using System.Collections.Generic;
using System.Common;
using System.Security.Cryptography;

namespace System
{
    /// <summary>
    /// 线程安全的随机数生成
    /// <para>https://devblogs.microsoft.com/pfxteam/getting-random-numbers-in-a-thread-safe-way</para>
    /// </summary>
    public static class Random2
    {
        static readonly RNGCryptoServiceProvider _global = new RNGCryptoServiceProvider();

        [ThreadStatic]
        static Random? _local;

        static Random GetRandom()
        {
            var inst = _local;
            if (inst == null)
            {
                byte[] buffer = new byte[4];
                _global.GetBytes(buffer);
                _local = inst = new Random(BitConverter.ToInt32(buffer, 0));
            }
            return inst;
        }

        /// <inheritdoc cref="RANDOM.Next"/>
        public static int Next() => GetRandom().Next();

        /// <inheritdoc cref="RANDOM.Next(int)"/>
        public static int Next(int maxValue) => GetRandom().Next(maxValue);

        /// <inheritdoc cref="RANDOM.Next(int, int)"/>
        public static int Next(int minValue, int maxValue) => GetRandom().Next(minValue, maxValue);

        /// <inheritdoc cref="RANDOM.NextDouble"/>
        public static double NextDouble() => GetRandom().NextDouble();

        static char RandomCharAt(string s, int index)
        {
            if (index == s.Length) index = 0;
            else if (index > s.Length) index %= s.Length;
            return s[index];
        }

        /// <summary>
        /// 生成随机字符串
        /// </summary>
        /// <param name="length">要生成的字符串长度</param>
        /// <param name="randomChars">随机字符串字符集</param>
        /// <returns></returns>
        public static string GenerateRandomString(int length = 6, string randomChars = Constants.DigitsLetters)
        {
            var random = GetRandom();
            var result = new char[length];
            if (random.Next(256) % 2 == 0)
                for (var i = length - 1; i >= 0; i--) // 5 4 3 2 1 0
                    EachGenerate(i);
            else
                for (var i = 0; i < length; i++) // 0 1 2 3 4 5
                    EachGenerate(i);
            return new string(result);
            void EachGenerate(int i)
            {
                var index = random.Next(0, randomChars.Length);
                var temp = RandomCharAt(randomChars, index);
                result[i] = temp;
            }
        }

        /// <summary>
        /// 生成随机数字
        /// </summary>
        /// <param name="length">要生成的字符串长度</param>
        /// <param name="endIsZero">生成的数字最后一位是否能够为0，默认不能为0(<see langword="false"/>)</param>
        /// <returns></returns>
        public static int GenerateRandomNum(int length = 6, bool endIsZero = false)
        {
            var random = GetRandom();
            var result = 0;
            var lastNum = 0;
            if (random.Next(256) % 2 == 0)
                for (int i = length - 1; i >= 0; i--) // 5 4 3 2 1 0
                    EachGenerate(i);
            else
                for (int i = 0; i < length; i++) // 0 1 2 3 4 5
                    EachGenerate(i);
            return result;
            void EachGenerate(int i)
            {
                var bit = (int)(i == 0 ? 1 : Math.Pow(10, i));
                // 100,000  10,000  1,000   100     10      1
                // 1        10      100     1,000   10,000  100,000
                var current = random.Next(lastNum + 1, lastNum + 10);
                lastNum = current % 10;
                if (lastNum == 0)
                {
                    // i != 0 &&  i!=5 末尾和开头不能有零
                    if ((i != 0 || endIsZero) && i != length - 1)
                        return;
                    lastNum = random.Next(1, 10);
                }
                result += lastNum * bit;
            }
        }

        /// <summary>
        /// 从多个元素中随机获取一个元素
        /// </summary>
        /// <typeparam name="T"></typeparam>
        /// <param name="items"></param>
        /// <returns></returns>
        public static T GetRandomItem<T>(this IReadOnlyList<T> items) => items[Next(items.Count)];
    }
}