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refactor: replace TryGetOtherEndpoint's dead bool with a structural loop bound

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TryGetOtherEndpoint's bool return was dead control flow: when a mutant
flipped its `return false` guard to `return true`, `other` stayed a clone
of `coord` (only one component would have been incremented on the true
path that never runs for that guard), so the caller compared a sample
against itself and behaved identically either way. That produced the
equivalent survivors at Extract's :92 continue and the guard's :198
return.

Bound the edge loop with the same condition (coord[axis] < cellCounts[axis])
directly, computing `other` inline once the bound holds. One path instead
of two that happened to agree; the helper is gone, and so is the `continue`
that used to depend on it. Behaviour is unchanged: verified with a
before/after run of the existing suite plus a temporary harness comparing
Extract's Positions/Quads/QuadEdges against ca220a5's code on sphere,
non-cubic, open-plane (each axis), checkerboard, and minimal 2x2x2
fixtures - all byte-for-byte identical.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
This commit is contained in:
Meta Vens 2026-09-25 20:30:19 +02:00
commit bef3d3eba3
1 changed files with 36 additions and 43 deletions
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79
src/GameCult.Geometry/CultGeometrySurfaceNets.cs
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@ -57,7 +57,6 @@ namespace GameCult.Geometry
var sizeX = samples.GetLength(0);
var sizeY = samples.GetLength(1);
var sizeZ = samples.GetLength(2);
var sizes = new[] { sizeX, sizeY, sizeZ };
var cellCounts = new[] { sizeX - 1, sizeY - 1, sizeZ - 1 };
var originValue = new float3(origin.X, origin.Y, origin.Z);
@ -87,39 +86,42 @@ namespace GameCult.Geometry
for (byte axis = 0; axis < 3; axis++)
{
if (!TryGetOtherEndpoint(sizes, coord, axis, out var other))
// Only an edge whose far endpoint is still inside the grid can cross into a
// quad. Bounding the loop with this one condition, instead of a helper that
// returned a bool for the caller to branch on, leaves one path through the
// rest of the body instead of two that happened to produce the same result.
if (coord[axis] < cellCounts[axis])
{
continue;
var other = (int[])coord.Clone();
other[axis] += 1;
var inside = IsInside(samples[x, y, z], isoValue);
var otherInside = IsInside(samples[other[0], other[1], other[2]], isoValue);
if (inside != otherInside && TryGetQuadCells(cellCounts, coord, axis, out var cells))
{
// The four cells around a crossing edge each contain that same crossing
// edge among their own twelve, so each of them is mixed and has a
// vertex: this indexer cannot miss without the extraction invariant
// already being broken.
var v = new uint[4];
for (var i = 0; i < 4; i++) v[i] = vertexIndex[cells[i]];
// Orientation comes only from which endpoint is inside and the axis's
// fixed handedness (OrientationSign) - never from the quad's own
// geometry, so degenerate (zero-area) quads still orient consistently
// with their neighbours.
if (inside == (OrientationSign[axis] < 0))
{
(v[1], v[3]) = (v[3], v[1]);
}
quads.Add(v[0]);
quads.Add(v[1]);
quads.Add(v[2]);
quads.Add(v[3]);
quadEdges.Add(new CultGeometryGridEdge(x, y, z, axis));
}
}
var inside = IsInside(samples[x, y, z], isoValue);
var otherInside = IsInside(samples[other[0], other[1], other[2]], isoValue);
if (inside == otherInside) continue;
if (!TryGetQuadCells(cellCounts, coord, axis, out var cells))
{
continue;
}
// The four cells around a crossing edge each contain that same crossing edge
// among their own twelve, so each of them is mixed and has a vertex: this
// indexer cannot miss without the extraction invariant already being broken.
var v = new uint[4];
for (var i = 0; i < 4; i++) v[i] = vertexIndex[cells[i]];
// Orientation comes only from which endpoint is inside and the axis's fixed
// handedness (OrientationSign) - never from the quad's own geometry, so
// degenerate (zero-area) quads still orient consistently with their neighbours.
if (inside == (OrientationSign[axis] < 0))
{
(v[1], v[3]) = (v[3], v[1]);
}
quads.Add(v[0]);
quads.Add(v[1]);
quads.Add(v[2]);
quads.Add(v[3]);
quadEdges.Add(new CultGeometryGridEdge(x, y, z, axis));
}
}
@ -192,20 +194,11 @@ namespace GameCult.Geometry
return math.lerp(first, second, amount);
}
private static bool TryGetOtherEndpoint(int[] sizes, int[] coord, byte axis, out int[] other)
{
other = (int[])coord.Clone();
if (coord[axis] + 1 >= sizes[axis]) return false;
other[axis] += 1;
return true;
}
// The four cells that share the grid edge from (x, y, z) to (x, y, z) + e_axis, listed as a
// perimeter loop around the edge (not crossed diagonally) so the caller can build a quad
// directly from them. The edge's own coordinate is unchanged for all four cells: it is
// guaranteed to already index a valid cell there because TryGetOtherEndpoint verified
// coord[axis] + 1 < sizes[axis], i.e. coord[axis] < cellCounts[axis].
// guaranteed to already index a valid cell there because the caller's own loop bound
// already verified coord[axis] < cellCounts[axis].
private static bool TryGetQuadCells(int[] cellCounts, int[] coord, byte axis, out (int, int, int)[] cells)
{
cells = Array.Empty<(int, int, int)>();