Add LineScanner and SpanScanner classes to string_scanner.

pkg/string_scanner/lib/src/line_scanner.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library string_scanner.line_scanner;
+
+import 'string_scanner.dart';
+
+/// A subclass of [StringScanner] that tracks line and column information.
+class LineScanner extends StringScanner {
+  /// The scanner's current (zero-based) line number.

[Bob Nystrom, 2014/05/28 21:28:57]

Almost every other line/column reporter I've seen (or written) has been
one-based. One-based offends me morally, but it seems to be the standard here.

[nweiz, 2014/05/28 23:56:34]

I want to match the source_maps package here, which uses zero-based numbering.

+  int get line => _line;
+  int _line = 0;
+
+  /// The scanner's current (zero-based) column number.
+  int get column => _column;
+  int _column = 0;
+
+  /// The scanner's state, including line and column information.
+  ///
+  /// This can be used to efficiently save and restore the state of the scanner
+  /// when backtracking. A given [LineScannerState] is only valid for the
+  /// [LineScanner] that created it.
+  LineScannerState get state =>
+      new LineScannerState._(this, position, line, column);
+
+  set state(LineScannerState state) {
+    if (!identical(state._scanner, this)) {
+      throw new ArgumentError("The given LineScannerState was not returned by "
+          "this LineScanner.");
+    }

[Bob Nystrom, 2014/05/28 21:28:57]

Nice.

+
+    super.position = state.position;
+    _line = state.line;
+    _column = state.column;
+  }
+
+  set position(int newPosition) {
+    var oldPosition = position;
+    super.position = newPosition;
+
+    if (newPosition > oldPosition) {
+      var newlines = "\n".allMatches(string.substring(oldPosition, newPosition))
+          .toList();
+      _line += newlines.length;
+      if (newlines.isEmpty) {
+        _column += newPosition - oldPosition;
+      } else {
+        _column = newPosition - newlines.last.end;
+      }
+    } else {
+      var newlines = "\n".allMatches(string.substring(newPosition, oldPosition))
+          .toList();
+      _line -= newlines.length;
+      if (newlines.isEmpty) {
+        _column -= oldPosition - newPosition;
+      } else {
+        _column = newPosition - string.lastIndexOf("\n", newPosition) - 1;
+      }
+    }
+  }
+
+  LineScanner(String string, {sourceUrl, int position})
+      : super(string, sourceUrl: sourceUrl, position: position);
+
+  int readChar() {
+    var char = super.readChar();
+    if (char == 0xA) {
+      _line += 1;
+      _column = 0;
+    } else {
+      _column += 1;
+    }
+    return char;
+  }
+
+  bool scan(Pattern pattern) {
+    var oldPosition = position;
+    if (!super.scan(pattern)) return false;
+
+    var newlines = "\n".allMatches(lastMatch[0]).toList();
+    _line = newlines.length;

[Bob Nystrom, 2014/05/28 21:28:57]

+=?

Does this take into account newlines after the current position but before the
matched text?

[nweiz, 2014/05/28 23:56:34]

Done.
lastMatch[0] is the entire text from the previous position to the current one.

+    if (newlines.isEmpty) {
+      _column += lastMatch[0].length;
+    } else {
+      _column = lastMatch[0].length - newlines.last.end;
+    }
+
+    return true;
+  }
+}
+
+/// A class representing the state of a [LineScanner].
+class LineScannerState {
+  /// The [LineScanner] that created this.
+  final LineScanner _scanner;
+
+  /// The position of the scanner in this state.
+  final int position;
+
+  /// The zero-based line number of the scanner in this state.
+  final int line;
+
+  /// The zero-based column number of the scanner in this state.
+  final int column;
+
+  LineScannerState._(this._scanner, this.position, this.line, this.column);
+}