fix(PatternGenerator): keep randomWalk within the value range

Tone/event/PatternGenerator.test.ts

@@ -99,5 +99,23 @@ describe("PatternGenerator", () => {
 				currentIndex = nextIndex;
 			}
 		});
+
+		it("never randomly walks outside of the range of values", () => {
+			[1, 2, 3, 4].forEach((numValues) => {
+				const pattern = PatternGenerator(numValues, "randomWalk");
+				for (let i = 0; i < 100; i++) {
+					expect(pattern.next().value)
+						.to.be.at.least(0)
+						.and.at.most(numValues - 1);
+				}
+			});
+		});
+
+		it("randomly walks in place when there is only one value", () => {
+			const pattern = PatternGenerator(1, "randomWalk");
+			expect(getArrayValues(pattern, 10)).to.deep.equal([
+				0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+			]);
+		});
 	});
 });

Tone/event/PatternGenerator.ts

@@ -138,15 +138,18 @@ function* randomWalk(numValues: number): IterableIterator<number> {
 	// randomly choose a starting index
 	let index = Math.floor(Math.random() * numValues);
 	while (true) {
-		if (index === 0) {
-			index++; // at bottom, so force upward step
-		} else if (index === numValues - 1) {
-			index--; // at top, so force downward step
-		} else if (Math.random() < 0.5) {
-			// else choose random downward or upward step
-			index--;
-		} else {
-			index++;
+		// with a single value there is nowhere to step to
+		if (numValues > 1) {
+			if (index === 0) {
+				index++; // at bottom, so force upward step
+			} else if (index === numValues - 1) {
+				index--; // at top, so force downward step
+			} else if (Math.random() < 0.5) {
+				// else choose random downward or upward step
+				index--;
+			} else {
+				index++;
+			}
 		}
 		yield index;
 	}