parsnip_plucker/src/physics.rs

use std::ops::DerefMut;

/// This module is intended to be used as a fixed-point physics system.
/// The hope is that this will make deterministic physics easier,
/// and therefore networking a bit less of a pain.
use bevy::{math::I64Vec2, prelude::*};

const FIXED_UPDATE_INTERVAL_MS: u128 = 10; // 100 fps physics

#[derive(Default, Debug)]
pub struct Physics2DPlugin;

#[derive(Default, Debug, Resource)]
pub struct Physics2DWorld {
    pub last_update: u128,
}

impl Plugin for Physics2DPlugin {
    fn build(&self, app: &mut bevy::app::App) {
        app.insert_resource(Physics2DWorld::default())
            .add_systems(Update, (tick_physics, resolve_aabb_collisions).chain());
    }
}

fn tick_physics(
    mut query: Query<&mut PhysicsBody2D>,
    mut p_world: ResMut<Physics2DWorld>,
    time: Res<Time>,
) {
    let t = time.elapsed().as_millis();
    let delta_t = t - p_world.last_update;

    if delta_t < FIXED_UPDATE_INTERVAL_MS {
        return;
    }

    p_world.last_update = t;

    query.iter_mut().for_each(|mut pb| {
        pb.tick(delta_t as i64);
    });
}

fn resolve_aabb_collisions(mut query: Query<(&mut PhysicsBody2D, &AABBCollider)>) {
    let bcs: Vec<_> = query.iter_mut().collect();

    let len = bcs.len();

    let mut updated_bcs: Vec<(usize, PhysicsBody2D)> = Vec::new();

    for i in 0..len {
        let (b, c) = &bcs[i];

        // don't bother collision checking immovables,
        // instead movable entities will check against them
        if b.immovable {
            continue;
        }

        for j in 0..len {
            if i == j {
                continue;
            }
            let (other_b, other_c) = &bcs[j];

            let bminx = b.pos.x;
            let bmaxx = b.pos.x + c.width as i64;
            let bminy = b.pos.y;
            let bmaxy = b.pos.y + c.height as i64;

            let obminx = other_b.pos.x;
            let obmaxx = other_b.pos.x + other_c.width as i64;
            let obminy = other_b.pos.y;
            let obmaxy = other_b.pos.y + other_c.height as i64;

            if bminx >= obmaxx || bminy >= obmaxy || bmaxx <= obminx || bmaxy <= obminy {
                // no collision
                continue;
            }

            info!("Collide!");

            let dx = b.pos.x - other_b.pos.x;
            let dy = b.pos.y - other_b.pos.y;

            if other_b.immovable {
                let new_x = match dx {
                    ..0 => other_b.pos.x - (c.width / 2 + other_c.width / 2) as i64,
                    0 => b.pos.x,
                    1.. => other_b.pos.x + (c.width / 2 + other_c.width / 2) as i64,
                };
                let new_y = match dy {
                    ..0 => other_b.pos.y - (c.height / 2 + other_c.height / 2) as i64,
                    0 => b.pos.y,
                    1.. => other_b.pos.y + (c.height / 2 + other_c.height / 2) as i64,
                };

                let updated_b = PhysicsBody2D {
                    pos: I64Vec2 { x: new_x, y: new_y },
                    vel: b.vel,
                    immovable: false,
                };
                updated_bcs.push((i, updated_b));
            } else {
                let updated_b = PhysicsBody2D {
                    pos: b.pos - I64Vec2::new(dx / 2, dy / 2),
                    vel: b.vel,
                    immovable: false,
                };

                let other_updated_b = PhysicsBody2D {
                    pos: other_b.pos + I64Vec2::new(dx / 2, dy / 2),
                    vel: other_b.vel,
                    immovable: false,
                };

                updated_bcs.push((i, updated_b));

                updated_bcs.push((i, other_updated_b));
            }
        }
    }

    for (i, (mut b, c)) in query.iter_mut().enumerate() {
        let Some((_, updateb)) = updated_bcs.iter().find(|(j, _)| i == *j) else {
            continue;
        };

        *b = updateb.clone();
    }
}

#[derive(Default, Debug, Component, Clone, Copy)]
pub struct PhysicsBody2D {
    pub pos: I64Vec2,
    pub vel: I64Vec2,
    pub immovable: bool,
}

impl PhysicsBody2D {
    pub fn tick(&mut self, delta_t: i64) {
        self.pos += self.vel * delta_t;
    }

    pub fn apply_force(&mut self, force: I64Vec2, term_speed: Option<i64>) {
        let mut vel = self.vel + force;
        let Some(max_speed) = term_speed else {
            return;
        };
        if vel.length_squared() >= max_speed.pow(2) {
            let l = vel.length_squared();
            let x = vel.x * vel.x;
            let y = vel.y * vel.y;

            vel.x = match vel.x {
                ..0 => (x * -max_speed) / l,
                _ => (x * max_speed) / l,
            };
            vel.y = match vel.y {
                ..0 => (y * -max_speed) / l,
                _ => (y * max_speed) / l,
            };
        }
        self.vel = vel;
    }
}

#[derive(Default, Component)]
pub struct Collideable;

#[derive(Default, Debug, Component)]
#[require(Collideable)]
pub struct AABBCollider {
    pub width: u32,
    pub height: u32,
    pub offset: IVec2,
}

impl AABBCollider {
    pub fn new(w: u32, h: u32) -> Self {
        Self {
            width: w,
            height: h,
            offset: IVec2::ZERO,
        }
    }
}