shinespark/src/fighter/attack/directional_influence.rs

use fixed::traits::ToFixed;
use fixed_math::SinCos;

use crate::math::{
    int::{FGWide, FGi32},
    vec::FGVec2,
};

/// Applies DI to a given attack direction
pub fn apply_directional_influence(
    receiver_stick: FGVec2,
    attack_angle: FGi32,
    di_max_angle: FGi32,
) -> FGVec2 {
    let (sin, cos) = attack_angle.sin_cos();
    let base_launch_direction = FGVec2::new(cos, sin);

    let Some(base_launch_direction) = base_launch_direction.normalize() else {
        return FGVec2::ZERO;
    };

    if receiver_stick.length_squared().is_zero() {
        return base_launch_direction;
    }

    let normal = FGVec2::new(-base_launch_direction.y, base_launch_direction.x);
    let di_amount = receiver_stick.dot(normal);
    let angle_change = di_amount * di_max_angle.to_fixed::<FGWide>();
    let angle_change = angle_change.clamp(
        -di_max_angle.to_fixed::<FGWide>(),
        di_max_angle.to_fixed::<FGWide>(),
    );
    let (sin, cos) = (attack_angle + angle_change.to_fixed::<FGi32>()).sin_cos();
    FGVec2::new(cos, sin).normalize_or_zero()
}

#[cfg(test)]
mod test {
    use crate::{
        fighter::attack::directional_influence::apply_directional_influence,
        math::{int::FGi32, vec::FGVec2},
    };

    #[test]
    fn di_zero_passes_unmodified() {
        let di = apply_directional_influence(FGVec2::ZERO, FGi32::lit("0.0"), FGi32::lit("18.0"));
        const EPSILON: FGi32 = FGi32::from_bits(4);
        assert!(di.x.abs_diff(FGi32::ONE) < EPSILON);
        assert!(di.y.abs_diff(FGi32::ZERO) < EPSILON);
    }

    #[test]
    fn di_applies_on_the_correct_side() {
        let di = apply_directional_influence(
            FGVec2::lit("0.0", "1.0"),
            FGi32::lit("0.0"),
            FGi32::lit("18.0"),
        );
        assert!(di.x > FGi32::ZERO);
        assert!(di.y > FGi32::ZERO);
        let di = apply_directional_influence(
            FGVec2::lit("0.0", "-1.0"),
            FGi32::lit("0.0"),
            FGi32::lit("18.0"),
        );
        assert!(di.x > FGi32::ZERO);
        assert!(di.y < FGi32::ZERO);
    }
}