1 files changed, 244 insertions, 0 deletions
diff --git a/src/image/jpeg/huffman.go b/src/image/jpeg/huffman.go
new file mode 100644
index 000000000..d4ff4cfa0
--- /dev/null
+++ b/src/image/jpeg/huffman.go
@@ -0,0 +1,244 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package jpeg
+
+import (
+	"io"
+)
+
+// maxCodeLength is the maximum (inclusive) number of bits in a Huffman code.
+const maxCodeLength = 16
+
+// maxNCodes is the maximum (inclusive) number of codes in a Huffman tree.
+const maxNCodes = 256
+
+// lutSize is the log-2 size of the Huffman decoder's look-up table.
+const lutSize = 8
+
+// huffman is a Huffman decoder, specified in section C.
+type huffman struct {
+	// length is the number of codes in the tree.
+	nCodes int32
+	// lut is the look-up table for the next lutSize bits in the bit-stream.
+	// The high 8 bits of the uint16 are the encoded value. The low 8 bits
+	// are 1 plus the code length, or 0 if the value is too large to fit in
+	// lutSize bits.
+	lut [1 << lutSize]uint16
+	// vals are the decoded values, sorted by their encoding.
+	vals [maxNCodes]uint8
+	// minCodes[i] is the minimum code of length i, or -1 if there are no
+	// codes of that length.
+	minCodes [maxCodeLength]int32
+	// maxCodes[i] is the maximum code of length i, or -1 if there are no
+	// codes of that length.
+	maxCodes [maxCodeLength]int32
+	// valsIndices[i] is the index into vals of minCodes[i].
+	valsIndices [maxCodeLength]int32
+}
+
+// errShortHuffmanData means that an unexpected EOF occurred while decoding
+// Huffman data.
+var errShortHuffmanData = FormatError("short Huffman data")
+
+// ensureNBits reads bytes from the byte buffer to ensure that d.bits.n is at
+// least n. For best performance (avoiding function calls inside hot loops),
+// the caller is the one responsible for first checking that d.bits.n < n.
+func (d *decoder) ensureNBits(n int32) error {
+	for {
+		c, err := d.readByteStuffedByte()
+		if err != nil {
+			if err == io.EOF {
+				return errShortHuffmanData
+			}
+			return err
+		}
+		d.bits.a = d.bits.a<<8 | uint32(c)
+		d.bits.n += 8
+		if d.bits.m == 0 {
+			d.bits.m = 1 << 7
+		} else {
+			d.bits.m <<= 8
+		}
+		if d.bits.n >= n {
+			break
+		}
+	}
+	return nil
+}
+
+// receiveExtend is the composition of RECEIVE and EXTEND, specified in section
+// F.2.2.1.
+func (d *decoder) receiveExtend(t uint8) (int32, error) {
+	if d.bits.n < int32(t) {
+		if err := d.ensureNBits(int32(t)); err != nil {
+			return 0, err
+		}
+	}
+	d.bits.n -= int32(t)
+	d.bits.m >>= t
+	s := int32(1) << t
+	x := int32(d.bits.a>>uint8(d.bits.n)) & (s - 1)
+	if x < s>>1 {
+		x += ((-1) << t) + 1
+	}
+	return x, nil
+}
+
+// processDHT processes a Define Huffman Table marker, and initializes a huffman
+// struct from its contents. Specified in section B.2.4.2.
+func (d *decoder) processDHT(n int) error {
+	for n > 0 {
+		if n < 17 {
+			return FormatError("DHT has wrong length")
+		}
+		if err := d.readFull(d.tmp[:17]); err != nil {
+			return err
+		}
+		tc := d.tmp[0] >> 4
+		if tc > maxTc {
+			return FormatError("bad Tc value")
+		}
+		th := d.tmp[0] & 0x0f
+		if th > maxTh || !d.progressive && th > 1 {
+			return FormatError("bad Th value")
+		}
+		h := &d.huff[tc][th]
+
+		// Read nCodes and h.vals (and derive h.nCodes).
+		// nCodes[i] is the number of codes with code length i.
+		// h.nCodes is the total number of codes.
+		h.nCodes = 0
+		var nCodes [maxCodeLength]int32
+		for i := range nCodes {
+			nCodes[i] = int32(d.tmp[i+1])
+			h.nCodes += nCodes[i]
+		}
+		if h.nCodes == 0 {
+			return FormatError("Huffman table has zero length")
+		}
+		if h.nCodes > maxNCodes {
+			return FormatError("Huffman table has excessive length")
+		}
+		n -= int(h.nCodes) + 17
+		if n < 0 {
+			return FormatError("DHT has wrong length")
+		}
+		if err := d.readFull(h.vals[:h.nCodes]); err != nil {
+			return err
+		}
+
+		// Derive the look-up table.
+		for i := range h.lut {
+			h.lut[i] = 0
+		}
+		var x, code uint32
+		for i := uint32(0); i < lutSize; i++ {
+			code <<= 1
+			for j := int32(0); j < nCodes[i]; j++ {
+				// The codeLength is 1+i, so shift code by 8-(1+i) to
+				// calculate the high bits for every 8-bit sequence
+				// whose codeLength's high bits matches code.
+				// The high 8 bits of lutValue are the encoded value.
+				// The low 8 bits are 1 plus the codeLength.
+				base := uint8(code << (7 - i))
+				lutValue := uint16(h.vals[x])<<8 | uint16(2+i)
+				for k := uint8(0); k < 1<<(7-i); k++ {
+					h.lut[base|k] = lutValue
+				}
+				code++
+				x++
+			}
+		}
+
+		// Derive minCodes, maxCodes, and valsIndices.
+		var c, index int32
+		for i, n := range nCodes {
+			if n == 0 {
+				h.minCodes[i] = -1
+				h.maxCodes[i] = -1
+				h.valsIndices[i] = -1
+			} else {
+				h.minCodes[i] = c
+				h.maxCodes[i] = c + n - 1
+				h.valsIndices[i] = index
+				c += n
+				index += n
+			}
+			c <<= 1
+		}
+	}
+	return nil
+}
+
+// decodeHuffman returns the next Huffman-coded value from the bit-stream,
+// decoded according to h.
+func (d *decoder) decodeHuffman(h *huffman) (uint8, error) {
+	if h.nCodes == 0 {
+		return 0, FormatError("uninitialized Huffman table")
+	}
+
+	if d.bits.n < 8 {
+		if err := d.ensureNBits(8); err != nil {
+			if err != errMissingFF00 && err != errShortHuffmanData {
+				return 0, err
+			}
+			// There are no more bytes of data in this segment, but we may still
+			// be able to read the next symbol out of the previously read bits.
+			// First, undo the readByte that the ensureNBits call made.
+			d.unreadByteStuffedByte()
+			goto slowPath
+		}
+	}
+	if v := h.lut[(d.bits.a>>uint32(d.bits.n-lutSize))&0xff]; v != 0 {
+		n := (v & 0xff) - 1
+		d.bits.n -= int32(n)
+		d.bits.m >>= n
+		return uint8(v >> 8), nil
+	}
+
+slowPath:
+	for i, code := 0, int32(0); i < maxCodeLength; i++ {
+		if d.bits.n == 0 {
+			if err := d.ensureNBits(1); err != nil {
+				return 0, err
+			}
+		}
+		if d.bits.a&d.bits.m != 0 {
+			code |= 1
+		}
+		d.bits.n--
+		d.bits.m >>= 1
+		if code <= h.maxCodes[i] {
+			return h.vals[h.valsIndices[i]+code-h.minCodes[i]], nil
+		}
+		code <<= 1
+	}
+	return 0, FormatError("bad Huffman code")
+}
+
+func (d *decoder) decodeBit() (bool, error) {
+	if d.bits.n == 0 {
+		if err := d.ensureNBits(1); err != nil {
+			return false, err
+		}
+	}
+	ret := d.bits.a&d.bits.m != 0
+	d.bits.n--
+	d.bits.m >>= 1
+	return ret, nil
+}
+
+func (d *decoder) decodeBits(n int32) (uint32, error) {
+	if d.bits.n < n {
+		if err := d.ensureNBits(n); err != nil {
+			return 0, err
+		}
+	}
+	ret := d.bits.a >> uint32(d.bits.n-n)
+	ret &= (1 << uint32(n)) - 1
+	d.bits.n -= n
+	d.bits.m >>= uint32(n)
+	return ret, nil
+}