Clang-tidy: Fixed math on shard-id validator.

tools/clang_tidy/lib/src/options.dart

@@ -120,7 +120,7 @@ class Options {
     }
     final String? shardIdString = argResults['shard-id'] as String?;
     final int? shardId = shardIdString == null ? null : int.parse(shardIdString);
-    if (shardId != null && (shardId >= shardCommands.length || shardId < 0)) {
+    if (shardId != null && (shardId > shardCommands.length || shardId < 0)) {
       return Options._error('Invalid shard-id value: $shardId.', errSink: errSink);
     }
     return Options._fromArgResults(

tools/clang_tidy/test/clang_tidy_test.dart

@@ -8,6 +8,7 @@ import 'package:clang_tidy/clang_tidy.dart';
 import 'package:clang_tidy/src/command.dart';
 import 'package:clang_tidy/src/options.dart';
 import 'package:litetest/litetest.dart';
+import 'package:path/path.dart' as path;
 import 'package:process_runner/process_runner.dart';
 
 // Recorded locally from clang-tidy.
@@ -115,6 +116,44 @@ Future<int> main(List<String> args) async {
     ));
   });
 
+  test('shard-id valid', () async {
+    final StringBuffer outBuffer = StringBuffer();
+    final StringBuffer errBuffer = StringBuffer();
+    final String variant = path.basename(io.File(buildCommands).parent.path);
+    final ClangTidy clangTidy = ClangTidy.fromCommandLine(
+      <String>[
+        '--shard-variants=$variant',
+        '--shard-id=1',
+      ],
+      outSink: outBuffer,
+      errSink: errBuffer,
+    );
+
+    final int result = await clangTidy.run();
+
+    expect(clangTidy.options.help, isFalse);
+    expect(result, equals(0));
+  });
+
+  test('shard-id invalid', () async {
+    final StringBuffer outBuffer = StringBuffer();
+    final StringBuffer errBuffer = StringBuffer();
+    final String variant = path.basename(io.File(buildCommands).parent.path);
+    final ClangTidy clangTidy = ClangTidy.fromCommandLine(
+      <String>[
+        '--shard-variants=$variant',
+        '--shard-id=2',
+      ],
+      outSink: outBuffer,
+      errSink: errBuffer,
+    );
+
+    final int result = await clangTidy.run();
+
+    expect(clangTidy.options.help, isFalse);
+    expect(result, equals(1));
+  });
+
   test('Error when --compile-commands path does not exist', () async {
     final StringBuffer outBuffer = StringBuffer();
     final StringBuffer errBuffer = StringBuffer();