Add new AddContainingCharacterMutator(#411)

AddContainingCharacterMutator helps with scenarios of going from squat candidate, to popular package name, and then confirming that said squat candidate is a squat on the popular package.

src/oss-find-squats-lib/MutateExtension.cs

@@ -25,6 +25,7 @@ public static class MutateExtension
         internal static IEnumerable<IMutator> BaseMutators { get; } = new List<IMutator>()
         {
             new AddCharacterMutator(),
+            new AddContainingCharacterMutator(),
             new AsciiHomoglyphMutator(),
             new BitFlipMutator(),
             new CloseLettersMutator(),

src/oss-find-squats-lib/Mutators/AddContainingCharacterMutator.cs

@@ -0,0 +1,38 @@
+// Copyright (c) Microsoft Corporation. Licensed under the MIT License.
+
+namespace Microsoft.CST.OpenSource.FindSquats.Mutators
+{
+    using System.Collections.Generic;
+    using System.Linq;
+
+    /// <summary>
+    /// Generates mutations for adding a character in the string, where the character being added already exists in the string.
+    /// </summary>
+    /// <example>requests -> reuquests. But won't generate something like requests -> regquests</example>
+    public class AddContainingCharacterMutator : IMutator
+    {
+        public MutatorType Kind { get; } = MutatorType.AddContainingCharacter;
+
+        public IEnumerable<Mutation> Generate(string arg)
+        {
+            var chars = arg.Distinct().ToArray();
+            for (int i = 0; i < arg.Length+1; i++)
+            {
+                // Can't add a character before an @ for a scoped package.
+                if (i == 0 && arg[i] == '@')
+                {
+                    continue;
+                }
+
+                foreach (var c in chars)
+                {
+                    yield return new Mutation(
+                        mutated: $"{arg[..i]}{c}{arg[i..]}",
+                        original: arg,
+                        mutator: Kind,
+                        reason: $"Containing Character Added: {c}");
+                }
+            }
+        }
+    }
+}

src/oss-find-squats-lib/Mutators/MutatorType.cs

@@ -20,6 +20,10 @@ public enum MutatorType
         /// </summary>
         AddCharacter,
         /// <summary>
+        /// The <see cref="AddContainingCharacterMutator"/> mutator.
+        /// </summary>
+        AddContainingCharacter,
+        /// <summary>
         /// The <see cref="AsciiHomoglyphMutator"/> mutator.
         /// </summary>
         AsciiHomoglyph,

src/oss-tests/FindSquatsTests.cs

@@ -565,6 +565,41 @@ public async Task NewtonsoftMutations_Succeeds_Async()
             CollectionAssert.AreEquivalent(squattingPackages, resultingMutationNames);
         }
 
+        [TestMethod]
+        public async Task RequestsMutations_Succeeds_Async()
+        {
+            // arrange
+            PackageURL requests = new("pkg:pypi/[email protected]");
+
+            string[] squattingPackages = new[]
+            {
+                "pkg:pypi/reuquests", // AddContainingCharacter
+                "pkg:pypi/requestss", // DoubleHit
+                "pkg:pypi/reqests", // RemovedCharacter
+                "pkg:pypi/request", // RemovedCharacter
+                "pkg:pypi/requets", // RemovedCharacter
+            };
+
+            IHttpClientFactory httpClientFactory =
+                FindSquatsHelper.SetupHttpCalls(purl: requests, validSquats: squattingPackages);
+
+            IManagerPackageActions<NuGetPackageVersionMetadata> packageActions = PackageActionsHelper<NuGetPackageVersionMetadata>.SetupPackageActions(requests, validSquats: squattingPackages) ?? throw new InvalidOperationException();
+            Dictionary<string, ProjectManagerFactory.ConstructProjectManager> overrideDict = ProjectManagerFactory.GetDefaultManagers(httpClientFactory);
+
+            overrideDict[NuGetProjectManager.Type] = directory =>
+                new NuGetProjectManager(directory, packageActions, httpClientFactory);
+
+            FindPackageSquats findPackageSquats = new(new ProjectManagerFactory(overrideDict), requests);
+
+            // act
+            IDictionary<string, IList<Mutation>>? squatCandidates = findPackageSquats.GenerateSquatCandidates();
+            List<FindPackageSquatResult> existingMutations = await findPackageSquats.FindExistingSquatsAsync(squatCandidates, new MutateOptions(){UseCache = false}).ToListAsync();
+            Assert.IsNotNull(existingMutations);
+            Assert.IsTrue(existingMutations.Any());
+            string[] resultingMutationNames = existingMutations.Select(m => m.MutatedPackageUrl.ToString()).ToArray();
+            CollectionAssert.AreEquivalent(squattingPackages, resultingMutationNames);
+        }
+
         [TestMethod]
         public async Task ScopedPackage_Succeeds_Async()
         {