Add no-threads ParallelFor implementation.

- When Ceres was configured without any multithreading enabled (actually
  the default) it would fail to build due to undefined symbols for
  ParallelFor() as there was no implementation for that case.
- This adds a trivial single-threaded implementation for that case,
  taken from the C++11 threads implementation.

Change-Id: Ib3cfd620842b09f1acafe95b8b0ba3aee4587581

internal/ceres/CMakeLists.txt

diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index a22b734..0532073 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -36,6 +36,8 @@
   set(CERES_PARALLEL_FOR_SRC parallel_for_openmp.cc)
 elseif (TBB)
   set(CERES_PARALLEL_FOR_SRC parallel_for_tbb.cc)
+else()
+  set(CERES_PARALLEL_FOR_SRC parallel_for_nothreads.cc)
 endif()
 
 set(CERES_INTERNAL_SRC

internal/ceres/parallel_for_nothreads.cc

diff --git a/internal/ceres/parallel_for_nothreads.cc b/internal/ceres/parallel_for_nothreads.cc
new file mode 100644
index 0000000..63b8f2a
--- /dev/null
+++ b/internal/ceres/parallel_for_nothreads.cc
@@ -0,0 +1,72 @@
+// 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: alexs.mac@gmail.com (Alex Stewart)
+
+// This include must come before any #ifndef check on Ceres compile options.
+#include "ceres/internal/port.h"
+
+#include "ceres/parallel_for.h"
+#include "glog/logging.h"
+
+namespace ceres {
+namespace internal {
+
+void ParallelFor(ContextImpl* context,
+                 int start,
+                 int end,
+                 int num_threads,
+                 const std::function<void(int)>& function) {
+  CHECK_GT(num_threads, 0);
+  CHECK(context != NULL);
+  if (end <= start) {
+    return;
+  }
+  for (int i = start; i < end; ++i) {
+    function(i);
+  }
+}
+
+void ParallelFor(ContextImpl* context,
+                 int start,
+                 int end,
+                 int num_threads,
+                 const std::function<void(int thread_id, int i)>& function) {
+  CHECK_GT(num_threads, 0);
+  CHECK(context != NULL);
+  if (end <= start) {
+    return;
+  }
+  const int thread_id = 0;
+  for (int i = start; i < end; ++i) {
+    function(thread_id, i);
+  }
+}
+
+}
+}