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Fix flaky central-difference assertions: use absolute tolerance, not rounding

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dotnet test found two failures where a fixed-decimal-place Assert.Equal
snapped two numbers that already agreed to within about 2e-5 to different
rounded digits, because their true difference straddled a rounding boundary
(0.824987829 vs 0.825006127 -> 0.82 vs 0.83). Both were central-difference
checks (smin_grad's own invariant-8 property test, and cellular's gradient-
matches-central-differences test), not a correctness bug in the primitives.

AssertWithinTolerance replaces the precision-based assertion for both central-
difference comparisons with an explicit absolute-difference check, which
states the actual claim (agreement within the finite-difference step's own
truncation error) without the rounding-boundary artifact. Also dropped an
unused local function (F2At) the build flagged.

Found by the third ygg-verify.sh test run on 77c3580.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
This commit is contained in:
Meta Vens 2026-09-25 19:28:36 +02:00
commit 08b0dced53
1 changed files with 14 additions and 7 deletions
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21
packages/cultmath/tests/CultMath.Tests/CellularAndSminGradTests.cs
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@ -88,9 +88,9 @@ public sealed class CellularAndSminGradTests
(Smin(p + new float3(0.0f, eps, 0.0f)) - Smin(p - new float3(0.0f, eps, 0.0f))) / (2.0f * eps),
(Smin(p + new float3(0.0f, 0.0f, eps)) - Smin(p - new float3(0.0f, 0.0f, eps))) / (2.0f * eps));
Assert.Equal(numeric.x, analytic.x, precision: 2);
Assert.Equal(numeric.y, analytic.y, precision: 2);
Assert.Equal(numeric.z, analytic.z, precision: 2);
AssertWithinTolerance(numeric.x, analytic.x, 0.02f);
AssertWithinTolerance(numeric.y, analytic.y, 0.02f);
AssertWithinTolerance(numeric.z, analytic.z, 0.02f);
}
}
@ -196,7 +196,6 @@ public sealed class CellularAndSminGradTests
var result = math.cellular(p);
const float eps = 0.0005f;
float F1At(float3 q) => FindF1F2(q).f1;
float F2At(float3 q) => FindF1F2(q).f2;
AssertGradientMatchesCentralDifference(F1At, p, eps, result.nearest);
var edgeValueFn = new Func<float3, float>(q => { var (a, b) = FindF1F2(q); return b - a; });
@ -302,11 +301,19 @@ public sealed class CellularAndSminGradTests
(scalarField(p + new float3(0.0f, eps, 0.0f)) - scalarField(p - new float3(0.0f, eps, 0.0f))) / (2.0f * eps),
(scalarField(p + new float3(0.0f, 0.0f, eps)) - scalarField(p - new float3(0.0f, 0.0f, eps))) / (2.0f * eps));
Assert.Equal(numeric.x, valueAndGradient.x, precision: 2);
Assert.Equal(numeric.y, valueAndGradient.y, precision: 2);
Assert.Equal(numeric.z, valueAndGradient.z, precision: 2);
AssertWithinTolerance(numeric.x, valueAndGradient.x, 0.02f);
AssertWithinTolerance(numeric.y, valueAndGradient.y, 0.02f);
AssertWithinTolerance(numeric.z, valueAndGradient.z, 0.02f);
}
// A fixed decimal-place Assert.Equal snaps two numbers that agree to within a few times 1e-5 to
// different rounded digits whenever their true difference straddles a rounding boundary (seen:
// 0.824987829 vs 0.825006127, agreeing to 2e-5, rounding to 0.82 and 0.83). An absolute-tolerance
// check states the actual claim (central differences agree with the analytic gradient within the
// finite-difference step's own truncation error) without that artifact.
private static void AssertWithinTolerance(float expected, float actual, float tolerance) =>
Assert.True(MathF.Abs(expected - actual) <= tolerance, $"expected {expected}, actual {actual}, tolerance {tolerance}");
private static float3 RandomUnit(System.Random random) =>
new(random.NextSingle() * 2.0f - 1.0f, random.NextSingle() * 2.0f - 1.0f, random.NextSingle() * 2.0f - 1.0f);