diff --git a/js/binary/arith.js b/js/binary/arith.js
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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+/**
+ * @fileoverview This file contains helper code used by jspb.utils to
+ * handle 64-bit integer conversion to/from strings.
+ *
+ * @author cfallin@google.com (Chris Fallin)
+ *
+ * TODO(haberman): move this to javascript/closure/math?
+ */
+
+goog.provide('jspb.arith.Int64');
+goog.provide('jspb.arith.UInt64');
+
+/**
+ * UInt64 implements some 64-bit arithmetic routines necessary for properly
+ * handling 64-bit integer fields. It implements lossless integer arithmetic on
+ * top of JavaScript's number type, which has only 53 bits of precision, by
+ * representing 64-bit integers as two 32-bit halves.
+ *
+ * @param {number} lo The low 32 bits.
+ * @param {number} hi The high 32 bits.
+ * @constructor
+ */
+jspb.arith.UInt64 = function(lo, hi) {
+  /**
+   * The low 32 bits.
+   * @public {number}
+   */
+  this.lo = lo;
+  /**
+   * The high 32 bits.
+   * @public {number}
+   */
+  this.hi = hi;
+};
+
+
+/**
+ * Compare two 64-bit numbers. Returns -1 if the first is
+ * less, +1 if the first is greater, or 0 if both are equal.
+ * @param {!jspb.arith.UInt64} other
+ * @return {number}
+ */
+jspb.arith.UInt64.prototype.cmp = function(other) {
+  if (this.hi < other.hi || (this.hi == other.hi && this.lo < other.lo)) {
+    return -1;
+  } else if (this.hi == other.hi && this.lo == other.lo) {
+    return 0;
+  } else {
+    return 1;
+  }
+};
+
+
+/**
+ * Right-shift this number by one bit.
+ * @return {!jspb.arith.UInt64}
+ */
+jspb.arith.UInt64.prototype.rightShift = function() {
+  var hi = this.hi >>> 1;
+  var lo = (this.lo >>> 1) | ((this.hi & 1) << 31);
+  return new jspb.arith.UInt64(lo >>> 0, hi >>> 0);
+};
+
+
+/**
+ * Left-shift this number by one bit.
+ * @return {!jspb.arith.UInt64}
+ */
+jspb.arith.UInt64.prototype.leftShift = function() {
+  var lo = this.lo << 1;
+  var hi = (this.hi << 1) | (this.lo >>> 31);
+  return new jspb.arith.UInt64(lo >>> 0, hi >>> 0);
+};
+
+
+/**
+ * Test the MSB.
+ * @return {boolean}
+ */
+jspb.arith.UInt64.prototype.msb = function() {
+  return !!(this.hi & 0x80000000);
+};
+
+
+/**
+ * Test the LSB.
+ * @return {boolean}
+ */
+jspb.arith.UInt64.prototype.lsb = function() {
+  return !!(this.lo & 1);
+};
+
+
+/**
+ * Test whether this number is zero.
+ * @return {boolean}
+ */
+jspb.arith.UInt64.prototype.zero = function() {
+  return this.lo == 0 && this.hi == 0;
+};
+
+
+/**
+ * Add two 64-bit numbers to produce a 64-bit number.
+ * @param {!jspb.arith.UInt64} other
+ * @return {!jspb.arith.UInt64}
+ */
+jspb.arith.UInt64.prototype.add = function(other) {
+  var lo = ((this.lo + other.lo) & 0xffffffff) >>> 0;
+  var hi =
+      (((this.hi + other.hi) & 0xffffffff) >>> 0) +
+      (((this.lo + other.lo) >= 0x100000000) ? 1 : 0);
+  return new jspb.arith.UInt64(lo >>> 0, hi >>> 0);
+};
+
+
+/**
+ * Subtract two 64-bit numbers to produce a 64-bit number.
+ * @param {!jspb.arith.UInt64} other
+ * @return {!jspb.arith.UInt64}
+ */
+jspb.arith.UInt64.prototype.sub = function(other) {
+  var lo = ((this.lo - other.lo) & 0xffffffff) >>> 0;
+  var hi =
+      (((this.hi - other.hi) & 0xffffffff) >>> 0) -
+      (((this.lo - other.lo) < 0) ? 1 : 0);
+  return new jspb.arith.UInt64(lo >>> 0, hi >>> 0);
+};
+
+
+/**
+ * Multiply two 32-bit numbers to produce a 64-bit number.
+ * @param {number} a The first integer:  must be in [0, 2^32-1).
+ * @param {number} b The second integer: must be in [0, 2^32-1).
+ * @return {!jspb.arith.UInt64}
+ */
+jspb.arith.UInt64.mul32x32 = function(a, b) {
+  // Directly multiplying two 32-bit numbers may produce up to 64 bits of
+  // precision, thus losing precision because of the 53-bit mantissa of
+  // JavaScript numbers. So we multiply with 16-bit digits (radix 65536)
+  // instead.
+  var aLow = (a & 0xffff);
+  var aHigh = (a >>> 16);
+  var bLow = (b & 0xffff);
+  var bHigh = (b >>> 16);
+  var productLow =
+      // 32-bit result, result bits 0-31, take all 32 bits
+      (aLow * bLow) +
+      // 32-bit result, result bits 16-47, take bottom 16 as our top 16
+      ((aLow * bHigh) & 0xffff) * 0x10000 +
+      // 32-bit result, result bits 16-47, take bottom 16 as our top 16
+      ((aHigh * bLow) & 0xffff) * 0x10000;
+  var productHigh =
+      // 32-bit result, result bits 32-63, take all 32 bits
+      (aHigh * bHigh) +
+      // 32-bit result, result bits 16-47, take top 16 as our bottom 16
+      ((aLow * bHigh) >>> 16) +
+      // 32-bit result, result bits 16-47, take top 16 as our bottom 16
+      ((aHigh * bLow) >>> 16);
+
+  // Carry. Note that we actually have up to *two* carries due to addition of
+  // three terms.
+  while (productLow >= 0x100000000) {
+    productLow -= 0x100000000;
+    productHigh += 1;
+  }
+
+  return new jspb.arith.UInt64(productLow >>> 0, productHigh >>> 0);
+};
+
+
+/**
+ * Multiply this number by a 32-bit number, producing a 96-bit number, then
+ * truncate the top 32 bits.
+ * @param {number} a The multiplier.
+ * @return {!jspb.arith.UInt64}
+ */
+jspb.arith.UInt64.prototype.mul = function(a) {
+  // Produce two parts: at bits 0-63, and 32-95.
+  var lo = jspb.arith.UInt64.mul32x32(this.lo, a);
+  var hi = jspb.arith.UInt64.mul32x32(this.hi, a);
+  // Left-shift hi by 32 bits, truncating its top bits. The parts will then be
+  // aligned for addition.
+  hi.hi = hi.lo;
+  hi.lo = 0;
+  return lo.add(hi);
+};
+
+
+/**
+ * Divide a 64-bit number by a 32-bit number to produce a
+ * 64-bit quotient and a 32-bit remainder.
+ * @param {number} _divisor
+ * @return {Array.<jspb.arith.UInt64>} array of [quotient, remainder],
+ * unless divisor is 0, in which case an empty array is returned.
+ */
+jspb.arith.UInt64.prototype.div = function(_divisor) {
+  if (_divisor == 0) {
+    return [];
+  }
+
+  // We perform long division using a radix-2 algorithm, for simplicity (i.e.,
+  // one bit at a time). TODO: optimize to a radix-2^32 algorithm, taking care
+  // to get the variable shifts right.
+  var quotient = new jspb.arith.UInt64(0, 0);
+  var remainder = new jspb.arith.UInt64(this.lo, this.hi);
+  var divisor = new jspb.arith.UInt64(_divisor, 0);
+  var unit = new jspb.arith.UInt64(1, 0);
+
+  // Left-shift the divisor and unit until the high bit of divisor is set.
+  while (!divisor.msb()) {
+    divisor = divisor.leftShift();
+    unit = unit.leftShift();
+  }
+
+  // Perform long division one bit at a time.
+  while (!unit.zero()) {
+    // If divisor < remainder, add unit to quotient and subtract divisor from
+    // remainder.
+    if (divisor.cmp(remainder) <= 0) {
+      quotient = quotient.add(unit);
+      remainder = remainder.sub(divisor);
+    }
+    // Right-shift the divisor and unit.
+    divisor = divisor.rightShift();
+    unit = unit.rightShift();
+  }
+
+  return [quotient, remainder];
+};
+
+
+/**
+ * Convert a 64-bit number to a string.
+ * @return {string}
+ * @override
+ */
+jspb.arith.UInt64.prototype.toString = function() {
+  var result = '';
+  var num = this;
+  while (!num.zero()) {
+    var divResult = num.div(10);
+    var quotient = divResult[0], remainder = divResult[1];
+    result = remainder.lo + result;
+    num = quotient;
+  }
+  if (result == '') {
+    result = '0';
+  }
+  return result;
+};
+
+
+/**
+ * Parse a string into a 64-bit number. Returns `null` on a parse error.
+ * @param {string} s
+ * @return {?jspb.arith.UInt64}
+ */
+jspb.arith.UInt64.fromString = function(s) {
+  var result = new jspb.arith.UInt64(0, 0);
+  // optimization: reuse this instance for each digit.
+  var digit64 = new jspb.arith.UInt64(0, 0);
+  for (var i = 0; i < s.length; i++) {
+    if (s[i] < '0' || s[i] > '9') {
+      return null;
+    }
+    var digit = parseInt(s[i], 10);
+    digit64.lo = digit;
+    result = result.mul(10).add(digit64);
+  }
+  return result;
+};
+
+
+/**
+ * Make a copy of the uint64.
+ * @return {!jspb.arith.UInt64}
+ */
+jspb.arith.UInt64.prototype.clone = function() {
+  return new jspb.arith.UInt64(this.lo, this.hi);
+};
+
+
+/**
+ * Int64 is like UInt64, but modifies string conversions to interpret the stored
+ * 64-bit value as a twos-complement-signed integer. It does *not* support the
+ * full range of operations that UInt64 does: only add, subtract, and string
+ * conversions.
+ *
+ * N.B. that multiply and divide routines are *NOT* supported. They will throw
+ * exceptions. (They are not necessary to implement string conversions, which
+ * are the only operations we really need in jspb.)
+ *
+ * @param {number} lo The low 32 bits.
+ * @param {number} hi The high 32 bits.
+ * @constructor
+ */
+jspb.arith.Int64 = function(lo, hi) {
+  /**
+   * The low 32 bits.
+   * @public {number}
+   */
+  this.lo = lo;
+  /**
+   * The high 32 bits.
+   * @public {number}
+   */
+  this.hi = hi;
+};
+
+
+/**
+ * Add two 64-bit numbers to produce a 64-bit number.
+ * @param {!jspb.arith.Int64} other
+ * @return {!jspb.arith.Int64}
+ */
+jspb.arith.Int64.prototype.add = function(other) {
+  var lo = ((this.lo + other.lo) & 0xffffffff) >>> 0;
+  var hi =
+      (((this.hi + other.hi) & 0xffffffff) >>> 0) +
+      (((this.lo + other.lo) >= 0x100000000) ? 1 : 0);
+  return new jspb.arith.Int64(lo >>> 0, hi >>> 0);
+};
+
+
+/**
+ * Subtract two 64-bit numbers to produce a 64-bit number.
+ * @param {!jspb.arith.Int64} other
+ * @return {!jspb.arith.Int64}
+ */
+jspb.arith.Int64.prototype.sub = function(other) {
+  var lo = ((this.lo - other.lo) & 0xffffffff) >>> 0;
+  var hi =
+      (((this.hi - other.hi) & 0xffffffff) >>> 0) -
+      (((this.lo - other.lo) < 0) ? 1 : 0);
+  return new jspb.arith.Int64(lo >>> 0, hi >>> 0);
+};
+
+
+/**
+ * Make a copy of the int64.
+ * @return {!jspb.arith.Int64}
+ */
+jspb.arith.Int64.prototype.clone = function() {
+  return new jspb.arith.Int64(this.lo, this.hi);
+};
+
+
+/**
+ * Convert a 64-bit number to a string.
+ * @return {string}
+ * @override
+ */
+jspb.arith.Int64.prototype.toString = function() {
+  // If the number is negative, find its twos-complement inverse.
+  var sign = (this.hi & 0x80000000) != 0;
+  var num = new jspb.arith.UInt64(this.lo, this.hi);
+  if (sign) {
+    num = new jspb.arith.UInt64(0, 0).sub(num);
+  }
+  return (sign ? '-' : '') + num.toString();
+};
+
+
+/**
+ * Parse a string into a 64-bit number. Returns `null` on a parse error.
+ * @param {string} s
+ * @return {?jspb.arith.Int64}
+ */
+jspb.arith.Int64.fromString = function(s) {
+  var hasNegative = (s.length > 0 && s[0] == '-');
+  if (hasNegative) {
+    s = s.substring(1);
+  }
+  var num = jspb.arith.UInt64.fromString(s);
+  if (num === null) {
+    return null;
+  }
+  if (hasNegative) {
+    num = new jspb.arith.UInt64(0, 0).sub(num);
+  }
+  return new jspb.arith.Int64(num.lo, num.hi);
+};