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<ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../ch00/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch01/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="../ch01/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="../ch01/volatile.html"><strong aria-hidden="true">3.2.</strong> Volatile</a></li><li><a href="../ch01/io_registers.html"><strong aria-hidden="true">3.3.</strong> IO Registers</a></li><li><a href="../ch01/the_display_control_register.html"><strong aria-hidden="true">3.4.</strong> The Display Control Register</a></li><li><a href="../ch01/video_memory_intro.html"><strong aria-hidden="true">3.5.</strong> Video Memory Intro</a></li><li><a href="../ch01/hello2.html"><strong aria-hidden="true">3.6.</strong> hello2</a></li></ol></li><li><a href="../ch02/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="../ch02/the_key_input_register.html" class="active"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="../ch02/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="../ch02/light_cycle.html"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li><li><a href="../ch03/index.html"><strong aria-hidden="true">5.</strong> Ch 3: Memory and Objects</a></li><li><ol class="section"><li><a href="../ch03/gba_memory.html"><strong aria-hidden="true">5.1.</strong> GBA Memory</a></li><li><a href="../ch03/tiled_backgrounds.html"><strong aria-hidden="true">5.2.</strong> Tiled Backgrounds</a></li><li><a href="../ch03/object_basics.html"><strong aria-hidden="true">5.3.</strong> Object Basics</a></li><li><a href="../ch03/gba_rng.html"><strong aria-hidden="true">5.4.</strong> GBA RNG</a></li><li><a href="../ch03/memory_game.html"><strong aria-hidden="true">5.5.</strong> memory_game</a></li></ol></li></ol>
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<a class="header" href="#the-key-input-register" id="the-key-input-register"><h1>The Key Input Register</h1></a>
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<p>The Key Input Register is our next IO register. Its shorthand name is
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<a href="http://problemkaputt.de/gbatek.htm#gbakeypadinput">KEYINPUT</a> and it's a <code>u16</code>
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at <code>0x4000130</code>. The entire register is obviously read only, you can't tell the
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GBA what buttons are pressed.</p>
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<p>Each button is exactly one bit:</p>
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<table><thead><tr><th align="center"> Bit </th><th align="center"> Button </th></tr></thead><tbody>
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<tr><td align="center"> 0 </td><td align="center"> A </td></tr>
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<tr><td align="center"> 1 </td><td align="center"> B </td></tr>
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<tr><td align="center"> 2 </td><td align="center"> Select </td></tr>
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<tr><td align="center"> 3 </td><td align="center"> Start </td></tr>
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<tr><td align="center"> 4 </td><td align="center"> Right </td></tr>
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<tr><td align="center"> 5 </td><td align="center"> Left </td></tr>
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<tr><td align="center"> 6 </td><td align="center"> Up </td></tr>
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<tr><td align="center"> 7 </td><td align="center"> Down </td></tr>
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<tr><td align="center"> 8 </td><td align="center"> R </td></tr>
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<tr><td align="center"> 9 </td><td align="center"> L </td></tr>
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</tbody></table>
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<p>The higher bits above are not used at all.</p>
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<p>Similar to other old hardware devices, the convention here is that a button's
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bit is <strong>clear when pressed, active when released</strong>. In other words, when the
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user is not touching the device at all the KEYINPUT value will read
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<code>0b0000_0011_1111_1111</code>. There's similar values for when the user is pressing as
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many buttons as possible, but since the left/right and up/down keys are on an
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arrow pad the value can never be 0 since you can't ever press every single key
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at once.</p>
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<p>When dealing with key input, the register always shows the exact key values at
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any moment you read it. Obviously that's what it should do, but what it means to
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you as a programmer is that you should usually gather input once at the top of a
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game frame and then use that single input poll as the input values across the
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whole game frame.</p>
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<p>Of course, you might want to know if a user's key state changed from frame to
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frame. That's fairly easy too: We just store the last frame keys as well as the
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current frame keys (it's only a <code>u16</code>) and then we can xor the two values.
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Anything that shows up in the xor result is a key that changed. If it's changed
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and it's now down, that means it was pushed this frame. If it's changed and it's
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now up, that means it was released this frame.</p>
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<p>The other major thing you might frequently want is to know "which way" the arrow
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pad is pointing: Up/Down/None and Left/Right/None. Sounds like an enum to me.
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Except that often time we'll have situations where the direction just needs to
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be multiplied by a speed and applied as a delta to a position. We want to
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support that as well as we can too.</p>
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<a class="header" href="#key-input-code" id="key-input-code"><h2>Key Input Code</h2></a>
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<p>Let's get down to some code. First we want to make a way to read the address as
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a <code>u16</code> and then wrap that in our newtype which will implement methods for
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reading and writing the key bits.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub const KEYINPUT: VolatilePtr<u16> = VolatilePtr(0x400_0130 as *mut u16);
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/// A newtype over the key input state of the GBA.
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#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
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#[repr(transparent)]
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pub struct KeyInputSetting(u16);
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pub fn key_input() -> KeyInputSetting {
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unsafe { KeyInputSetting(KEYINPUT.read()) }
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}
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#}</code></pre></pre>
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<p>Now we want a way to check if a key is <em>being pressed</em>, since that's normally
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how we think of things as a game designer and even as a player. That is, usually
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you'd say "if you press A, then X happens" instead of "if you don't press A,
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then X does not happen".</p>
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<p>Normally we'd pick a constant for the bit we want, <code>&</code> it with our value, and
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then check for <code>val != 0</code>. Since the bit we're looking for is <code>0</code> in the "true"
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state we still pick the same constant and we still do the <code>&</code>, but we test with
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<code>== 0</code>. Practically the same, right? Well, since I'm asking a rhetorical
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question like that you can probably already guess that it's not the same. I was
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shocked to learn this too.</p>
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<p>All we have to do is ask our good friend
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<a href="https://rust.godbolt.org/z/d-8oCe">Godbolt</a> what's gonna happen when the code
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compiles. The link there has the page set for the <code>stable</code> 1.30 compiler just so
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that the link results stay consistent if you read this book in a year or
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something. Also, we've set the target to <code>thumbv6m-none-eabi</code>, which is a
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slightly later version of ARM than the actual GBA, but it's close enough for
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just checking. Of course, in a full program small functions like these will
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probably get inlined into the calling code and disappear entirely as they're
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folded and refolded by the compiler, but we can just check.</p>
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<p>It turns out that the <code>!=0</code> test is 4 instructions and the <code>==0</code> test is 6
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instructions. Since we want to get savings where we can, and we'll probably
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check the keys of an input often enough, we'll just always use a <code>!=0</code> test and
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then adjust how we initially read the register to compensate. By using xor with
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a mask for only the 10 used bits we can flip the "low when pressed" values so
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that the entire result has active bits in all positions where a key is pressed.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub fn key_input() -> KeyInputSetting {
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unsafe { KeyInputSetting(KEYINPUT.read_volatile() ^ 0b0000_0011_1111_1111) }
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}
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#}</code></pre></pre>
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<p>Now we add a method for seeing if a key is pressed. In the full library there's
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a more advanced version of this that's built up via macro, but for this example
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we'll just name a bunch of <code>const</code> values and then have a method that takes a
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value and says if that bit is on.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub const KEY_A: u16 = 1 << 0;
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pub const KEY_B: u16 = 1 << 1;
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pub const KEY_SELECT: u16 = 1 << 2;
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pub const KEY_START: u16 = 1 << 3;
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pub const KEY_RIGHT: u16 = 1 << 4;
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pub const KEY_LEFT: u16 = 1 << 5;
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pub const KEY_UP: u16 = 1 << 6;
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pub const KEY_DOWN: u16 = 1 << 7;
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pub const KEY_R: u16 = 1 << 8;
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pub const KEY_L: u16 = 1 << 9;
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impl KeyInputSetting {
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pub fn contains(&self, key: u16) -> bool {
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(self.0 & key) != 0
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}
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}
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#}</code></pre></pre>
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<p>Because each key is a unique bit you can even check for more than one key at
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once by just adding two key values together.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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let input_contains_a_and_l = input.contains(KEY_A + KEY_L);
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#}</code></pre></pre>
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<p>And we wanted to save the state of an old frame and compare it to the current
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frame to see what was different:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub fn difference(&self, other: KeyInputSetting) -> KeyInputSetting {
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KeyInputSetting(self.0 ^ other.0)
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}
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#}</code></pre></pre>
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<p>Anything that's "in" the difference output is a key that <em>changed</em>, and then if
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the key reads as pressed this frame that means it was just pressed. The exact
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mechanics of all the ways you might care to do something based on new key
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presses is obviously quite varied, but it might be something like this:</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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let this_frame_diff = this_frame_input.difference(last_frame_input);
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if this_frame_diff.contains(KEY_B) && this_frame_input.contains(KEY_B) {
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// the user just pressed B, react in some way
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}
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#}</code></pre></pre>
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<p>And for the arrow pad, we'll make an enum that easily casts into <code>i32</code>. Whenever
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we're working with stuff we can try to use <code>i32</code> / <code>isize</code> as often as possible
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just because it's easier on the GBA's CPU if we stick to its native number size.
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Having it be an enum lets us use <code>match</code> and be sure that we've covered all our
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cases.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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/// A "tribool" value helps us interpret the arrow pad.
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#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
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#[repr(i32)]
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pub enum TriBool {
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Minus = -1,
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Neutral = 0,
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Plus = +1,
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}
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#}</code></pre></pre>
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<p>Now, how do we determine <em>which way</em> is plus or minus? Well... I don't know.
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Really. I'm not sure what the best one is because the GBA really wants the
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origin at 0,0 with higher rows going down and higher cols going right. On the
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other hand, all the normal math you and I learned in school is oriented with
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increasing Y being upward on the page. So, at least for this demo, we're going
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to go with what the GBA wants us to do and give it a try. If we don't end up
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confusing ourselves then we can stick with that. Maybe we can cover it over
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somehow later on.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub fn column_direction(&self) -> TriBool {
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if self.contains(KEY_RIGHT) {
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TriBool::Plus
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} else if self.contains(KEY_LEFT) {
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TriBool::Minus
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} else {
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TriBool::Neutral
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}
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}
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pub fn row_direction(&self) -> TriBool {
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if self.contains(KEY_DOWN) {
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TriBool::Plus
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} else if self.contains(KEY_UP) {
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TriBool::Minus
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} else {
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TriBool::Neutral
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}
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}
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#}</code></pre></pre>
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<p>So then in our game, every frame we can check for <code>column_direction</code> and
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<code>row_direction</code> and then apply those to the player's current position to make
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them move around the screen.</p>
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<p>With that settled I think we're all done with user input for now. There's some
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other things to eventually know about like key interrupts that you can set and
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stuff, but we'll cover that later on because it's not necessary right now.</p>
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