diff --git a/internal/ceres/thread_pool.h b/internal/ceres/thread_pool.h
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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)
+
+#ifndef CERES_INTERNAL_THREAD_POOL_H_
+#define CERES_INTERNAL_THREAD_POOL_H_
+
+#include <mutex>
+#include <thread>
+#include <vector>
+
+#include "ceres/concurrent_queue.h"
+
+namespace ceres {
+namespace internal {
+
+// A thread-safe thread pool with an unbounded task queue and a resizable number
+// of workers.  The size of the thread pool can be increased by never decreased
+// in order to support the largest number of threads requested.  The ThreadPool
+// has three states:
+//
+//  (1) The thread pool size is zero.  Tasks may be added to the thread pool via
+//  AddTask but they will not be executed until the thread pool is resized.
+//
+//  (2) The thread pool size is greater than zero.  Tasks may be added to the
+//  thread pool and will be executed as soon as a worker is available.  The
+//  thread pool may be resized while the thread pool is running.
+//
+//  (3) The thread pool is destructing.  The thread pool will signal all the
+//  workers to stop.  The workers will finish all of the tasks that have already
+//  been added to the thread pool.
+//
+class ThreadPool {
+ public:
+  // Default constructor with no active threads.  We allow instantiating a
+  // thread pool with no threads to support the use case of single threaded
+  // Ceres where everything will be executed on the main thread. For single
+  // threaded execution this has two benefits: avoid any overhead as threads
+  // are expensive to create, and no unused threads shown in the debugger.
+  ThreadPool();
+
+  // Instantiates a thread pool with min(num_hardware_threads, num_threads)
+  // number of threads.
+  explicit ThreadPool(int num_threads);
+
+  // Signals the workers to stop and waits for them to finish any tasks that
+  // have been scheduled.
+  ~ThreadPool();
+
+  // Resizes the thread pool if it is currently less than the requested number
+  // of threads.  The thread pool will be resized to min(num_hardware_threads,
+  // num_threads) number of threads.  Resize does not support reducing the
+  // thread pool size.  If a smaller number of threads is requested, the thread
+  // pool remains the same size.  The thread pool is reused within Ceres with
+  // different number of threads, and we need to ensure we can support the
+  // largest number of threads requested.  It is safe to resize the thread pool
+  // while the workers are executing tasks, and the resizing is guaranteed to
+  // complete upon return.
+  void Resize(int num_threads);
+
+  // Adds a task to the queue and wakes up a blocked thread.  If the thread pool
+  // size is greater than zero, then the task will be executed by a currently
+  // idle thread or when a thread becomes available.  If the thread pool has no
+  // threads, then the task will never be executed and the user should use
+  // Resize() to create a non-empty thread pool.
+  void AddTask(const std::function<void()>& func);
+
+  // Returns the current size of the thread pool.
+  int Size();
+
+ private:
+  // Main loop for the threads which blocks on the task queue until work becomes
+  // available.  It will return if and only if Stop has been called.
+  void ThreadMainLoop();
+
+  // Signal all the threads to stop.  It does not block until the threads are
+  // finished.
+  void Stop();
+
+  // The queue that stores the units of work available for the thread pool.  The
+  // task queue maintains its own thread safety.
+  ConcurrentQueue<std::function<void()>> task_queue_;
+  std::vector<std::thread> thread_pool_;
+  std::mutex thread_pool_mutex_;
+};
+
+}  // namespace internal
+}  // namespace ceres
+
+#endif  // CERES_INTERNAL_THREAD_POOL_H_