Keep unison in place when varying randomly

ref xenharmonic-devs/scale-workshop#613

src/__tests__/scale.spec.ts

@@ -3,7 +3,7 @@ import {describe, it, expect} from 'vitest';
 import {ExtendedMonzo} from '../monzo';
 import {Scale} from '../scale';
 import {Interval, IntervalOptions} from '../interval';
-import {arraysEqual, Fraction} from 'xen-dev-utils';
+import {arraysEqual, Fraction, valueToCents} from 'xen-dev-utils';
 
 describe('Scale', () => {
   it('supports just intonation', () => {
@@ -1054,6 +1054,30 @@ describe('Scale', () => {
     expect(scale.getMonzo(2).cents).toBeTruthy();
   });
 
+  it('keeps the unison in place during random variance', () => {
+    const scale = Scale.fromIntervalArray([
+      new Interval(
+        ExtendedMonzo.fromEqualTemperament(
+          new Fraction(3, 15),
+          new Fraction(9),
+          2
+        ),
+        'equal temperament',
+        undefined,
+        {preferredEtDenominator: 15, preferredEtEquave: new Fraction(9)}
+      ),
+      new Interval(ExtendedMonzo.fromFraction(9, 2), 'ratio'),
+    ]).vary(10, false);
+    expect(scale.intervals[0].totalCents()).toBe(0);
+    expect(scale.getCents(1)).toBeGreaterThanOrEqual(
+      (valueToCents(9) * 3) / 15 - 10
+    );
+    expect(scale.getCents(1)).toBeLessThanOrEqual(
+      (valueToCents(9) * 3) / 15 + 10
+    );
+    expect(scale.getCents(2)).toBeCloseTo(valueToCents(9));
+  });
+
   it('respells the equave when approximated in equal temperament', () => {
     const scale = Scale.fromIntervalArray([
       new Interval(

src/scale.ts

@@ -846,7 +846,9 @@ export class Scale {
    * @returns A new scale with variance added.
    */
   vary(maxCents: number, varyEquave = false) {
-    const intervals = this.intervals.map(interval => interval.vary(maxCents));
+    const intervals = this.intervals.map((interval, i) =>
+      i ? interval.vary(maxCents) : interval.clone()
+    );
     const result = this.variant(intervals);
     if (varyEquave) {
       result.equave = this.equave.vary(maxCents);