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68 lines
2.7 KiB
68 lines
2.7 KiB
3 years ago
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// Ceres Solver - A fast non-linear least squares minimizer
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// Copyright 2018 Google Inc. All rights reserved.
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// http://ceres-solver.org/
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//
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// Redistribution and use in source and binary forms, with or without
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// modification, are permitted provided that the following conditions are met:
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//
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// * Redistributions of source code must retain the above copyright notice,
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// this list of conditions and the following disclaimer.
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// * Redistributions in binary form must reproduce the above copyright notice,
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// this list of conditions and the following disclaimer in the documentation
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// and/or other materials provided with the distribution.
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// * Neither the name of Google Inc. nor the names of its contributors may be
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// used to endorse or promote products derived from this software without
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// specific prior written permission.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
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// AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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// ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
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// LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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// CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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// SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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// CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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// ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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// POSSIBILITY OF SUCH DAMAGE.
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//
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// Author: wjr@google.com (William Rucklidge)
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#ifndef CERES_INTERNAL_PARALLEL_UTILS_H_
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#define CERES_INTERNAL_PARALLEL_UTILS_H_
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namespace ceres {
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namespace internal {
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// Converts a linear iteration order into a triangular iteration order.
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// Suppose you have nested loops that look like
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// for (int i = 0; i < n; i++) {
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// for (int j = i; j < n; j++) {
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// ... use i and j
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// }
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// }
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// Naively using ParallelFor to parallelise those loops might look like
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// ParallelFor(..., 0, n * n, num_threads,
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// [](int thread_id, int k) {
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// int i = k / n, j = k % n;
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// if (j < i) return;
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// ...
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// });
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// but these empty work items can lead to very unbalanced threading. Instead,
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// do this:
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// int actual_work_items = (n * (n + 1)) / 2;
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// ParallelFor(..., 0, actual_work_items, num_threads,
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// [](int thread_id, int k) {
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// int i, j;
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// UnfoldIteration(k, n, &i, &j);
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// ...
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// });
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// which in each iteration will produce i and j satisfying
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// 0 <= i <= j < n
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void LinearIndexToUpperTriangularIndex(int k, int n, int* i, int* j);
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} // namespace internal
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} // namespace ceres
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#endif // CERES_INTERNAL_PARALLEL_UTILS_H_
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