Check-in [9dfd3cb1a2]
Overview
Comment:Extracted the predictor's hash function as a method of the context struct. Minor changes to the decompressor's variables.
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SHA1:9dfd3cb1a202cf8747262b5d7e6dc243268f9b8e
User & Date: spaskalev on 2014-12-22 19:28:49
Other Links: manifest | tags
Context
2014-12-22
19:52
Added documentation for the decompressor check-in: 89bfe97384 user: spaskalev tags: trunk
19:28
Extracted the predictor's hash function as a method of the context struct. Minor changes to the decompressor's variables. check-in: 9dfd3cb1a2 user: spaskalev tags: trunk
17:15
Calculate the decompressed block length outside of the predictor loop check-in: 27ecac81d3 user: spaskalev tags: trunk
Changes

Modified src/0dev.org/predictor/predictor.go from [46cd9d5cef] to [a4885df9dd].

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)

type context struct {
	table [1 << 16]byte
	input []byte
	hash  uint16
}










type compressor func([]byte) error

func (w compressor) Write(data []byte) (int, error) {
	return len(data), w(data)
}

................................................................................
					// Guess was right - don't output
					buf[0] |= 1 << uint(i)
				} else {
					// Guess was wrong, output char
					ctx.table[ctx.hash] = current
					buf = append(buf, current)
				}
				ctx.hash = (ctx.hash << 4) ^ uint16(current)
			}

			if _, err := writer.Write(buf); err != nil {
				return err
			}

			// Reset the flags and buffer for the next iteration
................................................................................
// 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, total int
		)

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

................................................................................
			return total, 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 r int
			r, err = reader.Read(ctx.input[predicted+rc:])
			rc += r
			goto retryData
		} // Continue on any error, try to decompress and return it along the result

		// rc now contains the amount of actual bytes in this cycle (usually 8)
		rc += predicted

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

		// Copy the decompressed data to the output
		ctx.input = ctx.input[:rc]
		available = copy(output, ctx.input)

		total += available

		// 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]

			output = output[available:]
			if len(output) > 0 && err == nil {
				goto readHeader
			}
		}

		return total, err
	})
}







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)

type context struct {
	table [1 << 16]byte
	input []byte
	hash  uint16
}

// The following hash code is the heart of the algorithm:
// It builds a sliding hash sum of the previous 3-and-a-bit
// characters which will be used to index the guess table.
// A better hash function would result in additional compression,
// at the expense of time.
func (ctx *context) update(val uint16) {
	ctx.hash = (ctx.hash << 4) ^ val
}

type compressor func([]byte) error

func (w compressor) Write(data []byte) (int, error) {
	return len(data), w(data)
}

................................................................................
					// Guess was right - don't output
					buf[0] |= 1 << uint(i)
				} else {
					// Guess was wrong, output char
					ctx.table[ctx.hash] = current
					buf = append(buf, current)
				}
				ctx.update(uint16(current))
			}

			if _, err := writer.Write(buf); err != nil {
				return err
			}

			// Reset the flags and buffer for the next iteration
................................................................................
// 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, predicted  byte
			rc, total, copied int
		)

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

................................................................................
			return total, 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 = bits.Hamming(flags)


		// 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(8-predicted) && err == nil {
			// Retry the read if we have fewer bytes than what the prediction header indicates
			var r int
			r, err = reader.Read(ctx.input[int(predicted)+rc:])
			rc += r
			goto retryData
		} // Continue on any error, try to decompress and return it along the result

		// rc now contains the amount of actual bytes in this cycle (usually 8)
		rc += int(predicted)

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

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

		total += copied

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

			output = output[copied:]
			if len(output) > 0 && err == nil {
				goto readHeader
			}
		}

		return total, err
	})
}