Documented and tested Axis<T> (#3276)

# Objective

Fixes #3247 

## Solution

- Added docs to `Axis<T>`. This one can be extended @alice-i-cecile 
- Added unit tests
- Added `min` and `max` values to the struct. @alice-i-cecile Do we need to these variables?
- Limited `set` method usage with `min` and `max` values


Co-authored-by: CrazyRoka <rokarostuk@gmail.com>

crates/bevy_input/src/axis.rs

@@ -1,6 +1,10 @@
 use bevy_utils::HashMap;
 use std::hash::Hash;
 
+/// Stores the position data of input devices of type T
+///
+/// Values are stored as `f32` values, which range from `min` to `max`.
+/// The valid range is from -1.0 to 1.0, inclusive.
 #[derive(Debug)]
 pub struct Axis<T> {
     axis_data: HashMap<T, f32>,
@@ -21,15 +25,82 @@ impl<T> Axis<T>
 where
     T: Copy + Eq + Hash,
 {
-    pub fn set(&mut self, axis: T, value: f32) -> Option<f32> {
-        self.axis_data.insert(axis, value)
+    pub const MIN: f32 = -1.0;
+    pub const MAX: f32 = 1.0;
+
+    /// Inserts a position data for an input device,
+    /// restricting the position data to an interval `min..=max`.
+    ///
+    /// If the input device wasn't present before, [None] is returned.
+    ///
+    /// If the input device was present, the position data is updated, and the old value is returned.
+    pub fn set(&mut self, input_device: T, position_data: f32) -> Option<f32> {
+        let new_position_data = position_data.clamp(Self::MIN, Self::MAX);
+        self.axis_data.insert(input_device, new_position_data)
     }
 
-    pub fn get(&self, axis: T) -> Option<f32> {
-        self.axis_data.get(&axis).copied()
+    /// Returns a position data corresponding to the input device.
+    pub fn get(&self, input_device: T) -> Option<f32> {
+        self.axis_data.get(&input_device).copied()
     }
 
-    pub fn remove(&mut self, axis: T) -> Option<f32> {
-        self.axis_data.remove(&axis)
+    /// Removes the position data of the input device,
+    /// returning the position data if the input device was previously set.
+    pub fn remove(&mut self, input_device: T) -> Option<f32> {
+        self.axis_data.remove(&input_device)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::{
+        gamepad::{Gamepad, GamepadButton, GamepadButtonType},
+        Axis,
+    };
+
+    #[test]
+    fn test_axis_set() {
+        let cases = [
+            (-1.5, Some(-1.0)),
+            (-1.1, Some(-1.0)),
+            (-1.0, Some(-1.0)),
+            (-0.9, Some(-0.9)),
+            (-0.1, Some(-0.1)),
+            (0.0, Some(0.0)),
+            (0.1, Some(0.1)),
+            (0.9, Some(0.9)),
+            (1.0, Some(1.0)),
+            (1.1, Some(1.0)),
+            (1.6, Some(1.0)),
+        ];
+
+        for (value, expected) in cases {
+            let gamepad_button = GamepadButton(Gamepad(1), GamepadButtonType::RightTrigger);
+            let mut axis = Axis::<GamepadButton>::default();
+
+            axis.set(gamepad_button, value);
+
+            let actual = axis.get(gamepad_button);
+            assert_eq!(expected, actual);
+        }
+    }
+
+    #[test]
+    fn test_axis_remove() {
+        let cases = [-1.0, -0.9, -0.1, 0.0, 0.1, 0.9, 1.0];
+
+        for value in cases {
+            let gamepad_button = GamepadButton(Gamepad(1), GamepadButtonType::RightTrigger);
+            let mut axis = Axis::<GamepadButton>::default();
+
+            axis.set(gamepad_button, value);
+            assert!(axis.get(gamepad_button).is_some());
+
+            axis.remove(gamepad_button);
+            let actual = axis.get(gamepad_button);
+            let expected = None;
+
+            assert_eq!(expected, actual);
+        }
     }
 }