1 files changed, 163 insertions, 0 deletions
diff --git a/DomainDigTests/DiffServiceTests.swift b/DomainDigTests/DiffServiceTests.swift
new file mode 100644
index 0000000..a3a367c
--- /dev/null
+++ b/DomainDigTests/DiffServiceTests.swift
@@ -0,0 +1,163 @@
+import XCTest
+@testable import DomainDig
+
+/// Characterization tests for the field-level diff engine. `DiffService` is a
+/// pure function over two `DomainReport`s, so every case here pins observable
+/// behavior (change classification, normalization, summary phrasing) against a
+/// deterministic fixture.
+final class DiffServiceTests: XCTestCase {
+
+    func testIdenticalReportsProduceNoChanges() {
+        let report = SnapshotFixture.report(
+            availability: .registered,
+            ownership: DomainOwnership(registrar: "Example Registrar")
+        )
+
+        let diff = DiffService.compare(from: report, to: report)
+
+        XCTAssertEqual(diff.changeCount, 0)
+        XCTAssertTrue(diff.changedSectionTitles.isEmpty)
+        XCTAssertFalse(diff.sections.contains { $0.hasChanges })
+    }
+
+    func testAvailabilityTransitionIsReportedAsChanged() {
+        let old = SnapshotFixture.report(availability: .available)
+        let new = SnapshotFixture.report(availability: .registered)
+
+        let diff = DiffService.compare(from: old, to: new)
+
+        let availability = try? XCTUnwrap(diff.sections.first { $0.id == "availability" })
+        let item = availability?.items.first { $0.id == "availability" }
+        XCTAssertEqual(item?.changeType, .changed)
+        XCTAssertEqual(item?.oldValue, "Available")
+        XCTAssertEqual(item?.newValue, "Registered")
+        XCTAssertTrue(diff.changedSectionTitles.contains("Domain / Availability"))
+    }
+
+    func testPrimaryIPChangeSurfacesInAvailabilitySection() {
+        let old = SnapshotFixture.report(
+            availability: .registered,
+            dnsSections: [SnapshotFixture.dnsSection(type: .A, values: ["203.0.113.10"])]
+        )
+        let new = SnapshotFixture.report(
+            availability: .registered,
+            dnsSections: [SnapshotFixture.dnsSection(type: .A, values: ["203.0.113.20"])]
+        )
+
+        let diff = DiffService.compare(from: old, to: new)
+        let item = diff.sections
+            .first { $0.id == "availability" }?
+            .items.first { $0.id == "primary-ip" }
+
+        XCTAssertEqual(item?.changeType, .changed)
+        XCTAssertEqual(item?.oldValue, "203.0.113.10")
+        XCTAssertEqual(item?.newValue, "203.0.113.20")
+    }
+
+    func testCaseAndWhitespaceDifferencesAreNotChanges() {
+        let old = SnapshotFixture.report(ownership: DomainOwnership(registrar: "GoDaddy"))
+        let new = SnapshotFixture.report(ownership: DomainOwnership(registrar: "  godaddy  "))
+
+        let diff = DiffService.compare(from: old, to: new)
+        let item = diff.sections
+            .first { $0.id == "ownership" }?
+            .items.first { $0.id == "registrar" }
+
+        XCTAssertEqual(item?.changeType, .unchanged, "case- and whitespace-only differences must not register as changes")
+    }
+
+    func testAddedAndRemovedOwnershipFields() {
+        let absent = SnapshotFixture.report(ownership: nil)
+        let present = SnapshotFixture.report(ownership: DomainOwnership(registrar: "Example Registrar"))
+
+        let added = DiffService.compare(from: absent, to: present)
+            .sections.first { $0.id == "ownership" }?
+            .items.first { $0.id == "registrar" }
+        XCTAssertEqual(added?.changeType, .added)
+        XCTAssertNil(added?.oldValue)
+        XCTAssertEqual(added?.newValue, "Example Registrar")
+
+        let removed = DiffService.compare(from: present, to: absent)
+            .sections.first { $0.id == "ownership" }?
+            .items.first { $0.id == "registrar" }
+        XCTAssertEqual(removed?.changeType, .removed)
+        XCTAssertEqual(removed?.oldValue, "Example Registrar")
+        XCTAssertNil(removed?.newValue)
+    }
+
+    func testDNSRecordValueChangeIsDetected() {
+        let old = SnapshotFixture.report(
+            dnsSections: [SnapshotFixture.dnsSection(type: .NS, values: ["ns1.example.com", "ns2.example.com"])]
+        )
+        let new = SnapshotFixture.report(
+            dnsSections: [SnapshotFixture.dnsSection(type: .NS, values: ["ns1.example.com", "ns3.example.com"])]
+        )
+
+        let dns = DiffService.compare(from: old, to: new).sections.first { $0.id == "dns" }
+        let records = dns?.items.first { $0.id == "dns-ns-records" }
+        XCTAssertEqual(records?.changeType, .changed)
+    }
+
+    func testDNSRecordReorderIsNotAChange() {
+        // Values are normalized (sorted, lowercased) before comparison, so a pure
+        // reorder must diff as unchanged.
+        let old = SnapshotFixture.report(
+            dnsSections: [SnapshotFixture.dnsSection(type: .NS, values: ["ns1.example.com", "ns2.example.com"])]
+        )
+        let new = SnapshotFixture.report(
+            dnsSections: [SnapshotFixture.dnsSection(type: .NS, values: ["NS2.example.com", "NS1.example.com"])]
+        )
+
+        let records = DiffService.compare(from: old, to: new)
+            .sections.first { $0.id == "dns" }?
+            .items.first { $0.id == "dns-ns-records" }
+        XCTAssertEqual(records?.changeType, .unchanged)
+    }
+
+    func testSummaryMessagePhrasing() {
+        XCTAssertEqual(DiffService.summaryMessage(from: [], changeCount: 0), "No meaningful changes")
+        XCTAssertEqual(DiffService.summaryMessage(from: ["DNS"], changeCount: 1), "DNS changed")
+        XCTAssertEqual(
+            DiffService.summaryMessage(from: ["DNS", "Ownership"], changeCount: 3),
+            "DNS and ownership changed (3 items)"
+        )
+    }
+
+    func testContextNoteFlagsDifferentResolvers() {
+        let old = SnapshotFixture.report(resolverURLString: "https://one.example/dns-query")
+        let new = SnapshotFixture.report(resolverURLString: "https://two.example/dns-query")
+
+        let note = DiffService.comparisonContextNote(from: old, to: new)
+        XCTAssertEqual(note, "Compared snapshots used different DNS resolvers.")
+    }
+
+    func testContextNoteNilWhenResolversMatch() {
+        let report = SnapshotFixture.report()
+        XCTAssertNil(DiffService.comparisonContextNote(from: report, to: report))
+    }
+
+    func testCertificateWarningLevelThresholds() {
+        func level(daysUntilExpiry days: Int?) -> CertificateWarningLevel {
+            let ssl = days.map { SnapshotFixture.certificate(daysUntilExpiry: $0) }
+            return DiffService.certificateWarningLevel(for: SnapshotFixture.snapshot(sslInfo: ssl))
+        }
+
+        XCTAssertEqual(level(daysUntilExpiry: nil), .none)
+        XCTAssertEqual(level(daysUntilExpiry: 45), .none)
+        XCTAssertEqual(level(daysUntilExpiry: 29), .warning)
+        XCTAssertEqual(level(daysUntilExpiry: 14), .warning)
+        XCTAssertEqual(level(daysUntilExpiry: 13), .critical)
+        XCTAssertEqual(level(daysUntilExpiry: 0), .critical)
+    }
+
+    func testCrossDomainComparisonPairsBothDomains() {
+        let a = SnapshotFixture.report(domain: "alpha.example", availability: .registered)
+        let b = SnapshotFixture.report(domain: "beta.example", availability: .available)
+
+        let result = DiffService.compare(domainA: a, domainB: b)
+
+        XCTAssertEqual(result.domainA, "alpha.example")
+        XCTAssertEqual(result.domainB, "beta.example")
+        XCTAssertTrue(result.changeCount > 0)
+    }
+}