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

namespace Demo.LearnByDoing.Tests.RandomStuff
{
    /// <summary>
    /// https://dev.to/peter/write-a-script-to-find-permutable-prime-numbers-1gg0
    /// </summary>
    public class PermutablePrimesTest
    {
        private const int UPTO = 1000;

        [Fact]
        public void TestSieveOf()
        {
            var expected = new[]
                {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61
                , 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139
                , 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229
                , 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317
                , 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421
                , 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521
                , 523, 541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619
                , 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733
                , 739, 743, 751, 757, 761, 769, 773, 787, 797, 809, 811, 821, 823, 827, 829, 839
                , 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953
                , 967, 971, 977, 983, 991, 997};
            var actual = GetPrimeNumbers(UPTO).ToList();

            Assert.True(expected.SequenceEqual(actual));
        }

        [Fact]
        public void TestPermutablePrimes()
        {
            var expected = new[] {2, 3, 5, 7, 11, 13, 17, 31, 37, 71, 73, 79, 97, 113, 131, 199, 311, 337, 373, 733, 919, 991};
            var actual = GetPermutablePrimes(UPTO).ToList();
            Assert.True(expected.SequenceEqual(actual));
        }

        private IEnumerable<int> GetPermutablePrimes(int upto)
        {
            // For O(1) lookup
            var primes = new HashSet<int>(GetPrimeNumbers(upto));

            foreach (var prime in primes)
            {
                List<int> perms = GetPermutations(prime.ToString()).ToList();
                if (perms.All(perm => primes.Contains(perm))) yield return prime;
            }
        }

        private IEnumerable<int> GetPermutations(string input)
        {
            return FillPermutations(input).Select(int.Parse);
        }

        private IEnumerable<string> FillPermutations(string input)
        {
            if (input.Length == 1) yield return input;
            else if (input.Length == 2)
            {
                yield return input;
                yield return new string(new[] { input[1], input[0] });
            }
            else
            {
                for (int i = 0; i < input.Length; i++)
                {
                    string prefix = input[i].ToString();
                    string left = input.Substring(0, i);
                    string right = input.Substring(i + 1);
                    string newInput = left + right;

                    foreach (var rest in FillPermutations(newInput))
                    {
                        yield return prefix + rest;
                    }
                }
            }
        }

        private IEnumerable<int> GetPrimeNumbers(int upto)
        {
            // Initialize all numbers as true to "upto + 1" since array is 0-based.
            var primes = Enumerable.Repeat(true, upto + 1).ToArray();

            for (int i = 2; i * i <= upto; i++)
            {
                // It's already processed.
                if (!primes[i]) continue;

                for (int j = i * 2; j <= upto; j += i)
                {
                    primes[j] = false;
                }
            }

            // Skip 0 & 1
            // Return indexes, which are prime numbers
            const int skipBy = 2;
            return primes
                .Skip(skipBy)
                .Select((_, i) => new { IsPrime = _, Value = i + skipBy })
                .Where(obj => obj.IsPrime)
                .Select(obj => obj.Value);
        }
    }
}