
#pragma once
#include "vector2.hpp"
namespace cavc
{
template <typename Real>
struct IntrLineSeg2Circle2Result
{
    // number of interescts found (0, 1, or 2)
    int numIntersects;
    // parametric value for first intersect (if numIntersects > 0) otherwise undefined
    Real t0;
    // parametric value for second intersect (if numintersects > 1) otherwise undefined
    Real t1;
};

// Gets the intersect between a segment and a circle, returning the parametric solution t to the
// segment equation P(t) = v1 + t * (v2 - v1) for t = 0 to t = 1, if t < 0 or t > 1 then intersect
// occurs only when extending the segment out past the points given (if t < 0 intersect nearest v1,
// if t > 0 then intersect nearest v2), intersects are "sticky" and "snap" to tangent points, e.g. a
// segment very close to being a tangent will be returned as a single intersect point
template <typename Real>
IntrLineSeg2Circle2Result<Real> intrLineSeg2Circle2(Vector2<Real> const& p0,
                                                    Vector2<Real> const& p1, Real radius,
                                                    Vector2<Real> const& circleCenter)
{
    // This function solves for t by substituting the parametric equations for the segment x = v1.X +
    // t * (v2.X - v1.X) and y = v1.Y + t * (v2.Y - v1.Y) for t = 0 to t = 1 into the circle equation
    // (x-h)^2 + (y-k)^2 = r^2 and then solving the resulting equation in the form a*t^2 + b*t + c = 0
    // using the quadratic formula
    IntrLineSeg2Circle2Result<Real> result;
    Real dx = p1.x() - p0.x();
    Real dy = p1.y() - p0.y();
    Real h = circleCenter.x();
    Real k = circleCenter.y();

    Real a = dx * dx + dy * dy;
    if (std::abs(a) < utils::realThreshold<Real>()) {
        // v1 = v2, test if point is on the circle
        Real xh = p0.x() - h;
        Real yk = p0.y() - k;
        if (utils::fuzzyEqual(xh * xh + yk * yk, radius * radius)) {
            result.numIntersects = 1;
            result.t0 = Real(0);
        }
        else {
            result.numIntersects = 0;
        }
    }
    else {
        Real b = Real(2) * (dx * (p0.x() - h) + dy * (p0.y() - k));
        Real c = (p0.x() * p0.x() - 2.0 * h * p0.x() + h * h) +
                 (p0.y() * p0.y() - 2.0 * k * p0.y() + k * k) - radius * radius;
        Real discr = b * b - 4.0 * a * c;

        if (std::abs(discr) < utils::realThreshold<Real>()) {
            // 1 solution (tangent line)
            result.numIntersects = 1;
            result.t0 = -b / (Real(2) * a);
        }
        else if (discr < Real(0)) {
            result.numIntersects = 0;
        }
        else {
            result.numIntersects = 2;
            std::pair<Real, Real> sols = utils::quadraticSolutions(a, b, c, discr);
            result.t0 = sols.first;
            result.t1 = sols.second;
        }
    }

    CAVC_ASSERT(result.numIntersects >= 0 && result.numIntersects <= 2, "invalid intersect count");
    return result;
}
}  // namespace cavc
