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Loop validate() fails to adequately detect duplicate vertices #108

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#135
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Loop validate() fails to adequately detect duplicate vertices#108
#135
@missinglink

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@missinglink
missinglink
opened on Sep 20, 2024

The current code only checks for duplicate adjacent vertices:

geo/s2/loop.go

Lines 251 to 260 in 6adc566

// Loops are not allowed to have any duplicate vertices or edge crossings.
// We split this check into two parts. First we check that no edge is
// degenerate (identical endpoints). Then we check that there are no
// intersections between non-adjacent edges (including at vertices). The
// second check needs the ShapeIndex, so it does not fall within the scope
// of this method.
for i, v := range l.vertices {
if v == l.Vertex(i+1) {
return fmt.Errorf("edge %d is degenerate (duplicate vertex)", i)
}

While the comments say:

Loops are not allowed to have any duplicate vertices (whether adjacent or not)

geo/s2/loop.go

Lines 34 to 35 in 6adc566

// Loops are not allowed to have any duplicate vertices (whether adjacent or
// not). Non-adjacent edges are not allowed to intersect, and furthermore edges

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