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diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc
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+// Copyright 2017 The Abseil 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.
+
+#include "absl/numeric/int128.h"
+
+#include <cassert>
+#include <cstdlib>
+#include <iomanip>
+#include <iostream>  // NOLINT(readability/streams)
+#include <sstream>
+
+namespace absl {
+
+const uint128 kuint128max = MakeUint128(std::numeric_limits<uint64_t>::max(),
+                                        std::numeric_limits<uint64_t>::max());
+
+namespace {
+
+// Returns the 0-based position of the last set bit (i.e., most significant bit)
+// in the given uint64_t. The argument may not be 0.
+//
+// For example:
+//   Given: 5 (decimal) == 101 (binary)
+//   Returns: 2
+#define STEP(T, n, pos, sh)                   \
+  do {                                        \
+    if ((n) >= (static_cast<T>(1) << (sh))) { \
+      (n) = (n) >> (sh);                      \
+      (pos) |= (sh);                          \
+    }                                         \
+  } while (0)
+static inline int Fls64(uint64_t n) {
+  assert(n != 0);
+  int pos = 0;
+  STEP(uint64_t, n, pos, 0x20);
+  uint32_t n32 = static_cast<uint32_t>(n);
+  STEP(uint32_t, n32, pos, 0x10);
+  STEP(uint32_t, n32, pos, 0x08);
+  STEP(uint32_t, n32, pos, 0x04);
+  return pos + ((uint64_t{0x3333333322221100} >> (n32 << 2)) & 0x3);
+}
+#undef STEP
+
+// Like Fls64() above, but returns the 0-based position of the last set bit
+// (i.e., most significant bit) in the given uint128. The argument may not be 0.
+static inline int Fls128(uint128 n) {
+  if (uint64_t hi = Uint128High64(n)) {
+    return Fls64(hi) + 64;
+  }
+  return Fls64(Uint128Low64(n));
+}
+
+// Long division/modulo for uint128 implemented using the shift-subtract
+// division algorithm adapted from:
+// http://stackoverflow.com/questions/5386377/division-without-using
+void DivModImpl(uint128 dividend, uint128 divisor, uint128* quotient_ret,
+                uint128* remainder_ret) {
+  assert(divisor != 0);
+
+  if (divisor > dividend) {
+    *quotient_ret = 0;
+    *remainder_ret = dividend;
+    return;
+  }
+
+  if (divisor == dividend) {
+    *quotient_ret = 1;
+    *remainder_ret = 0;
+    return;
+  }
+
+  uint128 denominator = divisor;
+  uint128 quotient = 0;
+
+  // Left aligns the MSB of the denominator and the dividend.
+  const int shift = Fls128(dividend) - Fls128(denominator);
+  denominator <<= shift;
+
+  // Uses shift-subtract algorithm to divide dividend by denominator. The
+  // remainder will be left in dividend.
+  for (int i = 0; i <= shift; ++i) {
+    quotient <<= 1;
+    if (dividend >= denominator) {
+      dividend -= denominator;
+      quotient |= 1;
+    }
+    denominator >>= 1;
+  }
+
+  *quotient_ret = quotient;
+  *remainder_ret = dividend;
+}
+
+template <typename T>
+uint128 Initialize128FromFloat(T v) {
+  // Rounding behavior is towards zero, same as for built-in types.
+
+  // Undefined behavior if v is NaN or cannot fit into uint128.
+  assert(!std::isnan(v) && v > -1 && v < std::ldexp(static_cast<T>(1), 128));
+
+  if (v >= std::ldexp(static_cast<T>(1), 64)) {
+    uint64_t hi = static_cast<uint64_t>(std::ldexp(v, -64));
+    uint64_t lo = static_cast<uint64_t>(v - std::ldexp(static_cast<T>(hi), 64));
+    return MakeUint128(hi, lo);
+  }
+
+  return MakeUint128(0, static_cast<uint64_t>(v));
+}
+}  // namespace
+
+uint128::uint128(float v) : uint128(Initialize128FromFloat(v)) {}
+uint128::uint128(double v) : uint128(Initialize128FromFloat(v)) {}
+uint128::uint128(long double v) : uint128(Initialize128FromFloat(v)) {}
+
+uint128& uint128::operator/=(const uint128& divisor) {
+  uint128 quotient = 0;
+  uint128 remainder = 0;
+  DivModImpl(*this, divisor, &quotient, &remainder);
+  *this = quotient;
+  return *this;
+}
+uint128& uint128::operator%=(const uint128& divisor) {
+  uint128 quotient = 0;
+  uint128 remainder = 0;
+  DivModImpl(*this, divisor, &quotient, &remainder);
+  *this = remainder;
+  return *this;
+}
+
+std::ostream& operator<<(std::ostream& o, const uint128& b) {
+  std::ios_base::fmtflags flags = o.flags();
+
+  // Select a divisor which is the largest power of the base < 2^64.
+  uint128 div;
+  int div_base_log;
+  switch (flags & std::ios::basefield) {
+    case std::ios::hex:
+      div = 0x1000000000000000;  // 16^15
+      div_base_log = 15;
+      break;
+    case std::ios::oct:
+      div = 01000000000000000000000;  // 8^21
+      div_base_log = 21;
+      break;
+    default:  // std::ios::dec
+      div = 10000000000000000000u;  // 10^19
+      div_base_log = 19;
+      break;
+  }
+
+  // Now piece together the uint128 representation from three chunks of
+  // the original value, each less than "div" and therefore representable
+  // as a uint64_t.
+  std::ostringstream os;
+  std::ios_base::fmtflags copy_mask =
+      std::ios::basefield | std::ios::showbase | std::ios::uppercase;
+  os.setf(flags & copy_mask, copy_mask);
+  uint128 high = b;
+  uint128 low;
+  DivModImpl(high, div, &high, &low);
+  uint128 mid;
+  DivModImpl(high, div, &high, &mid);
+  if (Uint128Low64(high) != 0) {
+    os << Uint128Low64(high);
+    os << std::noshowbase << std::setfill('0') << std::setw(div_base_log);
+    os << Uint128Low64(mid);
+    os << std::setw(div_base_log);
+  } else if (Uint128Low64(mid) != 0) {
+    os << Uint128Low64(mid);
+    os << std::noshowbase << std::setfill('0') << std::setw(div_base_log);
+  }
+  os << Uint128Low64(low);
+  std::string rep = os.str();
+
+  // Add the requisite padding.
+  std::streamsize width = o.width(0);
+  if (static_cast<size_t>(width) > rep.size()) {
+    if ((flags & std::ios::adjustfield) == std::ios::left) {
+      rep.append(width - rep.size(), o.fill());
+    } else {
+      rep.insert(0, width - rep.size(), o.fill());
+    }
+  }
+
+  // Stream the final representation in a single "<<" call.
+  return o << rep;
+}
+
+}  // namespace absl