// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

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

namespace Tests
{
    public class GroupBy : AsyncEnumerableTests
    {
        [Fact]
        public void GroupBy_Null()
        {
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int>(default(IAsyncEnumerable<int>), x => x));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int>(Return42, default(Func<int, int>)));

            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int>(default(IAsyncEnumerable<int>), x => x, EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int>(Return42, default(Func<int, int>), EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int>(Return42, x => x, default(IEqualityComparer<int>)));

            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(default(IAsyncEnumerable<int>), x => x, x => x));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, default(Func<int, int>), x => x));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, x => x, default(Func<int, int>)));

            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(default(IAsyncEnumerable<int>), x => x, x => x, EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, default(Func<int, int>), x => x, EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, x => x, default(Func<int, int>), EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, x => x, x => x, default(IEqualityComparer<int>)));

            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(default(IAsyncEnumerable<int>), x => x, (x, ys) => x));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, default(Func<int, int>), (x, ys) => x));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, x => x, default(Func<int, IAsyncEnumerable<int>, int>)));

            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(default(IAsyncEnumerable<int>), x => x, (x, ys) => x, EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, default(Func<int, int>), (x, ys) => x, EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, x => x, default(Func<int, IAsyncEnumerable<int>, int>), EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int>(Return42, x => x, (x, ys) => x, default(IEqualityComparer<int>)));

            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(default(IAsyncEnumerable<int>), x => x, x => x, (x, ys) => x));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(Return42, default(Func<int, int>), x => x, (x, ys) => x));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(Return42, x => x, default(Func<int, int>), (x, ys) => x));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(Return42, x => x, x => x, default(Func<int, IAsyncEnumerable<int>, int>)));

            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(default(IAsyncEnumerable<int>), x => x, x => x, (x, ys) => x, EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(Return42, default(Func<int, int>), x => x, (x, ys) => x, EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(Return42, x => x, default(Func<int, int>), (x, ys) => x, EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(Return42, x => x, x => x, default(Func<int, IAsyncEnumerable<int>, int>), EqualityComparer<int>.Default));
            AssertThrows<ArgumentNullException>(() => AsyncEnumerable.GroupBy<int, int, int, int>(Return42, x => x, x => x, (x, ys) => x, default(IEqualityComparer<int>)));
        }

        [Fact]
        public void GroupBy1()
        {
            var xs = new[] {
                new { Name = "Bart", Age = 27 },
                new { Name = "John", Age = 62 },
                new { Name = "Eric", Age = 27 },
                new { Name = "Lisa", Age = 14 },
                new { Name = "Brad", Age = 27 },
                new { Name = "Lisa", Age = 23 },
                new { Name = "Eric", Age = 42 },
            };

            var ys = xs.ToAsyncEnumerable();

            var res = ys.GroupBy(x => x.Age / 10);

            var e = res.GetAsyncEnumerator();

            Assert.True(e.MoveNextAsync().Result);
            Assert.Equal(2, e.Current.Key);
            var g1 = e.Current.GetAsyncEnumerator();
            HasNext(g1, xs[0]);
            HasNext(g1, xs[2]);
            HasNext(g1, xs[4]);
            HasNext(g1, xs[5]);
            NoNext(g1);

            Assert.True(e.MoveNextAsync().Result);
            Assert.Equal(6, e.Current.Key);
            var g2 = e.Current.GetAsyncEnumerator();
            HasNext(g2, xs[1]);
            NoNext(g2);

            Assert.True(e.MoveNextAsync().Result);
            Assert.Equal(1, e.Current.Key);
            var g3 = e.Current.GetAsyncEnumerator();
            HasNext(g3, xs[3]);
            NoNext(g3);

            Assert.True(e.MoveNextAsync().Result);
            Assert.Equal(4, e.Current.Key);
            var g4 = e.Current.GetAsyncEnumerator();
            HasNext(g4, xs[6]);
            NoNext(g4);

            NoNext(e);
        }

        [Fact]
        public void GroupBy2()
        {
            var xs = new[] {
                new { Name = "Bart", Age = 27 },
                new { Name = "John", Age = 62 },
                new { Name = "Eric", Age = 27 },
                new { Name = "Lisa", Age = 14 },
                new { Name = "Brad", Age = 27 },
                new { Name = "Lisa", Age = 23 },
                new { Name = "Eric", Age = 42 },
            };

            var ys = xs.ToAsyncEnumerable();

            var res = ys.GroupBy(x => x.Age / 10);

            var e = res.GetAsyncEnumerator();

            Assert.True(e.MoveNextAsync().Result);
            var g1 = e.Current;
            Assert.Equal(2, g1.Key);

            Assert.True(e.MoveNextAsync().Result);
            var g2 = e.Current;
            Assert.Equal(6, g2.Key);

            Assert.True(e.MoveNextAsync().Result);
            var g3 = e.Current;
            Assert.Equal(1, g3.Key);

            Assert.True(e.MoveNextAsync().Result);
            var g4 = e.Current;
            Assert.Equal(4, g4.Key);

            NoNext(e);

            var g1e = g1.GetAsyncEnumerator();
            HasNext(g1e, xs[0]);
            HasNext(g1e, xs[2]);
            HasNext(g1e, xs[4]);
            HasNext(g1e, xs[5]);
            NoNext(g1e);

            var g2e = g2.GetAsyncEnumerator();
            HasNext(g2e, xs[1]);
            NoNext(g2e);

            var g3e = g3.GetAsyncEnumerator();
            HasNext(g3e, xs[3]);
            NoNext(g3e);

            var g4e = g4.GetAsyncEnumerator();
            HasNext(g4e, xs[6]);
            NoNext(g4e);
        }

        [Fact]
        public void GroupBy3()
        {
            var xs = AsyncEnumerable.Empty<int>();
            var ys = xs.GroupBy(x => x);

            var e = ys.GetAsyncEnumerator();
            NoNext(e);
        }

        [Fact]
        public void GroupBy4()
        {
            var ex = new Exception("Bang!");
            var xs = AsyncEnumerable.Throw<int>(ex);
            var ys = xs.GroupBy(x => x);

            var e = ys.GetAsyncEnumerator();
            AssertThrows(() => e.MoveNextAsync().Wait(WaitTimeoutMs), (Exception ex_) => ((AggregateException)ex_).Flatten().InnerExceptions.Single() == ex);
        }

        [Fact]
        public void GroupBy5()
        {
            var xs = GetXs().ToAsyncEnumerable();
            var ys = xs.GroupBy(x => x);

            var e = ys.GetAsyncEnumerator();

            AssertThrows(() => e.MoveNextAsync().Wait(WaitTimeoutMs), (Exception ex_) => ((AggregateException)ex_).Flatten().InnerExceptions.Single().Message == "Bang!");

            //Assert.True(e.MoveNext().Result);
            //var g1 = e.Current;
            //Assert.Equal(g1.Key, 42);
            //var g1e = g1.GetEnumerator();
            //HasNext(g1e, 42);

            //Assert.True(e.MoveNext().Result);
            //var g2 = e.Current;
            //Assert.Equal(g2.Key, 43);
            //var g2e = g2.GetEnumerator();
            //HasNext(g2e, 43);


            //AssertThrows<Exception>(() => g1e.MoveNext().Wait(WaitTimeoutMs), ex_ => ((AggregateException)ex_).Flatten().InnerExceptions.Single().Message == "Bang!");
            //AssertThrows<Exception>(() => g2e.MoveNext().Wait(WaitTimeoutMs), ex_ => ((AggregateException)ex_).Flatten().InnerExceptions.Single().Message == "Bang!");
        }

        [Fact]
        public void GroupBy6()
        {
            var xs = GetXs().ToAsyncEnumerable();
            var ys = xs.GroupBy(x => x);

            var e = ys.GetAsyncEnumerator();

            AssertThrows(() => e.MoveNextAsync().Wait(WaitTimeoutMs), (Exception ex_) => ((AggregateException)ex_).Flatten().InnerExceptions.Single().Message == "Bang!");

            //Assert.True(e.MoveNext().Result);
            //var g1 = e.Current;
            //Assert.Equal(g1.Key, 42);
            //var g1e = g1.GetEnumerator();
            //HasNext(g1e, 42);
            //AssertThrows<Exception>(() => g1e.MoveNext().Wait(WaitTimeoutMs), ex_ => ((AggregateException)ex_).Flatten().InnerExceptions.Single().Message == "Bang!");


        }

        static IEnumerable<int> GetXs()
        {
            yield return 42;
            yield return 43;
            throw new Exception("Bang!");
        }

        [Fact]
        public void GroupBy7()
        {
            var ex = new Exception("Bang!");
            var xs = Return42;
            var ys = xs.GroupBy<int, int>(new Func<int, int>(x => { throw ex; }));

            var e = ys.GetAsyncEnumerator();
            AssertThrows(() => e.MoveNextAsync().Wait(WaitTimeoutMs), (Exception ex_) => ((AggregateException)ex_).Flatten().InnerExceptions.Single() == ex);
        }

        [Fact]
        public void GroupBy8()
        {
            var ex = new Exception("Bang!");
            var xs = new[] { 1, 2, 3 }.ToAsyncEnumerable();
            var ys = xs.GroupBy<int, int>(x => { if (x == 3) throw ex; return x; });

            var e = ys.GetAsyncEnumerator();

            AssertThrows(() => e.MoveNextAsync().Wait(WaitTimeoutMs), (Exception ex_) => ((AggregateException)ex_).Flatten().InnerExceptions.Single() == ex);

            //Assert.True(e.MoveNext().Result);
            //var g1 = e.Current;
            //Assert.Equal(g1.Key, 1);
            //var g1e = g1.GetEnumerator();
            //HasNext(g1e, 1);

            //Assert.True(e.MoveNext().Result);
            //var g2 = e.Current;
            //Assert.Equal(g2.Key, 2);
            //var g2e = g2.GetEnumerator();
            //HasNext(g2e, 2);


            //AssertThrows<Exception>(() => g1e.MoveNext().Wait(WaitTimeoutMs), ex_ => ((AggregateException)ex_).Flatten().InnerExceptions.Single() == ex);
            //AssertThrows<Exception>(() => g2e.MoveNext().Wait(WaitTimeoutMs), ex_ => ((AggregateException)ex_).Flatten().InnerExceptions.Single() == ex);
        }

        [Fact]
        public void GroupBy9()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x));

            var e = ys.GetAsyncEnumerator();

            Assert.True(e.MoveNextAsync().Result);
            var g1 = e.Current;
            Assert.Equal(0, g1.Key);
            var g1e = g1.GetAsyncEnumerator();
            HasNext(g1e, 'a');
            HasNext(g1e, 'd');
            HasNext(g1e, 'g');
            HasNext(g1e, 'j');
            NoNext(g1e);

            Assert.True(e.MoveNextAsync().Result);
            var g2 = e.Current;
            Assert.Equal(1, g2.Key);
            var g2e = g2.GetAsyncEnumerator();
            HasNext(g2e, 'b');
            HasNext(g2e, 'e');
            HasNext(g2e, 'h');
            NoNext(g2e);

            Assert.True(e.MoveNextAsync().Result);
            var g3 = e.Current;
            Assert.Equal(2, g3.Key);
            var g3e = g3.GetAsyncEnumerator();
            HasNext(g3e, 'c');
            HasNext(g3e, 'f');
            HasNext(g3e, 'i');
            NoNext(g3e);

            NoNext(e);
        }

        [Fact]
        public void GroupBy10()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x), (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result);

            var e = ys.GetAsyncEnumerator();
            HasNext(e, "0 - adgj");
            HasNext(e, "1 - beh");
            HasNext(e, "2 - cfi");
            NoNext(e);
        }

        [Fact]
        public void GroupBy11()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result);

            var e = ys.GetAsyncEnumerator();
            HasNext(e, "0 - 0369");
            HasNext(e, "1 - 147");
            HasNext(e, "2 - 258");
            NoNext(e);
        }

        [Fact]
        public void GroupBy12()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, new EqMod(3));

            var e = ys.GetAsyncEnumerator();

            Assert.True(e.MoveNextAsync().Result);
            var g1 = e.Current;
            Assert.Equal(0, g1.Key);
            var g1e = g1.GetAsyncEnumerator();
            HasNext(g1e, 0);
            HasNext(g1e, 3);
            HasNext(g1e, 6);
            HasNext(g1e, 9);
            NoNext(g1e);

            Assert.True(e.MoveNextAsync().Result);
            var g2 = e.Current;
            Assert.Equal(1, g2.Key);
            var g2e = g2.GetAsyncEnumerator();
            HasNext(g2e, 1);
            HasNext(g2e, 4);
            HasNext(g2e, 7);
            NoNext(g2e);

            Assert.True(e.MoveNextAsync().Result);
            var g3 = e.Current;
            Assert.Equal(2, g3.Key);
            var g3e = g3.GetAsyncEnumerator();
            HasNext(g3e, 2);
            HasNext(g3e, 5);
            HasNext(g3e, 8);
            NoNext(g3e);

            NoNext(e);
        }

        [Fact]
        public void GroupBy13()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, x => (char)('a' + x), new EqMod(3));

            var e = ys.GetAsyncEnumerator();

            Assert.True(e.MoveNextAsync().Result);
            var g1 = e.Current;
            Assert.Equal(0, g1.Key);
            var g1e = g1.GetAsyncEnumerator();
            HasNext(g1e, 'a');
            HasNext(g1e, 'd');
            HasNext(g1e, 'g');
            HasNext(g1e, 'j');
            NoNext(g1e);

            Assert.True(e.MoveNextAsync().Result);
            var g2 = e.Current;
            Assert.Equal(1, g2.Key);
            var g2e = g2.GetAsyncEnumerator();
            HasNext(g2e, 'b');
            HasNext(g2e, 'e');
            HasNext(g2e, 'h');
            NoNext(g2e);

            Assert.True(e.MoveNextAsync().Result);
            var g3 = e.Current;
            Assert.Equal(2, g3.Key);
            var g3e = g3.GetAsyncEnumerator();
            HasNext(g3e, 'c');
            HasNext(g3e, 'f');
            HasNext(g3e, 'i');
            NoNext(g3e);

            NoNext(e);
        }

        [Fact]
        public void GroupBy14()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, x => (char)('a' + x), (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result, new EqMod(3));

            var e = ys.GetAsyncEnumerator();
            HasNext(e, "0 - adgj");
            HasNext(e, "1 - beh");
            HasNext(e, "2 - cfi");
            NoNext(e);
        }

        [Fact]
        public void GroupBy15()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result, new EqMod(3));

            var e = ys.GetAsyncEnumerator();
            HasNext(e, "0 - 0369");
            HasNext(e, "1 - 147");
            HasNext(e, "2 - 258");
            NoNext(e);
        }

        [Fact]
        public async Task GroupBy16()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, x => (char)('a' + x), new EqMod(3));

            var e = ys.GetAsyncEnumerator();

            Assert.True(e.MoveNextAsync().Result);
            var g1 = e.Current;
            Assert.Equal(0, g1.Key);
            var g1e = g1.GetAsyncEnumerator();
            HasNext(g1e, 'a');
            HasNext(g1e, 'd');
            HasNext(g1e, 'g');
            HasNext(g1e, 'j');
            NoNext(g1e);
            await g1e.DisposeAsync();

            Assert.True(e.MoveNextAsync().Result);
            var g2 = e.Current;
            Assert.Equal(1, g2.Key);
            var g2e = g2.GetAsyncEnumerator();
            HasNext(g2e, 'b');
            HasNext(g2e, 'e');
            HasNext(g2e, 'h');
            NoNext(g2e);
            await g2e.DisposeAsync();

            Assert.True(e.MoveNextAsync().Result);
            var g3 = e.Current;
            Assert.Equal(2, g3.Key);
            var g3e = g3.GetAsyncEnumerator();
            HasNext(g3e, 'c');
            HasNext(g3e, 'f');
            HasNext(g3e, 'i');
            NoNext(g3e);
            await g3e.DisposeAsync();

            NoNext(e);

            await e.DisposeAsync();
        }

        [Fact]
        public async Task GroupBy17()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, x => (char)('a' + x), new EqMod(3));

            var e = ys.GetAsyncEnumerator();
            await e.DisposeAsync();

            Assert.False(e.MoveNextAsync().Result);
        }

        [Fact]
        public async Task GroupBy18()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, x => (char)('a' + x), new EqMod(3));

            var e = ys.GetAsyncEnumerator();

            Assert.True(e.MoveNextAsync().Result);
            var g1 = e.Current;
            Assert.Equal(0, g1.Key);
            var g1e = g1.GetAsyncEnumerator();
            HasNext(g1e, 'a');

            await e.DisposeAsync();

            HasNext(g1e, 'd');
            HasNext(g1e, 'g');
            HasNext(g1e, 'j');
            NoNext(g1e);
            await g1e.DisposeAsync();

            Assert.False(e.MoveNextAsync().Result);
        }

        [Fact]
        public async Task GroupBy19()
        {
            // We're using Kvp here because the types will eval as equal for this test
            var xs = new[]
            {
                new Kvp("Bart", 27),
                new Kvp("John", 62),
                new Kvp("Eric", 27),
                new Kvp("Lisa", 14),
                new Kvp("Brad", 27),
                new Kvp("Lisa", 23),
                new Kvp("Eric", 42)
            };

            var ys = xs.ToAsyncEnumerable();

            var res = ys.GroupBy(x => x.Item / 10);

            await SequenceIdentity(res);
        }


        [Fact]
        public async Task GroupBy20()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x), (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result);

            var arr = new[] { "0 - adgj", "1 - beh", "2 - cfi" };

            Assert.Equal(arr, await ys.ToArray());
        }

        [Fact]
        public async Task GroupBy21()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x), (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result);

            var arr = new List<string> { "0 - adgj", "1 - beh", "2 - cfi" };

            Assert.Equal(arr, await ys.ToList());
        }

        [Fact]
        public async Task GroupBy22()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x), (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result);

            Assert.Equal(3, await ys.Count());
        }

        [Fact]
        public async Task GroupBy23()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x), (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result);

            await SequenceIdentity(ys);
        }

        [Fact]
        public async Task GroupBy24()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x));

            var g1a = new[] { 'a', 'd', 'g', 'j' };
            var g2a = new[] { 'b', 'e', 'h' };
            var g3a = new[] { 'c', 'f', 'i' };

            var gar = await ys.ToArray();

            Assert.Equal(3, gar.Length);

            var gg1 = gar[0];
            var gg1a = await gg1.ToArray();

            Assert.Equal(g1a, gg1a);

            var gg2 = gar[1];
            var gg2a = await gg2.ToArray();

            Assert.Equal(g2a, gg2a);

            var gg3 = gar[2];
            var gg3a = await gg3.ToArray();
            Assert.Equal(g3a, gg3a);
        }

        [Fact]
        public async Task GroupBy25()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x));

            var g1a = new List<char> { 'a', 'd', 'g', 'j' };
            var g2a = new List<char> { 'b', 'e', 'h' };
            var g3a = new List<char> { 'c', 'f', 'i' };

            var gar = await ys.ToList();

            Assert.Equal(3, gar.Count);

            var gg1 = gar[0];
            var gg1a = await gg1.ToList();
            Assert.Equal(g1a, gg1a);

            var gg2 = gar[1];
            var gg2a = await gg2.ToList();
            Assert.Equal(g2a, gg2a);

            var gg3 = gar[2];
            var gg3a = await gg3.ToList();
            Assert.Equal(g3a, gg3a);
        }

        [Fact]
        public async Task GroupBy26()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x));

            var gar = await ys.ToList();

            Assert.Equal(3, gar.Count);

            var gg1 = gar[0];
            var gg1a = await gg1.Count();
            Assert.Equal(4, gg1a);

            var gg2 = gar[1];
            var gg2a = await gg2.Count();
            Assert.Equal(3, gg2a);

            var gg3 = gar[2];
            var gg3a = await gg3.Count();
            Assert.Equal(3, gg3a);
        }

        [Fact]
        public async Task GroupBy27()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x));

            var gar = await ys.Count();

            Assert.Equal(3, gar);
        }

        [Fact]
        public async Task GroupBy28()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x % 3, x => (char)('a' + x));

            await SequenceIdentity(ys);
        }

        [Fact]
        public async Task GroupBy29()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, new EqMod(3));

            var g1a = new List<int> { 0, 3, 6, 9 };
            var g2a = new List<int> { 1, 4, 7 };
            var g3a = new List<int> { 2, 5, 8 };

            var gar = await ys.ToList();

            Assert.Equal(3, gar.Count);

            var gg1 = gar[0];
            var gg1a = await gg1.ToList();
            Assert.Equal(g1a, gg1a);

            var gg2 = gar[1];
            var gg2a = await gg2.ToList();
            Assert.Equal(g2a, gg2a);

            var gg3 = gar[2];
            var gg3a = await gg3.ToList();
            Assert.Equal(g3a, gg3a);
        }

        [Fact]
        public async Task GroupBy30()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, new EqMod(3));


            var gar = await ys.ToList();

            Assert.Equal(3, gar.Count);

            var gg1 = gar[0];
            var gg1a = await gg1.Count();
            Assert.Equal(4, gg1a);

            var gg2 = gar[1];
            var gg2a = await gg2.Count();
            Assert.Equal(3, gg2a);

            var gg3 = gar[2];
            var gg3a = await gg3.Count();
            Assert.Equal(3, gg3a);
        }

        [Fact]
        public async Task GroupBy31()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, new EqMod(3));

            var g1a = new[] { 0, 3, 6, 9 };
            var g2a = new[] { 1, 4, 7 };
            var g3a = new[] { 2, 5, 8 };

            var gar = await ys.ToArray();

            Assert.Equal(3, gar.Length);

            var gg1 = gar[0];
            var gg1a = await gg1.ToArray();

            Assert.Equal(g1a, gg1a);

            var gg2 = gar[1];
            var gg2a = await gg2.ToArray();

            Assert.Equal(g2a, gg2a);

            var gg3 = gar[2];
            var gg3a = await gg3.ToArray();
            Assert.Equal(g3a, gg3a);
        }

        [Fact]
        public async Task GroupBy32()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, new EqMod(3));

            var gar = await ys.Count();

            Assert.Equal(3, gar);
        }

        [Fact]
        public async Task GroupBy33()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, new EqMod(3));

            await SequenceIdentity(ys);
        }

        [Fact]
        public async Task GroupBy34()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result, new EqMod(3));

            var arr = new[] { "0 - 0369", "1 - 147", "2 - 258" };

            Assert.Equal(arr, await ys.ToArray());
        }

        [Fact]
        public async Task GroupBy35()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result, new EqMod(3));

            var arr = new List<string> { "0 - 0369", "1 - 147", "2 - 258" };

            Assert.Equal(arr, await ys.ToList());
        }

        [Fact]
        public async Task GroupBy36()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result, new EqMod(3));

            Assert.Equal(3, await ys.Count());
        }

        [Fact]
        public async Task GroupBy37()
        {
            var xs = AsyncEnumerable.Range(0, 10);
            var ys = xs.GroupBy(x => x, (k, cs) => k + " - " + cs.Aggregate("", (a, c) => a + c).Result, new EqMod(3));

            await SequenceIdentity(ys);
        }

        private sealed class EqMod : IEqualityComparer<int>
        {
            private readonly int _d;

            public EqMod(int d)
            {
                _d = d;
            }

            public bool Equals(int x, int y)
            {
                return EqualityComparer<int>.Default.Equals(x % _d, y % _d);
            }

            public int GetHashCode(int obj)
            {
                return EqualityComparer<int>.Default.GetHashCode(obj % _d);
            }
        }

        private sealed class Kvp : IEquatable<Kvp>
        {
            public Kvp(string key, int item)
            {
                Key = key;
                Item = item;
            }

            public string Key { get; }
            public int Item { get; }

            public bool Equals(Kvp other)
            {
                if (ReferenceEquals(null, other)) return false;
                if (ReferenceEquals(this, other)) return true;
                return string.Equals(Key, other.Key) && Item == other.Item;
            }

            public override bool Equals(object obj)
            {
                if (ReferenceEquals(null, obj)) return false;
                if (ReferenceEquals(this, obj)) return true;
                if (obj.GetType() != GetType()) return false;
                return Equals((Kvp)obj);
            }

            public override int GetHashCode()
            {
                unchecked
                {
                    return ((Key != null ? Key.GetHashCode() : 0) * 397) ^ Item;
                }
            }

            public static bool operator ==(Kvp left, Kvp right)
            {
                return Equals(left, right);
            }

            public static bool operator !=(Kvp left, Kvp right)
            {
                return !Equals(left, right);
            }
        }
    }
}