lapack/gonum: fix ldwork in Dgeqrf and update its test

lapack/gonum/dgeqrf.go

@@ -59,7 +59,6 @@ func (impl Implementation) Dgeqrf(m, n int, a []float64, lda int, tau, work []fl
 	nbmin := 2 // Minimal block size.
 	var nx int // Use unblocked (unless changed in the next for loop)
 	iws := n
-	ldwork := nb
 	// Only consider blocked if the suggested block size is > 1 and the
 	// number of rows or columns is sufficiently large.
 	if 1 < nb && nb < k {
@@ -67,7 +66,7 @@ func (impl Implementation) Dgeqrf(m, n int, a []float64, lda int, tau, work []fl
 		// to unblocked.
 		nx = max(0, impl.Ilaenv(3, "DGEQRF", " ", m, n, -1, -1))
 		if k > nx {
-			iws = ldwork * n
+			iws = n * nb
 			if lwork < iws {
 				// Not enough workspace to use the optimal block
 				// size. Get the minimum block size instead.
@@ -80,6 +79,7 @@ func (impl Implementation) Dgeqrf(m, n int, a []float64, lda int, tau, work []fl
 	// Compute QR using a blocked algorithm.
 	var i int
 	if nbmin <= nb && nb < k && nx < k {
+		ldwork := nb
 		for i = 0; i < k-nx; i += nb {
 			ib := min(k-i, nb)
 			// Compute the QR factorization of the current block.

lapack/testlapack/dgeqrf.go

@@ -18,6 +18,7 @@ type Dgeqrfer interface {
 }
 
 func DgeqrfTest(t *testing.T, impl Dgeqrfer) {
+	const tol = 1e-12
 	rnd := rand.New(rand.NewSource(1))
 	for c, test := range []struct {
 		m, n, lda int
@@ -49,48 +50,53 @@ func DgeqrfTest(t *testing.T, impl Dgeqrfer) {
 		if lda == 0 {
 			lda = test.n
 		}
+
+		// Allocate m×n matrix A and fill it with random numbers.
 		a := make([]float64, m*lda)
-		for i := 0; i < m; i++ {
-			for j := 0; j < n; j++ {
-				a[i*lda+j] = rnd.Float64()
-			}
+		for i := range a {
+			a[i] = rnd.NormFloat64()
 		}
+		// Store a copy of A for later comparison.
+		aCopy := make([]float64, len(a))
+		copy(aCopy, a)
+
+		// Allocate a slice for scalar factors of elementary reflectors
+		// and fill it with random numbers.
 		tau := make([]float64, n)
 		for i := 0; i < n; i++ {
 			tau[i] = rnd.Float64()
 		}
-		aCopy := make([]float64, len(a))
-		copy(aCopy, a)
-		ans := make([]float64, len(a))
-		copy(ans, a)
-		work := make([]float64, n)
-		// Compute unblocked QR.
-		impl.Dgeqr2(m, n, ans, lda, tau, work)
-		// Compute blocked QR with small work.
-		impl.Dgeqrf(m, n, a, lda, tau, work, len(work))
-		if !floats.EqualApprox(ans, a, 1e-12) {
-			t.Errorf("Case %v, mismatch small work.", c)
-		}
-		// Try the full length of work.
-		impl.Dgeqrf(m, n, a, lda, tau, work, -1)
-		lwork := int(work[0])
-		work = make([]float64, lwork)
-		copy(a, aCopy)
-		impl.Dgeqrf(m, n, a, lda, tau, work, lwork)
-		if !floats.EqualApprox(ans, a, 1e-12) {
-			t.Errorf("Case %v, mismatch large work.", c)
-		}
 
-		// Try a slightly smaller version of work to test blocking.
-		if len(work) <= n {
-			continue
-		}
-		work = work[1:]
-		lwork--
+		// Compute the expected result using unblocked QR algorithm and
+		// store it in want.
+		want := make([]float64, len(a))
+		copy(want, a)
+		impl.Dgeqr2(m, n, want, lda, tau, make([]float64, n))
+
+		for _, wl := range []worklen{minimumWork, mediumWork, optimumWork} {
 			copy(a, aCopy)
-		impl.Dgeqrf(m, n, a, lda, tau, work, lwork)
-		if !floats.EqualApprox(ans, a, 1e-12) {
-			t.Errorf("Case %v, mismatch large work.", c)
+
+			var lwork int
+			switch wl {
+			case minimumWork:
+				lwork = n
+			case mediumWork:
+				work := make([]float64, 1)
+				impl.Dgeqrf(m, n, a, lda, tau, work, -1)
+				lwork = int(work[0]) - 2*n
+			case optimumWork:
+				work := make([]float64, 1)
+				impl.Dgeqrf(m, n, a, lda, tau, work, -1)
+				lwork = int(work[0])
+			}
+			work := make([]float64, lwork)
+
+			// Compute the QR factorization of A.
+			impl.Dgeqrf(m, n, a, lda, tau, work, len(work))
+			// Compare the result with Dgeqr2.
+			if !floats.EqualApprox(want, a, tol) {
+				t.Errorf("Case %v, workspace %v, unexpected result.", c, wl)
+			}
 		}
 	}
 }