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// 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 gonum
import "math"
// Dlassq updates a sum of squares in scaled form. The input parameters scale and
// sumsq represent the current scale and total sum of squares. These values are
// updated with the information in the first n elements of the vector specified
// by x and incX.
//
// Dlassq is an internal routine. It is exported for testing purposes.
func (impl Implementation) Dlassq(n int, x []float64, incx int, scale float64, sumsq float64) (scl, smsq float64) {
switch {
case n < 0:
panic(nLT0)
case incx <= 0:
panic(badIncX)
case len(x) < 1+(n-1)*incx:
panic(shortX)
}
if n == 0 {
return scale, sumsq
}
for ix := 0; ix <= (n-1)*incx; ix += incx {
absxi := math.Abs(x[ix])
if absxi > 0 || math.IsNaN(absxi) {
if scale < absxi {
sumsq = 1 + sumsq*(scale/absxi)*(scale/absxi)
scale = absxi
} else {
sumsq += (absxi / scale) * (absxi / scale)
}
}
}
return scale, sumsq
}