using System.Collections.Generic;
using System.Linq;

namespace Dbscan
{

	/// <summary>
	/// An implementation of the <see cref="T:Dbscan.ISpatialIndex`1" /> using a simple <see cref="T:System.Collections.Generic.List`1" />
	/// to hold the elements and a linear search of all items to <see cref="M:Dbscan.ListSpatialIndex`1.Search(Dbscan.IPointData@,System.Double)" />
	/// for nearby items.
	/// </summary>
	/// <typeparam name="T">The type of items in the index.</typeparam>
	public class ListSpatialIndex<T> : ISpatialIndex<T> where T : IPointData
	{
		private readonly IReadOnlyList<T> _list;

		private readonly DistanceFunction _distanceFunction;

		/// <summary>
		/// Initializes a <see cref="T:Dbscan.ListSpatialIndex`1" /> with a collection of data, using the
		/// Euclidean distance between two points as the distance function to search for points 
		/// in a given neighborhood.
		/// </summary>
		/// <param name="data">The collection of data to put into the index</param>
		public ListSpatialIndex(IEnumerable<T> data)
			: this(data, (DistanceFunction)DistanceFunctions.EuclideanDistance)
		{
		}

		/// <summary>
		/// Initializes a <see cref="T:Dbscan.ListSpatialIndex`1" /> with a collection of data, using the
		/// specified distanct function to search for points in a given neighborhood.
		/// </summary>
		/// <param name="data">The collection of data to put into the index</param>
		/// <param name="distanceFunction">The function used to determine if a point is within a specified distance of a given point.</param>
		public ListSpatialIndex(IEnumerable<T> data, DistanceFunction distanceFunction)
		{
			_list = data.ToList();
			_distanceFunction = distanceFunction;
		}

		/// <summary>
		/// Get all of the elements within the current <see cref="T:Dbscan.ListSpatialIndex`1" />.
		/// </summary>
		/// <returns>
		/// A list of every element contained in the <see cref="T:Dbscan.ListSpatialIndex`1" />.
		/// </returns>
		public IReadOnlyList<T> Search()
		{
			return _list;
		}

		/// <summary>
		/// Get all of the elements from this <see cref="T:Dbscan.ListSpatialIndex`1" />
		/// within a circle centered at the point <see cref="P:Dbscan.IPointData.Point" />
		/// with a radius of <paramref name="epsilon" />.
		/// </summary>
		/// <param name="p">The center of the search circle.</param>
		/// <param name="epsilon">The radius of the search circle.</param>
		/// <returns>
		/// A list of the points that are within the search area.
		/// </returns>
		public IReadOnlyList<T> Search(in IPointData p, double epsilon)
		{
			List<T> list = new List<T>();
			foreach (T item in _list)
			{
				DistanceFunction distanceFunction = _distanceFunction;
				IPointData p2 = item;
				if (distanceFunction(in p, in p2) < epsilon)
				{
					list.Add(item);
				}
			}
			return list;
		}

		IReadOnlyList<T> ISpatialIndex<T>.Search(in IPointData p, double epsilon)
		{
			return Search(in p, epsilon);
		}
	}
}