add BloomFilter class

packages/firestore/src/remote/bloom_filter.ts

@@ -0,0 +1,110 @@
+/**
+ * @license
+ * Copyright 2022 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.
+ */
+import { Md5, Integer } from '@firebase/webchannel-wrapper';
+
+import { newTextEncoder } from '../platform/serializer';
+import { debugAssert } from '../util/assert';
+
+const MAX_64_BIT_UNSIGNED_INTEGER = new Integer([0xffffffff, 0xffffffff], 0);
+
+// Hash a string using md5 hashing algorithm.
+function getMd5HashValue(value: string): Uint8Array {
+  const encodedValue = newTextEncoder().encode(value);
+  const md5 = new Md5();
+  md5.update(encodedValue);
+  return new Uint8Array(md5.digest());
+}
+
+// Interpret the 16 bytes array as two 64-bit unsigned integers, encoded using
+// 2’s complement using little endian.
+function get64BitUints(Bytes: Uint8Array): [Integer, Integer] {
+  const dataView = new DataView(Bytes.buffer);
+  const chunk1 = dataView.getUint32(0, /* littleEndian= */ true);
+  const chunk2 = dataView.getUint32(4, /* littleEndian= */ true);
+  const chunk3 = dataView.getUint32(8, /* littleEndian= */ true);
+  const chunk4 = dataView.getUint32(12, /* littleEndian= */ true);
+  const integer1 = new Integer([chunk1, chunk2], 0);
+  const integer2 = new Integer([chunk3, chunk4], 0);
+  return [integer1, integer2];
+}
+
+export class BloomFilter {
+  readonly size: number;
+  private readonly sizeInInteger: Integer;
+
+  constructor(
+    private readonly bitmap: Uint8Array,
+    padding: number,
+    private readonly hashCount: number
+  ) {
+    debugAssert(padding >= 0 && padding < 8, `Invalid padding: ${padding}`);
+    if (bitmap.length > 0) {
+      debugAssert(this.hashCount > 0, `Invalid hash count: ${hashCount}`);
+    } else {
+      // Only empty bloom filter can have 0 hash count.
+      debugAssert(this.hashCount >= 0, `Invalid hash count: ${hashCount}`);
+
+      // Empty bloom filter should have 0 padding.
+      debugAssert(
+        padding === 0,
+        `Invalid padding when bitmap length is 0: ${padding}`
+      );
+    }
+
+    this.size = bitmap.length * 8 - padding;
+    // Set the size in Integer to avoid repeated calculation in mightContain().
+    this.sizeInInteger = Integer.fromNumber(this.size);
+  }
+
+  // Calculate the ith hash value based on the hashed 64bit integers,
+  // and calculate its corresponding bit index in the bitmap to be checked.
+  private getBitIndex(num1: Integer, num2: Integer, index: number): number {
+    // Calculate hashed value h(i) = h1 + (i * h2).
+    let hashValue = num1.add(num2.multiply(Integer.fromNumber(index)));
+    // Wrap if hash value overflow 64bit.
+    if (hashValue.compare(MAX_64_BIT_UNSIGNED_INTEGER) === 1) {
+      hashValue = new Integer([hashValue.getBits(0), hashValue.getBits(1)], 0);
+    }
+    return hashValue.modulo(this.sizeInInteger).toNumber();
+  }
+
+  // Return whether the bit on the given index in the bitmap is set to 1.
+  private isBitSet(index: number): boolean {
+    // To retrieve bit n, calculate: (bitmap[n / 8] & (0x01 << (n % 8))).
+    const byte = this.bitmap[Math.floor(index / 8)];
+    const offset = index % 8;
+    return (byte & (0x01 << offset)) !== 0;
+  }
+
+  mightContain(value: string): boolean {
+    // Empty bitmap and empty value should always return false on membership
+    // check.
+    if (this.size === 0 || value === '') {
+      return false;
+    }
+
+    const md5HashedValue = getMd5HashValue(value);
+    const [hash1, hash2] = get64BitUints(md5HashedValue);
+    for (let i = 0; i < this.hashCount; i++) {
+      const index = this.getBitIndex(hash1, hash2, i);
+      if (!this.isBitSet(index)) {
+        return false;
+      }
+    }
+    return true;
+  }
+}