Add LineScanner and SpanScanner classes to string_scanner.

pkg/string_scanner/lib/src/string_scanner.dart

Index: pkg/string_scanner/lib/src/string_scanner.dart
diff --git a/pkg/string_scanner/lib/string_scanner.dart b/pkg/string_scanner/lib/src/string_scanner.dart
similarity index 69%
copy from pkg/string_scanner/lib/string_scanner.dart
copy to pkg/string_scanner/lib/src/string_scanner.dart
index 3e3913b50169ba5796102c6314aba658341457b8..c9e74594a6f5a7ee08f53e0c59adaba7e82c21c4 100644
--- a/pkg/string_scanner/lib/string_scanner.dart
+++ b/pkg/string_scanner/lib/src/string_scanner.dart
@@ -2,19 +2,26 @@
 // 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.
 
-/// A library for parsing strings using a sequence of patterns.
-library string_scanner;
+library string_scanner.string_scanner;
 
-import 'dart:math' as math;
+import 'package:source_maps/source_maps.dart';
+
+import 'exception.dart';
+import 'utils.dart';
 
 /// When compiled to JS, forward slashes are always escaped in [RegExp.pattern].
 ///
 /// See issue 17998.
 final _slashAutoEscape = new RegExp("/").pattern == "\\/";
 
-// TODO(nweiz): Add some integration between this and source maps.
 /// A class that scans through a string using [Pattern]s.
 class StringScanner {
+  /// The URL of the source of the string being scanned.
+  ///
+  /// This is used for error reporting. It may be `null`, indicating that the
+  /// source URL is unknown or unavailable.
+  final Uri sourceUrl;
+
   /// The string being scanned through.
   final String string;
 
@@ -43,11 +50,37 @@ class StringScanner {
 
   /// Creates a new [StringScanner] that starts scanning from [position].
   ///
-  /// [position] defaults to 0, the beginning of the string.
-  StringScanner(this.string, {int position}) {
+  /// [position] defaults to 0, the beginning of the string. [sourceUrl] is the
+  /// URL of the source of the string being scanned, if available. It can be
+  /// either a [String] or a [Uri].
+  StringScanner(this.string, {sourceUrl, int position})
+      : sourceUrl = sourceUrl is String ? Uri.parse(sourceUrl) : sourceUrl {
     if (position != null) this.position = position;
   }
 
+  /// Consumes a single character and returns its character code.
+  ///
+  /// This throws a [FormatException] if the string has been fully consumed. It
+  /// doesn't affect [lastMatch].
+  int readChar() {
+    if (isDone) _fail("more input");
+    return string.codeUnitAt(_position++);
+  }
+
+  /// Returns the character code of the character [offset] away from [position].
+  ///
+  /// [offset] defaults to zero, and may be negative to inspect already-consumed
+  /// characters.
+  ///
+  /// This returns `null` if [offset] points outside the string. It doesn't
+  /// affect [lastMatch].
+  int peekChar([int offset]) {
+    if (offset == null) offset = 0;
+    var index = position + offset;
+    if (index < 0 || index >= string.length) return null;
+    return string.codeUnitAt(index);
+  }
+
   /// If [pattern] matches at the current position of the string, scans forward
   /// until the end of the match.
   ///
@@ -112,17 +145,7 @@ class StringScanner {
   ///
   /// It's an error to pass [match] at the same time as [position] or [length].
   void error(String message, {Match match, int position, int length}) {
-    if (match != null && (position != null || length != null)) {
-      throw new ArgumentError("Can't pass both match and position/length.");
-    }
-
-    if (position != null && position < 0) {
-      throw new RangeError("position must be greater than or equal to 0.");
-    }
-
-    if (length != null && length < 1) {
-      throw new RangeError("length must be greater than or equal to 0.");
-    }
+    validateErrorArgs(string, match, position, length);
 
     if (match == null && position == null && length == null) match = lastMatch;
     if (position == null) {
@@ -130,40 +153,16 @@ class StringScanner {
     }
     if (length == null) length = match == null ? 1 : match.end - match.start;
 
-    var newlines = "\n".allMatches(string.substring(0, position)).toList();
-    var line = newlines.length + 1;
-    var column;
-    var lastLine;
-    if (newlines.isEmpty) {
-      column = position + 1;
-      lastLine = string.substring(0, position);
-    } else {
-      column = position - newlines.last.end + 1;
-      lastLine = string.substring(newlines.last.end, position);
-    }
-
-    var remaining = string.substring(position);
-    var nextNewline = remaining.indexOf("\n");
-    if (nextNewline == -1) {
-      lastLine += remaining;
-    } else {
-      length = math.min(length, nextNewline);
-      lastLine += remaining.substring(0, nextNewline);
-    }
-
-    var spaces = new List.filled(column - 1, ' ').join();
-    var underline = new List.filled(length, '^').join();
-
-    throw new FormatException(
-        "Error on line $line, column $column: $message\n"
-        "$lastLine\n"
-        "$spaces$underline");
+    var url = sourceUrl == null ? null : sourceUrl.toString();
+    var sourceFile = new SourceFile.text(url, string);
+    var span = sourceFile.span(position, position + length);
+    throw new StringScannerException(message, string, sourceUrl, span);
   }
 
   // TODO(nweiz): Make this handle long lines more gracefully.
   /// Throws a [FormatException] describing that [name] is expected at the
   /// current position in the string.
   void _fail(String name) {
-    error("expected $name.", position: this.position, length: 1);
+    error("expected $name.", position: this.position, length: 0);
   }
 }