Make the resampler window symmetric & center it

resample.go

@@ -57,7 +57,7 @@ func ResampleRatio(quality int, ratio float64, s Streamer) *Resampler {
 		ratio: ratio,
 		buf1:  make([][2]float64, resamplerSingleBufferSize),
 		buf2:  make([][2]float64, resamplerSingleBufferSize),
-		pts:   make([]point, quality*2),
+		pts:   make([]point, quality*2+1),
 		off:   -resamplerSingleBufferSize,
 		pos:   0,
 		end:   math.MaxInt,
@@ -85,9 +85,8 @@ func (r *Resampler) Stream(samples [][2]float64) (n int, ok bool) {
 
 		// Determine the quality*2 closest sample positions for the interpolation.
 		// The window has length len(r.pts) and is centered around wantPos.
-		// todo: check if this is actually centered around wantPos
-		windowStart := int(wantPos) - len(r.pts)/2 + 1 // (inclusive)
-		windowEnd := int(wantPos) + len(r.pts)/2 + 1   // (exclusive)
+		windowStart := int(wantPos) - len(r.pts)/2   // (inclusive)
+		windowEnd := int(wantPos) + len(r.pts)/2 + 1 // (exclusive)
 
 		// Prepare the buffers.
 		if windowEnd >= r.off+resamplerSingleBufferSize {

resample_test.go

@@ -32,7 +32,7 @@ func TestResample(t *testing.T) {
 
 func resampleCorrect(quality int, old, new beep.SampleRate, p [][2]float64) [][2]float64 {
 	ratio := float64(old) / float64(new)
-	pts := make([]point, quality*2)
+	pts := make([]point, quality*2+1)
 	var resampled [][2]float64
 	for i := 0; ; i++ {
 		j := float64(i) * ratio
@@ -42,7 +42,7 @@ func resampleCorrect(quality int, old, new beep.SampleRate, p [][2]float64) [][2
 		var sample [2]float64
 		for c := range sample {
 			for k := range pts {
-				l := int(j) + k - len(pts)/2 + 1
+				l := int(j) + k - (len(pts)-1)/2
 				if l >= 0 && l < len(p) {
 					pts[k] = point{X: float64(l), Y: p[l][c]}
 				} else {