#![warn(missing_docs)]

// TODO: add more precise crate documentation
//! Simple 2D collision system, inspired by Godot Engine.

use bevy::math::bounding::IntersectsVolume;

use bevy::{math::bounding::{Aabb2d, BoundingCircle}, prelude::*};

#[derive(Component, PartialEq, Debug)]
/// Enum of available shapes
pub enum CollisionShape {
    /// Rectangle shape
    Aabb {
        /// The shape itself, tied to [`Transform`]
        shape: Aabb2d,
        /// Start offset, used to calculate current position inside shape.
        offset: Vec2,
        /// Size of the shape, used to calculate global position.
        size: Vec2,
    },
    /// Circle shape
    Circle {
        /// The shape itself, tied to [`Transform`]
        shape: BoundingCircle,
        /// Start offset, used to calculate current position inside shape.
        offset: Vec2,
    },
}

impl CollisionShape {
    /// Returns true if shapes intersect
    pub fn intersects(&self, other: &CollisionShape) -> bool {
        match self {
            Self::Aabb { shape: aabb, .. } => {
                match other {
                    CollisionShape::Aabb { shape: other_aabb, .. } => aabb.intersects(other_aabb),
                    CollisionShape::Circle { shape: circle, ..  } => aabb.intersects(circle),
                }
            },
            Self::Circle{ shape: circle, .. } => {
                match other {
                    CollisionShape::Aabb { shape: aabb, .. } => circle.intersects(aabb),
                    CollisionShape::Circle { shape: other_circle, .. } => circle.intersects(other_circle),
                }
            },
        }
    }
    
    /// Create [`CollisionShape::Aabb`]
    pub fn new_aabb(offset: Vec2, size: Vec2) -> Self {
        let half_size = Vec2 { x: size.x / 2f32, y: size.y / 2f32 };
        Self::Aabb { shape: Aabb2d::new(offset, half_size), offset, size }
    }

    /// Create [`CollisionShape::Circle`]
    pub fn new_circle(offset: Vec2, radius: f32) -> Self {
        Self::Circle { shape: BoundingCircle::new(offset, radius), offset }
    }

    /// Returns current centered offset for the shape
    pub fn current_offset(&self) -> Vec2 {
        match self {
            Self::Aabb { shape, .. } => {
                Vec2::new(
                    (shape.min.x + shape.max.x) / 2.,
                    (shape.min.y + shape.max.y) / 2.,
                )
            },
            Self::Circle { shape, .. } => {
                shape.center
            },
        }
    }
}

#[derive(Component, PartialEq, Debug)]
#[require(Transform)]
/// Component, handling multiple collision shapes for single entity
pub struct Collider {
    /// Shapes of the collider
    pub shapes: Vec<CollisionShape>,
    /// Bitmask of collider groups.
    /// Setting this to 0 makes collider invisible for others
    pub mask: usize,
    /// Bitmask of groups which will be checked for collision.
    /// Setting this to 0 makes collider ignorant to others
    pub check_mask: usize,
}

impl Collider {
    #[inline]
    /// Returns true if any of self shapes intersect other's shapes, ignoring groups
    pub fn intersects_unchecked(&self, other: &Collider) -> bool {
        for shape in self.shapes.iter() {
            for other_shape in other.shapes.iter() {
                if shape.intersects(other_shape) {
                    return true;
                }
            }
        }
        false
    }

    /// Returns true if any of self shapes intersect other's shapes, respecting groups.
    /// Wrapper around [intersects_unchecked](Self::intersects_unchecked)
    pub fn intersects(&self, other: &Collider) -> bool {
        if self.check_mask & other.mask == 0 {
            return false;
        }
        self.intersects_unchecked(other)
    }

    /// Update shapes' current position
    pub fn update_shapes_position(&mut self, new_position: &Vec2) {
        for shape in self.shapes.iter_mut() {
            match shape {
                CollisionShape::Aabb { shape, offset, size } => {
                    shape.min.x = new_position.x + offset.x - size.x / 2.;
                    shape.min.y = new_position.y + offset.y - size.y / 2.;
                    shape.max.x = shape.min.x + size.x;
                    shape.max.y = shape.min.y + size.y;
                },
                CollisionShape::Circle { shape, offset } => {
                    shape.center.x = new_position.x + offset.x;
                    shape.center.y = new_position.y + offset.y;
                },
            }
        }
    }
}

/// Update collider shapes to match new [`Transform`]
pub fn update_collider_shapes(
    colliders: Query<(&mut Collider, &Transform), Changed<Transform>>,
) {
    for (mut collider, transform) in colliders {
        collider.update_shapes_position(&transform.translation.xy());
    }
}

#[derive(EntityEvent, Debug)]
/// Emitted when colliders intersect in [`check_for_collisions`] system
pub struct CollisionEvent {
    /// Entity, whose [`Collider`] detected another [`Collider`]
    pub entity: Entity,
    /// Entity, whose [`Collider`] was detected as intersecting
    pub detected: Entity,
}

fn collider_filter(checker: &&Collider, checkable: &&Collider) -> bool {
    checker != checkable && checker.check_mask != 0
}

/// Check each [`Collider`] for collisions with another colliders.
/// Triggers [`CollisionEvent`] on collider intersection
pub fn check_for_collisions(
    mut commands: Commands,
    colliders: Query<(&Collider, Entity)>,
) {
    for (collider, entity) in colliders.iter() {
        for (other_collider, other_entity) in colliders.iter().filter(|(c, _)| collider_filter(&collider, c)) {
            if collider.intersects(other_collider) {
                commands.trigger(CollisionEvent { entity, detected: other_entity });
            }
        }
    }
}

/// Plugin that adds collision systems
pub struct CollisionPlugin;

impl Plugin for CollisionPlugin {
    fn build(&self, app: &mut App) {
        app.add_systems(PostUpdate, (update_collider_shapes, check_for_collisions).chain());
    }
}