/******************************************************************************
 * Copyright (C) Ultraleap, Inc. 2011-2021.                                   *
 *                                                                            *
 * Use subject to the terms of the Apache License 2.0 available at            *
 * http://www.apache.org/licenses/LICENSE-2.0, or another agreement           *
 * between Ultraleap and you, your company or other organization.             *
 ******************************************************************************/

using Leap.Unity.Query;
using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

namespace Leap.Unity
{

    public static class ReadonlySliceExtensions
    {

        /// <summary>
        /// Creates a readonlySlice into the ReadonlyList with an inclusive beginIdx and an _exclusive_
        /// endIdx. A readonlySlice with identical begin and end indices would be an empty readonlySlice.
        /// 
        /// A readonlySlice whose endIdx is smaller than its beginIdx will index backwards along the
        /// underlying ReadonlyList.
        /// 
        /// Not providing either index argument will simply refer to the beginning of the
        /// list (for beginIdx) or to the end of the list (for endIdx).
        /// 
        /// ReadonlySlices do not allocate, and they provide an enumerator definition so they can be
        /// used in a <code>foreach</code> statement.
        /// </summary>
        public static ReadonlySlice<T> ReadonlySlice<T>(this ReadonlyList<T> list, int beginIdx = -1, int endIdx = -1)
        {
            if (beginIdx == -1 && endIdx == -1)
            {
                return new ReadonlySlice<T>(list, 0, list.Count);
            }
            else if (beginIdx == -1 && endIdx != -1)
            {
                return new ReadonlySlice<T>(list, 0, endIdx);
            }
            else if (endIdx == -1 && beginIdx != -1)
            {
                return new ReadonlySlice<T>(list, beginIdx, list.Count);
            }
            else
            {
                return new ReadonlySlice<T>(list, beginIdx, endIdx);
            }
        }

        public static ReadonlySlice<T> FromIndex<T>(this ReadonlyList<T> list, int fromIdx)
        {
            return ReadonlySlice(list, fromIdx);
        }

    }

    public struct ReadonlySlice<T> : IIndexableStruct<T, ReadonlySlice<T>>
    {

        private ReadonlyList<T> _list;

        private int _beginIdx;
        private int _endIdx;

        private int _direction;

        /// <summary>
        /// Creates a readonlySlice into the ReadonlyList with an inclusive beginIdx and an _exclusive_
        /// endIdx. A readonlySlice with identical begin and end indices would be an empty readonlySlice.
        /// 
        /// A readonlySlice whose endIdx is smaller than its beginIdx will index backwards along the
        /// underlying ReadonlyList.
        /// </summary>
        public ReadonlySlice(ReadonlyList<T> list, int beginIdx, int endIdx)
        {
            _list = list;
            _beginIdx = beginIdx;
            _endIdx = endIdx;
            _direction = beginIdx <= endIdx ? 1 : -1;
        }

        public T this[int index]
        {
            get
            {
                if (index < 0 || index > Count - 1) { throw new IndexOutOfRangeException(); }
                return _list[_beginIdx + index * _direction];
            }
        }

        public int Count
        {
            get
            {
                return (_endIdx - _beginIdx) * _direction;
            }
        }

        #region foreach and Query()

        public IndexableStructEnumerator<T, ReadonlySlice<T>> GetEnumerator()
        {
            return new IndexableStructEnumerator<T, ReadonlySlice<T>>(this);
        }

        public Query<T> Query()
        {
            T[] array = ArrayPool<T>.Spawn(Count);
            for (int i = 0; i < Count; i++)
            {
                array[i] = this[i];
            }
            return new Query<T>(array, Count);
        }

        #endregion

    }

}