Upsample via time domain.

resample.go

package fourier

import "errors"

func Upsample(in []float64, scale int) ([]float64, error) {
	if scale < 1 {
		return nil, errors.New("scale cannot be less than one")
	}

	var (
		l       = len(in)
		ln      = l * scale
		cin     = make([]complex128, l)
		cpadded = make([]complex128, ln)
		padded  = make([]float64, ln)
	)

	for i, v := range in {
		cin[i] = complex(v*float64(scale), 0)
	}
	if err := Forward(cin); err != nil {
		return nil, err
	}

	// Split the spectral content down the middle and copy both ends into the
	// ends of a new upscaled buffer. This will leave the zeros in the center.
	for i := 0; i < l/2; i++ {
		cpadded[i] = cin[i]
		cpadded[len(cpadded)-1-i] = cin[len(cin)-1-i]
	}
	if err := Inverse(cpadded); err != nil {
		return nil, err
	}
	for i, v := range cpadded {
		padded[i] = real(v)
	}
	return padded, nil
}

resample_test.go

package fourier

import (
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestResample(t *testing.T) {
	for _, tt := range []struct {
		src      []float64
		scale    int
		expected []float64
	}{
		{
			src:      []float64{1, 0.5, 1.0, 0.5, 1.0, 0.5, 1.0, 0.5},
			scale:    1,
			expected: []float64{1, 0.5, 1.0, 0.5, 1.0, 0.5, 1.0, 0.5},
		},
		{
			src:      []float64{1, 0.5, 1.0, 0.5, 1.0, 0.5, 1.0, 0.5},
			scale:    2,
			expected: []float64{1, 0.75, 0.5, 0.75, 1.0, 0.75, 0.5, 0.75, 1.0, 0.75, 0.5, 0.75, 1.0, 0.75, 0.5, 0.75},
		},
		{
			src:      []float64{1, 0.5, 1.0, 0.5, 1.0, 0.5, 1.0, 0.5},
			scale:    8,
			expected: []float64{1, 0.9809698831278217, 0.9267766952966369, 0.8456708580912724, 0.75, 0.6543291419087276, 0.5732233047033631, 0.5190301168721783, 0.5, 0.5190301168721783, 0.5732233047033631, 0.6543291419087276, 0.75, 0.8456708580912724, 0.9267766952966369, 0.9809698831278217, 1, 0.9809698831278217, 0.9267766952966369, 0.8456708580912724, 0.75, 0.6543291419087276, 0.5732233047033631, 0.5190301168721783, 0.5, 0.5190301168721783, 0.5732233047033631, 0.6543291419087276, 0.75, 0.8456708580912724, 0.9267766952966369, 0.9809698831278217, 1, 0.9809698831278217, 0.9267766952966369, 0.8456708580912724, 0.75, 0.6543291419087276, 0.5732233047033631, 0.5190301168721783, 0.5, 0.5190301168721783, 0.5732233047033631, 0.6543291419087276, 0.75, 0.8456708580912724, 0.9267766952966369, 0.9809698831278217, 1, 0.9809698831278217, 0.9267766952966369, 0.8456708580912724, 0.75, 0.6543291419087276, 0.5732233047033631, 0.5190301168721783, 0.5, 0.5190301168721783, 0.5732233047033631, 0.6543291419087276, 0.75, 0.8456708580912724, 0.9267766952966369, 0.9809698831278217},
		},
	} {
		out, err := Upsample(tt.src, tt.scale)
		assert.NoError(t, err)
		assert.Equal(t, tt.expected, out)
	}
}