package stats

import (
	"errors"
	"log"

	"gonum.org/v1/gonum/mat"
)

func rotateFloats(f []float64, n int) []float64 {
	flen := len(f)
	if n < 0 || flen == 0 {
		return f
	}

	r := flen - n%flen
	f = append(f[r:], f[:r]...)

	return f
}

func constructMatrix(f []float64) ([][]float64, error) {
	flen := len(f)

	if flen < 1 {
		return nil, errors.New("ErrSliceZeroLength")
	}

	m := make([][]float64, 0, flen)

	// add first row unchanged.
	m = append(m, f)

	rot := rotateFloats(f, 1)

	for i := 0; i < flen-1; i++ {
		m = append(m, rot)
		rot = rotateFloats(rot, 1)
	}

	return m, nil
}

func invertMatrix(m [][]float64, size int) (*mat.Dense, error) {
	flM := []float64{}
	for i := range m {
		flM = append(flM, m[i]...)
	}

	a := mat.NewDense(size, size, flM)

	// compute the inverse of A, if possible.
	var aInv mat.Dense

	err := aInv.Inverse(a)
	if err != nil {
		log.Print("matrix is not invertible")
		return nil, errors.New("matrix is not invertible")
	}

	return &aInv, nil
}

func estimateImpulseFunc(gMat mat.Dense, lruu int) ([]float64, error) {
	g := gMat.RawMatrix().Data
	lenG := len(g)

	if lenG != lruu {
		return nil, errors.New("ErrLenGDiffersFromLRuu")
	}

	h := make([]float64, lenG)

	// set first elem manually.
	h[0] = g[0]

	for i := 1; i < lenG; i++ {
		h[i] = h[i-1] + g[i]
	}

	return h, nil
}