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Comment:Predictor's compressor and decompressor structures now implement io.Writer/io.Reader in order to deal away with function indirection but they do not follow the required semantics. Those are provided by the SizedWriter/SizedReader wrappers returned by the constructor functions.
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SHA1: 4dfcff962c763b560a5af8101c2f3f3ad059ae8c
User & Date: spaskalev on 2014-12-25 00:55:43
Other Links: manifest | tags
Context
2014-12-25
01:25
Removed pdc's output buffer when decompressing as io.Copy uses a sufficiently-large buffer internally. check-in: 2b1ed8e45e user: spaskalev tags: trunk
00:55
Predictor's compressor and decompressor structures now implement io.Writer/io.Reader in order to deal away with function indirection but they do not follow the required semantics. Those are provided by the SizedWriter/SizedReader wrappers returned by the constructor functions. check-in: 4dfcff962c user: spaskalev tags: trunk
00:43
Extracted predictor's compressor and decompressor code into separate structs that embed Sized{Writer,Reader} check-in: 50507bd510 user: spaskalev tags: trunk
Changes
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Modified src/0dev.org/predictor/predictor.go from [cb877636af] to [3161320c11].

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// Returns an io.Writer implementation that wraps the provided io.Writer
// and compresses data according to the predictor algorithm
//
// It can buffer data as the predictor mandates 8-byte blocks with a header.
// A call with no data will force a flush.
func Compressor(writer io.Writer) io.Writer {
	var cmp compressor
	cmp.Writer = iou.SizedWriter(iou.WriterFunc(cmp.compress), 8)
	cmp.target = writer
	return &cmp
	return iou.SizedWriter(&cmp, 8)
}

type compressor struct {
	context
	io.Writer
	target io.Writer
}

// Note: this method does not implement the full io.Writer's Write() semantics
func (ctx *compressor) compress(data []byte) (int, error) {
func (ctx *compressor) Write(data []byte) (int, error) {
	var (
		blockSize  int = 8
		datalength int = len(data)
	)

	if datalength == 0 {
		return 0, nil
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	return datalength, nil
}

// Returns an io.Reader implementation that wraps the provided io.Reader
// and decompresses data according to the predictor algorithm
func Decompressor(reader io.Reader) io.Reader {
	var dcmp decompressor
	dcmp.Reader = iou.SizedReader(iou.ReaderFunc(dcmp.decompress), 8)
	dcmp.source = reader
	dcmp.input = make([]byte, 0, 8)
	return &dcmp
	dcmp.input = make([]byte, 8)
	return iou.SizedReader(&dcmp, 8)
}

type decompressor struct {
	context
	io.Reader
	source io.Reader
	input  []byte
}

// Note: this method does not implement the full io.Reader's Read() semantics
func (ctx *decompressor) decompress(output []byte) (int, error) {
func (ctx *decompressor) Read(output []byte) (int, error) {
	var (
		err               error
		flags, predicted  byte
		rc, total, copied int
	)

	// Read the next prediction header
readHeader:
	rc, err = ctx.source.Read(ctx.input[:1])
	// Fail on error unless it is EOF
	if err != nil && err != io.EOF {
		return total, err
	} else if rc == 0 {
		return total, err
	}

	// Extend the buffer, copy the prediction header
	//  and calculate the number of subsequent bytes to read
	// Copy the prediction header and calculate the number of subsequent bytes to read
	ctx.input = ctx.input[:8]
	flags = ctx.input[0]
	predicted = bits.Hamming(flags)

	// Read the non-predicted bytes and place them in the end of the buffer
	rc, err = ctx.source.Read(ctx.input[predicted:])
retryData:
	if rc < int(8-predicted) && err == nil {
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		ctx.update(ctx.input[i])
	}

	// Copy the decompressed data to the output and accumulate the count
	copied = copy(output, ctx.input[:rc])
	total += copied

	// Clear the buffer
	ctx.input = ctx.input[:0]

	// Loop for another pass if there is available space in the output
	output = output[copied:]
	if len(output) > 0 && err == nil {
		goto readHeader
	}

	return total, err
}

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