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Fix boundary condition in SMCK #2524

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Fix boundary condition in SMCK #2524

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2 changes: 1 addition & 1 deletion lib/msprime.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7690,7 +7690,7 @@ msp_smc_k_common_ancestor_event(

/* find second hull */
search = &x_hull->left_avl_node;

@petrelharp petrelharp May 5, 2026

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I don't know quite what's going on, but I'd want to check that we're not getting search == NULL here and remaining_mass == 0 before the loop even starts. LMK if you'd like me to get my head around all this enough to double-check the correctness.

@currocam currocam May 6, 2026

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In the problematic example I posted, I think that’s not the case.

Before entering the loop, search is not null and remaining_mass is exactly 1.0 (up to floating-point error, I guess).

At the end of the first iteration, remaining_mass is decreased by one (which makes remaining_mass==0) but search is still not null. That's the hull we chose when doing >0 instead of >=0.

The assertion error is triggered in the 3070th iteration when the loop finishes traversing the linked list and finds the NULL.

for (search = search->prev; remaining_mass >= 0; search = search->prev) {
for (search = search->prev; remaining_mass > 0; search = search->prev) {
tsk_bug_assert(search != NULL);
y_hull = (hull_t *) search->item;
if (y_hull->left == x_hull->left || y_hull->right > x_hull->left) {
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