1 files changed, 57 insertions, 0 deletions
diff --git a/source/blender/blenlib/intern/index_mask.cc b/source/blender/blenlib/intern/index_mask.cc
new file mode 100644
index 00000000000..cba985b8a44
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+++ b/source/blender/blenlib/intern/index_mask.cc
@@ -0,0 +1,57 @@
+/*
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ */
+
+#include "BLI_index_mask.hh"
+
+namespace blender {
+
+/**
+ * Create a sub-mask that is also shifted to the beginning. The shifting to the beginning allows
+ * code to work with smaller indices, which is more memory efficient.
+ *
+ * \return New index mask with the size of #slice. It is either empty or starts with 0. It might
+ * reference indices that have been appended to #r_new_indices.
+ *
+ * Example:
+ *  this:   [2, 3, 5, 7, 8, 9, 10]
+ *  slice:      ^--------^
+ *  output: [0, 2, 4, 5]
+ *
+ *  All the indices in the sub-mask are shifted by 3 towards zero, so that the first index in the
+ *  output is zero.
+ */
+IndexMask IndexMask::slice_and_offset(const IndexRange slice, Vector<int64_t> &r_new_indices) const
+{
+  const int slice_size = slice.size();
+  if (slice_size == 0) {
+    return {};
+  }
+  IndexMask sliced_mask{indices_.slice(slice)};
+  if (sliced_mask.is_range()) {
+    return IndexMask(slice_size);
+  }
+  const int64_t offset = sliced_mask.indices().first();
+  if (offset == 0) {
+    return sliced_mask;
+  }
+  r_new_indices.resize(slice_size);
+  for (const int i : IndexRange(slice_size)) {
+    r_new_indices[i] = sliced_mask[i] - offset;
+  }
+  return IndexMask(r_new_indices.as_span());
+}
+
+}  // namespace blender