BLI: add high level documentation for core data structures #25
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# Disjoint Set
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# Disjoint-Set
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A [disjoint-set](https://en.wikipedia.org/wiki/Disjoint-set_data_structure) data structure can be used to find connected and disconnected pieces in a graph efficiently. A typical use-case in Blender is to detect mesh islands.
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Blender currently has two implementations of this data structure. `AtomicDisjointSet` ([source](https://projects.blender.org/blender/blender/src/branch/main/source/blender/blenlib/BLI_atomic_disjoint_set.hh)) which is thread-safe and `DisjointSet` ([source](https://projects.blender.org/blender/blender/src/branch/main/source/blender/blenlib/BLI_disjoint_set.hh)) which is not. There is a small overhead to using the atomic version when multi-threading is not used.
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```cpp
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/* Create disjoint set. Initially, every vertex is disjoint from the others. */
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DisjointSet<int> disjoint_set(verts_num);
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/* Merge sets when there is an edge between them. */
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for (const int2 edge : edges) {
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disjoint_set.join(edge[0], edge[1]);
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}
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/* Check if two vertices are in the same set. */
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bool is_joined = disjoint_set.in_same_set(vert_1, vert_2);
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/* Get representative element for all vertices in an island. */
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int element = disjoint_set.find_root(vert_index);
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```
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