fix(linalg): refuse non-finite input in dense Cholesky, Eigen and rank-one updates

Assisted-by: GLM 5.3 Flash

linalg/cholupdate_test.go

@@ -5,8 +5,10 @@ package linalg
 
 import (
 	"math"
-	"sourcedock.dev/petrbalvin/tensor/internal/core"
+	"strings"
 	"testing"
+
+	"sourcedock.dev/petrbalvin/tensor/internal/core"
 )
 
 // spdSample builds a deterministic symmetric positive definite matrix:
@@ -156,3 +158,25 @@ func TestCholeskyRankOneErrors(t *testing.T) {
 		t.Fatal("expected an error for a complex vector")
 	}
 }
+
+// TestCholeskyRankOneRefusesNonFinite pins the refusal of a poisoned
+// factor or vector: the diagonal test cannot see a NaN, so the sweep
+// would answer an all-NaN factor with a nil error, where the sparse
+// twin refuses the same modification.
+func TestCholeskyRankOneRefusesNonFinite(t *testing.T) {
+	l, err := Cholesky(mustFromFloats(t, []float64{4, 0, 0, 1}, 2, 2))
+	if err != nil {
+		t.Fatalf("Cholesky: %v", err)
+	}
+	badVec := floatsToArray([]float64{math.NaN(), 0.1}, []int{2})
+	if _, err := CholeskyUpdate(l, badVec); err == nil || !strings.Contains(err.Error(), "not finite") {
+		t.Fatalf("CholeskyUpdate(NaN vector): %v", err)
+	}
+	if _, err := CholeskyDowndate(l, badVec); err == nil || !strings.Contains(err.Error(), "not finite") {
+		t.Fatalf("CholeskyDowndate(NaN vector): %v", err)
+	}
+	badFac := mustFromFloats(t, []float64{4, 0, math.Inf(1), 1}, 2, 2)
+	if _, err := CholeskyUpdate(badFac, floatsToArray([]float64{0.1, 0.1}, []int{2})); err == nil || !strings.Contains(err.Error(), "not finite") {
+		t.Fatalf("CholeskyUpdate(Inf factor): %v", err)
+	}
+}