Avoid treating /r as newline on it (#81)

Fixes https://github.com/dart-lang/string_scanner/issues/80

This fixes an error when scanning a zero-length match when between a CR and LF. 


* Fix typo

* comment nits

* Fix some bugs when setting position if the current position is between \r\n

---------

Co-authored-by: Nate Bosch <nbosch1@gmail.com>

CHANGELOG.md

@@ -1,3 +1,11 @@
+## 1.4.0
+
+* Fix `LineScanner`'s handling of `\r\n`'s to preventing errors scanning
+  zero-length matches when between CR and LF. CR is treated as a new line only
+  if not immediately followed by a LF.
+* Fix `LineScanner`'s updating of `column` when setting `position` if the
+  current position is not `0`.
+
 ## 1.3.0
 
 * Require Dart 3.1.0

lib/src/line_scanner.dart

@@ -8,8 +8,9 @@ import 'utils.dart';
 
 // Note that much of this code is duplicated in eager_span_scanner.dart.
 
-/// A regular expression matching newlines across platforms.
-final _newlineRegExp = RegExp(r'\r\n?|\n');
+/// A regular expression matching newlines. A newline is either a `\n`, a `\r\n`
+/// or a `\r` that is not immediately followed by a `\n`.
+final _newlineRegExp = RegExp(r'\n|\r\n|\r(?!\n)');
 
 /// A subclass of [StringScanner] that tracks line and column information.
 class LineScanner extends StringScanner {
@@ -48,27 +49,57 @@ class LineScanner extends StringScanner {
 
   @override
   set position(int newPosition) {
+    if (newPosition == position) {
+      return;
+    }
+
     final oldPosition = position;
     super.position = newPosition;
 
-    if (newPosition > oldPosition) {
-      final newlines = _newlinesIn(string.substring(oldPosition, newPosition));
+    if (newPosition == 0) {
+      _line = 0;
+      _column = 0;
+    } else if (newPosition > oldPosition) {
+      final newlines = _newlinesIn(string.substring(oldPosition, newPosition),
+          endPosition: newPosition);
       _line += newlines.length;
       if (newlines.isEmpty) {
         _column += newPosition - oldPosition;
       } else {
-        _column = newPosition - newlines.last.end;
+        // The regex got a substring, so we need to account for where it started
+        // in the string.
+        final offsetOfLastNewline = oldPosition + newlines.last.end;
+        _column = newPosition - offsetOfLastNewline;
       }
-    } else {
-      final newlines = _newlinesIn(string.substring(newPosition, oldPosition));
-      if (_betweenCRLF) newlines.removeLast();
+    } else if (newPosition < oldPosition) {
+      final newlines = _newlinesIn(string.substring(newPosition, oldPosition),
+          endPosition: oldPosition);
 
       _line -= newlines.length;
       if (newlines.isEmpty) {
         _column -= oldPosition - newPosition;
       } else {
-        _column =
-            newPosition - string.lastIndexOf(_newlineRegExp, newPosition) - 1;
+        // To compute the new column, we need to locate the last newline before
+        // the new position. When searching, we must exclude the CR if we're
+        // between a CRLF because it's not considered a newline.
+        final crOffset = _betweenCRLF ? -1 : 0;
+        // Additionally, if we use newPosition as the end of the search and the
+        // character at that position itself (the next character) is a newline
+        // we should not use it, so also offset to account for that.
+        const currentCharOffset = -1;
+        final lastNewline = string.lastIndexOf(
+            _newlineRegExp, newPosition + currentCharOffset + crOffset);
+
+        // Now we need to know the offset after the newline. This is the index
+        // above plus the length of the newline (eg. if we found `\r\n`) we need
+        // to add two. However if no newline was found, that index is 0.
+        final offsetAfterLastNewline = lastNewline == -1
+            ? 0
+            : string[lastNewline] == '\r' && string[lastNewline + 1] == '\n'
+                ? lastNewline + 2
+                : lastNewline + 1;
+
+        _column = newPosition - offsetAfterLastNewline;
       }
     }
   }
@@ -103,7 +134,7 @@ class LineScanner extends StringScanner {
   bool scan(Pattern pattern) {
     if (!super.scan(pattern)) return false;
 
-    final newlines = _newlinesIn(lastMatch![0]!);
+    final newlines = _newlinesIn(lastMatch![0]!, endPosition: position);
     _line += newlines.length;
     if (newlines.isEmpty) {
       _column += lastMatch![0]!.length;
@@ -115,10 +146,21 @@ class LineScanner extends StringScanner {
   }
 
   /// Returns a list of [Match]es describing all the newlines in [text], which
-  /// is assumed to end at [position].
-  List<Match> _newlinesIn(String text) {
+  /// ends at [endPosition].
+  ///
+  /// If [text] ends with `\r`, it will only be treated as a newline if the next
+  /// character at [position] is not a `\n`.
+  List<Match> _newlinesIn(String text, {required int endPosition}) {
     final newlines = _newlineRegExp.allMatches(text).toList();
-    if (_betweenCRLF) newlines.removeLast();
+    // If the last character is a `\r` it will have been treated as a newline,
+    // but this is only valid if the next character is not a `\n`.
+    if (endPosition < string.length &&
+        text.endsWith('\r') &&
+        string[endPosition] == '\n') {
+      // newlines should never be empty here, because if `text` ends with `\r`
+      // it would have matched `\r(?!\n)` in the newline regex.
+      newlines.removeLast();
+    }
     return newlines;
   }
 }

pubspec.yaml

@@ -1,5 +1,5 @@
 name: string_scanner
-version: 1.3.0
+version: 1.4.0
 description: A class for parsing strings using a sequence of patterns.
 repository: https://github.com/dart-lang/string_scanner