About the Pure pursuit algorithm in the code

Software
1

Hi,
Can someone explain how the code relate to Pure pursuit algorithm?
Why need to calculate point on ABLine closest to current position?
// ** Pure pursuit ** - calc point on ABLine closest to current position
double U = (((pivot.easting - currentABLineP1.easting) * dx)
+ ((pivot.northing - currentABLineP1.northing) * dy))
/ ((dx * dx) + (dy * dy));
Is there any picture to show the algorithm with the parameters in the code? It will give me a better understanding of the algorithm.

// ** Pure pursuit ** - calc point on ABLine closest to current position
                double U = (((pivot.easting - currentABLineP1.easting) * dx)
                            + ((pivot.northing - currentABLineP1.northing) * dy))
                            / ((dx * dx) + (dy * dy));

                //point on AB line closest to pivot axle point
                rEastAB = currentABLineP1.easting + (U * dx);
                rNorthAB = currentABLineP1.northing + (U * dy);

                //update base on autosteer settings and distance from line
                double goalPointDistance = mf.vehicle.UpdateGoalPointDistance();

                if (mf.isReverse ? isHeadingSameWay : !isHeadingSameWay)
                {
                    goalPointAB.easting = rEastAB - (Math.Sin(abHeading) * goalPointDistance);
                    goalPointAB.northing = rNorthAB - (Math.Cos(abHeading) * goalPointDistance);
                }
                else
                {
                    goalPointAB.easting = rEastAB + (Math.Sin(abHeading) * goalPointDistance);
                    goalPointAB.northing = rNorthAB + (Math.Cos(abHeading) * goalPointDistance);
                }

                //calc "D" the distance from pivot axle to lookahead point
                double goalPointDistanceDSquared
                    = glm.DistanceSquared(goalPointAB.northing, goalPointAB.easting, pivot.northing, pivot.easting);

                //calculate the the new x in local coordinates and steering angle degrees based on wheelbase
                double localHeading;

                if (isHeadingSameWay) localHeading = glm.twoPI - mf.fixHeading + inty;
                else localHeading = glm.twoPI - mf.fixHeading - inty;

                ppRadiusAB = goalPointDistanceDSquared / (2 * (((goalPointAB.easting - pivot.easting) * Math.Cos(localHeading))
                    + ((goalPointAB.northing - pivot.northing) * Math.Sin(localHeading))));

                steerAngleAB = glm.toDegrees(Math.Atan(2 * (((goalPointAB.easting - pivot.easting) * Math.Cos(localHeading))
                    + ((goalPointAB.northing - pivot.northing) * Math.Sin(localHeading))) * mf.vehicle.wheelbase
                    / goalPointDistanceDSquared));

image

Thanks a lot!