diff --git a/packages/firestore/src/index/ordered_code_writer.ts b/packages/firestore/src/index/ordered_code_writer.ts
new file mode 100644
index 00000000000..7e6676bc4d0
--- /dev/null
+++ b/packages/firestore/src/index/ordered_code_writer.ts
@@ -0,0 +1,158 @@
+/**
+ * @license
+ * Copyright 2021 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 | agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES | CONDITIONS OF ANY KIND, either express | implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+const LONG_SIZE = 64;
+const BYTE_SIZE = 8;
+
+/**
+ * The default size of the buffer. This is arbitrary, but likely larger than
+ * most index values so that less copies of the underlying buffer will be made.
+ * For large values, a single copy will made to double the buffer length.
+ */
+const DEFAULT_BUFFER_SIZE = 1024;
+
+/**
+ * Converts a JavaScript number to a byte array (using little endian
+ * encoding).
+ */
+function doubleToLongBits(value: number): Uint8Array {
+  const dv = new DataView(new ArrayBuffer(8));
+  dv.setFloat64(0, value, false);
+  return new Uint8Array(dv.buffer);
+}
+
+/** Counts the number of zeros in a byte. */
+export function numberOfLeadingZerosInByte(x: number): number {
+  if (x === 0) {
+    return 8;
+  }
+
+  let zeros = 0;
+  if (x >> 4 === 0) {
+    // Test if the first four bits are zero.
+    zeros += 4;
+    x = x << 4;
+  }
+  if (x >> 6 === 0) {
+    // Test if the first two (or next two) bits are zero.
+    zeros += 2;
+    x = x << 2;
+  }
+  if (x >> 7 === 0) {
+    // Test if the remaining bit is zero.
+    zeros += 1;
+  }
+  return zeros;
+}
+
+/** Counts the number of leading zeros in the given byte array. */
+function numberOfLeadingZeros(bytes: Uint8Array): number {
+  let leadingZeros = 0;
+  for (let i = 0; i < bytes.length; ++i) {
+    const zeros = numberOfLeadingZerosInByte(bytes[i] & 0xff);
+    leadingZeros += zeros;
+    if (zeros !== 8) {
+      break;
+    }
+  }
+  return leadingZeros;
+}
+
+/**
+ * Returns of bytes required to store "value". Leading zero bytes are skipped.
+ */
+function unsignedNumLength(value: Uint8Array): number {
+  // This is just the number of bytes for the unsigned representation of the number.
+  const numBits = LONG_SIZE - numberOfLeadingZeros(value);
+  return Math.ceil(numBits / BYTE_SIZE);
+}
+
+/**
+ * OrderedCodeWriter is a minimal-allocation implementation of the writing
+ * behavior defined by the backend.
+ *
+ * The code is ported from its Java counterpart.
+ */
+export class OrderedCodeWriter {
+  buffer = new Uint8Array(DEFAULT_BUFFER_SIZE);
+  position = 0;
+
+  writeNumberAscending(val: number): void {
+    const value = this.toOrderedBits(val);
+    const len = unsignedNumLength(value);
+    this.ensureAvailable(1 + len);
+    this.buffer[this.position++] = len & 0xff; // Write the length
+    for (let i = value.length - len; i < value.length; ++i) {
+      this.buffer[this.position++] = value[i] & 0xff;
+    }
+  }
+
+  writeNumberDescending(val: number): void {
+    const value = this.toOrderedBits(val);
+    const len = unsignedNumLength(value);
+    this.ensureAvailable(1 + ~len);
+    this.buffer[this.position++] = ~(len & 0xff); // Write the length
+    for (let i = value.length - len; i < value.length; ++i) {
+      this.buffer[this.position++] = ~(value[i] & 0xff);
+    }
+  }
+
+  /**
+   * Encodes `val` into an encoding that has the following properties:
+   * The order matches the IEEE 754 floating-point comparison results with the
+   * following exceptions:
+   *   -0.0 < 0.0
+   *   all non-NaN < NaN
+   *   NaN = NaN
+   */
+  private toOrderedBits(val: number): Uint8Array {
+    const value = doubleToLongBits(val);
+    const isNegative = (value[0] & 0x80) !== 0;
+    value[0] ^= isNegative ? 0xff : 0x80;
+    for (let i = 1; i < value.length; ++i) {
+      value[i] ^= isNegative ? 0xff : 0x00;
+    }
+    return value;
+  }
+
+  /** Resets the buffer such that it is the same as when it was newly constructed.  */
+  reset(): void {
+    this.position = 0;
+  }
+
+  /** Makes a copy of the encoded bytes in this buffer.  */
+  encodedBytes(): Uint8Array {
+    return this.buffer.slice(0, this.position);
+  }
+
+  private ensureAvailable(bytes: number): void {
+    const minCapacity = bytes + this.position;
+    if (minCapacity <= this.buffer.length) {
+      return;
+    }
+    // Try doubling.
+    let newLength = this.buffer.length * 2;
+    // Still not big enough? Just allocate the right size.
+    if (newLength < minCapacity) {
+      newLength = minCapacity;
+    }
+    // Create the new buffer.
+    const newBuffer = new Uint8Array(newLength);
+    newBuffer.set(this.buffer); // copy old data
+    this.buffer = newBuffer;
+  }
+}
diff --git a/packages/firestore/test/unit/index/ordered_code_writer.test.ts b/packages/firestore/test/unit/index/ordered_code_writer.test.ts
new file mode 100644
index 00000000000..fe1aa188a00
--- /dev/null
+++ b/packages/firestore/test/unit/index/ordered_code_writer.test.ts
@@ -0,0 +1,217 @@
+/**
+ * @license
+ * Copyright 2017 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0x00 (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-2.0x00
+ *
+ * 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 { expect } from 'chai';
+
+import {
+  numberOfLeadingZerosInByte,
+  OrderedCodeWriter
+} from '../../../src/index/ordered_code_writer';
+
+class ValueTestCase<T> {
+  constructor(
+    readonly val: T,
+    readonly ascEncoding: Uint8Array,
+    readonly descEncoding: Uint8Array
+  ) {}
+}
+
+const NUMBER_TEST_CASES: Array<ValueTestCase<number>> = [
+  new ValueTestCase(
+    Number.NEGATIVE_INFINITY,
+    // Note: This values are taken from the Android reference implementation
+    new Uint8Array([0x07, 0x0f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff]),
+    new Uint8Array([0xf8, 0xf0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+  ),
+  new ValueTestCase(
+    Number.MIN_SAFE_INTEGER,
+    new Uint8Array([0x08, 0x3c, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0xc3, 0x3f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    -2,
+    new Uint8Array([0x08, 0x3f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff]),
+    new Uint8Array([0xf7, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+  ),
+  new ValueTestCase(
+    -1,
+    new Uint8Array([0x08, 0x40, 0x0f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff]),
+    new Uint8Array([0xf7, 0xbf, 0xf0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+  ),
+  new ValueTestCase(
+    -0.1,
+    new Uint8Array([0x08, 0x40, 0x46, 0x66, 0x66, 0x66, 0x66, 0x66, 0x65]),
+    new Uint8Array([0xf7, 0xbf, 0xb9, 0x99, 0x99, 0x99, 0x99, 0x99, 0x9a])
+  ),
+  new ValueTestCase(
+    0,
+    new Uint8Array([0x08, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x7f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    Number.MIN_VALUE,
+    new Uint8Array([0x08, 0x80, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x01]),
+    new Uint8Array([0xf7, 0x7f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xfe])
+  ),
+  new ValueTestCase(
+    0.1,
+    new Uint8Array([0x08, 0xbf, 0xb9, 0x99, 0x99, 0x99, 0x99, 0x99, 0x9a]),
+    new Uint8Array([0xf7, 0x40, 0x46, 0x66, 0x66, 0x66, 0x66, 0x66, 0x65])
+  ),
+  new ValueTestCase(
+    1,
+    new Uint8Array([0x08, 0xbf, 0xf0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x40, 0x0f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    2,
+    new Uint8Array([0x08, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    4,
+    new Uint8Array([0x08, 0xc0, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0xef, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    0x08,
+    new Uint8Array([0x08, 0xc0, 0x20, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0xdf, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    0x10,
+    new Uint8Array([0x08, 0xc0, 0x30, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0xcf, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    0x20,
+    new Uint8Array([0x08, 0xc0, 0x40, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0xbf, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    0x40,
+    new Uint8Array([0x08, 0xc0, 0x50, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0xaf, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    0x80,
+    new Uint8Array([0x08, 0xc0, 0x60, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0x9f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    0xff,
+    new Uint8Array([0x08, 0xc0, 0x6f, 0xe0, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0x90, 0x1f, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    256,
+    new Uint8Array([0x08, 0xc0, 0x70, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0x8f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    257,
+    new Uint8Array([0x08, 0xc0, 0x70, 0x10, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x3f, 0x8f, 0xef, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    Number.MAX_SAFE_INTEGER,
+    new Uint8Array([0x08, 0xc3, 0x3f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff]),
+    new Uint8Array([0xf7, 0x3c, 0xc0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00])
+  ),
+  new ValueTestCase(
+    Number.POSITIVE_INFINITY,
+    new Uint8Array([0x08, 0xff, 0xf0, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x00, 0x0f, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  ),
+  new ValueTestCase(
+    Number.NaN,
+    new Uint8Array([0x08, 0xff, 0xf8, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]),
+    new Uint8Array([0xf7, 0x00, 0x07, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff])
+  )
+];
+
+describe('Ordered Code Writer', () => {
+  it('computes number of leading zeros', () => {
+    for (let i = 0; i < 0xff; ++i) {
+      let zeros = 0;
+      for (let bit = 7; bit >= 0; --bit) {
+        if ((i & (1 << bit)) === 0) {
+          ++zeros;
+        } else {
+          break;
+        }
+      }
+      expect(numberOfLeadingZerosInByte(i)).to.equal(zeros, `for number ${i}`);
+    }
+  });
+
+  it('converts numbers to bits', () => {
+    for (let i = 0; i < NUMBER_TEST_CASES.length; ++i) {
+      const bytes = getBytes(NUMBER_TEST_CASES[i].val);
+      expect(bytes.asc).to.deep.equal(
+        NUMBER_TEST_CASES[i].ascEncoding,
+        'Ascending for ' + NUMBER_TEST_CASES[i].val
+      );
+      expect(bytes.desc).to.deep.equal(
+        NUMBER_TEST_CASES[i].descEncoding,
+        'Descending for ' + NUMBER_TEST_CASES[i].val
+      );
+    }
+  });
+
+  it('orders numbers correctly', () => {
+    for (let i = 0; i < NUMBER_TEST_CASES.length; ++i) {
+      for (let j = i; j < NUMBER_TEST_CASES.length; ++j) {
+        const left = NUMBER_TEST_CASES[i].val;
+        const leftBytes = getBytes(left);
+        const right = NUMBER_TEST_CASES[j].val;
+        const rightBytes = getBytes(right);
+        expect(compare(leftBytes.asc, rightBytes.asc)).to.equal(
+          i === j ? 0 : -1,
+          `Ascending order: ${left} vs ${right}`
+        );
+        expect(compare(leftBytes.desc, rightBytes.desc)).to.equal(
+          i === j ? 0 : 1,
+          `Descending order: ${left} vs ${right}`
+        );
+      }
+    }
+  });
+});
+
+function compare(left: Uint8Array, right: Uint8Array): number {
+  for (let i = 0; i < Math.min(left.length, right.length); ++i) {
+    if (left[i] < right[i]) {
+      return -1;
+    }
+    if (left[i] > right[i]) {
+      return 1;
+    }
+  }
+  return left.length - right.length;
+}
+
+function getBytes(val: unknown): { asc: Uint8Array; desc: Uint8Array } {
+  const ascWriter = new OrderedCodeWriter();
+  const descWriter = new OrderedCodeWriter();
+  if (typeof val === 'number') {
+    ascWriter.writeNumberAscending(val);
+    descWriter.writeNumberDescending(val);
+  } else {
+    throw new Error('Encoding not yet supported for ' + val);
+  }
+  return { asc: ascWriter.encodedBytes(), desc: descWriter.encodedBytes() };
+}