// Copyright ©2015 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 (
	"math"

	"github.com/gonum/blas/blas64"
)

// Dlacn2 estimates the 1-norm of an n×n matrix A using sequential updates with
// matrix-vector products provided externally.
//
// Dlacn2 is called sequentially. In between calls, x should be overwritten by
//  A * X if kase == 1
//  A^T * X if kase == 2
// all other parameters should be unchanged during sequential calls, and the updated
// values of est and kase should be used. On the final return (when kase is returned
// as 0), V = A * W, where est = norm(V) / norm(W).
//
// isign, v, and x must all have length n and will panic otherwise. isave is used
// for temporary storage.
//
// Dlacn2 is an internal routine. It is exported for testing purposes.
func (impl Implementation) Dlacn2(n int, v, x []float64, isgn []int, est float64, kase int, isave *[3]int) (float64, int) {
	checkVector(n, x, 1)
	checkVector(n, v, 1)
	if len(isgn) < n {
		panic("lapack: insufficient isgn length")
	}
	if isave[0] < 0 || isave[0] > 5 {
		panic("lapack: bad isave value")
	}
	if isave[0] == 0 && kase != 0 {
		panic("lapack: bad isave value")
	}
	itmax := 5
	bi := blas64.Implementation()
	if kase == 0 {
		for i := 0; i < n; i++ {
			x[i] = 1 / float64(n)
		}
		kase = 1
		isave[0] = 1
		return est, kase
	}
	switch isave[0] {
	default:
		panic("unreachable")
	case 1:
		if n == 1 {
			v[0] = x[0]
			est = math.Abs(v[0])
			kase = 0
			return est, kase
		}
		est = bi.Dasum(n, x, 1)
		for i := 0; i < n; i++ {
			x[i] = math.Copysign(1, x[i])
			isgn[i] = int(x[i])
		}
		kase = 2
		isave[0] = 2
		return est, kase
	case 2:
		isave[1] = bi.Idamax(n, x, 1)
		isave[2] = 2
		for i := 0; i < n; i++ {
			x[i] = 0
		}
		x[isave[1]] = 1
		kase = 1
		isave[0] = 3
		return est, kase
	case 3:
		bi.Dcopy(n, x, 1, v, 1)
		estold := est
		est = bi.Dasum(n, v, 1)
		sameSigns := true
		for i := 0; i < n; i++ {
			if int(math.Copysign(1, x[i])) != isgn[i] {
				sameSigns = false
				break
			}
		}
		if !sameSigns && est > estold {
			for i := 0; i < n; i++ {
				x[i] = math.Copysign(1, x[i])
				isgn[i] = int(x[i])
			}
			kase = 2
			isave[0] = 4
			return est, kase
		}
	case 4:
		jlast := isave[1]
		isave[1] = bi.Idamax(n, x, 1)
		if x[jlast] != math.Abs(x[isave[1]]) && isave[2] < itmax {
			isave[2] += 1
			for i := 0; i < n; i++ {
				x[i] = 0
			}
			x[isave[1]] = 1
			kase = 1
			isave[0] = 3
			return est, kase
		}
	case 5:
		tmp := 2 * (bi.Dasum(n, x, 1)) / float64(3*n)
		if tmp > est {
			bi.Dcopy(n, x, 1, v, 1)
			est = tmp
		}
		kase = 0
		return est, kase
	}
	// Iteration complete. Final stage
	altsgn := 1.0
	for i := 0; i < n; i++ {
		x[i] = altsgn * (1 + float64(i)/float64(n-1))
		altsgn *= -1
	}
	kase = 1
	isave[0] = 5
	return est, kase
}