bevy/crates/bevy_sprite/src/collide_aabb.rs

use bevy_math::{Vec2, Vec3};

#[derive(Debug)]
pub enum Collision {
    Left,
    Right,
    Top,
    Bottom,
}

// TODO: ideally we can remove this once bevy gets a physics system
/// Axis-aligned bounding box collision with "side" detection
pub fn collide(a_pos: Vec3, a_size: Vec2, b_pos: Vec3, b_size: Vec2) -> Option<Collision> {
    let a_min = a_pos.truncate() - a_size / 2.0;
    let a_max = a_pos.truncate() + a_size / 2.0;

    let b_min = b_pos.truncate() - b_size / 2.0;
    let b_max = b_pos.truncate() + b_size / 2.0;

    // check to see if the two rectangles are intersecting
    if a_min.x() < b_max.x()
        && a_max.x() > b_min.x()
        && a_min.y() < b_max.y()
        && a_max.y() > b_min.y()
    {
        // check to see if we hit on the left or right side
        let (x_collision, x_depth) =
            if a_min.x() < b_min.x() && a_max.x() > b_min.x() && a_max.x() < b_max.x() {
                (Some(Collision::Left), b_min.x() - a_max.x())
            } else if a_min.x() > b_min.x() && a_min.x() < b_max.x() && a_max.x() > b_max.x() {
                (Some(Collision::Right), a_min.x() - b_max.x())
            } else {
                (None, 0.0)
            };

        // check to see if we hit on the top or bottom side
        let (y_collision, y_depth) =
            if a_min.y() < b_min.y() && a_max.y() > b_min.y() && a_max.y() < b_max.y() {
                (Some(Collision::Bottom), b_min.y() - a_max.y())
            } else if a_min.y() > b_min.y() && a_min.y() < b_max.y() && a_max.y() > b_max.y() {
                (Some(Collision::Top), a_min.y() - b_max.y())
            } else {
                (None, 0.0)
            };

        // if we had an "x" and a "y" collision, pick the "primary" side using penetration depth
        match (x_collision, y_collision) {
            (Some(x_collision), Some(y_collision)) => {
                if y_depth < x_depth {
                    Some(y_collision)
                } else {
                    Some(x_collision)
                }
            }
            (Some(x_collision), None) => Some(x_collision),
            (None, Some(y_collision)) => Some(y_collision),
            (None, None) => None,
        }
    } else {
        None
    }
}