perf(sql): offset scanner for leading comments in sql_limit (#1515)

lib/core/database/sql_limit.dart

@@ -20,25 +20,55 @@ int _blockCommentEnd(String sql, int start) {
   return sql.length;
 }
 
-/// Removes leading whitespace, `--` line comments and `/* */` block comments.
-String stripLeadingWhitespaceAndLineComments(String sql) {
-  var s = sql.trimLeft();
+/// The characters `String.trimLeft` removes: Unicode White_Space and the BOM.
+bool _isTrimWhitespace(int c) =>
+    (c >= 0x09 && c <= 0x0D) ||
+    c == 0x20 ||
+    c == 0x85 ||
+    c == 0xA0 ||
+    c == 0x1680 ||
+    (c >= 0x2000 && c <= 0x200A) ||
+    c == 0x2028 ||
+    c == 0x2029 ||
+    c == 0x202F ||
+    c == 0x205F ||
+    c == 0x3000 ||
+    c == 0xFEFF;
+
+/// Offset of the first character of [sql] that is not whitespace, a `--` line
+/// comment or a `/* */` block comment; `sql.length` when nothing else is there.
+///
+/// Walks an index, so a header of many comment lines in front of a large script
+/// costs one pass and no copies (#1515).
+int _skipLeadingCommentsOffset(String sql) {
+  final length = sql.length;
+  var i = 0;
   while (true) {
-    if (s.isEmpty) return s;
-    if (s.startsWith('--')) {
-      final nl = s.indexOf('\n');
-      if (nl == -1) return '';
-      s = s.substring(nl + 1).trimLeft();
+    while (i < length && _isTrimWhitespace(sql.codeUnitAt(i))) {
+      i++;
+    }
+    if (i >= length) return length;
+    if (sql.startsWith('--', i)) {
+      final nl = sql.indexOf('\n', i);
+      if (nl == -1) return length;
+      i = nl + 1;
       continue;
     }
-    if (s.startsWith('/*')) {
-      s = s.substring(_blockCommentEnd(s, 0)).trimLeft();
+    if (sql.startsWith('/*', i)) {
+      i = _blockCommentEnd(sql, i);
       continue;
     }
-    return s;
+    return i;
   }
 }
 
+/// Removes leading whitespace, `--` line comments and `/* */` block comments.
+String stripLeadingWhitespaceAndLineComments(String sql) {
+  final start = _skipLeadingCommentsOffset(sql);
+  if (start >= sql.length) return '';
+  return start == 0 ? sql : sql.substring(start);
+}
+
 final _limitAll = RegExp(r'\bLIMIT\s+ALL\b', caseSensitive: false);
 final _limitCount = RegExp(
   r'\bLIMIT\s+(\d+)(\s+OFFSET\s+\d+)?',

test/core/database/sql_limit_strip_test.dart

@@ -0,0 +1,108 @@
+import 'dart:math';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:querya_desktop/core/database/sql_limit.dart';
+
+/// `stripLeadingWhitespaceAndLineComments` before #1515: one substring + trimLeft
+/// per comment, the reference the offset scanner has to match.
+String _reference(String sql) {
+  int blockEnd(String s, int start) {
+    var depth = 0;
+    var i = start;
+    while (i < s.length) {
+      if (s.startsWith('/*', i)) {
+        depth++;
+        i += 2;
+      } else if (s.startsWith('*/', i)) {
+        depth--;
+        i += 2;
+        if (depth == 0) return i;
+      } else {
+        i++;
+      }
+    }
+    return s.length;
+  }
+
+  var s = sql.trimLeft();
+  while (true) {
+    if (s.isEmpty) return s;
+    if (s.startsWith('--')) {
+      final nl = s.indexOf('\n');
+      if (nl == -1) return '';
+      s = s.substring(nl + 1).trimLeft();
+      continue;
+    }
+    if (s.startsWith('/*')) {
+      s = s.substring(blockEnd(s, 0)).trimLeft();
+      continue;
+    }
+    return s;
+  }
+}
+
+void main() {
+  group('stripLeadingWhitespaceAndLineComments offset scanner (#1515)', () {
+    test('agrees with the previous implementation on listed scripts', () {
+      const cases = <String>[
+        '',
+        ' ',
+        '\n\t \r\n',
+        'SELECT 1',
+        '  SELECT 1',
+        '-- c\nSELECT 1',
+        '-- c\r\nSELECT 1',
+        '-- c\n-- d\n\n  -- e\nSELECT 1',
+        '-- only a comment',
+        '-- c\n',
+        '/* c */SELECT 1',
+        '/* a */ /* b */\n  SELECT 1',
+        '/* a /* nested */ still */ SELECT 1',
+        '/* unterminated',
+        '/* a */ -- b\n/* c */ SELECT 1',
+        '--\nSELECT 1',
+        '-',
+        '/',
+        'SELECT 1 -- trailing\nFROM t',
+        '  -- c\nSELECT 1',
+        '\u0085SELECT 1',
+        '᠎SELECT 1',
+        '/*/ x */ SELECT 1',
+        '/**/SELECT 1',
+      ];
+      for (final sql in cases) {
+        expect(
+          stripLeadingWhitespaceAndLineComments(sql),
+          _reference(sql),
+          reason: sql,
+        );
+      }
+    });
+
+    test('agrees with the previous implementation on random scripts', () {
+      const pieces = <String>[
+        '--', '/*', '*/', '\n', '\r\n', ' ', '\t', ' ', '',
+        'SELECT', '1', 'x', '-', '/', '*',
+      ];
+      final random = Random(1515);
+      for (var n = 0; n < 4000; n++) {
+        final sql = [
+          for (var k = random.nextInt(24); k > 0; k--)
+            pieces[random.nextInt(pieces.length)],
+        ].join();
+        expect(
+          stripLeadingWhitespaceAndLineComments(sql),
+          _reference(sql),
+          reason: sql,
+        );
+      }
+    });
+
+    test('a long comment header in front of a large script is one pass', () {
+      final header = List.generate(500, (i) => '-- header line $i').join('\n');
+      final body = 'SELECT ${List.filled(20000, 'col').join(', ')} FROM t';
+      final sql = '$header\n$body';
+      expect(stripLeadingWhitespaceAndLineComments(sql), body);
+    });
+  });
+}