feat(scale): add descendingIntervalSteps (#104)

* feat(scale): add `descendingIntervalSteps`

* test(scale): remove unnecessary `toList` call on `reversed`

* test(scale): add test case for descending `PositionedNote` scale

lib/src/scale/scale.dart

@@ -4,11 +4,15 @@ part of '../../music_notes.dart';
 ///
 /// See [Scale (music)](https://en.wikipedia.org/wiki/Scale_(music)).
 class Scale {
-  /// The interval steps that define the scale.
+  /// The interval steps that define this [Scale].
   final List<Interval> intervalSteps;
 
-  /// Creates a new [Scale] from [intervalSteps].
-  const Scale(this.intervalSteps);
+  /// The descending interval steps that define this [Scale] (if different).
+  final List<Interval>? _descendingIntervalSteps;
+
+  /// Creates a new [Scale] from [intervalSteps] and optional
+  /// [_descendingIntervalSteps].
+  const Scale(this.intervalSteps, [this._descendingIntervalSteps]);
 
   /// ![C Ionian scale](https://upload.wikimedia.org/score/p/2/p2fun2296uif26uyy61yxjli7ocfq9d/p2fun229.png).
   static const ionian = Scale([
@@ -105,15 +109,26 @@ class Scale {
   ]);
 
   /// ![C Melodic minor scale](https://upload.wikimedia.org/score/9/2/92i6sjg41ji8y1ab881a1pcq1u3hr0p/92i6sjg4.png).
-  static const melodicMinor = Scale([
-    Interval.majorSecond,
-    Interval.minorSecond,
-    Interval.majorSecond,
-    Interval.majorSecond,
-    Interval.majorSecond,
-    Interval.majorSecond,
-    Interval.minorSecond,
-  ]);
+  static const melodicMinor = Scale(
+    [
+      Interval.majorSecond,
+      Interval.minorSecond,
+      Interval.majorSecond,
+      Interval.majorSecond,
+      Interval.majorSecond,
+      Interval.majorSecond,
+      Interval.minorSecond,
+    ],
+    [
+      Interval.majorSecond,
+      Interval.majorSecond,
+      Interval.minorSecond,
+      Interval.majorSecond,
+      Interval.majorSecond,
+      Interval.minorSecond,
+      Interval.majorSecond,
+    ],
+  );
 
   /// See [Chromatic scale](https://en.wikipedia.org/wiki/Chromatic_scale).
   ///
@@ -183,6 +198,10 @@ class Scale {
     Interval.minorSecond,
   ]);
 
+  /// The descending interval steps that define this [Scale].
+  List<Interval> get descendingIntervalSteps =>
+      _descendingIntervalSteps ?? intervalSteps.reversed.toList();
+
   /// Returns the scale of notes starting from [transposable].
   ///
   /// Example:
@@ -198,13 +217,27 @@ class Scale {
   /// Scale.melodicMinor.fromNote(Note.c)
   ///   == const [Note.c, Note.d, Note.eFlat, Note.f, Note.g, Note.a, Note.b,
   ///        Note.c]
+  ///
+  /// Scale.melodicMinor.fromNote(Note.c, isDescending: true)
+  ///   == const [Note.c, Note.bFlat, Note.aFlat, Note.g, Note.f, Note.eFlat,
+  ///        Note.d, Note.c]
   /// ```
-  List<Transposable<T>> fromNote<T>(Transposable<T> transposable) =>
-      intervalSteps.fold(
-        [transposable],
-        (scaleNotes, interval) =>
-            [...scaleNotes, scaleNotes.last.transposeBy(interval)],
-      );
+  List<Transposable<T>> fromNote<T>(
+    Transposable<T> transposable, {
+    bool isDescending = false,
+  }) {
+    final steps = isDescending ? descendingIntervalSteps : intervalSteps;
+
+    return steps.fold(
+      [transposable],
+      (scaleNotes, interval) => [
+        ...scaleNotes,
+        scaleNotes.last.transposeBy(
+          interval.descending(isDescending: isDescending),
+        ),
+      ],
+    );
+  }
 
   /// Returns the mirrored scale version of this [Scale].
   ///

test/src/scale/scale_test.dart

@@ -57,6 +57,19 @@ void main() {
             Note.d.inOctave(3),
           ],
         );
+        expect(
+          Scale.major.fromNote(Note.cSharp, isDescending: true),
+          const [
+            Note.cSharp,
+            Note(Notes.b, Accidental.sharp),
+            Note.aSharp,
+            Note.gSharp,
+            Note.fSharp,
+            Note(Notes.e, Accidental.sharp),
+            Note.dSharp,
+            Note.cSharp,
+          ],
+        );
       });
 
       test(
@@ -88,6 +101,19 @@ void main() {
               Note.d,
             ],
           );
+          expect(
+            Scale.naturalMinor.fromNote(Note.aFlat, isDescending: true),
+            const [
+              Note.aFlat,
+              Note.gFlat,
+              Note(Notes.f, Accidental.flat),
+              Note.eFlat,
+              Note.dFlat,
+              Note(Notes.c, Accidental.flat),
+              Note.bFlat,
+              Note.aFlat,
+            ],
+          );
         },
       );
 
@@ -120,6 +146,20 @@ void main() {
               Note.d,
             ],
           );
+          expect(
+            Scale.harmonicMinor
+                .fromNote(Note.e.inOctave(3), isDescending: true),
+            [
+              Note.e.inOctave(3),
+              Note.dSharp.inOctave(3),
+              Note.c.inOctave(3),
+              Note.b.inOctave(2),
+              Note.a.inOctave(2),
+              Note.g.inOctave(2),
+              Note.fSharp.inOctave(2),
+              Note.e.inOctave(2),
+            ],
+          );
         },
       );
 
@@ -152,6 +192,10 @@ void main() {
               Note.gSharp.inOctave(2),
             ],
           );
+          expect(
+            Scale.melodicMinor.fromNote(Note.f, isDescending: true),
+            Scale.naturalMinor.fromNote(Note.f).reversed,
+          );
         },
       );