using System;
using System.Collections;
using System.Collections.Generic;
using AlgorithmCollection;

//https://zh.wikipedia.org/wiki/%E6%8E%92%E5%BA%8F%E7%AE%97%E6%B3%95
namespace AlgorithmCollection.Sorting
{

    // 排序主要关注稳定性和效率
    // *稳定性，相同值的元素相对位置在排序前后是否改变
    // *效率，时间复杂度和空间复杂度
    public static class SortingHelper
    {

        // 选择合适的算法排序
        public static void Sort<T>(T[] data, bool bDescent = false, bool bStable = false) where T : IComparable, IEquatable<T>
        {
            int n = data.Length;
        }

        #region 归并排序
        // 归并排序 | 稳定 | 分治法
        // T(n) = O(nlogn) 渐进最优
        // S(n) = O(n) 需要大小为n的额外空间
        public static void MergeSort<T>(T[] data, bool descent = false) where T : IComparable, IEquatable<T>
        {
            T[] temp = new T[data.Length];// 需要额外的临时空间
            _MergeSort(data, 0, data.Length - 1, descent, temp);
        }

        // 归并排序，非递归
        public static void MergeSort_NoneRecursion<T>(T[] data, bool descent = false) where T : IComparable, IEquatable<T>
        {
            T[] temp = new T[data.Length];// 需要额外的临时空间
            int s = 1;
            int n = data.Length;
            for(int seg = 1; seg < n; seg += seg) //步长1,2,4,8...
            {
                for(int start = 0; start < n; start += seg * 2)
                {
                    int left = start, right = Algorithms.Min(start + seg * 2 - 1, n - 1);
                    int mid = Algorithms.Min(start + seg - 1, n - 1);
                    _Merge(data, left, mid, right, descent, temp);
                    _CopyTo(temp, data, left, right);
                }
            }
        }

        private static void _MergeSort<T>(T[] data, int left, int right, bool descent, T[] temp) where T : IComparable, IEquatable<T>
        {
            // 这是一个O(logn)的分治算法，对于每此分治，会有一个O(n)的合并算法，所以归并排序复杂度是O(nlogn)
            if (left < right)
            {
                int middle = (left + right) / 2;
                _MergeSort(data, left, middle, descent, temp);
                _MergeSort(data, middle + 1, right, descent, temp);
                _Merge(data, left, middle, right, descent, temp);// 把left~right部分合并到temp中
                _CopyTo(temp, data, left, right); // 把合并后的temp部分复制回去
            }
        }

        private static T[] _Merge<T>(T[]data, int left, int middle, int right, bool descent, T[] temp) where T : IComparable, IEquatable<T>
        {
            // 这是一个O(n)的合并算法
            int l = left, r = middle + 1;
            int i = 0;
            while(l <= middle || r <= right)
            {
                if(l > middle)
                {
                    temp[i++] = data[r++];
                    continue;
                }
                if(r > right)
                {
                    temp[i++] = data[l++];
                    continue;
                }
                if (descent && data[l].CompareTo(data[r]) > 0 || !descent && data[l].CompareTo(data[r]) < 0)
                {
                    temp[i++] = data[l];
                    l++;
                }
                else
                {
                    temp[i++] = data[r];
                    r++;
                }
            }
            return temp;
        }

        private static void _CopyTo<T>(T[] src, T[] dst, int from, int to)
        {
            for (int i = from; i <= to; ++i)
                dst[i] = src[i - from];
        }

        #endregion

        #region 快速排序
        // 快速排序 | 不稳定 | 原地排序 | 分治法
        // 平均T(n)=O(nlogn)，最坏T(n)=O(n²)
        // S(n)=O(1)
        public static void QuickSort<T>(T[] data, bool descent = false) where T : IComparable, IEquatable<T>
        {
            _QuickSort(data, descent, 0, data.Length - 1);
        }
        private static void _QuickSort<T>(T[] data, bool descent, int start, int end) where T : IComparable, IEquatable<T>
        {
            if(start < end)
            {
                int p = _Partion(data, descent, start, end);
                _QuickSort(data, descent, start, p - 1);
                _QuickSort(data, descent, p + 1, end);
            }
        }
        private static int _Partion<T>(T[] data, bool descent, int start, int end) where T : IComparable
        {
            T v = data[start];
            int i = start;
            int j = end + 1;
            while(true)
            {
                while ((descent && data[++i].CompareTo(v) >= 0
                    || !descent && data[++i].CompareTo(v) <= 0) && i < end) ;
                while ((descent && data[--j].CompareTo(v) <= 0
                    || !descent && data[--j].CompareTo(v) >= 0) && j > start) ;
                if (i >= j)
                    break;
                Algorithms.Swap(ref data, i, j);
            }
            Algorithms.Swap(ref data, start, j);
            return j;
        }
        #endregion

        // 冒泡排序，稳定
        // O(n²)
        public static void BubbleSort<T>(T[] data, bool descent = false) where T : IComparable, IEquatable<T>
        {
            int n = data.Length;
            for(int i = 0; i < n; ++i)
            {
                for(int j = 0; j < n - i - 1;  ++j)
                {
                    if (descent && data[j].CompareTo(data[j + 1]) < 0
                    || !descent && data[j].CompareTo(data[j + 1]) > 0)
                    {
                        Algorithms.Swap(ref data, j, j + 1);
                    }
                }
            }
        }

    }

}