Check-in [ae0940d072]
Overview
Comment:Read available bytes and the end of the buffer and decompress in a singe pass
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SHA1:ae0940d072f9767465179b78fccf2e835b753f24
User & Date: spaskalev on 2014-12-22 14:23:31
Other Links: manifest | tags
Context
2014-12-22
15:34
Fixed a nasty variable shadowing bug :) check-in: e9b80a705b user: spaskalev tags: decompressor2
14:23
Read available bytes and the end of the buffer and decompress in a singe pass check-in: ae0940d072 user: spaskalev tags: decompressor2
2014-12-21
23:26
Closing the decompressor2 branch as this implementation is slower than the naive one. check-in: 52e14c83da user: spaskalev tags: decompressor2
Changes

Modified src/0dev.org/predictor/predictor.go from [84146b7c8c] to [c018a46296].

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// and decompresses data according to the predictor algorithm
func Decompressor(reader io.Reader) io.Reader {
	var ctx context
	ctx.input = make([]byte, 0, 8)

	return decompressor(func(output []byte) (int, error) {
		var (
			err                  error
			flags                byte
			readCount, available int
		)

		// Sanity check for space to read into
		if len(output) == 0 {
			return 0, nil
		}

		// Check whether we have leftover data in the buffer
		if len(ctx.input) > 0 {
			readCount = copy(output, ctx.input)

			// Check whether we still have leftover data in the buffer :)
			if readCount < len(ctx.input) {
				ctx.input = ctx.input[:copy(ctx.input, ctx.input[readCount:])]
			}
			return readCount, nil
		}

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

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


		// Read the non-predicted bytes according to header.
		readCount, err = reader.Read(ctx.input[:available])
	retryData:
		if readCount < int(available) && err == nil {
			// Retry the read if we have fewer bytes than what the prediction header indicates
			var rc int
			rc, err = reader.Read(ctx.input[readCount:available])
			readCount += rc
			goto retryData
		} // Continue on any error, try to decompress and return it along the result

		// Spread the read bytes right to left to avoid overlapping
		for i, a := 7, available-1; i >= 0; i-- {
			if ((flags >> uint(i)) & 1) == 0 {
				ctx.input[i] = ctx.input[a]
				a--
			}
		}

		// Walk the buffer, fill in the predicted blanks and update the guess table

		for i := uint(0); i < 8; i++ {
			if (flags & (1 << i)) > 0 {
				// Guess succeeded, fill in from the table
				ctx.input[i] = ctx.table[ctx.hash]
				readCount++
			} else {


				// Guess failed, update the table
				ctx.table[ctx.hash] = ctx.input[i]
			}
			// Update the hash
			ctx.hash = (ctx.hash << 4) ^ uint16(ctx.input[i])
		}

		// readCount now contains the precise amount of populated data
		ctx.input = ctx.input[:readCount]
		available = copy(output, ctx.input)

		// Check for remaining bytes that dont fit in the output buffer
		if available < readCount {
			ctx.input = ctx.input[:copy(ctx.input, ctx.input[available:])]
		} else {
			// Clear the buffer
			ctx.input = ctx.input[:0]
		}

		return available, err
	})
}







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// and decompresses data according to the predictor algorithm
func Decompressor(reader io.Reader) io.Reader {
	var ctx context
	ctx.input = make([]byte, 0, 8)

	return decompressor(func(output []byte) (int, error) {
		var (
			err                      error
			flags                    byte
			rc, available, predicted int
		)

		// Sanity check for space to read into
		if len(output) == 0 {
			return 0, nil
		}

		// Check whether we have leftover data in the buffer
		if len(ctx.input) > 0 {
			rc = copy(output, ctx.input)

			// Check whether we still have leftover data in the buffer :)
			if rc < len(ctx.input) {
				ctx.input = ctx.input[:copy(ctx.input, ctx.input[rc:])]
			}
			return rc, nil
		}

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

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

		// Read the non-predicted bytes and place them in the end of the buffer
		rc, err = reader.Read(ctx.input[predicted:])
	retryData:
		if rc < int(available) && err == nil {
			// Retry the read if we have fewer bytes than what the prediction header indicates
			var rc int
			rc, err = reader.Read(ctx.input[predicted+rc:])
			rc += rc
			goto retryData
		} // Continue on any error, try to decompress and return it along the result









		// Walk the buffer, filling in the predicted blanks,
		// relocating read bytes and and updating the guess table
		for i, a := uint(0), predicted; i < 8; i++ {
			if (flags & (1 << i)) > 0 {
				// Guess succeeded, fill in from the table
				ctx.input[i] = ctx.table[ctx.hash]
				rc++
			} else {
				// Relocate a read byte
				ctx.input[i], a = ctx.input[a], a+1
				// Guess failed, update the table
				ctx.table[ctx.hash] = ctx.input[i]
			}
			// Update the hash
			ctx.hash = (ctx.hash << 4) ^ uint16(ctx.input[i])
		}

		// rc now contains the precise amount of populated data
		ctx.input = ctx.input[:rc]
		available = copy(output, ctx.input)

		// Check for remaining bytes that dont fit in the output buffer
		if available < rc {
			ctx.input = ctx.input[:copy(ctx.input, ctx.input[available:])]
		} else {
			// Clear the buffer
			ctx.input = ctx.input[:0]
		}

		return available, err
	})
}