Implement Compact DiffFormat

Sources/CustomDump/Diff.swift

@@ -111,6 +111,7 @@ public func diff<T>(_ lhs: T, _ rhs: T, format: DiffFormat = .default) -> String
       elementIndent: Int,
       elementSeparator: String,
       collapseUnchanged: Bool,
+      hideUnchanged: Bool = false,
       filter isIncluded: (Mirror.Child) -> Bool = { _ in true },
       areEquivalent: (Mirror.Child, Mirror.Child) -> Bool = { $0.label == $1.label },
       areInIncreasingOrder: ((Mirror.Child, Mirror.Child) -> Bool)? = nil,
@@ -220,6 +221,11 @@ public func diff<T>(_ lhs: T, _ rhs: T, format: DiffFormat = .default) -> String
 
       func flushUnchanged() {
         guard collapseUnchanged else { return }
+        // When the caller opted into silent-discard mode, drop without printing.
+        if hideUnchanged {
+          unchangedBuffer.removeAll()
+          return
+        }
         if areInIncreasingOrder == nil && unchangedBuffer.count == 1 {
           let child = unchangedBuffer[0]
           print(
@@ -390,7 +396,8 @@ public func diff<T>(_ lhs: T, _ rhs: T, format: DiffFormat = .default) -> String
             suffix: ")",
             elementIndent: 2,
             elementSeparator: ",",
-            collapseUnchanged: false,
+            collapseUnchanged: format.hideUnchangedChildren,
+            hideUnchanged: format.hideUnchangedChildren,
             filter: macroPropertyFilter(for: lhs)
           )
           tracker.visitedItems.insert(lhsItem)
@@ -426,7 +433,8 @@ public func diff<T>(_ lhs: T, _ rhs: T, format: DiffFormat = .default) -> String
             suffix: ")",
             elementIndent: 2,
             elementSeparator: ",",
-            collapseUnchanged: false,
+            collapseUnchanged: format.hideUnchangedChildren,
+            hideUnchanged: format.hideUnchangedChildren,
             filter: macroPropertyFilter(for: lhs)
           )
         } else {
@@ -599,7 +607,8 @@ public func diff<T>(_ lhs: T, _ rhs: T, format: DiffFormat = .default) -> String
         suffix: ")",
         elementIndent: 2,
         elementSeparator: ",",
-        collapseUnchanged: false,
+        collapseUnchanged: format.hideUnchangedChildren,
+        hideUnchanged: format.hideUnchangedChildren,
         map: { child, _ in
           if child.label?.first == "." {
             child.label = nil
@@ -671,7 +680,8 @@ public func diff<T>(_ lhs: T, _ rhs: T, format: DiffFormat = .default) -> String
         suffix: ")",
         elementIndent: 2,
         elementSeparator: ",",
-        collapseUnchanged: false,
+        collapseUnchanged: format.hideUnchangedChildren,
+        hideUnchanged: format.hideUnchangedChildren,
         filter: macroPropertyFilter(for: lhs)
       )
 
@@ -683,7 +693,8 @@ public func diff<T>(_ lhs: T, _ rhs: T, format: DiffFormat = .default) -> String
         suffix: ")",
         elementIndent: 2,
         elementSeparator: ",",
-        collapseUnchanged: false,
+        collapseUnchanged: format.hideUnchangedChildren,
+        hideUnchanged: format.hideUnchangedChildren,
         map: { child, _ in
           if child.label?.first == "." {
             child.label = nil
@@ -766,14 +777,21 @@ public struct DiffFormat: Sendable {
   /// something "unchanged."
   public var both: String
 
+  /// When `true`, unchanged children of struct, class, tuple, enum, and `_CustomDiffObject` types
+  /// are silently omitted from the diff output, while the parent container name is still shown.
+  /// Collection, dictionary, set, and multi-line string diffs are unaffected.
+  public var hideUnchangedChildren: Bool
+
   public init(
     first: String,
     second: String,
-    both: String
+    both: String,
+    hideUnchangedChildren: Bool = false
   ) {
     self.first = first
     self.second = second
     self.both = both
+    self.hideUnchangedChildren = hideUnchangedChildren
   }
 
   /// The default format for ``diff(_:_:format:)`` output, appropriate for where monospaced fonts
@@ -790,6 +808,10 @@ public struct DiffFormat: Sendable {
   /// figure space (" ") for unchanged. These three characters are more likely to render with equal
   /// widths in proportional fonts.
   public static let proportional = Self(first: "\u{2212}", second: "+", both: "\u{2007}")
+
+  /// A compact diff format that hides unchanged properties, showing only changed ones with parent
+  /// context. Useful for deep or wide types to reduce noise when only a few fields change.
+  public static let compact = Self(first: "-", second: "+", both: " ", hideUnchangedChildren: true)
 }
 
 private struct Line: CustomDumpStringConvertible, Identifiable {

Tests/CustomDumpTests/DiffTests.swift

@@ -728,6 +728,106 @@ final class DiffTests: XCTestCase {
     )
   }
 
+  func testHideUnchangedChildren() {
+    // Test 1: Struct — unchanged property hidden (last element changes)
+    expectNoDifference(
+      diff(User(id: 42, name: "Blob"), User(id: 42, name: "Blob, Jr."), format: .compact),
+      """
+        User(
+      -   name: "Blob"
+      +   name: "Blob, Jr."
+        )
+      """
+    )
+
+    // Test 2: Struct — changed property in the middle (trailing comma expected)
+    struct Info: Equatable {
+      var age: Int
+      var name: String
+      var email: String
+    }
+    expectNoDifference(
+      diff(
+        Info(age: 30, name: "Blob", email: "blob@example.com"),
+        Info(age: 30, name: "Blob, Jr.", email: "blob@example.com"),
+        format: .compact
+      ),
+      """
+        DiffTests.Info(
+      -   name: "Blob",
+      +   name: "Blob, Jr.",
+        )
+      """
+    )
+
+    // Test 3: Enum — unchanged associated value hidden
+    expectNoDifference(
+      diff(Enum.fizz(42, buzz: "Blob"), Enum.fizz(42, buzz: "Glob"), format: .compact),
+      """
+        Enum.fizz(
+      -   buzz: "Blob"
+      +   buzz: "Glob"
+        )
+      """
+    )
+
+    // Test 4: Tuple — unchanged elements hidden
+    expectNoDifference(
+      diff((42, "Blob"), (42, "Blob, Jr."), format: .compact),
+      """
+        (
+      -   "Blob"
+      +   "Blob, Jr."
+        )
+      """
+    )
+
+    // Test 5: Nested struct — unchanged at both levels hidden
+    expectNoDifference(
+      diff(
+        Pair(driver: User(id: 1, name: "Blob"), passenger: User(id: 2, name: "Blob, Jr.")),
+        Pair(driver: User(id: 1, name: "Blob"), passenger: User(id: 2, name: "Blob, Sr.")),
+        format: .compact
+      ),
+      """
+        Pair(
+          passenger: User(
+      -     name: "Blob, Jr."
+      +     name: "Blob, Sr."
+          )
+        )
+      """
+    )
+
+    // Test 6: Collection is NOT silently dropped — still shows `… (N unchanged)`
+    expectNoDifference(
+      diff([1, 2, 3, 4, 5], [1, 2, 99, 4, 5], format: .compact),
+      """
+        [
+          … (2 unchanged),
+      -   [2]: 3,
+      +   [2]: 99,
+          … (2 unchanged)
+        ]
+      """
+    )
+
+    // Test 7: Default format still shows all unchanged properties (regression)
+    expectNoDifference(
+      diff(User(id: 42, name: "Blob"), User(id: 42, name: "Blob, Jr.")),
+      """
+        User(
+          id: 42,
+      -   name: "Blob"
+      +   name: "Blob, Jr."
+        )
+      """
+    )
+
+    // Test 8: Equal values return `nil` with `.compact`
+    XCTAssertEqual(diff(User(id: 1, name: "Blob"), User(id: 1, name: "Blob"), format: .compact), nil)
+  }
+
   #if !os(WASI)
     func testNestedCustomMirror() {
       #if compiler(>=5.4)