Add tests for out of bounds error

Differ.xcodeproj/project.pbxproj

@@ -7,6 +7,7 @@
 	objects = {
 
 /* Begin PBXBuildFile section */
+		6F5641F027B09338009F0F6F /* ExtendedPatchTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 6F5641EF27B09338009F0F6F /* ExtendedPatchTests.swift */; };
 		900E03A41DE7C6C60033A799 /* Differ.h in Headers */ = {isa = PBXBuildFile; fileRef = C92178BB1CD0023E004642C7 /* Differ.h */; settings = {ATTRIBUTES = (Public, ); }; };
 		900E03A51DE7C6D60033A799 /* Differ.framework in Frameworks */ = {isa = PBXBuildFile; fileRef = C9EE87161CCFCA83006BD90E /* Differ.framework */; };
 		90382F122075D23E006EFFBF /* NestedBatchUpdate.swift in Sources */ = {isa = PBXBuildFile; fileRef = 90382F112075D23E006EFFBF /* NestedBatchUpdate.swift */; };
@@ -44,6 +45,7 @@
 /* End PBXContainerItemProxy section */
 
 /* Begin PBXFileReference section */
+		6F5641EF27B09338009F0F6F /* ExtendedPatchTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ExtendedPatchTests.swift; sourceTree = "<group>"; };
 		900E039F1DE7C3370033A799 /* Universal-Framework-Target.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; path = "Universal-Framework-Target.xcconfig"; sourceTree = "<group>"; };
 		900E03A01DE7C3370033A799 /* Universal-Target-Base.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; path = "Universal-Target-Base.xcconfig"; sourceTree = "<group>"; };
 		900E03AF1DE7F1E80033A799 /* Deployment-Targets.xcconfig */ = {isa = PBXFileReference; lastKnownFileType = text.xcconfig; path = "Deployment-Targets.xcconfig"; sourceTree = "<group>"; };
@@ -138,6 +140,7 @@
 				C921BF701E15448300747566 /* NestedExtendedDiffTests.swift */,
 				C991189D1DDB4F1100067A60 /* PatchApplyTests.swift */,
 				C9C924791DBB97130006ACC4 /* PatchSortTests.swift */,
+				6F5641EF27B09338009F0F6F /* ExtendedPatchTests.swift */,
 			);
 			path = DifferTests;
 			sourceTree = "<group>";
@@ -279,6 +282,7 @@
 				C921BF711E15448300747566 /* NestedExtendedDiffTests.swift in Sources */,
 				C90CD7191DFB43C600BE9114 /* NestedDiffTests.swift in Sources */,
 				E2F1C8561E3A14DF00FBE786 /* BatchUpdateTests.swift in Sources */,
+				6F5641F027B09338009F0F6F /* ExtendedPatchTests.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};

Tests/DifferTests/ExtendedPatchTests.swift

@@ -0,0 +1,77 @@
+import XCTest
+import Differ
+
+class ExtendedPatchTests: XCTestCase {
+    func testDifferScenario1() throws {
+        let input  = [1, 3, 4]
+        let output = [1, 2, 3, 4]
+        let result = runDiffer(start: input, end: output)
+        XCTAssertEqual(output, result)
+    }
+
+    func testDifferScenario2() throws {
+        let input  = [1, 2, 3, 4]
+        let output = [1, 3, 4]
+        let result = runDiffer(start: input, end: output)
+        XCTAssertEqual(output, result)
+    }
+
+    func testDifferScenario3() throws {
+        let input  = [1, 2, 3, 4]
+        let output = [4, 3, 2, 1]
+        let result = runDiffer(start: input, end: output)
+        XCTAssertEqual(output, result)
+    }
+
+    /// This fails: Thread 1: Fatal error: Index out of range
+    /// MOVE number from 8 4 [4, 4, 3, 8, 2, 5, 6, 9]
+    /// The array is only 8 elements long, not 9
+    func testDifferScenarioFailure() throws {
+        let input  = [7, 2, 5, 6, 9]
+        let output = [4, 4, 3, 8, 9, 5, 2, 5]
+        let result = runDiffer(start: input, end: output)
+        XCTAssertEqual(output, result)
+    }
+
+    /// This is another example of a failed scenario
+    func testDifferScenarioFailure_alternative() throws {
+        let input  = [9, 0, 10, 5]
+        let output = [1, 2, 7, 5, 6, 5, 10, 6, 9, 10]
+        let result = runDiffer(start: input, end: output)
+        XCTAssertEqual(output, result)
+    }
+
+    func testDifferScenario5() throws {
+        let input  = [7, 2, 5, 6, 9]
+        let output = [4, 4, 3, 8, 9, 5, 2, 5]
+        let result = runDiffer(start: input, end: output)
+        XCTAssertEqual(output, result)
+    }
+}
+
+private func runDiffer<T: Equatable>(start: [T], end: [T]) -> [T] {
+    let patches = Differ.extendedPatch(from: start, to: end)
+    var workingSet = start
+
+    for patch in patches {
+        switch patch {
+        case .insertion(index: let index, element: let element):
+            print("INSERT \(element) at", index)
+            workingSet.insert(element, at: index)
+            print(workingSet)
+        case .deletion(index: let index):
+            print("REMOVE \(workingSet[index]) at", index)
+            workingSet.remove(at: index)
+            print(workingSet)
+        case .move(from: let from, to: let to):
+            print("MOVE number from", from, to, workingSet)
+            let val = workingSet[from]
+            print(val)
+            workingSet.remove(at: from)
+            workingSet.insert(val, at: to)
+            print(workingSet)
+        }
+    }
+
+    return workingSet
+}