Use character based columns

Towards #74

Use `Characters` to operate based on grapheme clusters instead of code
units.

lib/src/file.dart

@@ -5,6 +5,8 @@
 import 'dart:math' as math;
 import 'dart:typed_data';
 
+import 'package:characters/characters.dart';
+
 import 'location.dart';
 import 'location_mixin.dart';
 import 'span.dart';
@@ -184,7 +186,9 @@ class SourceFile {
  throw RangeError('Line $line comes after offset $offset.');
  }
 
- return offset - lineStart;
+ final content = _decodedChars.sublist(lineStart, offset);
+ final characters = Characters(String.fromCharCodes(content));
+ return characters.length;
  }
 
  /// Gets the offset for a [line] and [column].
@@ -202,11 +206,16 @@ class SourceFile {
  throw RangeError('Column may not be negative, was $column.');
  }
 
- final result = _lineStarts[line] + column;
- if (result > length ||
- (line + 1 < lines && result >= _lineStarts[line + 1])) {
+ final content = line < lines - 1
+ ? _decodedChars.sublist(_lineStarts[line], _lineStarts[line + 1])
+ : _decodedChars.sublist(_lineStarts[line]);
+ final characters = Characters(String.fromCharCodes(content));
+ if (characters.length < column) {
  throw RangeError("Line $line doesn't have $column columns.");
  }
+ final withinLineLength =
+ characters.take(column).toString().codeUnits.length;
+ final result = _lineStarts[line] + withinLineLength;
 
  return result;
  }

pubspec.yaml

@@ -7,6 +7,7 @@ environment:
  sdk: ">=2.14.0 <3.0.0"
 
 dependencies:
+ characters: ^1.2.0
  collection: ^1.15.0
  path: ^1.8.0
  term_glyph: ^1.2.0