Ensure no duplicate mutants are given

Author: danielctull

Package.resolved

@@ -11,6 +11,7 @@ let package = Package(
     .executable(name: "swift-mutation-testing", targets: ["MutationTesting"]),
   ],
   dependencies: [
+    .package(url: "https://github.com/apple/swift-algorithms.git", from: "1.2.0"),
     .package(url: "https://github.com/apple/swift-argument-parser.git", from: "1.6.0"),
     .package(url: "https://github.com/swiftlang/swift-subprocess.git", branch: "main"),
     .package(url: "https://github.com/swiftlang/swift-syntax.git", from: "601.0.0"),
@@ -31,6 +32,7 @@ let package = Package(
       name: "CoreMutation",
       dependencies: [
         "PackageKit",
+        .product(name: "Algorithms", package: "swift-algorithms"),
       ],
     ),
 

Package.swift

@@ -2,28 +2,8 @@ import Foundation
 import PackageKit
 
 public struct Mutant {
-
   public let mutation: Mutation.Name
   public let location: Source.Location
   public let original: Source.Code
-  private let mutate: () -> Source.Code
-
-  public init(
-    mutation: Mutation.Name,
-    location: Source.Location,
-    original: Source.Code,
-    mutate: @escaping () -> Source.Code
-  ) {
-    self.mutation = mutation
-    self.location = location
-    self.original = original
-    self.mutate = mutate
-  }
-}
-
-extension Mutant {
-
-  public var replacement: Source.Code {
-    mutate()
-  }
+  public let replacement: Source.Code
 }

Sources/CoreMutation/Mutant.swift

@@ -1,3 +1,4 @@
+import Algorithms
 import PackageKit
 
 public struct Mutation: Sendable {
@@ -17,19 +18,24 @@ public struct Mutation: Sendable {
 extension Mutation {
 
   public func mutants(for file: Source.File) -> [Mutant] {
-    changes(file).map {
-      Mutant(
-        mutation: name,
-        location: Source.Location(
-          name: file.name,
-          path: file.path,
-          start: $0.start,
-          end: $0.end,
-        ),
-        original: file.code,
-        mutate: $0.mutate
-      )
-    }
+
+    let mutants = changes(file)
+      .map {
+        Mutant(
+          mutation: name,
+          location: Source.Location(
+            name: file.name,
+            path: file.path,
+            start: $0.start,
+            end: $0.end,
+          ),
+          original: file.code,
+          replacement: $0.mutate()
+        )
+      }
+      .uniqued(on: \.replacement)
+
+    return Array(mutants)
   }
 }
 

Sources/CoreMutation/Mutation.swift

@@ -55,7 +55,7 @@ extension Source.Path: ExpressibleByStringLiteral {
 // MARK: - Source.Code
 
 extension Source {
-  public struct Code: Equatable, Sendable {
+  public struct Code: Equatable, Hashable, Sendable {
     fileprivate let data: Data
     public init(data: Data) {
       self.data = data

Sources/PackageKit/Source.swift

@@ -37,4 +37,37 @@ struct SyntaxMutationTests {
       let name = "Daniel"
       """)
   }
+
+  @Test("duplicate mutants are removed")
+  func duplicates() throws {
+
+    final class Visitor: MutationVisitor {
+
+      override func visit(_ node: CodeBlockItemListSyntax) -> SyntaxVisitorContinueKind {
+        record(before: node, after: ExprSyntax("""
+          let name = "Daniel"
+          """))
+        record(before: node, after: ExprSyntax("""
+          let name = "Daniel"
+          """))
+        return super.visit(node)
+      }
+    }
+
+    let mutation = Mutation(name: "Replace File", visitor: Visitor.self)
+    let file = Source.File(name: "name", path: "path", code: "")
+
+    let mutants = mutation.mutants(for: file)
+
+    try #require(mutants.count == 1)
+    #expect(mutants[0].original == file.code)
+    #expect(mutants[0].mutation == mutation.name)
+    #expect(mutants[0].location.name == file.name)
+    #expect(mutants[0].location.path == file.path)
+    #expect(mutants[0].location.start == Source.Position(line: 1, column: 1, offset: 0))
+    #expect(mutants[0].location.end == Source.Position(line: 1, column: 1, offset: 0))
+    #expect(mutants[0].replacement == """
+      let name = "Daniel"
+      """)
+  }
 }