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diff --git a/extern/ceres/internal/ceres/thread_pool.cc b/extern/ceres/internal/ceres/thread_pool.cc
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+++ b/extern/ceres/internal/ceres/thread_pool.cc
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+// Ceres Solver - A fast non-linear least squares minimizer
+// Copyright 2018 Google Inc. All rights reserved.
+// http://ceres-solver.org/
+//
+// 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.
+//
+// Author: vitus@google.com (Michael Vitus)
+
+// This include must come before any #ifndef check on Ceres compile options.
+#include "ceres/internal/port.h"
+
+#ifdef CERES_USE_CXX_THREADS
+
+#include "ceres/thread_pool.h"
+
+#include <cmath>
+#include <limits>
+
+namespace ceres {
+namespace internal {
+namespace {
+
+// Constrain the total number of threads to the amount the hardware can support.
+int GetNumAllowedThreads(int requested_num_threads) {
+  return std::min(requested_num_threads, ThreadPool::MaxNumThreadsAvailable());
+}
+
+}  // namespace
+
+int ThreadPool::MaxNumThreadsAvailable() {
+  const int num_hardware_threads = std::thread::hardware_concurrency();
+  // hardware_concurrency() can return 0 if the value is not well defined or not
+  // computable.
+  return num_hardware_threads == 0
+      ? std::numeric_limits<int>::max()
+      : num_hardware_threads;
+}
+
+ThreadPool::ThreadPool() { }
+
+ThreadPool::ThreadPool(int num_threads) {
+  Resize(num_threads);
+}
+
+ThreadPool::~ThreadPool() {
+  std::lock_guard<std::mutex> lock(thread_pool_mutex_);
+  // Signal the thread workers to stop and wait for them to finish all scheduled
+  // tasks.
+  Stop();
+  for (std::thread& thread : thread_pool_) {
+    thread.join();
+  }
+}
+
+void ThreadPool::Resize(int num_threads) {
+  std::lock_guard<std::mutex> lock(thread_pool_mutex_);
+
+  const int num_current_threads = thread_pool_.size();
+  if (num_current_threads >= num_threads) {
+    return;
+  }
+
+  const int create_num_threads =
+      GetNumAllowedThreads(num_threads) - num_current_threads;
+
+  for (int i = 0; i < create_num_threads; ++i) {
+    thread_pool_.push_back(std::thread(&ThreadPool::ThreadMainLoop, this));
+  }
+}
+
+void ThreadPool::AddTask(const std::function<void()>& func) {
+  task_queue_.Push(func);
+}
+
+int ThreadPool::Size() {
+  std::lock_guard<std::mutex> lock(thread_pool_mutex_);
+  return thread_pool_.size();
+}
+
+void ThreadPool::ThreadMainLoop() {
+  std::function<void()> task;
+  while (task_queue_.Wait(&task)) {
+    task();
+  }
+}
+
+void ThreadPool::Stop() {
+  task_queue_.StopWaiters();
+}
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif // CERES_USE_CXX_THREADS