audio/internal/convert: bug fix: StereoI16 didn't work in some cases

There are no such actual use cases, so this is not a critical bug.

audio/internal/convert/stereoi16.go

@@ -34,7 +34,7 @@ func NewStereoI16(source io.ReadSeeker, mono, eight bool) *StereoI16 {
 }
 
 func (s *StereoI16) Read(b []byte) (int, error) {
-	l := len(b)
+	l := len(b) / 4 * 4
 	if s.mono {
 		l /= 2
 	}
@@ -75,6 +75,8 @@ func (s *StereoI16) Read(b []byte) (int, error) {
 			b[4*i+2] = byte(v1)
 			b[4*i+3] = byte(v1 >> 8)
 		}
+	default:
+		copy(b[:n], s.buf[:n])
 	}
 	if s.mono {
 		n *= 2
@@ -86,6 +88,7 @@ func (s *StereoI16) Read(b []byte) (int, error) {
 }
 
 func (s *StereoI16) Seek(offset int64, whence int) (int64, error) {
+	offset = offset / 4 * 4
 	if s.mono {
 		offset /= 2
 	}

audio/internal/convert/stereoi16_test.go

@@ -0,0 +1,182 @@
+// Copyright 2024 The Ebitengine Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package convert_test
+
+import (
+	"bytes"
+	"fmt"
+	"io" // TODO: Use math/rand/v2 when the minimum supported version becomes Go 1.22.
+	"math/rand"
+	"testing"
+
+	"github.com/hajimehoshi/ebiten/v2/audio/internal/convert"
+)
+
+func TestStereoI16(t *testing.T) {
+	testCases := []struct {
+		Name string
+		In   []int16
+	}{
+		{
+			Name: "nil",
+			In:   nil,
+		},
+		{
+			Name: "-1, 0, 1, 0",
+			In:   []int16{-1, 0, 1, 0},
+		},
+		{
+			Name: "8 0s",
+			In:   []int16{0, 0, 0, 0, 0, 0, 0, 0},
+		},
+		{
+			Name: "random 256 values",
+			In:   randInt16s(256),
+		},
+		{
+			Name: "random 65536 values",
+			In:   randInt16s(65536),
+		},
+	}
+	for _, tc := range testCases {
+		tc := tc
+		t.Run(tc.Name, func(t *testing.T) {
+			for _, mono := range []bool{false, true} {
+				mono := mono
+				t.Run(fmt.Sprintf("mono=%t", mono), func(t *testing.T) {
+					var inBytes, outBytes []byte
+					for _, v := range tc.In {
+						inBytes = append(inBytes, byte(v), byte(v>>8))
+						if mono {
+							// As the source is mono, the output should be stereo.
+							outBytes = append(outBytes, byte(v), byte(v>>8), byte(v), byte(v>>8))
+						} else {
+							outBytes = append(outBytes, byte(v), byte(v>>8))
+						}
+					}
+					s := convert.NewStereoI16(bytes.NewReader(inBytes), mono, false)
+					var got []byte
+					for {
+						var buf [97]byte
+						n, err := s.Read(buf[:])
+						got = append(got, buf[:n]...)
+						if err != nil {
+							if err != io.EOF {
+								t.Fatal(err)
+							}
+							break
+						}
+						// Shifting by incomplete bytes should not affect the result.
+						for i := 0; i < 2*2; i++ {
+							if _, err := s.Seek(int64(i), io.SeekCurrent); err != nil {
+								if err != io.EOF {
+									t.Fatal(err)
+								}
+								break
+							}
+						}
+					}
+					want := outBytes
+					if !bytes.Equal(got, want) {
+						t.Errorf("got: %v, want: %v", got, want)
+					}
+				})
+			}
+		})
+	}
+}
+
+func randBytes(n int) []byte {
+	r := make([]byte, n)
+	for i := range r {
+		r[i] = byte(rand.Intn(256))
+	}
+	return r
+}
+
+func TestStereoI16EightBits(t *testing.T) {
+	testCases := []struct {
+		Name string
+		In   []byte
+	}{
+		{
+			Name: "nil",
+			In:   nil,
+		},
+		{
+			Name: "1, 0, 1, 0",
+			In:   []byte{1, 0, 1, 0},
+		},
+		{
+			Name: "8 0s",
+			In:   []byte{0, 0, 0, 0, 0, 0, 0, 0},
+		},
+		{
+			Name: "random 256 values",
+			In:   randBytes(256),
+		},
+		{
+			Name: "random 65536 values",
+			In:   randBytes(65536),
+		},
+	}
+	for _, tc := range testCases {
+		tc := tc
+		t.Run(tc.Name, func(t *testing.T) {
+			for _, mono := range []bool{false, true} {
+				mono := mono
+				t.Run(fmt.Sprintf("mono=%t", mono), func(t *testing.T) {
+					inBytes := tc.In
+					var outBytes []byte
+					for _, v := range tc.In {
+						v16 := int16(int(v)*0x101 - (1 << 15))
+						if mono {
+							// As the source is mono, the output should be stereo.
+							outBytes = append(outBytes, byte(v16), byte(v16>>8), byte(v16), byte(v16>>8))
+						} else {
+							outBytes = append(outBytes, byte(v16), byte(v16>>8))
+						}
+					}
+					s := convert.NewStereoI16(bytes.NewReader(inBytes), mono, true)
+					var got []byte
+					for {
+						var buf [97]byte
+						n, err := s.Read(buf[:])
+						got = append(got, buf[:n]...)
+						if err != nil {
+							if err != io.EOF {
+								t.Fatal(err)
+							}
+							break
+						}
+						// Shifting by incomplete bytes should not affect the result.
+						for i := 0; i < 2*2; i++ {
+							if _, err := s.Seek(int64(i), io.SeekCurrent); err != nil {
+								if err != io.EOF {
+									t.Fatal(err)
+								}
+								break
+							}
+						}
+					}
+					want := outBytes
+					if !bytes.Equal(got, want) {
+						t.Errorf("got: %v, want: %v", got, want)
+					}
+				})
+			}
+		})
+	}
+}