Add VersionConstraint.difference().

lib/src/utils.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 import 'version_range.dart';
 
 /// Returns whether [range1] is immediately next to, but not overlapping,
 /// [range2].
 bool areAdjacent(VersionRange range1, VersionRange range2) {
   if (range1.max != range2.min) return false;
 
   return (range1.includeMax && !range2.includeMin) ||
       (!range1.includeMax && range2.includeMin);
 }
 
-/// A [Comparator] that compares the maximum versions of [range1] and [range2].
-int compareMax(VersionRange range1, VersionRange range2) {
-  if (range1.max == null) {
-    if (range2.max == null) return 0;
-    return 1;
-  } else if (range2.max == null) {
-    return -1;
-  }
+/// Returns whether [range1] allows lower versions than [range2].
+bool allowsLower(VersionRange range1, VersionRange range2) {
+  if (range1.min == null) return range2.min != null;
+  if (range2.min == null) return false;
 
-  var result = range1.max.compareTo(range2.max);
-  if (result != 0) return result;
-  if (range1.includeMax != range2.includeMax) return range1.includeMax ? 1 : -1;
-  return 0;
+  var comparison = range1.min.compareTo(range2.min);
+  if (comparison == -1) return true;
+  if (comparison == 1) return false;
+  return range1.includeMin && !range2.includeMin;
 }
+
+/// Returns whether [range1] allows higher versions than [range2].
+bool allowsHigher(VersionRange range1, VersionRange range2) {
+  if (range1.max == null) return range2.max != null;
+  if (range2.max == null) return false;
+
+  var comparison = range1.max.compareTo(range2.max);
+  if (comparison == 1) return true;
+  if (comparison == -1) return false;
+  return range1.includeMax && !range2.includeMax;
+}
+
+/// Returns whether [range1] allows only versions lower than those allowed by
+/// [range2].
+bool strictlyLower(VersionRange range1, VersionRange range2) {
+  if (range1.max == null || range2.min == null) return false;
+
+  var comparison = range1.max.compareTo(range2.min);
+  if (comparison == -1) return true;
+  if (comparison == 1) return false;
+  return !range1.includeMax || !range2.includeMin;
+}
+
+/// Returns whether [range1] allows only versions higher than those allowed by
+/// [range2].
+bool strictlyHigher(VersionRange range1, VersionRange range2) {
+  if (range1.min == null || range2.max == null) return false;
+
+  var comparison = range1.min.compareTo(range2.max);
+  if (comparison == 1) return true;
+  if (comparison == -1) return false;
+  return !range1.includeMin || !range2.includeMax;
+}