native: implement dgerq2

Author: kortschak

native/dgerq2.go

@@ -0,0 +1,32 @@
+// Copyright ©2017 The gonum Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package native
+
+import "github.com/gonum/blas"
+
+func (impl Implementation) Dgerq2(m, n int, a []float64, lda int, tau, work []float64) {
+	checkMatrix(m, n, a, lda)
+	k := min(m, n)
+	if len(tau) < k {
+		panic(badTau)
+	}
+	if len(work) < m {
+		panic(badWork)
+	}
+
+	for i := k - 1; i >= 0; i-- {
+		// Generate elementary reflector H[i] to annihilate
+		// A[m-k+i, 0:n-k+i-1].
+		mki := m - k + i
+		nki := n - k + i
+		var aii float64
+		aii, tau[i] = impl.Dlarfg(nki, a[mki*lda+mki], a[mki*lda:], lda)
+
+		// Apply H[i] to A[0:m-k+i-1, 0:n-k+i] from the right.
+		a[mki*lda+nki] = 1
+		impl.Dlarf(blas.Right, mki, nki, a[mki*lda:], 1, tau[i], a, lda, work)
+		a[mki*lda+nki] = aii
+	}
+}