builder.put(element, element);
        }
        return builder.build();
      }
    };

    abstract Map<Element, Element> create(Collection<Element> contents);
  }

  @Param({"5", "50", "500", "5000", "50000"})
  private int size;

  // TODO: look at exact (==) hits vs. equals() hits?
  @Param("0.9")
  private double hitRate;

  @Param("true")
  private boolean isUserTypeFast;

    }
  }

  // Depends on the correctness of BigIntegerMath.factorial.
  @GwtIncompatible // TODO
  public void testFactorial() {
    for (int n = 0; n <= 50; n++) {
      BigInteger expectedBig = BigIntegerMath.factorial(n);
      long expectedLong = fitsInLong(expectedBig) ? expectedBig.longValue() : Long.MAX_VALUE;
      assertEquals(expectedLong, LongMath.factorial(n));
    }

                    + " trials");
          }
        }
      }
    }
  }

  /**
   * Test for avalanche with 2-bit deltas. Most probabilities of output bit(j) differing are well
   * within 50%.
   */
  static void check2BitAvalanche(HashFunction function, int trials, double epsilon) {
    Random rand = new Random(0);
    int keyBits = 32;
    int hashBits = function.bits();

          uniques++;
        }
      }
      Arrays.fill(sortedElements, uniques, length, null);
      if (maybeExpand && uniques * 4 > length * 3) {
        // lots of nonduplicated elements, expand the array by 50%
        sortedElements =
            Arrays.copyOf(sortedElements, IntMath.saturatedAdd(length, length / 2 + 1));
      }
      int[] sortedCounts = new int[sortedElements.length];
      for (int i = 0; i < length; i++) {