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use crate::math::{Point, Vector};
use na::{self, DVector, RealField};

/// The unique identifier of a fluid.
pub type FluidHandle = usize;

/// A fluid object.
///
/// A fluid object is composed of movable particles with additional properties like viscosity.
pub struct Fluid<N: RealField> {
    /// The world-space position of the fluid particles.
    pub positions: Vec<Point<N>>,
    /// The velocities of the fluid particles.
    pub velocities: Vec<Vector<N>>,
    /// The volume of the fluid particles.
    pub volumes: DVector<N>,
    /// The rest density of this fluid.
    pub density0: N,
    /// The viscosity coefficient of this fluid.
    pub viscosity: N,
}

impl<N: RealField> Fluid<N> {
    /// Initializes a new fluid object with the given particle positions, particle radius, density, and viscosity.
    ///
    /// The particle radius should be the same as the radius used to initialize the liquid world.
    pub fn new(
        particle_positions: Vec<Point<N>>,
        particle_radius: N, // XXX: remove this parameter since it is already defined by the liquid world.
        density0: N,
        viscosity: N,
    ) -> Self {
        let num_particles = particle_positions.len();
        let velocities = std::iter::repeat(Vector::zeros())
            .take(num_particles)
            .collect();
        // The volume of a fluid is computed as the volume of a cuboid of half-width equal to particle_radius.
        // It is multiplied by 0.8 so that there is no pressure when the cuboids are aligned on a grid.
        // This mass computation method is inspired from the SplishSplash project.
        #[cfg(feature = "dim2")]
        let particle_volume = particle_radius * particle_radius * na::convert(4.0 * 0.8);
        #[cfg(feature = "dim3")]
        let particle_volume =
            particle_radius * particle_radius * particle_radius * na::convert(8.0 * 0.8);

        Self {
            positions: particle_positions,
            velocities,
            volumes: DVector::repeat(num_particles, particle_volume),
            density0,
            viscosity,
        }
    }

    /// The number of particles on this fluid.
    pub fn num_particles(&self) -> usize {
        self.positions.len()
    }

    /// Computes the AABB of this fluid.
    #[cfg(feature = "nphysics")]
    pub fn compute_aabb(&self, particle_radius: N) -> ncollide::bounding_volume::AABB<N> {
        use ncollide::bounding_volume::{self, BoundingVolume};
        bounding_volume::local_point_cloud_aabb(&self.positions).loosened(particle_radius)
    }

    /// The mass of the `i`-th particle of this fluid.
    pub fn particle_mass(&self, i: usize) -> N {
        self.volumes[i] * self.density0
    }

    /// The inverse mass of the `i`-th particle of this fluid.
    ///
    /// Returns 0 if the `i`-th particle has a zero mass.
    pub fn particle_inv_mass(&self, i: usize) -> N {
        if self.volumes[i].is_zero() {
            N::zero()
        } else {
            N::one() / (self.volumes[i] * self.density0)
        }
    }
}