cscheck_sketch.cs

using System;
using System.CodeDom.Compiler;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
using Xunit;
using Xunit.Abstractions;
using Xunit.Sdk;

namespace AutoData
{
    [AttributeUsage(AttributeTargets.Method)]
    [XunitTestCaseDiscoverer("AutoData.PropertyTestCaseDiscoverer", "AutoData")]
    public sealed class PropertyAttribute : FactAttribute
    {
        public uint Iterations { get; set; } = 1000;

        public bool Exhaustive { get; set; }
    }

    public struct TestSettings
    {
        public TestSettings
            ( uint iterations
            , bool exhaustive
            )
        {
            Iterations = iterations;
            Exhaustive = exhaustive;
        }

        public uint Iterations { get; }

        public bool Exhaustive { get; }

        public void Serialize(IXunitSerializationInfo info)
        {
            info.AddValue("TestSettings.Iterations", Iterations);
            info.AddValue("TestSettings.Exhaustive", Exhaustive);
        }

        public static TestSettings Deserialize(IXunitSerializationInfo info)
        {
            return new TestSettings(
                info.GetValue<uint>("TestSettings.Iterations"),
                info.GetValue<bool>("TestSettings.Exhaustive")
                );
        }
    }

    public class PropertyTestCaseDiscoverer : IXunitTestCaseDiscoverer
    {

        private readonly IMessageSink _diagnosticMessageSink;

        private static IEnumerable<IEnumerable<T>> CartesianProduct<T>(IEnumerable<IEnumerable<T>> sequences)
        {
            IEnumerable<IEnumerable<T>> emptyProduct = new[] { Enumerable.Empty<T>() };
            return sequences.Aggregate(
                emptyProduct,
                (accumulator, sequence) =>
                    from accseq in accumulator
                    from item in sequence
                    select accseq.Concat(new[] { item })
                );
        }

        internal static Generator GetGenerator(ITypeInfo t)
        {
            var generator = GeneratorProvider.InstanceForType(t.ToRuntimeType());
            if (generator != null)
            {
                return generator;
            }

            throw new NotSupportedException($"No generator for type: {t}");
        }

        public PropertyTestCaseDiscoverer(IMessageSink diagnosticMessageSink)
        {
            _diagnosticMessageSink = diagnosticMessageSink;
        }

        public IEnumerable<IXunitTestCase> Discover(
            ITestFrameworkDiscoveryOptions discoveryOptions,
            ITestMethod testMethod,
            IAttributeInfo factAttribute)
        {
            var settings = GetTestSettings(factAttribute);

            List<Generator> generators = null;
            Exception ex = null;
            try
            {
                generators =
                    testMethod.Method.GetParameters()
                    .Select(p => GetGenerator(p.ParameterType))
                    .ToList();
            }
            catch (Exception e)
            {
                ex = e;
                // throwing here yields un-nice behaviour,
                // so we return a test that fails instead
            }

            if (ex != null)
            {
                //yield return new Whatever();
                yield break;
            }

            if (settings.Exhaustive || CalculatePotentialCombinations(generators) <= settings.Iterations)
            {
                // generate test cases up-front
                foreach (var row in CartesianProduct(generators.Select(g => g.AllObjects())))
                {
                    yield return
                        new XunitTestCase(
                            _diagnosticMessageSink,
                            discoveryOptions.MethodDisplayOrDefault(),
                            testMethod,
                            row.ToArray());
                }
            }
            else
            {
                // otherwise we will generate test cases later
                yield return
                    new PropertyTestCase(
                        _diagnosticMessageSink,
                        discoveryOptions.MethodDisplayOrDefault(),
                        testMethod,
                        generators,
                        settings);
            }
        }

        private static TestSettings GetTestSettings(IAttributeInfo factAttribute)
        {
            var iterations = factAttribute.GetNamedArgument<uint>("Iterations");
            var exhaustive = factAttribute.GetNamedArgument<bool>("Exhaustive");

            return new TestSettings(iterations, exhaustive);
        }

        private static long? CalculatePotentialCombinations(List<Generator> generators)
            => generators.Aggregate((long?)1L, (product, g) => g.PotentialValues() * product);
    }

    public class PropertyTestCase : XunitTestCase
    {
        private TestSettings _settings;
        private IReadOnlyList<Generator> _generators;

        [Obsolete("Only for serialization")]
        public PropertyTestCase()
        { }

        public PropertyTestCase
            ( IMessageSink diagnosticMessageSink
            , TestMethodDisplay defaultMethodDisplay
            , ITestMethod testMethod
            , IReadOnlyList<Generator> generators
            , TestSettings settings
            )
            : base
            ( diagnosticMessageSink
            , defaultMethodDisplay
            , testMethod
            )
        {
            _settings = settings;
            _generators = generators;
            // technically we should copy here but we control the callers
        }

        public override async Task<RunSummary> RunAsync(
            IMessageSink diagnosticMessageSink,
            IMessageBus messageBus,
            object[] constructorArguments,
            ExceptionAggregator aggregator,
            CancellationTokenSource cancellationTokenSource)
        {
            // we don't serialize this
            if (_generators == null)
            {
                _generators =
                    TestMethod.Method.GetParameters()
                    .Select(p => p.ParameterType.ToRuntimeType())
                    .Select(GeneratorProvider.InstanceForType)
                    .ToList();
            }

            TestMethodArguments = new object[_generators.Count];
            try
            {
                var r = new Random();
                var summary = new RunSummary();

                for (int i = 0; i < _settings.Iterations; ++i)
                {
                    FillTestMethodArguments(r);

                    var tSummary = await base.RunAsync(
                        diagnosticMessageSink,
                        messageBus,
                        constructorArguments,
                        aggregator,
                        cancellationTokenSource);

                    summary.Aggregate(tSummary);

                    if (summary.Failed > 0)
                    {
                        // short-circuit failure
                        break;
                    }
                }

                return summary;
            }
            finally
            {
                TestMethodArguments = null;
            }
        }

        private void FillTestMethodArguments(Random r)
        {
            for (int j = 0; j < _generators.Count; ++j)
            {
                TestMethodArguments[j] = _generators[j].GenerateObject(r);
            }
        }

        public override void Deserialize(IXunitSerializationInfo data)
        {
            base.Deserialize(data);

            _settings = TestSettings.Deserialize(data);
        }

        public override void Serialize(IXunitSerializationInfo data)
        {
            base.Serialize(data);

            _settings.Serialize(data);
        }
    }

    abstract class GeneratorProvider
    {
        public abstract Generator ForType(Type type);

        public static GeneratorProvider operator |(GeneratorProvider left, GeneratorProvider right)
            => left.OrFrom(right);

        // hmmmmmmmm

        private static readonly GeneratorProvider Instance =
            new BuiltinGeneratorProvider() | new EnumGeneratorProvider();

        public static Generator InstanceForType(Type type)
            => Instance.ForType(type);
    }

    class BuiltinGeneratorProvider : GeneratorProvider
    {
        private static readonly Dictionary<Type, Generator> Generators =
            new Dictionary<Type, Generator>
            {
                { typeof(bool), new BoolGenerator() },
                { typeof(int), new IntGenerator() },
            };

        public override Generator ForType(Type type)
        {
            Generator generator;
            Generators.TryGetValue(type, out generator);
            return generator;
        }
    }

    class EnumGeneratorProvider : GeneratorProvider
    {
        public override Generator ForType(Type type)
        {
            if (type.IsEnum)
            {
                return new EnumGenerator(type);
            }

            return null;
        }
    }

    internal static class GeneratorProviderExtensions
    {
        public static GeneratorProvider OrFrom(this GeneratorProvider left, GeneratorProvider right)
            => new Combined(left, right);

        private class Combined : GeneratorProvider
        {
            private readonly GeneratorProvider _left;
            private readonly GeneratorProvider _right;

            public Combined(GeneratorProvider left, GeneratorProvider right)
            {
                _left = left;
                _right = right;
            }

            public override Generator ForType(Type type)
                => _left.ForType(type) ?? _right.ForType(type);
        }
    }

    public abstract class Generator
    {
        public abstract object GenerateObject(Random random);

        public abstract int? PotentialValues();

        public abstract IEnumerable<object> AllObjects();

        public abstract IEnumerable<object> ShrinkObject(object o);
    }

    public abstract class Generator<T> : Generator
    {
        public sealed override object GenerateObject(Random random)
            => Generate(random);

        public sealed override IEnumerable<object> AllObjects()
            => AllValues().Cast<object>();

        public sealed override IEnumerable<object> ShrinkObject(object o)
            => Shrink((T)o).Cast<object>();

        protected abstract T Generate(Random random);

        protected abstract IEnumerable<T> Shrink(T input); 

        protected abstract IEnumerable<T> AllValues();

    }

    public sealed class BoolGenerator : Generator<bool>
    {
        public override int? PotentialValues() => 2;

        protected override bool Generate(Random random)
            => random.Next(2) == 0;

        protected override IEnumerable<bool> Shrink(bool input)
        {
            if (input)
            {
                yield return false;
            }
        }

        protected override IEnumerable<bool> AllValues()
        {
            yield return false;
            yield return true;
        }
    }

    public sealed class EnumGenerator : Generator
    {
        private readonly object[] _values;

        public EnumGenerator(Type type)
        {
            _values = type.GetEnumValues().Cast<object>().ToArray();
            if (_values.Length == 0)
            {
                // UHOH
            }
        }

        public override object GenerateObject(Random random)
        {
            return _values[random.Next(0, _values.Length)];
        }

        public override int? PotentialValues() => _values.Length;

        public override IEnumerable<object> AllObjects() => _values;

        public override IEnumerable<object> ShrinkObject(object o)
        {
            var ix = Array.IndexOf(_values, o);
            if (ix > 0)
            {
                yield return _values[ix - 1];
            }
        }
    }

    public sealed class IntGenerator : Generator<int>
    {
        public override int? PotentialValues() => null;

        protected override int Generate(Random random) => random.Next();

        protected override IEnumerable<int> Shrink(int input)
        {
            // shrink in magnitude
            if (input > 0)
            {
                yield return input - 1;
            }
            else if (input < 0)
            {
                yield return input + 1;
            }
        }

        protected override IEnumerable<int> AllValues()
        {
            throw new NotSupportedException();
        }
    }

    public class Tests
    {
        [Property]
        public void DeMorgans(bool x, bool y)
        {
            Assert.Equal(!(x && y), !x || !y);
        }

        [Property]
        public void DeMorgans2(bool x, bool y)
        {
            Assert.Equal(!(x || y), !x && !y);
        }

        [Property]
        public void EqualInts(int x, int y)
        {
            Assert.Equal(x, y);
        }
    }
}