| // Ceres Solver - A fast non-linear least squares minimizer |
| // Copyright 2023 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 |
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| // ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE |
| // POSSIBILITY OF SUCH DAMAGE. |
| // |
| // Author: wjr@google.com (William Rucklidge) |
| |
| #ifndef CERES_INTERNAL_PARALLEL_UTILS_H_ |
| #define CERES_INTERNAL_PARALLEL_UTILS_H_ |
| |
| #include "ceres/internal/export.h" |
| |
| namespace ceres::internal { |
| |
| // Converts a linear iteration order into a triangular iteration order. |
| // Suppose you have nested loops that look like |
| // for (int i = 0; i < n; i++) { |
| // for (int j = i; j < n; j++) { |
| // ... use i and j |
| // } |
| // } |
| // Naively using ParallelFor to parallelise those loops might look like |
| // ParallelFor(..., 0, n * n, num_threads, |
| // [](int thread_id, int k) { |
| // int i = k / n, j = k % n; |
| // if (j < i) return; |
| // ... |
| // }); |
| // but these empty work items can lead to very unbalanced threading. Instead, |
| // do this: |
| // int actual_work_items = (n * (n + 1)) / 2; |
| // ParallelFor(..., 0, actual_work_items, num_threads, |
| // [](int thread_id, int k) { |
| // int i, j; |
| // UnfoldIteration(k, n, &i, &j); |
| // ... |
| // }); |
| // which in each iteration will produce i and j satisfying |
| // 0 <= i <= j < n |
| CERES_NO_EXPORT void LinearIndexToUpperTriangularIndex(int k, |
| int n, |
| int* i, |
| int* j); |
| |
| } // namespace ceres::internal |
| |
| #endif // CERES_INTERNAL_PARALLEL_UTILS_H_ |
