diff --git a/Sources/SaplingEditor/DocumentLineIndex.swift b/Sources/SaplingEditor/DocumentLineIndex.swift
new file mode 100644
index 0000000..b0d9cb2
--- /dev/null
+++ b/Sources/SaplingEditor/DocumentLineIndex.swift
@@ -0,0 +1,160 @@
+import Foundation
+
+public enum LineEndingStrategy: String, Hashable, Sendable {
+    case lf
+    case crlf
+    case cr
+    case none
+
+    public var utf16Length: Int {
+        switch self {
+        case .lf, .cr:
+            return 1
+        case .crlf:
+            return 2
+        case .none:
+            return 0
+        }
+    }
+}
+
+public struct DocumentLineBoundary: Hashable, Sendable {
+    public var index: Int
+    public var contentRange: NSRange
+    public var lineEnding: LineEndingStrategy
+
+    public init(index: Int, contentRange: NSRange, lineEnding: LineEndingStrategy) {
+        self.index = index
+        self.contentRange = contentRange
+        self.lineEnding = lineEnding
+    }
+
+    public var lineEndingRange: NSRange {
+        NSRange(location: contentRange.upperBound, length: lineEnding.utf16Length)
+    }
+
+    public var nextLineLocation: Int {
+        contentRange.upperBound + lineEnding.utf16Length
+    }
+}
+
+public struct DocumentLineIndex: Hashable, Sendable {
+    public var source: String
+    public var boundaries: [DocumentLineBoundary]
+
+    public init(source: String) {
+        self.source = source
+        self.boundaries = Self.scanBoundaries(in: source)
+    }
+
+    public func lineIndex(containing location: Int) -> Int {
+        guard !boundaries.isEmpty else { return 0 }
+
+        let clampedLocation = max(0, min(location, source.utf16.count))
+        for boundary in boundaries.dropLast() {
+            if clampedLocation < boundary.nextLineLocation {
+                return boundary.index
+            }
+        }
+
+        return boundaries[boundaries.count - 1].index
+    }
+
+    public func editorLines(activeLineIndex: Int) -> [EditorLine] {
+        let nsSource = source as NSString
+        return boundaries.map { boundary in
+            EditorLine(
+                index: boundary.index,
+                source: nsSource.substring(with: boundary.contentRange),
+                range: boundary.contentRange,
+                mode: boundary.index == activeLineIndex ? .source : .rendered
+            )
+        }
+    }
+
+    private static func scanBoundaries(in source: String) -> [DocumentLineBoundary] {
+        let nsSource = source as NSString
+        let sourceLength = nsSource.length
+        guard sourceLength > 0 else {
+            return [
+                DocumentLineBoundary(
+                    index: 0,
+                    contentRange: NSRange(location: 0, length: 0),
+                    lineEnding: .none
+                )
+            ]
+        }
+
+        var boundaries: [DocumentLineBoundary] = []
+        var lineStart = 0
+        var lineIndex = 0
+        var endedWithLineEnding = false
+
+        while lineStart < sourceLength {
+            var cursor = lineStart
+            while cursor < sourceLength,
+                  !isLineEndingStart(nsSource.character(at: cursor)) {
+                cursor += 1
+            }
+
+            let contentRange = NSRange(location: lineStart, length: cursor - lineStart)
+            if cursor < sourceLength {
+                let lineEnding = lineEndingStrategy(at: cursor, in: nsSource)
+                boundaries.append(DocumentLineBoundary(
+                    index: lineIndex,
+                    contentRange: contentRange,
+                    lineEnding: lineEnding
+                ))
+                cursor += lineEnding.utf16Length
+                lineStart = cursor
+                endedWithLineEnding = cursor == sourceLength
+            } else {
+                boundaries.append(DocumentLineBoundary(
+                    index: lineIndex,
+                    contentRange: contentRange,
+                    lineEnding: .none
+                ))
+                lineStart = sourceLength
+                endedWithLineEnding = false
+            }
+
+            lineIndex += 1
+        }
+
+        if endedWithLineEnding {
+            boundaries.append(DocumentLineBoundary(
+                index: lineIndex,
+                contentRange: NSRange(location: sourceLength, length: 0),
+                lineEnding: .none
+            ))
+        }
+
+        return boundaries
+    }
+
+    private static func lineEndingStrategy(at location: Int, in source: NSString) -> LineEndingStrategy {
+        let character = source.character(at: location)
+        if character == carriageReturnUTF16 {
+            let nextLocation = location + 1
+            if nextLocation < source.length,
+               source.character(at: nextLocation) == lineFeedUTF16 {
+                return .crlf
+            }
+            return .cr
+        }
+        return .lf
+    }
+
+    private static func isLineEndingStart(_ character: unichar) -> Bool {
+        character == lineFeedUTF16 || character == carriageReturnUTF16
+    }
+
+    private static let lineFeedUTF16: unichar = 10
+    private static let carriageReturnUTF16: unichar = 13
+}
+
+private extension NSRange {
+    var upperBound: Int {
+        location + length
+    }
+}
diff --git a/Sources/SaplingEditor/EditorActiveLineTracker.swift b/Sources/SaplingEditor/EditorActiveLineTracker.swift
index 904ecd8..5b218bd 100644
--- a/Sources/SaplingEditor/EditorActiveLineTracker.swift
+++ b/Sources/SaplingEditor/EditorActiveLineTracker.swift
@@ -2,57 +2,11 @@ import Foundation
 
 public enum EditorActiveLineTracker {
     public static func lines(from source: String, activeLineIndex: Int) -> [EditorLine] {
-        var lines: [EditorLine] = []
-        var lineStart = source.startIndex
-        var utf16Location = 0
-        var index = 0
-
-        while lineStart < source.endIndex {
-            let lineEnd = source[lineStart...].firstIndex(of: "\n") ?? source.endIndex
-            let line = String(source[lineStart..<lineEnd])
-            let length = line.utf16.count
-            lines.append(EditorLine(
-                index: index,
-                source: line,
-                range: NSRange(location: utf16Location, length: length),
-                mode: index == activeLineIndex ? .source : .rendered
-            ))
-
-            if lineEnd == source.endIndex {
-                lineStart = lineEnd
-                utf16Location += length
-            } else {
-                lineStart = source.index(after: lineEnd)
-                utf16Location += length + 1
-            }
-            index += 1
-        }
-
-        if source.isEmpty || source.hasSuffix("\n") {
-            lines.append(EditorLine(
-                index: index,
-                source: "",
-                range: NSRange(location: utf16Location, length: 0),
-                mode: index == activeLineIndex ? .source : .rendered
-            ))
-        }
-
-        return lines
+        DocumentLineIndex(source: source).editorLines(activeLineIndex: activeLineIndex)
     }
 
     public static func lineIndex(containing location: Int, in source: String) -> Int {
-        let clampedLocation = max(0, min(location, source.utf16.count))
-        var currentLocation = 0
-
-        for (index, line) in source.split(separator: "\n", omittingEmptySubsequences: false).enumerated() {
-            let length = line.utf16.count
-            if clampedLocation <= currentLocation + length {
-                return index
-            }
-            currentLocation += length + 1
-        }
-
-        return 0
+        DocumentLineIndex(source: source).lineIndex(containing: location)
     }
 
     public static func clampedSelection(_ selection: EditorSelection, in source: String) -> EditorSelection {
diff --git a/Sources/SaplingEditor/EditorPerformanceProfiling.swift b/Sources/SaplingEditor/EditorPerformanceProfiling.swift
index cdf2bf4..f81189d 100644
--- a/Sources/SaplingEditor/EditorPerformanceProfiling.swift
+++ b/Sources/SaplingEditor/EditorPerformanceProfiling.swift
@@ -256,9 +256,11 @@ public enum EditorBenchmarkProfiler {
     }
 
     public static func documentProfile(fileName: String, source: String) -> EditorBenchmarkDocumentProfile {
-        let lines = source.components(separatedBy: "\n")
-        let lineLengths = lines.map { $0.utf16.count }
-        let lineCount = lines.count
+        let lineIndex = DocumentLineIndex(source: source)
+        let nsSource = source as NSString
+        let lineSources = lineIndex.boundaries.map { nsSource.substring(with: $0.contentRange) }
+        let lineLengths = lineIndex.boundaries.map(\.contentRange.length)
+        let lineCount = lineIndex.boundaries.count
         let maxLineLength = lineLengths.max() ?? 0
         let totalLineLength = lineLengths.reduce(0, +)
 
@@ -273,9 +275,9 @@ public enum EditorBenchmarkProfiler {
             orderedListItemCount: countMatches("(?m)^\\d+\\.\\s", in: source),
             blockquoteCount: countMatches("(?m)^>\\s", in: source),
             fencedCodeFenceCount: countMatches("(?m)^```", in: source),
-            inlineCodeLineCount: countMatchingLines("`[^`\\n]+`", in: lines),
-            boldLineCount: countMatchingLines("\\*\\*[^*\\n]+\\*\\*", in: lines),
-            italicLineCount: countMatchingLines("(?<!\\*)\\*[^*\\n]+\\*(?!\\*)", in: lines),
+            inlineCodeLineCount: countMatchingLines("`[^`\\n]+`", in: lineSources),
+            boldLineCount: countMatchingLines("\\*\\*[^*\\n]+\\*\\*", in: lineSources),
+            italicLineCount: countMatchingLines("(?<!\\*)\\*[^*\\n]+\\*(?!\\*)", in: lineSources),
             inlineLinkCount: countMatches("(?<!!)\\[[^\\]]+\\]\\([^\\)]+\\)", in: source),
             referenceLinkLikeCount: countMatches("(?<!!)\\[[A-Za-z0-9_-]+\\]", in: source),
             imageCount: countMatches("!\\[", in: source),
diff --git a/Tests/SaplingEditorTests/DocumentLineIndexTests.swift b/Tests/SaplingEditorTests/DocumentLineIndexTests.swift
new file mode 100644
index 0000000..c6c6700
--- /dev/null
+++ b/Tests/SaplingEditorTests/DocumentLineIndexTests.swift
@@ -0,0 +1,117 @@
+import XCTest
+@testable import SaplingEditor
+
+final class DocumentLineIndexTests: XCTestCase {
+    func testLFLineBoundaries() {
+        assertLineIndex(
+            source: "One\nTwo\nThree",
+            expectedSources: ["One", "Two", "Three"],
+            expectedRanges: [
+                NSRange(location: 0, length: 3),
+                NSRange(location: 4, length: 3),
+                NSRange(location: 8, length: 5)
+            ],
+            expectedEndings: [.lf, .lf, .none]
+        )
+    }
+
+    func testCRLFLineBoundaries() {
+        assertLineIndex(
+            source: "One\r\nTwo\r\nThree",
+            expectedSources: ["One", "Two", "Three"],
+            expectedRanges: [
+                NSRange(location: 0, length: 3),
+                NSRange(location: 5, length: 3),
+                NSRange(location: 10, length: 5)
+            ],
+            expectedEndings: [.crlf, .crlf, .none]
+        )
+    }
+
+    func testCRLineBoundaries() {
+        assertLineIndex(
+            source: "One\rTwo\rThree",
+            expectedSources: ["One", "Two", "Three"],
+            expectedRanges: [
+                NSRange(location: 0, length: 3),
+                NSRange(location: 4, length: 3),
+                NSRange(location: 8, length: 5)
+            ],
+            expectedEndings: [.cr, .cr, .none]
+        )
+    }
+
+    func testMixedLineBoundaries() {
+        assertLineIndex(
+            source: "One\nTwo\r\nThree\rFour",
+            expectedSources: ["One", "Two", "Three", "Four"],
+            expectedRanges: [
+                NSRange(location: 0, length: 3),
+                NSRange(location: 4, length: 3),
+                NSRange(location: 9, length: 5),
+                NSRange(location: 15, length: 4)
+            ],
+            expectedEndings: [.lf, .crlf, .cr, .none]
+        )
+    }
+
+    func testTrailingBlankLineForEveryLineEnding() {
+        XCTAssertEqual(DocumentLineIndex(source: "One\n").boundaries.map(\.contentRange.location), [0, 4])
+        XCTAssertEqual(DocumentLineIndex(source: "One\r\n").boundaries.map(\.contentRange.location), [0, 5])
+        XCTAssertEqual(DocumentLineIndex(source: "One\r").boundaries.map(\.contentRange.location), [0, 4])
+    }
+
+    func testActiveLineDetectionAcrossCRLFBoundaries() {
+        let source = "One\r\nTwo\r\nThree"
+
+        XCTAssertEqual(EditorActiveLineTracker.lineIndex(containing: 0, in: source), 0)
+        XCTAssertEqual(EditorActiveLineTracker.lineIndex(containing: 3, in: source), 0)
+        XCTAssertEqual(EditorActiveLineTracker.lineIndex(containing: 4, in: source), 0)
+        XCTAssertEqual(EditorActiveLineTracker.lineIndex(containing: 5, in: source), 1)
+        XCTAssertEqual(EditorActiveLineTracker.lineIndex(containing: 8, in: source), 1)
+        XCTAssertEqual(EditorActiveLineTracker.lineIndex(containing: 9, in: source), 1)
+        XCTAssertEqual(EditorActiveLineTracker.lineIndex(containing: 10, in: source), 2)
+    }
+
+    func testEditorLinesPreserveCRLFSourceRangesAndModes() {
+        let lines = EditorActiveLineTracker.lines(from: "One\r\nTwo\r\nThree", activeLineIndex: 1)
+
+        XCTAssertEqual(lines.count, 3)
+        XCTAssertEqual(lines.map(\.source), ["One", "Two", "Three"])
+        XCTAssertEqual(lines.map(\.range), [
+            NSRange(location: 0, length: 3),
+            NSRange(location: 5, length: 3),
+            NSRange(location: 10, length: 5)
+        ])
+        XCTAssertEqual(lines.map(\.mode), [.rendered, .source, .rendered])
+    }
+
+    func testBenchmarkDocumentSegmentsIntoPhysicalLines() throws {
+        let url = URL(fileURLWithPath: "/Users/feror/Sapling/Docs/Benchmarks/5mb.md")
+        let source = try String(contentsOf: url, encoding: .utf8)
+        let lines = EditorActiveLineTracker.lines(from: source, activeLineIndex: 0)
+
+        XCTAssertEqual(lines.count, 51_482)
+        XCTAssertEqual(lines[0].source, "# ExampleFile.com - Example Files")
+        XCTAssertEqual(lines[1].source, "")
+        XCTAssertEqual(lines[2].source, "- ✨ExampleFile ✨")
+        XCTAssertEqual(lines.filter { $0.mode == .source }.count, 1)
+        XCTAssertEqual(lines.filter { $0.mode == .rendered }.count, 51_481)
+    }
+
+    private func assertLineIndex(
+        source: String,
+        expectedSources: [String],
+        expectedRanges: [NSRange],
+        expectedEndings: [LineEndingStrategy],
+        file: StaticString = #filePath,
+        line: UInt = #line
+    ) {
+        let index = DocumentLineIndex(source: source)
+        let lines = index.editorLines(activeLineIndex: 0)
+
+        XCTAssertEqual(lines.map(\.source), expectedSources, file: file, line: line)
+        XCTAssertEqual(index.boundaries.map(\.contentRange), expectedRanges, file: file, line: line)
+        XCTAssertEqual(index.boundaries.map(\.lineEnding), expectedEndings, file: file, line: line)
+    }
+}