Update song key when changing modifier (#1599)

When changing a song's modifier, the song key should also be changed to
make sure chords a correctly rendered.

Author: martijnversluis

src/chord_sheet/song.ts

@@ -343,7 +343,14 @@ class Song extends MetadataAccessors {
    * @returns {Song} the changed song
    */
   useModifier(modifier: Modifier): Song {
-    return this.mapChordLyricsPairs((pair) => pair.useModifier(modifier));
+    const { currentKey } = this;
+    let changedSong = this.mapChordLyricsPairs((pair) => pair.useModifier(modifier));
+
+    if (currentKey && currentKey.modifier !== modifier) {
+      changedSong = changedSong.changeKey(currentKey.useModifier(modifier));
+    }
+
+    return changedSong;
   }
 
   /**
@@ -373,10 +380,14 @@ class Song extends MetadataAccessors {
     return this.mapChordLyricsPairs((pair) => pair.changeChord(func));
   }
 
+  get currentKey(): Key | null {
+    return Key.wrap(this.key);
+  }
+
   requireCurrentKey(): Key {
-    const wrappedKey = Key.wrap(this.key);
+    const { currentKey } = this;
 
-    if (!wrappedKey) {
+    if (!currentKey) {
       throw new Error(`
 Cannot change song key, the original key is unknown.
 
@@ -387,7 +398,7 @@ Or set the song key before changing key:
   \`song.setKey('C');\``.substring(1));
     }
 
-    return wrappedKey;
+    return currentKey;
   }
 
   /**

test/chord_sheet/song.test.ts

@@ -1,10 +1,14 @@
 import { ChordLyricsPair, ChordSheetSerializer, Tag } from '../../src';
 
 import {
+  chordLyricsPair,
   createChordDefinition,
   createChordLyricsPair,
   createLine,
-  createSong, createTag,
+  createSong,
+  createSongFromAst,
+  createTag,
+  tag,
 } from '../utilities';
 
 import { exampleSongSolfege, exampleSongSymbol } from '../fixtures/song';
@@ -481,4 +485,30 @@ describe('Song', () => {
       expect((normalizedSong.paragraphs[0].lines[0].items[0] as ChordLyricsPair).chords).toEqual('E(9)');
     });
   });
+
+  describe('#useModifier', () => {
+    it('changes the modifier of the chords in a song', () => {
+      const song = createSongFromAst([
+        [
+          chordLyricsPair('G#', 'let it'),
+          chordLyricsPair('F#', 'it'),
+        ],
+      ]);
+
+      const modifiedSong = song.useModifier('b');
+
+      expect((modifiedSong.paragraphs[0].lines[0].items[0] as ChordLyricsPair).chords).toEqual('Ab');
+      expect((modifiedSong.paragraphs[0].lines[0].items[1] as ChordLyricsPair).chords).toEqual('Gb');
+    });
+
+    it('updates the key of the song', () => {
+      const song = createSongFromAst([
+        [tag('key', 'F#')],
+      ]);
+
+      const modifiedSong = song.useModifier('b');
+
+      expect(modifiedSong.key).toEqual('Gb');
+    });
+  });
 });