function invertChar(char) {
  return char === "a" ? "b" : "a";
}

function last2EqualsChar(str, char) {
  return (
    str.length >= 2 &&
    str[str.length - 1] === char &&
    str[str.length - 2] === char
  );
}

function gen(A, B, acc = "") {
  if (A <= 0 && B <= 0) {
    return acc;
  }

  let char = A > B ? "a" : "b";

  char = last2EqualsChar(acc, char) ? invertChar(char) : char;

  const nextA = char === "a" ? A - 1 : A;
  const nextB = char === "b" ? B - 1 : B;

  return gen(nextA, nextB, acc + char);
};


describe("Uncle bob approach", () => {
  it("should return empty string with empty params", () => {
    expect(gen(0, 0)).toBe("");
  });

  it("should be `a` for A = 1 & B = 0", () => {
    expect(gen(1, 0)).toBe("a");
  });

  it("should be `b` for A = 0 & B = 1", () => {
    expect(gen(0, 1)).toBe("b");
  });

  it("result should have proper length", () => {
    expect(gen(1, 1)).toHaveLength(2);
    expect(gen(3, 5)).toHaveLength(8);
  });

  it("should use maximum available amount of symbol in a row", () => {
    expect(gen(1, 3)).toBe("bbab");
    expect(gen(4, 1)).toBe("aabaa");
  });
});