public struct FiringSolution
{
    public float timeToHit;
    public Vector3 projectileVelocity;
    public bool validSolutionExists;
}
public struct FiringParameters
{
    public float projectileSpeed;
    public Vector3 relativeTargetPosition;
    public Vector3 targetVelocity;
}

public static FiringSolution GetFiringSolution(FiringParameters parameters)
{
    var vDotDeltaP = Vector3.Dot(parameters.targetVelocity, parameters.relativeTargetPosition);
    var squarePMagnitude = parameters.relativeTargetPosition.sqrMagnitude;
    var squareVMagnitudeLessSquareSpeed = parameters.targetVelocity.sqrMagnitude - (parameters.projectileSpeed * parameters.projectileSpeed);

    var tSolution = new QuadraticSolution(squareVMagnitudeLessSquareSpeed, 2 * vDotDeltaP, squarePMagnitude);
    if (!tSolution.hasSolution)
    {
        return new FiringSolution
        {
            validSolutionExists = false
        };
    }
    var chosenT = math.max(tSolution.solutionOne, tSolution.solutionTwo);
    if (chosenT < 0)
    {
        return new FiringSolution
        {
            validSolutionExists = false
        };
    }

    var projectedTargetPosition = parameters.relativeTargetPosition + parameters.targetVelocity * chosenT;
    // firing origin is assumed to be (0, 0, 0)
    var firingVector = projectedTargetPosition.normalized * parameters.projectileSpeed;

    return new FiringSolution
    {
        timeToHit = chosenT,
        projectileVelocity = firingVector,
        validSolutionExists = true
    };
}


private struct QuadraticSolution
{
    public float solutionOne;
    public float solutionTwo;
    public bool hasSolution;

    public QuadraticSolution(float a, float b, float c)
    {
        var root = b * b - 4 * a * c;
        if (root < 0)
        {
            solutionOne = float.NaN;
            solutionTwo = float.NaN;
            hasSolution = false;
            return;
        }
        root = math.sqrt(root);

        solutionOne = (-b + root) / (2 * a);
        solutionTwo = (-b - root) / (2 * a);
        hasSolution = true;
    }
}