HashTestUtils.java

/*
 * Copyright (C) 2011 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.hash;

import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertFalse;

import com.google.common.base.Charsets;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Sets;
import com.google.common.primitives.Ints;
import com.google.common.testing.EqualsTester;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.charset.Charset;
import java.util.Arrays;
import java.util.Random;
import java.util.Set;
import org.junit.Assert;

/**
 * Various utilities for testing {@link HashFunction}s.
 *
 * @author Dimitris Andreou
 * @author Kurt Alfred Kluever
 */
final class HashTestUtils {
  private HashTestUtils() {}

  /** Converts a string, which should contain only ascii-representable characters, to a byte[]. */
  static byte[] ascii(String string) {
    byte[] bytes = new byte[string.length()];
    for (int i = 0; i < string.length(); i++) {
      bytes[i] = (byte) string.charAt(i);
    }
    return bytes;
  }

  interface HashFn {
    byte[] hash(byte[] input, int seed);
  }

  static void verifyHashFunction(HashFn hashFunction, int hashbits, int expected) {
    int hashBytes = hashbits / 8;

    byte[] key = new byte[256];
    byte[] hashes = new byte[hashBytes * 256];

    // Hash keys of the form {}, {0}, {0,1}, {0,1,2}... up to N=255,using 256-N as the seed
    for (int i = 0; i < 256; i++) {
      key[i] = (byte) i;
      int seed = 256 - i;
      byte[] hash = hashFunction.hash(Arrays.copyOf(key, i), seed);
      System.arraycopy(hash, 0, hashes, i * hashBytes, hash.length);
    }

    // Then hash the result array
    byte[] result = hashFunction.hash(hashes, 0);

    // interpreted in little-endian order.
    int verification = Integer.reverseBytes(Ints.fromByteArray(result));

    if (expected != verification) {
      throw new AssertionError(
          "Expected: "
              + Integer.toHexString(expected)
              + " got: "
              + Integer.toHexString(verification));
    }
  }

  static final Funnel<Object> BAD_FUNNEL =
      new Funnel<Object>() {
        @Override
        public void funnel(Object object, PrimitiveSink bytePrimitiveSink) {
          bytePrimitiveSink.putInt(object.hashCode());
        }
      };

  enum RandomHasherAction {
    PUT_BOOLEAN() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        boolean value = random.nextBoolean();
        for (PrimitiveSink sink : sinks) {
          sink.putBoolean(value);
        }
      }
    },
    PUT_BYTE() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        int value = random.nextInt();
        for (PrimitiveSink sink : sinks) {
          sink.putByte((byte) value);
        }
      }
    },
    PUT_SHORT() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        short value = (short) random.nextInt();
        for (PrimitiveSink sink : sinks) {
          sink.putShort(value);
        }
      }
    },
    PUT_CHAR() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        char value = (char) random.nextInt();
        for (PrimitiveSink sink : sinks) {
          sink.putChar(value);
        }
      }
    },
    PUT_INT() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        int value = random.nextInt();
        for (PrimitiveSink sink : sinks) {
          sink.putInt(value);
        }
      }
    },
    PUT_LONG() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        long value = random.nextLong();
        for (PrimitiveSink sink : sinks) {
          sink.putLong(value);
        }
      }
    },
    PUT_FLOAT() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        float value = random.nextFloat();
        for (PrimitiveSink sink : sinks) {
          sink.putFloat(value);
        }
      }
    },
    PUT_DOUBLE() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        double value = random.nextDouble();
        for (PrimitiveSink sink : sinks) {
          sink.putDouble(value);
        }
      }
    },
    PUT_BYTES() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        byte[] value = new byte[random.nextInt(128)];
        random.nextBytes(value);
        for (PrimitiveSink sink : sinks) {
          sink.putBytes(value);
        }
      }
    },
    PUT_BYTES_INT_INT() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        byte[] value = new byte[random.nextInt(128)];
        random.nextBytes(value);
        int off = random.nextInt(value.length + 1);
        int len = random.nextInt(value.length - off + 1);
        for (PrimitiveSink sink : sinks) {
          sink.putBytes(value, off, len);
        }
      }
    },
    PUT_BYTE_BUFFER() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        byte[] value = new byte[random.nextInt(128)];
        random.nextBytes(value);
        int pos = random.nextInt(value.length + 1);
        int limit = pos + random.nextInt(value.length - pos + 1);
        for (PrimitiveSink sink : sinks) {
          ByteBuffer buffer = ByteBuffer.wrap(value);
          Java8Compatibility.position(buffer, pos);
          Java8Compatibility.limit(buffer, limit);
          sink.putBytes(buffer);
          assertEquals(limit, buffer.limit());
          assertEquals(limit, buffer.position());
        }
      }
    },
    PUT_STRING() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        char[] value = new char[random.nextInt(128)];
        for (int i = 0; i < value.length; i++) {
          value[i] = (char) random.nextInt();
        }
        String s = new String(value);
        for (PrimitiveSink sink : sinks) {
          sink.putUnencodedChars(s);
        }
      }
    },
    PUT_STRING_LOW_SURROGATE() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        String s = new String(new char[] {randomLowSurrogate(random)});
        for (PrimitiveSink sink : sinks) {
          sink.putUnencodedChars(s);
        }
      }
    },
    PUT_STRING_HIGH_SURROGATE() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        String s = new String(new char[] {randomHighSurrogate(random)});
        for (PrimitiveSink sink : sinks) {
          sink.putUnencodedChars(s);
        }
      }
    },
    PUT_STRING_LOW_HIGH_SURROGATE() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        String s = new String(new char[] {randomLowSurrogate(random), randomHighSurrogate(random)});
        for (PrimitiveSink sink : sinks) {
          sink.putUnencodedChars(s);
        }
      }
    },
    PUT_STRING_HIGH_LOW_SURROGATE() {
      @Override
      void performAction(Random random, Iterable<? extends PrimitiveSink> sinks) {
        String s = new String(new char[] {randomHighSurrogate(random), randomLowSurrogate(random)});
        for (PrimitiveSink sink : sinks) {
          sink.putUnencodedChars(s);
        }
      }
    };

    abstract void performAction(Random random, Iterable<? extends PrimitiveSink> sinks);

    private static final RandomHasherAction[] actions = values();

    static RandomHasherAction pickAtRandom(Random random) {
      return actions[random.nextInt(actions.length)];
    }
  }

  /**
   * Test that the hash function contains no funnels. A funnel is a situation where a set of input
   * (key) bits 'affects' a strictly smaller set of output bits. Funneling is bad because it can
   * result in more-than-ideal collisions for a non-uniformly distributed key space. In practice,
   * most key spaces are ANYTHING BUT uniformly distributed. A bit(i) in the input is said to
   * 'affect' a bit(j) in the output if two inputs, identical but for bit(i), will differ at output
   * bit(j) about half the time
   *
   * <p>Funneling is pretty simple to detect. The key idea is to find example keys which
   * unequivocally demonstrate that funneling cannot be occurring. This is done bit-by-bit. For each
   * input bit(i) and output bit(j), two pairs of keys must be found with all bits identical except
   * bit(i). One pair must differ in output bit(j), and one pair must not. This proves that input
   * bit(i) can alter output bit(j).
   */
  static void checkNoFunnels(HashFunction function) {
    Random rand = new Random(0);
    int keyBits = 32;
    int hashBits = function.bits();

    // output loop tests input bit
    for (int i = 0; i < keyBits; i++) {
      int same = 0x0; // bitset for output bits with same values
      int diff = 0x0; // bitset for output bits with different values
      int count = 0;
      // originally was 2 * Math.log(...), making it try more times to avoid flakiness issues
      int maxCount = (int) (4 * Math.log(2 * keyBits * hashBits) + 1);
      while (same != 0xffffffff || diff != 0xffffffff) {
        int key1 = rand.nextInt();
        // flip input bit for key2
        int key2 = key1 ^ (1 << i);
        // get hashes
        int hash1 = function.hashInt(key1).asInt();
        int hash2 = function.hashInt(key2).asInt();
        // test whether the hash values have same output bits
        same |= ~(hash1 ^ hash2);
        // test whether the hash values have different output bits
        diff |= (hash1 ^ hash2);

        count++;
        // check whether we've exceeded the probabilistically
        // likely number of trials to have proven no funneling
        if (count > maxCount) {
          Assert.fail(
              "input bit("
                  + i
                  + ") was found not to affect all "
                  + hashBits
                  + " output bits; The unaffected bits are "
                  + "as follows: "
                  + ~(same & diff)
                  + ". This was "
                  + "determined after "
                  + count
                  + " trials.");
        }
      }
    }
  }

  /**
   * Test for avalanche. Avalanche means that output bits differ with roughly 1/2 probability on
   * different input keys. This test verifies that each possible 1-bit key delta achieves avalanche.
   *
   * <p>For more information: http://burtleburtle.net/bob/hash/avalanche.html
   */
  static void checkAvalanche(HashFunction function, int trials, double epsilon) {
    Random rand = new Random(0);
    int keyBits = 32;
    int hashBits = function.bits();
    for (int i = 0; i < keyBits; i++) {
      int[] same = new int[hashBits];
      int[] diff = new int[hashBits];
      // go through trials to compute probability
      for (int j = 0; j < trials; j++) {
        int key1 = rand.nextInt();
        // flip input bit for key2
        int key2 = key1 ^ (1 << i);
        // compute hash values
        int hash1 = function.hashInt(key1).asInt();
        int hash2 = function.hashInt(key2).asInt();
        for (int k = 0; k < hashBits; k++) {
          if ((hash1 & (1 << k)) == (hash2 & (1 << k))) {
            same[k] += 1;
          } else {
            diff[k] += 1;
          }
        }
      }
      // measure probability and assert it's within margin of error
      for (int j = 0; j < hashBits; j++) {
        double prob = (double) diff[j] / (double) (diff[j] + same[j]);
        Assert.assertEquals(0.50d, prob, epsilon);
      }
    }
  }

  /**
   * Test for 2-bit characteristics. A characteristic is a delta in the input which is repeated in
   * the output. For example, if f() is a block cipher and c is a characteristic, then f(x^c) =
   * f(x)^c with greater than expected probability. The test for funneling is merely a test for
   * 1-bit characteristics.
   *
   * <p>There is more general code provided by Bob Jenkins to test arbitrarily sized characteristics
   * using the magic of gaussian elimination: http://burtleburtle.net/bob/crypto/findingc.html.
   */
  static void checkNo2BitCharacteristics(HashFunction function) {
    Random rand = new Random(0);
    int keyBits = 32;

    // get every one of (keyBits choose 2) deltas:
    for (int i = 0; i < keyBits; i++) {
      for (int j = 0; j < keyBits; j++) {
        if (j <= i) continue;
        int count = 0;
        int maxCount = 20; // the probability of error here is miniscule
        boolean diff = false;

        while (!diff) {
          int delta = (1 << i) | (1 << j);
          int key1 = rand.nextInt();
          // apply delta
          int key2 = key1 ^ delta;

          // get hashes
          int hash1 = function.hashInt(key1).asInt();
          int hash2 = function.hashInt(key2).asInt();

          // this 2-bit candidate delta is not a characteristic
          // if deltas are different
          if ((hash1 ^ hash2) != delta) {
            diff = true;
            continue;
          }

          // check if we've exceeded the probabilistically
          // likely number of trials to have proven 2-bit candidate
          // is not a characteristic
          count++;
          if (count > maxCount) {
            Assert.fail(
                "2-bit delta ("
                    + i
                    + ", "
                    + j
                    + ") is likely a "
                    + "characteristic for this hash. This was "
                    + "determined after "
                    + count
                    + " 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();
    for (int bit1 = 0; bit1 < keyBits; bit1++) {
      for (int bit2 = 0; bit2 < keyBits; bit2++) {
        if (bit2 <= bit1) continue;
        int delta = (1 << bit1) | (1 << bit2);
        int[] same = new int[hashBits];
        int[] diff = new int[hashBits];
        // go through trials to compute probability
        for (int j = 0; j < trials; j++) {
          int key1 = rand.nextInt();
          // flip input bit for key2
          int key2 = key1 ^ delta;
          // compute hash values
          int hash1 = function.hashInt(key1).asInt();
          int hash2 = function.hashInt(key2).asInt();
          for (int k = 0; k < hashBits; k++) {
            if ((hash1 & (1 << k)) == (hash2 & (1 << k))) {
              same[k] += 1;
            } else {
              diff[k] += 1;
            }
          }
        }
        // measure probability and assert it's within margin of error
        for (int j = 0; j < hashBits; j++) {
          double prob = (double) diff[j] / (double) (diff[j] + same[j]);
          Assert.assertEquals(0.50d, prob, epsilon);
        }
      }
    }
  }

  /**
   * Checks that a Hasher returns the same HashCode when given the same input, and also that the
   * collision rate looks sane.
   */
  static void assertInvariants(HashFunction hashFunction) {
    int objects = 100;
    Set<HashCode> hashcodes = Sets.newHashSetWithExpectedSize(objects);
    Random random = new Random(314159);
    for (int i = 0; i < objects; i++) {
      int value = random.nextInt();
      HashCode hashcode1 = hashFunction.hashInt(value);
      HashCode hashcode2 = hashFunction.hashInt(value);
      Assert.assertEquals(hashcode1, hashcode2); // idempotent
      Assert.assertEquals(hashFunction.bits(), hashcode1.bits());
      Assert.assertEquals(hashFunction.bits(), hashcode1.asBytes().length * 8);
      hashcodes.add(hashcode1);
    }
    Assert.assertTrue(hashcodes.size() > objects * 0.95); // quite relaxed test

    assertHashBytesThrowsCorrectExceptions(hashFunction);
    assertIndependentHashers(hashFunction);
    assertShortcutsAreEquivalent(hashFunction, 512);
  }

  static void assertHashByteBufferInvariants(HashFunction hashFunction) {
    assertHashByteBufferMatchesBytes(hashFunction);
    assertHashByteBufferExhaustsBuffer(hashFunction);
    assertHashByteBufferPreservesByteOrder(hashFunction);
    assertHasherByteBufferPreservesByteOrder(hashFunction);
  }

  static void assertHashByteBufferMatchesBytes(HashFunction hashFunction) {
    Random rng = new Random(0L);
    byte[] bytes = new byte[rng.nextInt(256) + 1];
    rng.nextBytes(bytes);
    assertEquals(hashFunction.hashBytes(bytes), hashFunction.hashBytes(ByteBuffer.wrap(bytes)));
  }

  static void assertHashByteBufferExhaustsBuffer(HashFunction hashFunction) {
    Random rng = new Random(0L);
    byte[] bytes = new byte[rng.nextInt(256) + 1];
    rng.nextBytes(bytes);
    ByteBuffer buffer = ByteBuffer.wrap(bytes);
    HashCode unused = hashFunction.hashBytes(buffer);
    assertFalse(buffer.hasRemaining());
  }

  static void assertHashByteBufferPreservesByteOrder(HashFunction hashFunction) {
    Random rng = new Random(0L);
    byte[] bytes = new byte[rng.nextInt(256) + 1];
    rng.nextBytes(bytes);
    ByteBuffer littleEndian = ByteBuffer.wrap(bytes).order(ByteOrder.LITTLE_ENDIAN);
    ByteBuffer bigEndian = ByteBuffer.wrap(bytes).order(ByteOrder.BIG_ENDIAN);
    assertEquals(hashFunction.hashBytes(littleEndian), hashFunction.hashBytes(bigEndian));
    assertEquals(ByteOrder.LITTLE_ENDIAN, littleEndian.order());
    assertEquals(ByteOrder.BIG_ENDIAN, bigEndian.order());
  }

  static void assertHasherByteBufferPreservesByteOrder(HashFunction hashFunction) {
    Random rng = new Random(0L);
    byte[] bytes = new byte[rng.nextInt(256) + 1];
    rng.nextBytes(bytes);
    ByteBuffer littleEndian = ByteBuffer.wrap(bytes).order(ByteOrder.LITTLE_ENDIAN);
    ByteBuffer bigEndian = ByteBuffer.wrap(bytes).order(ByteOrder.BIG_ENDIAN);
    assertEquals(
        hashFunction.newHasher().putBytes(littleEndian).hash(),
        hashFunction.newHasher().putBytes(bigEndian).hash());
    assertEquals(ByteOrder.LITTLE_ENDIAN, littleEndian.order());
    assertEquals(ByteOrder.BIG_ENDIAN, bigEndian.order());
  }

  static void assertHashBytesThrowsCorrectExceptions(HashFunction hashFunction) {
    {
      HashCode unused = hashFunction.hashBytes(new byte[64], 0, 0);
    }

    try {
      hashFunction.hashBytes(new byte[128], -1, 128);
      Assert.fail();
    } catch (IndexOutOfBoundsException expected) {
    }
    try {
      hashFunction.hashBytes(new byte[128], 64, 256 /* too long len */);
      Assert.fail();
    } catch (IndexOutOfBoundsException expected) {
    }
    try {
      hashFunction.hashBytes(new byte[64], 0, -1);
      Assert.fail();
    } catch (IndexOutOfBoundsException expected) {
    }
  }

  static void assertIndependentHashers(HashFunction hashFunction) {
    int numActions = 100;
    // hashcodes from non-overlapping hash computations
    HashCode expected1 = randomHash(hashFunction, new Random(1L), numActions);
    HashCode expected2 = randomHash(hashFunction, new Random(2L), numActions);

    // equivalent, but overlapping, computations (should produce the same results as above)
    Random random1 = new Random(1L);
    Random random2 = new Random(2L);
    Hasher hasher1 = hashFunction.newHasher();
    Hasher hasher2 = hashFunction.newHasher();
    for (int i = 0; i < numActions; i++) {
      RandomHasherAction.pickAtRandom(random1).performAction(random1, ImmutableSet.of(hasher1));
      RandomHasherAction.pickAtRandom(random2).performAction(random2, ImmutableSet.of(hasher2));
    }

    Assert.assertEquals(expected1, hasher1.hash());
    Assert.assertEquals(expected2, hasher2.hash());
  }

  static HashCode randomHash(HashFunction hashFunction, Random random, int numActions) {
    Hasher hasher = hashFunction.newHasher();
    for (int i = 0; i < numActions; i++) {
      RandomHasherAction.pickAtRandom(random).performAction(random, ImmutableSet.of(hasher));
    }
    return hasher.hash();
  }

  private static void assertShortcutsAreEquivalent(HashFunction hashFunction, int trials) {
    Random random = new Random(42085L);
    for (int i = 0; i < trials; i++) {
      assertHashBytesEquivalence(hashFunction, random);
      assertHashByteBufferEquivalence(hashFunction, random);
      assertHashIntEquivalence(hashFunction, random);
      assertHashLongEquivalence(hashFunction, random);
      assertHashStringEquivalence(hashFunction, random);
      assertHashStringWithSurrogatesEquivalence(hashFunction, random);
    }
  }

  private static void assertHashBytesEquivalence(HashFunction hashFunction, Random random) {
    int size = random.nextInt(2048);
    byte[] bytes = new byte[size];
    random.nextBytes(bytes);
    assertEquals(
        hashFunction.hashBytes(bytes), hashFunction.newHasher(size).putBytes(bytes).hash());
    int off = random.nextInt(size);
    int len = random.nextInt(size - off);
    assertEquals(
        hashFunction.hashBytes(bytes, off, len),
        hashFunction.newHasher(size).putBytes(bytes, off, len).hash());
  }

  private static void assertHashByteBufferEquivalence(HashFunction hashFunction, Random random) {
    int size = random.nextInt(2048);
    byte[] bytes = new byte[size];
    random.nextBytes(bytes);
    assertEquals(
        hashFunction.hashBytes(ByteBuffer.wrap(bytes)),
        hashFunction.newHasher(size).putBytes(ByteBuffer.wrap(bytes)).hash());
    int off = random.nextInt(size);
    int len = random.nextInt(size - off);
    assertEquals(
        hashFunction.hashBytes(ByteBuffer.wrap(bytes, off, len)),
        hashFunction.newHasher(size).putBytes(ByteBuffer.wrap(bytes, off, len)).hash());
  }

  private static void assertHashIntEquivalence(HashFunction hashFunction, Random random) {
    int i = random.nextInt();
    assertEquals(hashFunction.hashInt(i), hashFunction.newHasher().putInt(i).hash());
  }

  private static void assertHashLongEquivalence(HashFunction hashFunction, Random random) {
    long l = random.nextLong();
    assertEquals(hashFunction.hashLong(l), hashFunction.newHasher().putLong(l).hash());
  }

  private static final ImmutableSet<Charset> CHARSETS =
      ImmutableSet.of(
          Charsets.ISO_8859_1,
          Charsets.US_ASCII,
          Charsets.UTF_16,
          Charsets.UTF_16BE,
          Charsets.UTF_16LE,
          Charsets.UTF_8);

  private static void assertHashStringEquivalence(HashFunction hashFunction, Random random) {
    // Test that only data and data-order is important, not the individual operations.
    new EqualsTester()
        .addEqualityGroup(
            hashFunction.hashUnencodedChars("abc"),
            hashFunction.newHasher().putUnencodedChars("abc").hash(),
            hashFunction.newHasher().putUnencodedChars("ab").putUnencodedChars("c").hash(),
            hashFunction.newHasher().putUnencodedChars("a").putUnencodedChars("bc").hash(),
            hashFunction
                .newHasher()
                .putUnencodedChars("a")
                .putUnencodedChars("b")
                .putUnencodedChars("c")
                .hash(),
            hashFunction.newHasher().putChar('a').putUnencodedChars("bc").hash(),
            hashFunction.newHasher().putUnencodedChars("ab").putChar('c').hash(),
            hashFunction.newHasher().putChar('a').putChar('b').putChar('c').hash())
        .testEquals();

    int size = random.nextInt(2048);
    byte[] bytes = new byte[size];
    random.nextBytes(bytes);
    String string = new String(bytes, Charsets.US_ASCII);
    assertEquals(
        hashFunction.hashUnencodedChars(string),
        hashFunction.newHasher().putUnencodedChars(string).hash());
    for (Charset charset : CHARSETS) {
      assertEquals(
          hashFunction.hashString(string, charset),
          hashFunction.newHasher().putString(string, charset).hash());
    }
  }

  /**
   * This verifies that putUnencodedChars(String) and hashUnencodedChars(String) are equivalent,
   * even for funny strings composed by (possibly unmatched, and mostly illegal) surrogate
   * characters. (But doesn't test that they do the right thing - just their consistency).
   */
  private static void assertHashStringWithSurrogatesEquivalence(
      HashFunction hashFunction, Random random) {
    int size = random.nextInt(8) + 1;
    char[] chars = new char[size];
    for (int i = 0; i < chars.length; i++) {
      chars[i] = random.nextBoolean() ? randomLowSurrogate(random) : randomHighSurrogate(random);
    }
    String string = new String(chars);
    assertEquals(
        hashFunction.hashUnencodedChars(string),
        hashFunction.newHasher().putUnencodedChars(string).hash());
  }

  static char randomLowSurrogate(Random random) {
    return (char)
        (Character.MIN_LOW_SURROGATE
            + random.nextInt(Character.MAX_LOW_SURROGATE - Character.MIN_LOW_SURROGATE + 1));
  }

  static char randomHighSurrogate(Random random) {
    return (char)
        (Character.MIN_HIGH_SURROGATE
            + random.nextInt(Character.MAX_HIGH_SURROGATE - Character.MIN_HIGH_SURROGATE + 1));
  }
}