compress/lzw: do not use background goroutines

Programs expect that Read and Write are synchronous.
The background goroutines make the implementation
a little easier, but they introduce asynchrony that
trips up calling code.  Remove them.

R=golang-dev, nigeltao
CC=golang-dev
https://golang.org/cl/4548080

2 files changed, 135 insertions, 96 deletions

diff --git a/src/pkg/compress/lzw/reader.go b/src/pkg/compress/lzw/reader.go
index a1cd2abc04..ccd882f88b 100644
--- a/src/pkg/compress/lzw/reader.go
+++ b/src/pkg/compress/lzw/reader.go
@@ -32,13 +32,48 @@ const (
 	MSB
 )
 
+const (
+	maxWidth           = 12
+	decoderInvalidCode = 0xffff
+	flushBuffer        = 1 << maxWidth
+)
+
 // decoder is the state from which the readXxx method converts a byte
 // stream into a code stream.
 type decoder struct {
-	r     io.ByteReader
-	bits  uint32
-	nBits uint
-	width uint
+	r        io.ByteReader
+	bits     uint32
+	nBits    uint
+	width    uint
+	read     func(*decoder) (uint16, os.Error) // readLSB or readMSB
+	litWidth int                               // width in bits of literal codes
+	err      os.Error
+
+	// The first 1<<litWidth codes are literal codes.
+	// The next two codes mean clear and EOF.
+	// Other valid codes are in the range [lo, hi] where lo := clear + 2,
+	// with the upper bound incrementing on each code seen.
+	// overflow is the code at which hi overflows the code width.
+	// last is the most recently seen code, or decoderInvalidCode.
+	clear, eof, hi, overflow, last uint16
+
+	// Each code c in [lo, hi] expands to two or more bytes. For c != hi:
+	//   suffix[c] is the last of these bytes.
+	//   prefix[c] is the code for all but the last byte.
+	//   This code can either be a literal code or another code in [lo, c).
+	// The c == hi case is a special case.
+	suffix [1 << maxWidth]uint8
+	prefix [1 << maxWidth]uint16
+	// buf is a scratch buffer for reconstituting the bytes that a code expands to.
+	// Code suffixes are written right-to-left from the end of the buffer.
+	buf [1 << maxWidth]byte
+
+	// output is the temporary output buffer.
+	// It is flushed when it contains >= 1<<maxWidth bytes,
+	// so that there is always room to copy buf into it while decoding.
+	output [2 * 1 << maxWidth]byte
+	o      int    // write index into output
+	toRead []byte // bytes to return from Read
 }
 
 // readLSB returns the next code for "Least Significant Bits first" data.
@@ -73,119 +108,113 @@ func (d *decoder) readMSB() (uint16, os.Error) {
 	return code, nil
 }
 
-// decode decompresses bytes from r and writes them to pw.
-// read specifies how to decode bytes into codes.
-// litWidth is the width in bits of literal codes.
-func decode(r io.Reader, read func(*decoder) (uint16, os.Error), litWidth int, pw *io.PipeWriter) {
-	br, ok := r.(io.ByteReader)
-	if !ok {
-		br = bufio.NewReader(r)
+func (d *decoder) Read(b []byte) (int, os.Error) {
+	for {
+		if len(d.toRead) > 0 {
+			n := copy(b, d.toRead)
+			d.toRead = d.toRead[n:]
+			return n, nil
+		}
+		if d.err != nil {
+			return 0, d.err
+		}
+		d.decode()
 	}
-	pw.CloseWithError(decode1(pw, br, read, uint(litWidth)))
+	panic("unreachable")
 }
 
-func decode1(pw *io.PipeWriter, r io.ByteReader, read func(*decoder) (uint16, os.Error), litWidth uint) os.Error {
-	const (
-		maxWidth    = 12
-		invalidCode = 0xffff
-	)
-	d := decoder{r, 0, 0, 1 + litWidth}
-	w := bufio.NewWriter(pw)
-	// The first 1<<litWidth codes are literal codes.
-	// The next two codes mean clear and EOF.
-	// Other valid codes are in the range [lo, hi] where lo := clear + 2,
-	// with the upper bound incrementing on each code seen.
-	clear := uint16(1) << litWidth
-	eof, hi := clear+1, clear+1
-	// overflow is the code at which hi overflows the code width.
-	overflow := uint16(1) << d.width
-	var (
-		// Each code c in [lo, hi] expands to two or more bytes. For c != hi:
-		//   suffix[c] is the last of these bytes.
-		//   prefix[c] is the code for all but the last byte.
-		//   This code can either be a literal code or another code in [lo, c).
-		// The c == hi case is a special case.
-		suffix [1 << maxWidth]uint8
-		prefix [1 << maxWidth]uint16
-		// buf is a scratch buffer for reconstituting the bytes that a code expands to.
-		// Code suffixes are written right-to-left from the end of the buffer.
-		buf [1 << maxWidth]byte
-	)
-
+// decode decompresses bytes from r and leaves them in d.toRead.
+// read specifies how to decode bytes into codes.
+// litWidth is the width in bits of literal codes.
+func (d *decoder) decode() {
 	// Loop over the code stream, converting codes into decompressed bytes.
-	last := uint16(invalidCode)
 	for {
-		code, err := read(&d)
+		code, err := d.read(d)
 		if err != nil {
 			if err == os.EOF {
 				err = io.ErrUnexpectedEOF
 			}
-			return err
+			d.err = err
+			return
 		}
 		switch {
-		case code < clear:
+		case code < d.clear:
 			// We have a literal code.
-			if err := w.WriteByte(uint8(code)); err != nil {
-				return err
-			}
-			if last != invalidCode {
+			d.output[d.o] = uint8(code)
+			d.o++
+			if d.last != decoderInvalidCode {
 				// Save what the hi code expands to.
-				suffix[hi] = uint8(code)
-				prefix[hi] = last
+				d.suffix[d.hi] = uint8(code)
+				d.prefix[d.hi] = d.last
 			}
-		case code == clear:
-			d.width = 1 + litWidth
-			hi = eof
-			overflow = 1 << d.width
-			last = invalidCode
+		case code == d.clear:
+			d.width = 1 + uint(d.litWidth)
+			d.hi = d.eof
+			d.overflow = 1 << d.width
+			d.last = decoderInvalidCode
 			continue
-		case code == eof:
-			return w.Flush()
-		case code <= hi:
-			c, i := code, len(buf)-1
-			if code == hi {
+		case code == d.eof:
+			d.flush()
+			d.err = os.EOF
+			return
+		case code <= d.hi:
+			c, i := code, len(d.buf)-1
+			if code == d.hi {
 				// code == hi is a special case which expands to the last expansion
 				// followed by the head of the last expansion. To find the head, we walk
 				// the prefix chain until we find a literal code.
-				c = last
-				for c >= clear {
-					c = prefix[c]
+				c = d.last
+				for c >= d.clear {
+					c = d.prefix[c]
 				}
-				buf[i] = uint8(c)
+				d.buf[i] = uint8(c)
 				i--
-				c = last
+				c = d.last
 			}
 			// Copy the suffix chain into buf and then write that to w.
-			for c >= clear {
-				buf[i] = suffix[c]
+			for c >= d.clear {
+				d.buf[i] = d.suffix[c]
 				i--
-				c = prefix[c]
+				c = d.prefix[c]
 			}
-			buf[i] = uint8(c)
-			if _, err := w.Write(buf[i:]); err != nil {
-				return err
-			}
-			if last != invalidCode {
+			d.buf[i] = uint8(c)
+			d.o += copy(d.output[d.o:], d.buf[i:])
+			if d.last != decoderInvalidCode {
 				// Save what the hi code expands to.
-				suffix[hi] = uint8(c)
-				prefix[hi] = last
+				d.suffix[d.hi] = uint8(c)
+				d.prefix[d.hi] = d.last
 			}
 		default:
-			return os.NewError("lzw: invalid code")
+			d.err = os.NewError("lzw: invalid code")
+			return
 		}
-		last, hi = code, hi+1
-		if hi >= overflow {
+		d.last, d.hi = code, d.hi+1
+		if d.hi >= d.overflow {
 			if d.width == maxWidth {
-				last = invalidCode
-				continue
+				d.last = decoderInvalidCode
+			} else {
+				d.width++
+				d.overflow <<= 1
 			}
-			d.width++
-			overflow <<= 1
+		}
+		if d.o >= flushBuffer {
+			d.flush()
+			return
 		}
 	}
 	panic("unreachable")
 }
 
+func (d *decoder) flush() {
+	d.toRead = d.output[:d.o]
+	d.o = 0
+}
+
+func (d *decoder) Close() os.Error {
+	d.err = os.EINVAL // in case any Reads come along
+	return nil
+}
+
 // NewReader creates a new io.ReadCloser that satisfies reads by decompressing
 // the data read from r.
 // It is the caller's responsibility to call Close on the ReadCloser when
@@ -193,21 +222,31 @@ func decode1(pw *io.PipeWriter, r io.ByteReader, read func(*decoder) (uint16, os
 // The number of bits to use for literal codes, litWidth, must be in the
 // range [2,8] and is typically 8.
 func NewReader(r io.Reader, order Order, litWidth int) io.ReadCloser {
-	pr, pw := io.Pipe()
-	var read func(*decoder) (uint16, os.Error)
+	d := new(decoder)
 	switch order {
 	case LSB:
-		read = (*decoder).readLSB
+		d.read = (*decoder).readLSB
 	case MSB:
-		read = (*decoder).readMSB
+		d.read = (*decoder).readMSB
 	default:
-		pw.CloseWithError(os.NewError("lzw: unknown order"))
-		return pr
+		d.err = os.NewError("lzw: unknown order")
+		return d
 	}
 	if litWidth < 2 || 8 < litWidth {
-		pw.CloseWithError(fmt.Errorf("lzw: litWidth %d out of range", litWidth))
-		return pr
+		d.err = fmt.Errorf("lzw: litWidth %d out of range", litWidth)
+		return d
 	}
-	go decode(r, read, litWidth, pw)
-	return pr
+	if br, ok := r.(io.ByteReader); ok {
+		d.r = br
+	} else {
+		d.r = bufio.NewReader(r)
+	}
+	d.litWidth = litWidth
+	d.width = 1 + uint(litWidth)
+	d.clear = uint16(1) << uint(litWidth)
+	d.eof, d.hi = d.clear+1, d.clear+1
+	d.overflow = uint16(1) << d.width
+	d.last = decoderInvalidCode
+
+	return d
 }
diff --git a/src/pkg/compress/lzw/writer_test.go b/src/pkg/compress/lzw/writer_test.go
index 82464ecd1b..4c5e522f94 100644
--- a/src/pkg/compress/lzw/writer_test.go
+++ b/src/pkg/compress/lzw/writer_test.go
@@ -77,13 +77,13 @@ func testFile(t *testing.T, fn string, order Order, litWidth int) {
 		t.Errorf("%s (order=%d litWidth=%d): %v", fn, order, litWidth, err1)
 		return
 	}
-	if len(b0) != len(b1) {
-		t.Errorf("%s (order=%d litWidth=%d): length mismatch %d versus %d", fn, order, litWidth, len(b0), len(b1))
+	if len(b1) != len(b0) {
+		t.Errorf("%s (order=%d litWidth=%d): length mismatch %d != %d", fn, order, litWidth, len(b1), len(b0))
 		return
 	}
 	for i := 0; i < len(b0); i++ {
-		if b0[i] != b1[i] {
-			t.Errorf("%s (order=%d litWidth=%d): mismatch at %d, 0x%02x versus 0x%02x\n", fn, order, litWidth, i, b0[i], b1[i])
+		if b1[i] != b0[i] {
+			t.Errorf("%s (order=%d litWidth=%d): mismatch at %d, 0x%02x != 0x%02x\n", fn, order, litWidth, i, b1[i], b0[i])
 			return
 		}
 	}