Implement BloomFilter class

firebase-firestore/src/main/java/com/google/firebase/firestore/remote/BloomFilter.java

@@ -0,0 +1,167 @@
+// Copyright 2023 Google LLC
+//
+// 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.firebase.firestore.remote;
+
+import android.util.Base64;
+import androidx.annotation.NonNull;
+import androidx.annotation.VisibleForTesting;
+import java.nio.charset.StandardCharsets;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+
+public class BloomFilter {
+  private final int bitCount;
+  private final byte[] bitmap;
+  private final int hashCount;
+  private final MessageDigest md5HashMessageDigest;
+
+  public BloomFilter(@NonNull byte[] bitmap, int padding, int hashCount) {
+    if (bitmap == null) {
+      throw new NullPointerException("Bitmap cannot be null.");
+    }
+    if (padding < 0 || padding >= 8) {
+      throw new IllegalArgumentException("Invalid padding: " + padding);
+    }
+    if (hashCount < 0) {
+      throw new IllegalArgumentException("Invalid hash count: " + hashCount);
+    }
+    if (bitmap.length > 0 && hashCount == 0) {
+      // Only empty bloom filter can have 0 hash count.
+      throw new IllegalArgumentException("Invalid hash count: " + hashCount);
+    }
+    if (bitmap.length == 0) {
+      // Empty bloom filter should have 0 padding.
+      if (padding != 0) {
+        throw new IllegalArgumentException(
+            "Expected padding of 0 when bitmap length is 0, but got " + padding);
+      }
+    }
+
+    this.bitmap = bitmap;
+    this.hashCount = hashCount;
+    this.bitCount = bitmap.length * 8 - padding;
+    this.md5HashMessageDigest = createMd5HashMessageDigest();
+  }
+
+  @VisibleForTesting
+  int getBitCount() {
+    return this.bitCount;
+  }
+
+  /**
+   * Check whether the given string is a possible member of the bloom filter. It might return false
+   * positive result, ie, the given string is not a member of the bloom filter, but the method
+   * returned true.
+   *
+   * @param value the string to be tested for membership.
+   * @return true if the given string might be contained in the bloom filter, or false if the given
+   *     string is definitely not contained in the bloom filter.
+   */
+  public boolean mightContain(@NonNull String value) {
+    // Empty bitmap should return false on membership check.
+    if (this.bitCount == 0) {
+      return false;
+    }
+
+    byte[] hashedValue = md5HashDigest(value);
+    if (hashedValue.length != 16) {
+      throw new RuntimeException(
+          "Invalid md5 hash array length: " + hashedValue.length + " (expected 16)");
+    }
+
+    long hash1 = getLongLittleEndian(hashedValue, 0);
+    long hash2 = getLongLittleEndian(hashedValue, 8);
+
+    for (int i = 0; i < this.hashCount; i++) {
+      int index = this.getBitIndex(hash1, hash2, i);
+      if (!this.isBitSet(index)) {
+        return false;
+      }
+    }
+    return true;
+  }
+
+  /** Hash a string using md5 hashing algorithm, and return an array of 16 bytes. */
+  @NonNull
+  private byte[] md5HashDigest(@NonNull String value) {
+    return md5HashMessageDigest.digest(value.getBytes(StandardCharsets.UTF_8));
+  }
+
+  @NonNull
+  private static MessageDigest createMd5HashMessageDigest() {
+    try {
+      return MessageDigest.getInstance("MD5");
+    } catch (NoSuchAlgorithmException e) {
+      throw new RuntimeException("Missing MD5 MessageDigest provider: ", e);
+    }
+  }
+
+  /** Interpret 8 bytes into a long, using little endian 2’s complement. */
+  private static long getLongLittleEndian(@NonNull byte[] bytes, int offset) {
+    long result = 0;
+    for (int i = 0; i < 8; i++) {
+      result |= (bytes[offset + i] & 0xFFL) << (i * 8);
+    }
+    return result;
+  }
+
+  /**
+   * Calculate the ith hash value based on the hashed 64 bit unsigned integers, and calculate its
+   * corresponding bit index in the bitmap to be checked.
+   */
+  private int getBitIndex(long hash1, long hash2, int hashIndex) {
+    // Calculate hashed value h(i) = h1 + (i * h2).
+    // Even though we are interpreting hash1 and hash2 as unsigned, the addition and multiplication
+    // operators still perform the correct operation and give the desired overflow behavior.
+    long combinedHash = hash1 + (hash2 * hashIndex);
+    long modulo = unsignedRemainder(combinedHash, this.bitCount);
+    return (int) modulo;
+  }
+
+  /**
+   * Calculate modulo, where the dividend and divisor are treated as unsigned 64-bit longs.
+   *
+   * <p>The implementation is taken from <a
+   * href="https://github.com/google/guava/blob/553037486901cc60820ab7dcb38a25b6f34eba43/android/guava/src/com/google/common/primitives/UnsignedLongs.java">Guava</a>,
+   * simplified to our needs.
+   *
+   * <p>
+   */
+  private static long unsignedRemainder(long dividend, long divisor) {
+    long quotient = ((dividend >>> 1) / divisor) << 1;
+    long remainder = dividend - quotient * divisor;
+    return remainder - (remainder >= divisor ? divisor : 0);
+  }
+
+  /** Return whether the bit at the given index in the bitmap is set to 1. */
+  private boolean isBitSet(int index) {
+    // To retrieve bit n, calculate: (bitmap[n / 8] & (0x01 << (n % 8))).
+    byte byteAtIndex = this.bitmap[index / 8];
+    int offset = index % 8;
+    return (byteAtIndex & (0x01 << offset)) != 0;
+  }
+
+  @Override
+  public String toString() {
+    return "BloomFilter{"
+        + "hashCount="
+        + hashCount
+        + ", size="
+        + bitCount
+        + ", bitmap=\""
+        + Base64.encodeToString(bitmap, Base64.NO_WRAP)
+        + "\"}";
+  }
+}