From 1f2611dda48ce522a9c0eec1fcba997b8b67b98c Mon Sep 17 00:00:00 2001
From: Savanni D'Gerinel <savanni@luminescent-dreams.com>
Date: Wed, 7 Feb 2024 08:16:06 -0500
Subject: [PATCH] Look into better collision resolution which will deflect
 objects

---
 falling-sand/src/physics.rs | 44 +++++++++++++++++++++++++------------
 1 file changed, 30 insertions(+), 14 deletions(-)

diff --git a/falling-sand/src/physics.rs b/falling-sand/src/physics.rs
index a95fdde..ffbe4e5 100644
--- a/falling-sand/src/physics.rs
+++ b/falling-sand/src/physics.rs
@@ -1,6 +1,6 @@
 use crate::profile;
 use std::{
-    ops::{Mul, Neg},
+    ops::{Mul, Neg, Sub},
     sync::{Arc, RwLock},
 };
 
@@ -35,6 +35,16 @@ struct Vector {
     dy: f64,
 }
 
+impl Sub for Vector {
+    type Output = Vector;
+    fn sub(self, rhs: Vector) -> Self::Output {
+        Self {
+            dx: rhs.dx - self.dx,
+            dy: rhs.dy - self.dy,
+        }
+    }
+}
+
 impl Neg for Vector {
     type Output = Vector;
     fn neg(self) -> Self {
@@ -100,51 +110,57 @@ impl World {
 
     pub fn next(&self, delta_t: std::time::Duration) {
         self.move_objects(delta_t);
-        self.resolve_collisions();
+        self.resolve_overlaps();
     }
 
     fn move_objects(&self, delta_t: std::time::Duration) {
+        /* First, need to resolve collisions between objects and ensure that they bounce in the correct direction. This will only resolve the initial collision in a step. Additional collisions are undetectable, so we will resolve those with the overlap code. */
         for grain in self.grains.write().unwrap().iter_mut() {
             grain.move_by(delta_t);
         }
     }
 
-    fn resolve_collisions(&self) {}
+    /* Once all movement has been calculated out, some objects are probably still going to be overlapping. */
+    fn resolve_overlaps(&self) {}
 }
 
 pub trait Collision {
     type Other;
-    fn overlap(&self, other: &Self::Other) -> Option<(f64, Vector)>;
+    fn overlap(&self, other: &Self::Other) -> Option<f64>;
     fn resolve_collision(&mut self, other: &mut Self::Other);
 }
 
 impl Collision for Grain {
     type Other = Grain;
 
-    fn overlap(&self, other: &Self::Other) -> Option<(f64, Vector)> {
+    fn overlap(&self, other: &Self::Other) -> Option<f64> {
         let radii = (self.radius + other.radius) * (self.radius + other.radius);
         let distance = self.location.distance_sq(&other.location);
         if distance < radii {
             let dx = other.location.x - self.location.x;
             let dy = other.location.y - self.location.y;
-            Some((radii.sqrt() - distance.sqrt(), Vector { dx, dy }))
+            Some(radii.sqrt() - distance.sqrt())
         } else {
             None
         }
     }
 
     fn resolve_collision(&mut self, other: &mut Self::Other) {
+        // between every two objects calculate out the point and angle of incidence for a collision.
+        // Start with i, the intersection distance, which is the addition of the radii of both objects.
+        // Then calculate t = i / |v1 - v2|
+        // I don't think that my equation takes into account two objects that don't collide
+        /*
         match self.overlap(other) {
-            Some((overlap, v)) => {
-                let max_overlap = self.radius + other.radius;
-                let resolution = overlap / max_overlap;
-                let v = v * resolution;
-                println!("{:?} {} {} {}", v, overlap, max_overlap, resolution);
-                self.translate(-v);
-                other.translate(v);
+            Some(overlap) => {
+                // a collision has occured. First, undo the overlap by moving both pieces backwards to their intersection points
+                // once reversed, calculate percentage of travel already done
+                // next, calculate the bounce vectors
+                // finally, apply the bounce vectors, scaled by remaining travel percentage
             }
             None => {}
         }
+        */
     }
 }
 
@@ -171,7 +187,7 @@ mod test {
             velocity: Vector { dx: 0., dy: 0. },
         };
 
-        assert_matches!(grain_1.overlap(&grain_2), Some((overlap, _)) => {
+        assert_matches!(grain_1.overlap(&grain_2), Some(overlap) => {
             assert!(within(overlap, 19., 0.1));
         } );
         assert_matches!(grain_1.overlap(&grain_3), None);