Fix str-slice(). (#70)

See sass/sass#2211

CHANGELOG.md

@@ -5,6 +5,12 @@
 * Fix conversions between numbers with `dpi`, `dpcm`, and `dppx` units.
   Previously these conversions were inverted.
 
+* Don't crash when calling `str-slice()` with an `$end-at` index lower than the
+  `$start-at` index.
+
+* `str-slice()` now correctly returns `""` when `$end-at` is negative and points
+  before the beginning of the string.
+
 ## 1.0.0-alpha.5
 
 * Fix bounds-checking for `opacify()`, `fade-in()`, `transparentize()`, and

lib/src/functions.dart

@@ -487,6 +487,7 @@ void defineCoreFunctions(Environment environment) {
 
     var codeUnitIndex = codepointIndexToCodeUnitIndex(string.text,
         _codepointForIndex(index.assertInt("index"), string.text.runes.length));
+
     return new SassString(
         string.text.replaceRange(codeUnitIndex, codeUnitIndex, insert.text),
         quotes: string.hasQuotes);
@@ -514,7 +515,13 @@ void defineCoreFunctions(Environment environment) {
     var lengthInCodepoints = string.text.runes.length;
     var startCodepoint =
         _codepointForIndex(start.assertInt(), lengthInCodepoints);
-    var endCodepoint = _codepointForIndex(end.assertInt(), lengthInCodepoints);
+    var endCodepoint = _codepointForIndex(end.assertInt(), lengthInCodepoints,
+        allowNegative: true);
+    if (endCodepoint == lengthInCodepoints) endCodepoint -= 1;
+    if (endCodepoint < startCodepoint) {
+      return new SassString.empty(quotes: string.hasQuotes);
+    }
+
     return new SassString(
         string.text.substring(
             codepointIndexToCodeUnitIndex(string.text, startCodepoint),
@@ -1018,10 +1025,17 @@ SassColor _transparentize(List<Value> arguments) {
 /// A Sass string index is one-based, and uses negative numbers to count
 /// backwards from the end of the string. A codepoint index is an index into
 /// [String.runes].
-int _codepointForIndex(int index, int lengthInCodepoints) {
+///
+/// If [index] is negative and it points before the beginning of
+/// [lengthInCodepoints], this will return `0` if [allowNegative] is `false` and
+/// the index if it's `true`.
+int _codepointForIndex(int index, int lengthInCodepoints,
+    {bool allowNegative: false}) {
   if (index == 0) return 0;
   if (index > 0) return math.min(index - 1, lengthInCodepoints);
-  return math.max(lengthInCodepoints + index, 0);
+  var result = lengthInCodepoints + index;
+  if (result < 0 && !allowNegative) return 0;
+  return result;
 }
 
 /// Returns a [BuiltInCallable] named [name] that transforms a number's value

lib/src/value/string.dart

@@ -8,6 +8,12 @@ import '../util/character.dart';
 import '../visitor/interface/value.dart';
 import '../value.dart';
 
+/// A quoted empty string, returned by [SassString.empty].
+final _emptyQuoted = new SassString("", quotes: true);
+
+/// An unquoted empty string, returned by [SassString.empty].
+final _emptyUnquoted = new SassString("", quotes: true);
+
 /// A SassScript string.
 ///
 /// Strings can either be quoted or unquoted. Unquoted strings are usually CSS
@@ -51,6 +57,9 @@ class SassString extends Value {
 
   bool get isBlank => !hasQuotes && text.isEmpty;
 
+  factory SassString.empty({bool quotes: false}) =>
+      quotes ? _emptyQuoted : _emptyUnquoted;
+
   SassString(this.text, {bool quotes: false}) : hasQuotes = quotes;
 
   /*=T*/ accept/*<T>*/(ValueVisitor/*<T>*/ visitor) =>