using System;

namespace GetNeighbors
{
    public static class CartesianCoordinates
    {
        /// <summary>
        /// Gets from a set of points only points that are h-neighbors for a point with integer coordinates x and y.
        /// </summary>
        /// <param name="point">Given point with integer coordinates x and y.</param>
        /// <param name="h">Distance around a given point.</param>
        /// <param name="points">A given set of points.</param>
        /// <returns>Only points that are h-neighbors for a point with integer coordinates x and y.</returns>
        /// <exception cref="ArgumentNullException">Throw when array points is null.</exception>
        /// <exception cref="ArgumentException">Throw when h-distance is less or equals zero.</exception>
        public static Point[] GetNeighbors(Point point, int h, params Point[] points)
        {
            if (points is null)
            {
                throw new ArgumentNullException(nameof(points));
            }

            if (h <= 0)
            {
                throw new ArgumentException("Distance can't be less or equals zero", nameof(h));
            }

            Point[] hNeighbors = Array.Empty<Point>();
            int count = 0;

            for (int i = 0; i < points.Length; i++)
            {
                if (Math.Abs(point.X - points[i].X) <= h && Math.Abs(point.Y - points[i].Y) <= h)
                {
                    count++;
                    if (count > hNeighbors.Length)
                    {
                        Array.Resize(ref hNeighbors, hNeighbors.Length == 0 ? 1 : hNeighbors.Length * 2);
                    }

                    hNeighbors[count - 1] = points[i];
                }
            }

            Array.Resize(ref hNeighbors, count);

            return hNeighbors;
        }
    }
}