SassNumber.assertIndexFor() -> Value.sassIndexToListIndex() (#211)

lib/src/functions.dart

@@ -696,17 +696,17 @@ final List<BuiltInCallable> coreFunctions = new UnmodifiableListView([
       (arguments) => new SassNumber(arguments[0].asList.length)),
 
   new BuiltInCallable("nth", r"$list, $n", (arguments) {
-    var list = arguments[0].asList;
-    var index = arguments[1].assertNumber("n");
-    return list[index.assertIndexFor(list, "n")];
+    var list = arguments[0];
+    var index = arguments[1];
+    return list.asList[list.sassIndexToListIndex(index, "n")];
   }),
 
   new BuiltInCallable("set-nth", r"$list, $n, $value", (arguments) {
-    var list = arguments[0].asList;
-    var index = arguments[1].assertNumber("n");
+    var list = arguments[0];
+    var index = arguments[1];
     var value = arguments[2];
-    var newList = list.toList();
-    newList[index.assertIndexFor(list, "n")] = value;
+    var newList = list.asList.toList();
+    newList[list.sassIndexToListIndex(index, "n")] = value;
     return arguments[0].changeListContents(newList);
   }),
 

lib/src/value.dart

@@ -2,6 +2,8 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'package:meta/meta.dart';
+
 import 'ast/selector.dart';
 import 'exception.dart';
 import 'value/boolean.dart';
@@ -36,6 +38,13 @@ abstract class Value implements ext.Value {
   bool get hasBrackets => false;
   List<Value> get asList => [this];
 
+  /// The length of [asList].
+  ///
+  /// This is used to compute [sassIndexToListIndex] without allocating a new
+  /// list.
+  @protected
+  int get lengthAsList => 1;
+
   /// Whether the value will be represented in CSS as the empty string.
   bool get isBlank => false;
 
@@ -61,6 +70,18 @@ abstract class Value implements ext.Value {
   /// It's not guaranteed to be stable across versions.
   T accept<T>(ValueVisitor<T> visitor);
 
+  int sassIndexToListIndex(ext.Value sassIndex, [String name]) {
+    var index = sassIndex.assertNumber(name).assertInt(name);
+    if (index == 0) throw _exception("List index may not be 0.", name);
+    if (index.abs() > lengthAsList) {
+      throw _exception(
+          "Invalid index $sassIndex for a list with ${lengthAsList} elements.",
+          name);
+    }
+
+    return index < 0 ? lengthAsList + index : index - 1;
+  }
+
   SassBoolean assertBoolean([String name]) =>
       throw _exception("$this is not a boolean.", name);
 

lib/src/value/external/number.dart

@@ -69,17 +69,6 @@ abstract class SassNumber extends Value {
   /// It's used for error reporting.
   int assertInt([String name]);
 
-  /// Asserts that this is a valid Sass-style index for [list], and returns the
-  /// Dart-style index.
-  ///
-  /// A Sass-style index is one-based, and uses negative numbers to count
-  /// backwards from the end of the list.
-  ///
-  /// Throws a [SassScriptException] if this isn't an integer or if it isn't a
-  /// valid index for [list]. If this came from a function argument, [name] is
-  /// the argument name (without the `$`). It's used for error reporting.
-  int assertIndexFor(List list, [String name]);
-
   /// If [value] is between [min] and [max], returns it.
   ///
   /// If [value] is [fuzzyEquals] to [min] or [max], it's clamped to the

lib/src/value/external/value.dart

@@ -58,6 +58,18 @@ abstract class Value {
   /// and all other values count as single-value lists.
   List<Value> get asList;
 
+  /// Converts [sassIndex] into a Dart-style index into the list returned by
+  /// [asList].
+  ///
+  /// Sass indexes are one-based, while Dart indexes are zero-based. Sass
+  /// indexes may also be negative in order to index from the end of the list.
+  ///
+  /// Throws a [SassScriptException] if [sassIndex] isn't a number, if that
+  /// number isn't an integer, or if that integer isn't a valid index for
+  /// [asList]. If [sassIndex] came from a function argument, [name] is the
+  /// argument name (without the `$`). It's used for error reporting.
+  int sassIndexToListIndex(Value sassIndex, [String name]);
+
   /// Throws a [SassScriptException] if [this] isn't a boolean.
   ///
   /// Note that generally, functions should use [isTruthy] rather than requiring

lib/src/value/list.dart

@@ -18,6 +18,8 @@ class SassList extends Value implements ext.SassList {
 
   List<Value> get asList => contents;
 
+  int get lengthAsList => contents.length;
+
   const SassList.empty({ListSeparator separator, bool brackets: false})
       : contents = const [],
         separator = separator ?? ListSeparator.undecided,

lib/src/value/map.dart

@@ -21,6 +21,8 @@ class SassMap extends Value implements ext.SassMap {
     return result;
   }
 
+  int get lengthAsList => contents.length;
+
   /// Returns an empty map.
   const SassMap.empty() : contents = const {};
 

lib/src/value/number.dart

@@ -200,17 +200,6 @@ class SassNumber extends Value implements ext.SassNumber {
     throw _exception("$this is not an int.", name);
   }
 
-  int assertIndexFor(List list, [String name]) {
-    var sassIndex = assertInt(name);
-    if (sassIndex == 0) throw _exception("List index may not be 0.");
-    if (sassIndex.abs() > list.length) {
-      throw _exception(
-          "Invalid index $this for a list with ${list.length} elements.");
-    }
-
-    return sassIndex < 0 ? list.length + sassIndex : sassIndex - 1;
-  }
-
   num valueInRange(num min, num max, [String name]) {
     var result = fuzzyCheckRange(value, min, max);
     if (result != null) return result;

test/dart_api/value/list_test.dart

@@ -62,6 +62,39 @@ main() {
               ListSeparator.comma))));
     });
 
+    group("sassIndexToListIndex()", () {
+      test("converts a positive index to a Dart index", () {
+        expect(value.sassIndexToListIndex(new SassNumber(1)), equals(0));
+        expect(value.sassIndexToListIndex(new SassNumber(2)), equals(1));
+        expect(value.sassIndexToListIndex(new SassNumber(3)), equals(2));
+      });
+
+      test("converts a negative index to a Dart index", () {
+        expect(value.sassIndexToListIndex(new SassNumber(-1)), equals(2));
+        expect(value.sassIndexToListIndex(new SassNumber(-2)), equals(1));
+        expect(value.sassIndexToListIndex(new SassNumber(-3)), equals(0));
+      });
+
+      test("rejects a non-number", () {
+        expect(() => value.sassIndexToListIndex(new SassString("foo")),
+            throwsSassScriptException);
+      });
+
+      test("rejects a non-integer", () {
+        expect(() => value.sassIndexToListIndex(new SassNumber(1.1)),
+            throwsSassScriptException);
+      });
+
+      test("rejects invalid indices", () {
+        expect(() => value.sassIndexToListIndex(new SassNumber(0)),
+            throwsSassScriptException);
+        expect(() => value.sassIndexToListIndex(new SassNumber(4)),
+            throwsSassScriptException);
+        expect(() => value.sassIndexToListIndex(new SassNumber(-4)),
+            throwsSassScriptException);
+      });
+    });
+
     test("isn't any other type", () {
       expect(value.assertBoolean, throwsSassScriptException);
       expect(value.assertColor, throwsSassScriptException);
@@ -133,6 +166,15 @@ main() {
       expect(value.assertNumber, throwsSassScriptException);
       expect(value.assertString, throwsSassScriptException);
     });
+
+    test("sassIndexToListIndex() rejects invalid indices", () {
+      expect(() => value.sassIndexToListIndex(new SassNumber(0)),
+          throwsSassScriptException);
+      expect(() => value.sassIndexToListIndex(new SassNumber(1)),
+          throwsSassScriptException);
+      expect(() => value.sassIndexToListIndex(new SassNumber(-1)),
+          throwsSassScriptException);
+    });
   });
 
   group("a scalar value", () {
@@ -152,6 +194,25 @@ main() {
       expect(list, hasLength(1));
       expect(list.first, same(value));
     });
+
+    group("sassIndexToListIndex()", () {
+      test("converts a positive index to a Dart index", () {
+        expect(value.sassIndexToListIndex(new SassNumber(1)), equals(0));
+      });
+
+      test("converts a negative index to a Dart index", () {
+        expect(value.sassIndexToListIndex(new SassNumber(-1)), equals(0));
+      });
+
+      test("rejects invalid indices", () {
+        expect(() => value.sassIndexToListIndex(new SassNumber(0)),
+            throwsSassScriptException);
+        expect(() => value.sassIndexToListIndex(new SassNumber(2)),
+            throwsSassScriptException);
+        expect(() => value.sassIndexToListIndex(new SassNumber(-2)),
+            throwsSassScriptException);
+      });
+    });
   });
 
   group("new SassList.empty()", () {

test/dart_api/value/map_test.dart

@@ -39,6 +39,27 @@ main() {
           ]));
     });
 
+    group("sassIndexToListIndex()", () {
+      test("converts a positive index to a Dart index", () {
+        expect(value.sassIndexToListIndex(new SassNumber(1)), equals(0));
+        expect(value.sassIndexToListIndex(new SassNumber(2)), equals(1));
+      });
+
+      test("converts a negative index to a Dart index", () {
+        expect(value.sassIndexToListIndex(new SassNumber(-1)), equals(1));
+        expect(value.sassIndexToListIndex(new SassNumber(-2)), equals(0));
+      });
+
+      test("rejects invalid indices", () {
+        expect(() => value.sassIndexToListIndex(new SassNumber(0)),
+            throwsSassScriptException);
+        expect(() => value.sassIndexToListIndex(new SassNumber(3)),
+            throwsSassScriptException);
+        expect(() => value.sassIndexToListIndex(new SassNumber(-3)),
+            throwsSassScriptException);
+      });
+    });
+
     test("equals the same map", () {
       expect(
           value,
@@ -128,6 +149,15 @@ main() {
     test("equals an empty list", () {
       expect(value, equalsWithHash(new SassList.empty()));
     });
+
+    test("sassIndexToListIndex() rejects invalid indices", () {
+      expect(() => value.sassIndexToListIndex(new SassNumber(0)),
+          throwsSassScriptException);
+      expect(() => value.sassIndexToListIndex(new SassNumber(1)),
+          throwsSassScriptException);
+      expect(() => value.sassIndexToListIndex(new SassNumber(-1)),
+          throwsSassScriptException);
+    });
   });
 
   test("new SassMap.empty() creates an empty map with default metadata", () {