Fix halved sample value for 16/24bit wav audio

[Eiton]

The original code is mapping sample value of 16/24bit wav audio to [-0.5xxx,0.5].
This commit changes the range to [-1,1).

[MarkKremer]

Thanks!

I'll review this after I'm back from holliday.

[MarkKremer]

The pipeline fails on gofmt but your changes look correct. Tnx!

Do you happen to have time to figure out the encoding as well? It's a bit more complexly written.

Testcase:

func TestEncodeDecodeRoundTrip(t *testing.T) {
	numChannelsS := []int{1, 2}
	precisions := []int{1, 2, 3}

	for _, numChannels := range numChannelsS {
		for _, precision := range precisions {
			name := fmt.Sprintf("%d_channel(s)_%d_precision", numChannels, precision)
			t.Run(name, func(t *testing.T) {
				var s beep.Streamer
				s, data := testtools.RandomDataStreamer(100)

				if numChannels == 1 {
					s = effects.Mono(s)
					for i := range data {
						mix := (data[i][0] + data[i][1]) / 2
						data[i][0] = mix
						data[i][1] = mix
					}
				}

				var w writerseeker.WriterSeeker

				format := beep.Format{SampleRate: 44100, NumChannels: numChannels, Precision: precision}

				err := Encode(&w, s, format)
				assert.NoError(t, err)

				s, decodedFormat, err := Decode(w.Reader())
				assert.NoError(t, err)
				assert.Equal(t, format, decodedFormat)
				assert.Equal(t, data, testtools.Collect(s))
			})
		}
	}
}

[Eiton]

I think it may not be an appropriate test case.
The random float64 data generated by RandomDataStreamer() loses precision when it is encoded to 8/16/24bit wav format.

[MarkKremer]

Ok, does this seem fairer?

func TestEncodeDecodeRoundTrip(t *testing.T) {
	numChannelsS := []int{1, 2}
	precisions := []int{1, 2, 3}

	for _, numChannels := range numChannelsS {
		for _, precision := range precisions {
			name := fmt.Sprintf("%d_channel(s)_%d_precision", numChannels, precision)
			t.Run(name, func(t *testing.T) {
				var s beep.Streamer
				s, data := testtools.RandomDataStreamer(1000)

				if numChannels == 1 {
					s = effects.Mono(s)
					for i := range data {
						mix := (data[i][0] + data[i][1]) / 2
						data[i][0] = mix
						data[i][1] = mix
					}
				}

				var w writerseeker.WriterSeeker

				format := beep.Format{SampleRate: 44100, NumChannels: numChannels, Precision: precision}

				err := Encode(&w, s, format)
				assert.NoError(t, err)

				s, decodedFormat, err := Decode(w.Reader())
				assert.NoError(t, err)
				assert.Equal(t, format, decodedFormat)

				actual := testtools.Collect(s)
				assert.Len(t, actual, 1000)

				// Delta is determined as follows:
				// The float values range from -1 to 1, which difference is 2.0.
				// For each byte of precision, there are 8 bits -> 2^(precision*8) different possible values.
				// So, fitting 2^(precision*8) values into a range of 2.0, each "step" must not
				// be bigger than 2.0 / math.Pow(2, float64(precision*8)).
				delta := 2.0 / math.Pow(2, float64(precision*8))
				for i := range actual {
					if actual[i][0] < data[i][0]-delta || actual[i][0] > data[i][0]+delta {
						t.Fatalf("encoded & decoded sample doesn't match orginal. expected: %v, actual: %v", data[i][0], actual[i][0])
					}
					if actual[i][1] < data[i][1]-delta || actual[i][1] > data[i][1]+delta {
						t.Fatalf("encoded & decoded sample doesn't match orginal. expected: %v, actual: %v", data[i][1], actual[i][1])
					}
				}
			})
		}
	}
}

[Eiton]

I updated the code. Now it should pass the test.

[MarkKremer]

Sorry for the delay, life got in the way & I want to make sure we have the encoding/decoding right this time.

[Eiton]

I updated the code.

[MarkKremer]

You're a star!