diff --git a/src/Application.php b/src/Application.php
index 45a0a0a..9827b56 100644
--- a/src/Application.php
+++ b/src/Application.php
@@ -83,12 +83,12 @@ public function run(): int
             $overridePaths = $this->resolveOverridePaths($overridePaths);
         }
 
-        $diffLines = null;
+        $diff = null;
 
         if ($options['diff'] !== null) {
             try {
                 $diffContent = $this->readDiff($options['diff']);
-                $diffLines = (new DiffParser())->parse($diffContent);
+                $diff = (new DiffParser())->parse($diffContent);
             } catch (\Throwable $e) {
                 $this->writeError('Error reading diff: ' . $e->getMessage() . PHP_EOL);
 
@@ -100,7 +100,7 @@ public function run(): int
 
         try {
             $runner = new SniffRunner($progress);
-            $report = $runner->run($config, $overridePaths, $diffLines);
+            $report = $runner->run($config, $overridePaths, $diff);
         } catch (\Throwable $e) {
             $this->writeError('Runtime error: ' . $e->getMessage() . PHP_EOL);
 
diff --git a/src/Diff/Diff.php b/src/Diff/Diff.php
new file mode 100644
index 0000000..83b3bd9
--- /dev/null
+++ b/src/Diff/Diff.php
@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DocbookCS\Diff;
+
+final readonly class Diff
+{
+    /** @param list<FileChange> $fileChanges */
+    public function __construct(public array $fileChanges)
+    {
+    }
+
+    public function changeFor(string $filePath): ?FileChange
+    {
+        $normalisedPath = str_replace('\\', '/', $filePath);
+
+        foreach ($this->fileChanges as $fileChange) {
+            $normalisedDiffPath = str_replace('\\', '/', $fileChange->filePath);
+
+            if (
+                $normalisedPath === $normalisedDiffPath
+                || str_ends_with($normalisedPath, '/' . ltrim($normalisedDiffPath, '/'))
+            ) {
+                return $fileChange;
+            }
+        }
+
+        return null;
+    }
+}
diff --git a/src/Diff/DiffParser.php b/src/Diff/DiffParser.php
index fc29ef2..9dbd2fe 100644
--- a/src/Diff/DiffParser.php
+++ b/src/Diff/DiffParser.php
@@ -8,10 +8,10 @@ final class DiffParser
 {
     private const string NO_FINAL_LINE_MARKER = '\ No newline at end of file';
 
-    /** @return array<string, list<int>> */
-    public function parse(string $diff): array
+    public function parse(string $diff): Diff
     {
-        $result = [];
+        /** @var array<string, list<int>> $changedLinesByFile */
+        $changedLinesByFile = [];
         $currentFile = null;
         $deleted = false;
         $newLineNumber = 0;
@@ -41,8 +41,8 @@ public function parse(string $diff): array
                 }
                 $currentFile = $path !== '/dev/null' ? $path : null;
                 $inHunk = false;
-                if ($currentFile !== null && !isset($result[$currentFile])) {
-                    $result[$currentFile] = [];
+                if ($currentFile !== null && !isset($changedLinesByFile[$currentFile])) {
+                    $changedLinesByFile[$currentFile] = [];
                 }
                 continue;
             }
@@ -67,7 +67,7 @@ public function parse(string $diff): array
             }
 
             if (str_starts_with($line, '+')) {
-                $result[$currentFile][] = $newLineNumber;
+                $changedLinesByFile[$currentFile][] = $newLineNumber;
                 $newLineNumber++;
                 $newLinesRemaining--;
             } elseif (str_starts_with($line, '-')) {
@@ -84,6 +84,12 @@ public function parse(string $diff): array
             }
         }
 
-        return $result;
+        $fileChanges = [];
+
+        foreach ($changedLinesByFile as $filePath => $lineNumbers) {
+            $fileChanges[] = new FileChange($filePath, $lineNumbers);
+        }
+
+        return new Diff($fileChanges);
     }
 }
diff --git a/src/Diff/FileChange.php b/src/Diff/FileChange.php
new file mode 100644
index 0000000..6b31ca2
--- /dev/null
+++ b/src/Diff/FileChange.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace DocbookCS\Diff;
+
+final readonly class FileChange
+{
+    /** @param list<int> $addedLineNumbers */
+    public function __construct(
+        public string $filePath,
+        public array $addedLineNumbers,
+    ) {
+    }
+}
diff --git a/src/Runner/SniffRunner.php b/src/Runner/SniffRunner.php
index 15e7e9a..2884155 100644
--- a/src/Runner/SniffRunner.php
+++ b/src/Runner/SniffRunner.php
@@ -6,6 +6,7 @@
 
 use DocbookCS\Config\ConfigData;
 use DocbookCS\Config\SniffEntry;
+use DocbookCS\Diff\Diff;
 use DocbookCS\Path\EntityResolver;
 use DocbookCS\Path\PathLoader;
 use DocbookCS\Path\PathMatcher;
@@ -25,11 +26,10 @@ public function __construct(?ProgressInterface $progress = null)
 
     /**
      * @param list<string>|null $overridePaths
-     * @param array<string, list<int>>|null $diffLines
      * @throws \RuntimeException if a sniff class cannot be found or does not implement SniffInterface.
      * @throws \UnexpectedValueException if no files are found to scan.
      */
-    public function run(ConfigData $config, ?array $overridePaths = null, ?array $diffLines = null): Report
+    public function run(ConfigData $config, ?array $overridePaths = null, ?Diff $diff = null): Report
     {
         $startTime = microtime(true);
 
@@ -45,8 +45,11 @@ public function run(ConfigData $config, ?array $overridePaths = null, ?array $di
         $pathLoader = new PathLoader($includePaths, $matcher);
         $files = $pathLoader->loadPaths();
 
-        if ($diffLines !== null) {
-            $files = $this->filterByDiff($files, array_keys($diffLines));
+        if ($diff !== null) {
+            $files = array_values(array_filter(
+                $files,
+                static fn(string $file): bool => $diff->changeFor($file) !== null,
+            ));
         }
 
         $report = new Report();
@@ -60,9 +63,7 @@ public function run(ConfigData $config, ?array $overridePaths = null, ?array $di
         foreach ($files as $index => $file) {
             $report->incrementFilesScanned();
 
-            $changedLines = $diffLines !== null
-                ? $this->getChangedLinesForFile($file, $diffLines)
-                : null;
+            $changedLines = $diff?->changeFor($file)?->addedLineNumbers;
 
             $fileReport = $processor->processFile(
                 $file,
@@ -125,40 +126,6 @@ private function instantiateSniffs(array $entries): array
         return $sniffs;
     }
 
-    /**
-     * @param list<string> $files
-     * @param list<string> $diffPaths
-     * @return list<string>
-     */
-    private function filterByDiff(array $files, array $diffPaths): array
-    {
-        return array_values(
-            array_filter(
-                $files,
-                fn(string $file) => $this->matchesDiffPath($file, $diffPaths),
-            )
-        );
-    }
-
-    /** @param list<string> $diffPaths */
-    private function matchesDiffPath(string $absolutePath, array $diffPaths): bool
-    {
-        $normalized = str_replace('\\', '/', $absolutePath);
-
-        foreach ($diffPaths as $diffPath) {
-            $normalizedDiff = str_replace('\\', '/', $diffPath);
-
-            if (
-                $normalized === $normalizedDiff
-                || str_ends_with($normalized, '/' . ltrim($normalizedDiff, '/'))
-            ) {
-                return true;
-            }
-        }
-
-        return false;
-    }
-
     private function makeRelative(string $absolutePath): string
     {
         $cwd = getcwd();
@@ -175,26 +142,4 @@ private function makeRelative(string $absolutePath): string
 
         return $absolutePath; // @codeCoverageIgnore
     }
-
-    /**
-     * @param array<string, list<int>> $diffLines
-     * @return list<int>
-     */
-    private function getChangedLinesForFile(string $absolutePath, array $diffLines): array
-    {
-        $normalized = str_replace('\\', '/', $absolutePath);
-
-        foreach ($diffLines as $diffPath => $lines) {
-            $normalizedDiff = str_replace('\\', '/', $diffPath);
-
-            if (
-                $normalized === $normalizedDiff
-                || str_ends_with($normalized, '/' . ltrim($normalizedDiff, '/'))
-            ) {
-                return $lines;
-            }
-        }
-
-        return []; // @codeCoverageIgnore
-    }
 }
diff --git a/tests/Unit/ApplicationTest.php b/tests/Unit/ApplicationTest.php
index 4233537..275288a 100644
--- a/tests/Unit/ApplicationTest.php
+++ b/tests/Unit/ApplicationTest.php
@@ -6,7 +6,9 @@
 
 use DocbookCS\Application;
 use DocbookCS\Config\ConfigData;
+use DocbookCS\Diff\Diff;
 use DocbookCS\Diff\DiffParser;
+use DocbookCS\Diff\FileChange;
 use DocbookCS\Config\ConfigParser;
 use DocbookCS\Config\ConfigParserException;
 use DocbookCS\Config\SniffEntry;
@@ -25,6 +27,7 @@
 use DocbookCS\Sniff\ExceptionNameSniff;
 use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\Attributes\UsesClass;
 use PHPUnit\Framework\TestCase;
 
 #[
@@ -47,6 +50,8 @@
     CoversClass(SniffEntry::class),
     CoversClass(SniffRunner::class),
     CoversClass(XmlFileProcessor::class),
+    UsesClass(Diff::class),
+    UsesClass(FileChange::class),
 ]
 final class ApplicationTest extends TestCase
 {
diff --git a/tests/Unit/Diff/DiffParserTest.php b/tests/Unit/Diff/DiffParserTest.php
index 298a3c6..ad3e5d2 100644
--- a/tests/Unit/Diff/DiffParserTest.php
+++ b/tests/Unit/Diff/DiffParserTest.php
@@ -4,13 +4,18 @@
 
 namespace DocbookCS\Tests\Unit\Diff;
 
+use DocbookCS\Diff\Diff;
 use DocbookCS\Diff\DiffParser;
+use DocbookCS\Diff\FileChange;
 use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\Attributes\UsesClass;
 use PHPUnit\Framework\TestCase;
 
 #[
     CoversClass(DiffParser::class),
+    UsesClass(Diff::class),
+    UsesClass(FileChange::class),
 ]
 final class DiffParserTest extends TestCase
 {
@@ -24,7 +29,7 @@ protected function setUp(): void
     #[Test]
     public function itReturnsEmptyArrayForEmptyDiff(): void
     {
-        self::assertSame([], $this->parser->parse(''));
+        self::assertSame([], $this->lineNumbersByFile($this->parser->parse('')));
     }
 
     #[Test]
@@ -41,7 +46,7 @@ public function itParsesAddedLineNumbers(): void
  line3
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         self::assertArrayHasKey('reference/file.xml', $result);
         self::assertSame([2], $result['reference/file.xml']);
@@ -62,7 +67,7 @@ public function itParsesMultipleAddedLines(): void
  last line
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         self::assertSame([6, 7], $result['doc/chapter.xml']);
     }
@@ -79,7 +84,7 @@ public function itStripsTheBPrefix(): void
 +added
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         self::assertArrayHasKey('src/file.xml', $result);
         self::assertArrayNotHasKey('b/src/file.xml', $result);
@@ -99,7 +104,7 @@ public function itExcludesDeletedFiles(): void
 -line3
 DIFF;
 
-        self::assertSame([], $this->parser->parse($diff));
+        self::assertSame([], $this->lineNumbersByFile($this->parser->parse($diff)));
     }
 
     #[Test]
@@ -116,7 +121,7 @@ public function itHandlesNewlyCreatedFiles(): void
 +line3
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         self::assertArrayHasKey('new.xml', $result);
         self::assertSame([1, 2, 3], $result['new.xml']);
@@ -142,7 +147,7 @@ public function itHandlesMultipleFilesInOneDiff(): void
  unchanged
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         self::assertArrayHasKey('first.xml', $result);
         self::assertArrayHasKey('second.xml', $result);
@@ -164,7 +169,7 @@ public function itIgnoresRemovedLines(): void
  line3
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         // No lines added, so the changed set is empty (not absent — the file is tracked).
         self::assertArrayHasKey('file.xml', $result);
@@ -185,7 +190,7 @@ public function itIgnoresTheMissingFinalNewlineMarker(): void
 +second
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         self::assertSame([1, 2], $result['file.xml']);
     }
@@ -209,7 +214,7 @@ public function itTracksLineNumbersAcrossMultipleHunks(): void
  line12
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         self::assertSame([2, 12], $result['file.xml']);
     }
@@ -225,8 +230,21 @@ public function itHandlesHunkWithNoContext(): void
 +only line
 DIFF;
 
-        $result = $this->parser->parse($diff);
+        $result = $this->lineNumbersByFile($this->parser->parse($diff));
 
         self::assertSame([1], $result['file.xml']);
     }
+
+    // TODO: avoids test diff churn; remove when fixers merged
+    /** @return array<string, list<int>> */
+    private function lineNumbersByFile(Diff $diff): array
+    {
+        $lineNumbersByFile = [];
+
+        foreach ($diff->fileChanges as $fileChange) {
+            $lineNumbersByFile[$fileChange->filePath] = $fileChange->addedLineNumbers;
+        }
+
+        return $lineNumbersByFile;
+    }
 }
diff --git a/tests/Unit/Runner/SniffRunnerTest.php b/tests/Unit/Runner/SniffRunnerTest.php
index af38188..47b6d25 100644
--- a/tests/Unit/Runner/SniffRunnerTest.php
+++ b/tests/Unit/Runner/SniffRunnerTest.php
@@ -6,6 +6,8 @@
 
 use DocbookCS\Config\ConfigData;
 use DocbookCS\Config\SniffEntry;
+use DocbookCS\Diff\Diff;
+use DocbookCS\Diff\FileChange;
 use DocbookCS\Path\EntityResolver;
 use DocbookCS\Path\PathLoader;
 use DocbookCS\Path\PathMatcher;
@@ -21,6 +23,7 @@
 use DocbookCS\Sniff\SniffInterface;
 use PHPUnit\Framework\Attributes\CoversClass;
 use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\Attributes\UsesClass;
 use PHPUnit\Framework\TestCase;
 
 #[
@@ -36,6 +39,8 @@
     CoversClass(SniffRunner::class),
     CoversClass(Violation::class),
     CoversClass(XmlFileProcessor::class),
+    UsesClass(Diff::class),
+    UsesClass(FileChange::class),
 ]
 final class SniffRunnerTest extends TestCase
 {
@@ -238,8 +243,8 @@ public function itFiltersFilesToOnlyThoseInTheDiff(): void
         $config = $this->createConfig();
         $runner = new SniffRunner();
 
-        $diffLines = ['sniff_runner/default/file_a.xml' => [1]];
-        $report = $runner->run($config, null, $diffLines);
+        $diff = new Diff([new FileChange('sniff_runner/default/file_a.xml', [1])]);
+        $report = $runner->run($config, null, $diff);
 
         self::assertSame(1, $report->getFilesScanned());
     }
@@ -250,8 +255,8 @@ public function itScansNoFilesWhenDiffContainsNoMatchingPaths(): void
         $config = $this->createConfig();
         $runner = new SniffRunner();
 
-        $diffLines = ['completely/different/file.xml' => [1, 2, 3]];
-        $report = $runner->run($config, null, $diffLines);
+        $diff = new Diff([new FileChange('completely/different/file.xml', [1, 2, 3])]);
+        $report = $runner->run($config, null, $diff);
 
         self::assertSame(0, $report->getFilesScanned());
     }
@@ -264,8 +269,8 @@ public function itMatchesWhenDiffPathEqualsDiscoveredPath(): void
 
         $discoveredPath = self::FIXTURE_DIR . '/file_a.xml';
 
-        $diffLines = [$discoveredPath => [1]];
-        $report = $runner->run($config, null, $diffLines);
+        $diff = new Diff([new FileChange($discoveredPath, [1])]);
+        $report = $runner->run($config, null, $diff);
 
         self::assertSame(1, $report->getFilesScanned());
     }
@@ -311,8 +316,8 @@ public function setProperty(string $name, string $value): void
         $config = $this->createConfig(sniffs: [new SniffEntry($sniff::class)]);
         $runner = new SniffRunner();
 
-        $diffLines = ['sniff_runner/default/file_a.xml' => []];
-        $report = $runner->run($config, null, $diffLines);
+        $diff = new Diff([new FileChange('sniff_runner/default/file_a.xml', [])]);
+        $report = $runner->run($config, null, $diff);
 
         self::assertSame(1, $report->getFilesScanned());
         self::assertFalse($report->hasViolations());