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Use path halving in unionfind to improve performance #404

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Merged
christian-rauch merged 1 commit into from
Oct 7, 2025
Merged

Use path halving in unionfind to improve performance #404

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20 changes: 7 additions & 13 deletions common/unionfind.h
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Original file line number Diff line number Diff line change
Expand Up @@ -82,26 +82,20 @@ static inline uint32_t unionfind_get_representative(unionfind_t *uf, uint32_t id
// version above.
static inline uint32_t unionfind_get_representative(unionfind_t *uf, uint32_t id)
{
uint32_t root = uf->parent[id];
// unititialized node, so set to self
if (root == 0xffffffff) {
if (uf->parent[id] == 0xffffffff) {
uf->parent[id] = id;
return id;
}

// chase down the root
while (uf->parent[root] != root) {
root = uf->parent[root];
// Path halving: make every node point to its grandparent (single pass)
// This is simpler and faster than full path compression while still effective
while (uf->parent[id] != id) {
uf->parent[id] = uf->parent[uf->parent[id]]; // Point to grandparent
id = uf->parent[id]; // Move to grandparent
}

// go back and collapse the tree.
while (uf->parent[id] != root) {
uint32_t tmp = uf->parent[id];
uf->parent[id] = root;
id = tmp;
}

return root;
return id;
}

static inline uint32_t unionfind_get_set_size(unionfind_t *uf, uint32_t id)
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