wiki/Application_programming_interface">API</a> into your
rules. That is, make methods/functions that you feed with, say, your Character and which skill you
want to check. That is, you want something similar to this:</p>
<p>You might need to make these functions more or less complex depending on your game. For example the
properties of the room might matter to the outcome of a roll (if the room is dark, burning etc).
Establishing just what you need to send into your game mechanic module is a great way to also get a
feel for what you need to add to your engine.</p>
</section>
<sectionid="coded-systems">
<h2>Coded systems<aclass="headerlink"href="#coded-systems"title="Permalink to this headline">¶</a></h2>
<p>Inspired by tabletop role playing games, most game systems mimic some sort of die mechanic. To this
end Evennia offers a full <aclass="reference external"href="https://github.com/evennia/evennia/blob/master/evennia/contrib/dice.py">dice
roller</a> in its <codeclass="docutils literal notranslate"><spanclass="pre">contrib</span></code>
folder. For custom implementations, Python offers many ways to randomize a result using its in-built
<codeclass="docutils literal notranslate"><spanclass="pre">random</span></code> module. No matter how it’s implemented, we will in this text refer to the action of
determining an outcome as a “roll”.</p>
<p>In a freeform system, the result of the roll is just compared with values and people (or the game
master) just agree on what it means. In a coded system the result now needs to be processed somehow.
There are many things that may happen as a result of rule enforcement:</p>
<ulclass="simple">
<li><p>Health may be added or deducted. This can effect the character in various ways.</p></li>
<li><p>Experience may need to be added, and if a level-based system is used, the player might need to be
informed they have increased a level.</p></li>
<li><p>Room-wide effects need to be reported to the room, possibly affecting everyone in the room.</p></li>
</ul>
<p>There are also a slew of other things that fall under “Coded systems”, including things like
weather, NPC artificial intelligence and game economy. Basically everything about the world that a
Game master would control in a tabletop role playing game can be mimicked to some level by coded
systems.</p>
</section>
<sectionid="example-of-rule-module">
<h2>Example of Rule module<aclass="headerlink"href="#example-of-rule-module"title="Permalink to this headline">¶</a></h2>
<p>Here is a simple example of a rule module. This is what we assume about our simple example game:</p>
<ulclass="simple">
<li><p>Characters have only four numerical values:</p>
<ul>
<li><p>Their <codeclass="docutils literal notranslate"><spanclass="pre">level</span></code>, which starts at 1.</p></li>
<li><p>A skill <codeclass="docutils literal notranslate"><spanclass="pre">combat</span></code>, which determines how good they are at hitting things. Starts between 5 and</p></li>
</ul>
</li>
</ul>
<olclass="simple">
<li><ulclass="simple">
<li><p>Their Strength, <codeclass="docutils literal notranslate"><spanclass="pre">STR</span></code>, which determine how much damage they do. Starts between 1 and 10.</p></li>
<li><p>Their Health points, <codeclass="docutils literal notranslate"><spanclass="pre">HP</span></code>, which starts at 100.</p></li>
</ul>
</li>
</ol>
<ulclass="simple">
<li><p>When a Character reaches <codeclass="docutils literal notranslate"><spanclass="pre">HP</span><spanclass="pre">=</span><spanclass="pre">0</span></code>, they are presumed “defeated”. Their HP is reset and they get a
failure message (as a stand-in for death code).</p></li>
<li><p>Abilities are stored as simple Attributes on the Character.</p></li>
<li><p>“Rolls” are done by rolling a 100-sided die. If the result is below the <codeclass="docutils literal notranslate"><spanclass="pre">combat</span></code> value, it’s a
success and damage is rolled. Damage is rolled as a six-sided die + the value of <codeclass="docutils literal notranslate"><spanclass="pre">STR</span></code> (for this
example we ignore weapons and assume <codeclass="docutils literal notranslate"><spanclass="pre">STR</span></code> is all that matters).</p></li>
<li><p>Every successful <codeclass="docutils literal notranslate"><spanclass="pre">attack</span></code> roll gives 1-3 experience points (<codeclass="docutils literal notranslate"><spanclass="pre">XP</span></code>). Every time the number of <codeclass="docutils literal notranslate"><spanclass="pre">XP</span></code>
reaches <codeclass="docutils literal notranslate"><spanclass="pre">(level</span><spanclass="pre">+</span><spanclass="pre">1)</span><spanclass="pre">**</span><spanclass="pre">2</span></code>, the Character levels up. When leveling up, the Character’s <codeclass="docutils literal notranslate"><spanclass="pre">combat</span></code>
value goes up by 2 points and <codeclass="docutils literal notranslate"><spanclass="pre">STR</span></code> by one (this is a stand-in for a real progression system).</p></li>
</ul>
<sectionid="character">
<h3>Character<aclass="headerlink"href="#character"title="Permalink to this headline">¶</a></h3>
<p>The Character typeclass is simple. It goes in <codeclass="docutils literal notranslate"><spanclass="pre">mygame/typeclasses/characters.py</span></code>. There is already
an empty <codeclass="docutils literal notranslate"><spanclass="pre">Character</span></code> class there that Evennia will look to and use.</p>
<p><codeclass="docutils literal notranslate"><spanclass="pre">@reload</span></code> the server to load up the new code. Doing <codeclass="docutils literal notranslate"><spanclass="pre">examine</span><spanclass="pre">self</span></code> will however <em>not</em> show the new
Attributes on yourself. This is because the <codeclass="docutils literal notranslate"><spanclass="pre">at_object_creation</span></code> hook is only called on <em>new</em>
Characters. Your Character was already created and will thus not have them. To force a reload, use
<p>The <codeclass="docutils literal notranslate"><spanclass="pre">examine</span><spanclass="pre">self</span></code> command will now show the new Attributes.</p>
</section>
<sectionid="rule-module">
<h3>Rule module<aclass="headerlink"href="#rule-module"title="Permalink to this headline">¶</a></h3>
<p>This is a module <codeclass="docutils literal notranslate"><spanclass="pre">mygame/world/rules.py</span></code>.</p>
<spanclass="n">character</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You fall down, defeated!"</span><spanclass="p">)</span>
<spanclass="n">character</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="sa">f</span><spanclass="s2">"You are now level </span><spanclass="si">{</span><spanclass="n">character</span><spanclass="o">.</span><spanclass="n">db</span><spanclass="o">.</span><spanclass="n">level</span><spanclass="si">}</span><spanclass="s2">!"</span><spanclass="p">)</span>
<spanclass="n">failtext_template</span><spanclass="o">=</span><spanclass="s2">"You are hit by </span><spanclass="si">{attacker}</span><spanclass="s2"> for </span><spanclass="si">{dmg}</span><spanclass="s2"> damage!"</span>
<spanclass="n">wintext_template</span><spanclass="o">=</span><spanclass="s2">"You hit </span><spanclass="si">{target}</span><spanclass="s2"> for </span><spanclass="si">{dmg}</span><spanclass="s2"> damage!"</span>
<spanclass="k">raise</span><spanclass="n">RunTimeError</span><spanclass="p">(</span><spanclass="sa">f</span><spanclass="s2">"Skillname </span><spanclass="si">{</span><spanclass="n">skillname</span><spanclass="si">}</span><spanclass="s2"> not found."</span><spanclass="p">)</span>
</pre></div>
</div>
<p>These few functions implement the entirety of our simple rule system. We have a function to check
the “defeat” condition and reset the <codeclass="docutils literal notranslate"><spanclass="pre">HP</span></code> back to 100 again. We define a generic “skill” function.
Multiple skills could all be added with the same signature; our <codeclass="docutils literal notranslate"><spanclass="pre">SKILLS</span></code> dictionary makes it easy to
look up the skills regardless of what their actual functions are called. Finally, the access
function <codeclass="docutils literal notranslate"><spanclass="pre">roll_challenge</span></code> just picks the skill and gets the result.</p>
<p>In this example, the skill function actually does a lot - it not only rolls results, it also informs
everyone of their results via <codeclass="docutils literal notranslate"><spanclass="pre">character.msg()</span></code> calls.</p>
<p>Here is an example of usage in a game command:</p>
<spanclass="n">caller</span><spanclass="o">.</span><spanclass="n">msg</span><spanclass="p">(</span><spanclass="s2">"You need to pick a target to attack."</span><spanclass="p">)</span>
<p>Note how simple the command becomes and how generic you can make it. It becomes simple to offer any
number of Combat commands by just extending this functionality - you can easily roll challenges and
pick different skills to check. And if you ever decided to, say, change how to determine hit chance,
you don’t have to change every command, but need only change the single <codeclass="docutils literal notranslate"><spanclass="pre">roll_hit</span></code> function inside
your <codeclass="docutils literal notranslate"><spanclass="pre">rules</span></code> module.</p>
