testlapack: add isIdentity helper

lapack/testlapack/general.go

@@ -1464,3 +1464,28 @@ func constructGSVPresults(n, p, m, k, l int, a, b blas64.General) (zeroA, zeroB
 
 	return zeroA, zeroB
 }
+
+// isIdentity returns whether an n×n matrix A is approximately equal to the
+// identity matrix.
+func isIdentity(n int, a []float64, lda int, tol float64) bool {
+	for i := 0; i < n; i++ {
+		for j := 0; j < n; j++ {
+			aij := a[i*lda+j]
+			if math.IsNaN(aij) {
+				return false
+			}
+			if i == j {
+				if math.Abs(aij-1) > tol {
+					fmt.Println(i, j, aij)
+					return false
+				}
+			} else {
+				if math.Abs(aij) > tol {
+					fmt.Println(i, j, aij)
+					return false
+				}
+			}
+		}
+	}
+	return true
+}