Make flac.NewSeek() use a buffered reader too (#72)

* Make flac.NewSeek() use a buffered reader

Which is similar to how flac.New() works.

* Test ReadSeeker

* Rewrite ReadSeeker tests

* Fix error message inconsistencies

flac.go

@@ -35,6 +35,7 @@ import (
 	"os"
 
 	"github.com/mewkiz/flac/frame"
+	"github.com/mewkiz/flac/internal/bufseekio"
 	"github.com/mewkiz/flac/meta"
 )
 
@@ -96,7 +97,8 @@ func New(r io.Reader) (stream *Stream, err error) {
 // will not be buffered, which might result in performance issues. Using an
 // in-memory buffer like *bytes.Reader should work well.
 func NewSeek(rs io.ReadSeeker) (stream *Stream, err error) {
-	stream = &Stream{r: rs, seekTableSize: defaultSeekTableSize}
+	br := bufseekio.NewReadSeeker(rs)
+	stream = &Stream{r: br, seekTableSize: defaultSeekTableSize}
 
 	// Verify FLAC signature and parse the StreamInfo metadata block.
 	block, err := stream.parseStreamInfo()
@@ -121,7 +123,7 @@ func NewSeek(rs io.ReadSeeker) (stream *Stream, err error) {
 	}
 
 	// Record file offset of the first frame header.
-	stream.dataStart, err = rs.Seek(0, io.SeekCurrent)
+	stream.dataStart, err = br.Seek(0, io.SeekCurrent)
 	return stream, err
 }
 

internal/bufseekio/readseeker.go

@@ -0,0 +1,152 @@
+package bufseekio
+
+import (
+	"errors"
+	"io"
+)
+
+const (
+	defaultBufSize = 4096
+)
+
+// ReadSeeker implements buffering for an io.ReadSeeker object.
+// ReadSeeker is based on bufio.Reader with Seek functionality added
+// and unneeded functionality removed.
+type ReadSeeker struct {
+	buf  []byte
+	pos  int64         // absolute start position of buf
+	rd   io.ReadSeeker // read-seeker provided by the client
+	r, w int           // buf read and write positions within buf
+	err  error
+}
+
+const minReadBufferSize = 16
+
+// NewReadSeekerSize returns a new ReadSeeker whose buffer has at least the specified
+// size. If the argument io.ReadSeeker is already a ReadSeeker with large enough
+// size, it returns the underlying ReadSeeker.
+func NewReadSeekerSize(rd io.ReadSeeker, size int) *ReadSeeker {
+	// Is it already a Reader?
+	b, ok := rd.(*ReadSeeker)
+	if ok && len(b.buf) >= size {
+		return b
+	}
+	if size < minReadBufferSize {
+		size = minReadBufferSize
+	}
+	r := new(ReadSeeker)
+	r.reset(make([]byte, size), rd)
+	return r
+}
+
+// NewReadSeeker returns a new ReadSeeker whose buffer has the default size.
+func NewReadSeeker(rd io.ReadSeeker) *ReadSeeker {
+	return NewReadSeekerSize(rd, defaultBufSize)
+}
+
+var errNegativeRead = errors.New("bufseekio: reader returned negative count from Read")
+
+func (b *ReadSeeker) reset(buf []byte, r io.ReadSeeker) {
+	*b = ReadSeeker{
+		buf: buf,
+		rd:  r,
+	}
+}
+
+func (b *ReadSeeker) readErr() error {
+	err := b.err
+	b.err = nil
+	return err
+}
+
+// Read reads data into p.
+// It returns the number of bytes read into p.
+// The bytes are taken from at most one Read on the underlying Reader,
+// hence n may be less than len(p).
+// To read exactly len(p) bytes, use io.ReadFull(b, p).
+// If the underlying Reader can return a non-zero count with io.EOF,
+// then this Read method can do so as well; see the [io.Reader] docs.
+func (b *ReadSeeker) Read(p []byte) (n int, err error) {
+	n = len(p)
+	if n == 0 {
+		if b.buffered() > 0 {
+			return 0, nil
+		}
+		return 0, b.readErr()
+	}
+	if b.r == b.w {
+		if b.err != nil {
+			return 0, b.readErr()
+		}
+		if len(p) >= len(b.buf) {
+			// Large read, empty buffer.
+			// Read directly into p to avoid copy.
+			n, b.err = b.rd.Read(p)
+			if n < 0 {
+				panic(errNegativeRead)
+			}
+			b.pos += int64(n)
+			return n, b.readErr()
+		}
+		// One read.
+		b.pos += int64(b.r)
+		b.r = 0
+		b.w = 0
+		n, b.err = b.rd.Read(b.buf)
+		if n < 0 {
+			panic(errNegativeRead)
+		}
+		if n == 0 {
+			return 0, b.readErr()
+		}
+		b.w += n
+	}
+
+	// copy as much as we can
+	// Note: if the slice panics here, it is probably because
+	// the underlying reader returned a bad count. See issue 49795.
+	n = copy(p, b.buf[b.r:b.w])
+	b.r += n
+	return n, nil
+}
+
+// buffered returns the number of bytes that can be read from the current buffer.
+func (b *ReadSeeker) buffered() int { return b.w - b.r }
+
+func (b *ReadSeeker) Seek(offset int64, whence int) (int64, error) {
+	// The stream.Seek() implementation makes heavy use of seeking with offset 0
+	// to obtain the current position; let's optimize for it.
+	if offset == 0 && whence == io.SeekCurrent {
+		return b.position(), nil
+	}
+	// When seeking from the end, the absolute position isn't known by ReadSeeker
+	// so the current buffer cannot be used. Seeking cannot be avoided.
+	if whence == io.SeekEnd {
+		return b.seek(offset, whence)
+	}
+	// Calculate the absolute offset.
+	abs := offset
+	if whence == io.SeekCurrent {
+		abs += b.position()
+	}
+	// Check if the offset is within buf.
+	if abs >= b.pos && abs < b.pos+int64(b.w) {
+		b.r = int(abs - b.pos)
+		return abs, nil
+	}
+
+	return b.seek(abs, io.SeekStart)
+}
+
+func (b *ReadSeeker) seek(offset int64, whence int) (int64, error) {
+	b.r = 0
+	b.w = 0
+	var err error
+	b.pos, err = b.rd.Seek(offset, whence)
+	return b.pos, err
+}
+
+// position returns the absolute read offset.
+func (b *ReadSeeker) position() int64 {
+	return b.pos + int64(b.r)
+}