gtsocial-umbx

reader.go (21235B)

---

 // Copyright 2019 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.
 
 //go:generate go run gen.go
 
 // Package ccitt implements a CCITT (fax) image decoder.
 package ccitt
 
 import (
 	"encoding/binary"
 	"errors"
 	"image"
 	"io"
 	"math/bits"
 )
 
 var (
 	errIncompleteCode          = errors.New("ccitt: incomplete code")
 	errInvalidBounds           = errors.New("ccitt: invalid bounds")
 	errInvalidCode             = errors.New("ccitt: invalid code")
 	errInvalidMode             = errors.New("ccitt: invalid mode")
 	errInvalidOffset           = errors.New("ccitt: invalid offset")
 	errMissingEOL              = errors.New("ccitt: missing End-of-Line")
 	errRunLengthOverflowsWidth = errors.New("ccitt: run length overflows width")
 	errRunLengthTooLong        = errors.New("ccitt: run length too long")
 	errUnsupportedMode         = errors.New("ccitt: unsupported mode")
 	errUnsupportedSubFormat    = errors.New("ccitt: unsupported sub-format")
 	errUnsupportedWidth        = errors.New("ccitt: unsupported width")
 )
 
 // Order specifies the bit ordering in a CCITT data stream.
 type Order uint32
 
 const (
 	// LSB means Least Significant Bits first.
 	LSB Order = iota
 	// MSB means Most Significant Bits first.
 	MSB
 )
 
 // SubFormat represents that the CCITT format consists of a number of
 // sub-formats. Decoding or encoding a CCITT data stream requires knowing the
 // sub-format context. It is not represented in the data stream per se.
 type SubFormat uint32
 
 const (
 	Group3 SubFormat = iota
 	Group4
 )
 
 // AutoDetectHeight is passed as the height argument to NewReader to indicate
 // that the image height (the number of rows) is not known in advance.
 const AutoDetectHeight = -1
 
 // Options are optional parameters.
 type Options struct {
 	// Align means that some variable-bit-width codes are byte-aligned.
 	Align bool
 	// Invert means that black is the 1 bit or 0xFF byte, and white is 0.
 	Invert bool
 }
 
 // maxWidth is the maximum (inclusive) supported width. This is a limitation of
 // this implementation, to guard against integer overflow, and not anything
 // inherent to the CCITT format.
 const maxWidth = 1 << 20
 
 func invertBytes(b []byte) {
 	for i, c := range b {
 		b[i] = ^c
 	}
 }
 
 func reverseBitsWithinBytes(b []byte) {
 	for i, c := range b {
 		b[i] = bits.Reverse8(c)
 	}
 }
 
 // highBits writes to dst (1 bit per pixel, most significant bit first) the
 // high (0x80) bits from src (1 byte per pixel). It returns the number of bytes
 // written and read such that dst[:d] is the packed form of src[:s].
 //
 // For example, if src starts with the 8 bytes [0x7D, 0x7E, 0x7F, 0x80, 0x81,
 // 0x82, 0x00, 0xFF] then 0x1D will be written to dst[0].
 //
 // If src has (8 * len(dst)) or more bytes then only len(dst) bytes are
 // written, (8 * len(dst)) bytes are read, and invert is ignored.
 //
 // Otherwise, if len(src) is not a multiple of 8 then the final byte written to
 // dst is padded with 1 bits (if invert is true) or 0 bits. If inverted, the 1s
 // are typically temporary, e.g. they will be flipped back to 0s by an
 // invertBytes call in the highBits caller, reader.Read.
 func highBits(dst []byte, src []byte, invert bool) (d int, s int) {
 	// Pack as many complete groups of 8 src bytes as we can.
 	n := len(src) / 8
 	if n > len(dst) {
 		n = len(dst)
 	}
 	dstN := dst[:n]
 	for i := range dstN {
 		src8 := src[i*8 : i*8+8]
 		dstN[i] = ((src8[0] & 0x80) >> 0) |
 			((src8[1] & 0x80) >> 1) |
 			((src8[2] & 0x80) >> 2) |
 			((src8[3] & 0x80) >> 3) |
 			((src8[4] & 0x80) >> 4) |
 			((src8[5] & 0x80) >> 5) |
 			((src8[6] & 0x80) >> 6) |
 			((src8[7] & 0x80) >> 7)
 	}
 	d, s = n, 8*n
 	dst, src = dst[d:], src[s:]
 
 	// Pack up to 7 remaining src bytes, if there's room in dst.
 	if (len(dst) > 0) && (len(src) > 0) {
 		dstByte := byte(0)
 		if invert {
 			dstByte = 0xFF >> uint(len(src))
 		}
 		for n, srcByte := range src {
 			dstByte |= (srcByte & 0x80) >> uint(n)
 		}
 		dst[0] = dstByte
 		d, s = d+1, s+len(src)
 	}
 	return d, s
 }
 
 type bitReader struct {
 	r io.Reader
 
 	// readErr is the error returned from the most recent r.Read call. As the
 	// io.Reader documentation says, when r.Read returns (n, err), "always
 	// process the n > 0 bytes returned before considering the error err".
 	readErr error
 
 	// order is whether to process r's bytes LSB first or MSB first.
 	order Order
 
 	// The high nBits bits of the bits field hold upcoming bits in MSB order.
 	bits  uint64
 	nBits uint32
 
 	// bytes[br:bw] holds bytes read from r but not yet loaded into bits.
 	br    uint32
 	bw    uint32
 	bytes [1024]uint8
 }
 
 func (b *bitReader) alignToByteBoundary() {
 	n := b.nBits & 7
 	b.bits <<= n
 	b.nBits -= n
 }
 
 // nextBitMaxNBits is the maximum possible value of bitReader.nBits after a
 // bitReader.nextBit call, provided that bitReader.nBits was not more than this
 // value before that call.
 //
 // Note that the decode function can unread bits, which can temporarily set the
 // bitReader.nBits value above nextBitMaxNBits.
 const nextBitMaxNBits = 31
 
 func (b *bitReader) nextBit() (uint64, error) {
 	for {
 		if b.nBits > 0 {
 			bit := b.bits >> 63
 			b.bits <<= 1
 			b.nBits--
 			return bit, nil
 		}
 
 		if available := b.bw - b.br; available >= 4 {
 			// Read 32 bits, even though b.bits is a uint64, since the decode
 			// function may need to unread up to maxCodeLength bits, putting
 			// them back in the remaining (64 - 32) bits. TestMaxCodeLength
 			// checks that the generated maxCodeLength constant fits.
 			//
 			// If changing the Uint32 call, also change nextBitMaxNBits.
 			b.bits = uint64(binary.BigEndian.Uint32(b.bytes[b.br:])) << 32
 			b.br += 4
 			b.nBits = 32
 			continue
 		} else if available > 0 {
 			b.bits = uint64(b.bytes[b.br]) << (7 * 8)
 			b.br++
 			b.nBits = 8
 			continue
 		}
 
 		if b.readErr != nil {
 			return 0, b.readErr
 		}
 
 		n, err := b.r.Read(b.bytes[:])
 		b.br = 0
 		b.bw = uint32(n)
 		b.readErr = err
 
 		if b.order != MSB {
 			reverseBitsWithinBytes(b.bytes[:b.bw])
 		}
 	}
 }
 
 func decode(b *bitReader, decodeTable [][2]int16) (uint32, error) {
 	nBitsRead, bitsRead, state := uint32(0), uint64(0), int32(1)
 	for {
 		bit, err := b.nextBit()
 		if err != nil {
 			if err == io.EOF {
 				err = errIncompleteCode
 			}
 			return 0, err
 		}
 		bitsRead |= bit << (63 - nBitsRead)
 		nBitsRead++
 
 		// The "&1" is redundant, but can eliminate a bounds check.
 		state = int32(decodeTable[state][bit&1])
 		if state < 0 {
 			return uint32(^state), nil
 		} else if state == 0 {
 			// Unread the bits we've read, then return errInvalidCode.
 			b.bits = (b.bits >> nBitsRead) | bitsRead
 			b.nBits += nBitsRead
 			return 0, errInvalidCode
 		}
 	}
 }
 
 // decodeEOL decodes the 12-bit EOL code 0000_0000_0001.
 func decodeEOL(b *bitReader) error {
 	nBitsRead, bitsRead := uint32(0), uint64(0)
 	for {
 		bit, err := b.nextBit()
 		if err != nil {
 			if err == io.EOF {
 				err = errMissingEOL
 			}
 			return err
 		}
 		bitsRead |= bit << (63 - nBitsRead)
 		nBitsRead++
 
 		if nBitsRead < 12 {
 			if bit&1 == 0 {
 				continue
 			}
 		} else if bit&1 != 0 {
 			return nil
 		}
 
 		// Unread the bits we've read, then return errMissingEOL.
 		b.bits = (b.bits >> nBitsRead) | bitsRead
 		b.nBits += nBitsRead
 		return errMissingEOL
 	}
 }
 
 type reader struct {
 	br        bitReader
 	subFormat SubFormat
 
 	// width is the image width in pixels.
 	width int
 
 	// rowsRemaining starts at the image height in pixels, when the reader is
 	// driven through the io.Reader interface, and decrements to zero as rows
 	// are decoded. Alternatively, it may be negative if the image height is
 	// not known in advance at the time of the NewReader call.
 	//
 	// When driven through DecodeIntoGray, this field is unused.
 	rowsRemaining int
 
 	// curr and prev hold the current and previous rows. Each element is either
 	// 0x00 (black) or 0xFF (white).
 	//
 	// prev may be nil, when processing the first row.
 	curr []byte
 	prev []byte
 
 	// ri is the read index. curr[:ri] are those bytes of curr that have been
 	// passed along via the Read method.
 	//
 	// When the reader is driven through DecodeIntoGray, instead of through the
 	// io.Reader interface, this field is unused.
 	ri int
 
 	// wi is the write index. curr[:wi] are those bytes of curr that have
 	// already been decoded via the decodeRow method.
 	//
 	// What this implementation calls wi is roughly equivalent to what the spec
 	// calls the a0 index.
 	wi int
 
 	// These fields are copied from the *Options (which may be nil).
 	align  bool
 	invert bool
 
 	// atStartOfRow is whether we have just started the row. Some parts of the
 	// spec say to treat this situation as if "wi = -1".
 	atStartOfRow bool
 
 	// penColorIsWhite is whether the next run is black or white.
 	penColorIsWhite bool
 
 	// seenStartOfImage is whether we've called the startDecode method.
 	seenStartOfImage bool
 
 	// truncated is whether the input is missing the final 6 consecutive EOL's
 	// (for Group3) or 2 consecutive EOL's (for Group4). Omitting that trailer
 	// (but otherwise padding to a byte boundary, with either all 0 bits or all
 	// 1 bits) is invalid according to the spec, but happens in practice when
 	// exporting from Adobe Acrobat to TIFF + CCITT. This package silently
 	// ignores the format error for CCITT input that has been truncated in that
 	// fashion, returning the full decoded image.
 	//
 	// Detecting trailer truncation (just after the final row of pixels)
 	// requires knowing which row is the final row, and therefore does not
 	// trigger if the image height is not known in advance.
 	truncated bool
 
 	// readErr is a sticky error for the Read method.
 	readErr error
 }
 
 func (z *reader) Read(p []byte) (int, error) {
 	if z.readErr != nil {
 		return 0, z.readErr
 	}
 	originalP := p
 
 	for len(p) > 0 {
 		// Allocate buffers (and decode any start-of-image codes), if
 		// processing the first or second row.
 		if z.curr == nil {
 			if !z.seenStartOfImage {
 				if z.readErr = z.startDecode(); z.readErr != nil {
 					break
 				}
 				z.atStartOfRow = true
 			}
 			z.curr = make([]byte, z.width)
 		}
 
 		// Decode the next row, if necessary.
 		if z.atStartOfRow {
 			if z.rowsRemaining < 0 {
 				// We do not know the image height in advance. See if the next
 				// code is an EOL. If it is, it is consumed. If it isn't, the
 				// bitReader shouldn't advance along the bit stream, and we
 				// simply decode another row of pixel data.
 				//
 				// For the Group4 subFormat, we may need to align to a byte
 				// boundary. For the Group3 subFormat, the previous z.decodeRow
 				// call (or z.startDecode call) has already consumed one of the
 				// 6 consecutive EOL's. The next EOL is actually the second of
 				// 6, in the middle, and we shouldn't align at that point.
 				if z.align && (z.subFormat == Group4) {
 					z.br.alignToByteBoundary()
 				}
 
 				if err := z.decodeEOL(); err == errMissingEOL {
 					// No-op. It's another row of pixel data.
 				} else if err != nil {
 					z.readErr = err
 					break
 				} else {
 					if z.readErr = z.finishDecode(true); z.readErr != nil {
 						break
 					}
 					z.readErr = io.EOF
 					break
 				}
 
 			} else if z.rowsRemaining == 0 {
 				// We do know the image height in advance, and we have already
 				// decoded exactly that many rows.
 				if z.readErr = z.finishDecode(false); z.readErr != nil {
 					break
 				}
 				z.readErr = io.EOF
 				break
 
 			} else {
 				z.rowsRemaining--
 			}
 
 			if z.readErr = z.decodeRow(z.rowsRemaining == 0); z.readErr != nil {
 				break
 			}
 		}
 
 		// Pack from z.curr (1 byte per pixel) to p (1 bit per pixel).
 		packD, packS := highBits(p, z.curr[z.ri:], z.invert)
 		p = p[packD:]
 		z.ri += packS
 
 		// Prepare to decode the next row, if necessary.
 		if z.ri == len(z.curr) {
 			z.ri, z.curr, z.prev = 0, z.prev, z.curr
 			z.atStartOfRow = true
 		}
 	}
 
 	n := len(originalP) - len(p)
 	if z.invert {
 		invertBytes(originalP[:n])
 	}
 	return n, z.readErr
 }
 
 func (z *reader) penColor() byte {
 	if z.penColorIsWhite {
 		return 0xFF
 	}
 	return 0x00
 }
 
 func (z *reader) startDecode() error {
 	switch z.subFormat {
 	case Group3:
 		if err := z.decodeEOL(); err != nil {
 			return err
 		}
 
 	case Group4:
 		// No-op.
 
 	default:
 		return errUnsupportedSubFormat
 	}
 
 	z.seenStartOfImage = true
 	return nil
 }
 
 func (z *reader) finishDecode(alreadySeenEOL bool) error {
 	numberOfEOLs := 0
 	switch z.subFormat {
 	case Group3:
 		if z.truncated {
 			return nil
 		}
 		// The stream ends with a RTC (Return To Control) of 6 consecutive
 		// EOL's, but we should have already just seen an EOL, either in
 		// z.startDecode (for a zero-height image) or in z.decodeRow.
 		numberOfEOLs = 5
 
 	case Group4:
 		autoDetectHeight := z.rowsRemaining < 0
 		if autoDetectHeight {
 			// Aligning to a byte boundary was already handled by reader.Read.
 		} else if z.align {
 			z.br.alignToByteBoundary()
 		}
 		// The stream ends with two EOL's. If the first one is missing, and we
 		// had an explicit image height, we just assume that the trailing two
 		// EOL's were truncated and return a nil error.
 		if err := z.decodeEOL(); err != nil {
 			if (err == errMissingEOL) && !autoDetectHeight {
 				z.truncated = true
 				return nil
 			}
 			return err
 		}
 		numberOfEOLs = 1
 
 	default:
 		return errUnsupportedSubFormat
 	}
 
 	if alreadySeenEOL {
 		numberOfEOLs--
 	}
 	for ; numberOfEOLs > 0; numberOfEOLs-- {
 		if err := z.decodeEOL(); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
 func (z *reader) decodeEOL() error {
 	return decodeEOL(&z.br)
 }
 
 func (z *reader) decodeRow(finalRow bool) error {
 	z.wi = 0
 	z.atStartOfRow = true
 	z.penColorIsWhite = true
 
 	if z.align {
 		z.br.alignToByteBoundary()
 	}
 
 	switch z.subFormat {
 	case Group3:
 		for ; z.wi < len(z.curr); z.atStartOfRow = false {
 			if err := z.decodeRun(); err != nil {
 				return err
 			}
 		}
 		err := z.decodeEOL()
 		if finalRow && (err == errMissingEOL) {
 			z.truncated = true
 			return nil
 		}
 		return err
 
 	case Group4:
 		for ; z.wi < len(z.curr); z.atStartOfRow = false {
 			mode, err := decode(&z.br, modeDecodeTable[:])
 			if err != nil {
 				return err
 			}
 			rm := readerMode{}
 			if mode < uint32(len(readerModes)) {
 				rm = readerModes[mode]
 			}
 			if rm.function == nil {
 				return errInvalidMode
 			}
 			if err := rm.function(z, rm.arg); err != nil {
 				return err
 			}
 		}
 		return nil
 	}
 
 	return errUnsupportedSubFormat
 }
 
 func (z *reader) decodeRun() error {
 	table := blackDecodeTable[:]
 	if z.penColorIsWhite {
 		table = whiteDecodeTable[:]
 	}
 
 	total := 0
 	for {
 		n, err := decode(&z.br, table)
 		if err != nil {
 			return err
 		}
 		if n > maxWidth {
 			panic("unreachable")
 		}
 		total += int(n)
 		if total > maxWidth {
 			return errRunLengthTooLong
 		}
 		// Anything 0x3F or below is a terminal code.
 		if n <= 0x3F {
 			break
 		}
 	}
 
 	if total > (len(z.curr) - z.wi) {
 		return errRunLengthOverflowsWidth
 	}
 	dst := z.curr[z.wi : z.wi+total]
 	penColor := z.penColor()
 	for i := range dst {
 		dst[i] = penColor
 	}
 	z.wi += total
 	z.penColorIsWhite = !z.penColorIsWhite
 
 	return nil
 }
 
 // The various modes' semantics are based on determining a row of pixels'
 // "changing elements": those pixels whose color differs from the one on its
 // immediate left.
 //
 // The row above the first row is implicitly all white. Similarly, the column
 // to the left of the first column is implicitly all white.
 //
 // For example, here's Figure 1 in "ITU-T Recommendation T.6", where the
 // current and previous rows contain black (B) and white (w) pixels. The a?
 // indexes point into curr, the b? indexes point into prev.
 //
 //                 b1 b2
 //                 v  v
 // prev: BBBBBwwwwwBBBwwwww
 // curr: BBBwwwwwBBBBBBwwww
 //          ^    ^     ^
 //          a0   a1    a2
 //
 // a0 is the "reference element" or current decoder position, roughly
 // equivalent to what this implementation calls reader.wi.
 //
 // a1 is the next changing element to the right of a0, on the "coding line"
 // (the current row).
 //
 // a2 is the next changing element to the right of a1, again on curr.
 //
 // b1 is the first changing element on the "reference line" (the previous row)
 // to the right of a0 and of opposite color to a0.
 //
 // b2 is the next changing element to the right of b1, again on prev.
 //
 // The various modes calculate a1 (and a2, for modeH):
 //  - modePass calculates that a1 is at or to the right of b2.
 //  - modeH    calculates a1 and a2 without considering b1 or b2.
 //  - modeV*   calculates a1 to be b1 plus an adjustment (between -3 and +3).
 
 const (
 	findB1 = false
 	findB2 = true
 )
 
 // findB finds either the b1 or b2 value.
 func (z *reader) findB(whichB bool) int {
 	// The initial row is a special case. The previous row is implicitly all
 	// white, so that there are no changing pixel elements. We return b1 or b2
 	// to be at the end of the row.
 	if len(z.prev) != len(z.curr) {
 		return len(z.curr)
 	}
 
 	i := z.wi
 
 	if z.atStartOfRow {
 		// a0 is implicitly at -1, on a white pixel. b1 is the first black
 		// pixel in the previous row. b2 is the first white pixel after that.
 		for ; (i < len(z.prev)) && (z.prev[i] == 0xFF); i++ {
 		}
 		if whichB == findB2 {
 			for ; (i < len(z.prev)) && (z.prev[i] == 0x00); i++ {
 			}
 		}
 		return i
 	}
 
 	// As per figure 1 above, assume that the current pen color is white.
 	// First, walk past every contiguous black pixel in prev, starting at a0.
 	oppositeColor := ^z.penColor()
 	for ; (i < len(z.prev)) && (z.prev[i] == oppositeColor); i++ {
 	}
 
 	// Then walk past every contiguous white pixel.
 	penColor := ^oppositeColor
 	for ; (i < len(z.prev)) && (z.prev[i] == penColor); i++ {
 	}
 
 	// We're now at a black pixel (or at the end of the row). That's b1.
 	if whichB == findB2 {
 		// If we're looking for b2, walk past every contiguous black pixel
 		// again.
 		oppositeColor := ^penColor
 		for ; (i < len(z.prev)) && (z.prev[i] == oppositeColor); i++ {
 		}
 	}
 
 	return i
 }
 
 type readerMode struct {
 	function func(z *reader, arg int) error
 	arg      int
 }
 
 var readerModes = [...]readerMode{
 	modePass: {function: readerModePass},
 	modeH:    {function: readerModeH},
 	modeV0:   {function: readerModeV, arg: +0},
 	modeVR1:  {function: readerModeV, arg: +1},
 	modeVR2:  {function: readerModeV, arg: +2},
 	modeVR3:  {function: readerModeV, arg: +3},
 	modeVL1:  {function: readerModeV, arg: -1},
 	modeVL2:  {function: readerModeV, arg: -2},
 	modeVL3:  {function: readerModeV, arg: -3},
 	modeExt:  {function: readerModeExt},
 }
 
 func readerModePass(z *reader, arg int) error {
 	b2 := z.findB(findB2)
 	if (b2 < z.wi) || (len(z.curr) < b2) {
 		return errInvalidOffset
 	}
 	dst := z.curr[z.wi:b2]
 	penColor := z.penColor()
 	for i := range dst {
 		dst[i] = penColor
 	}
 	z.wi = b2
 	return nil
 }
 
 func readerModeH(z *reader, arg int) error {
 	// The first iteration finds a1. The second finds a2.
 	for i := 0; i < 2; i++ {
 		if err := z.decodeRun(); err != nil {
 			return err
 		}
 	}
 	return nil
 }
 
 func readerModeV(z *reader, arg int) error {
 	a1 := z.findB(findB1) + arg
 	if (a1 < z.wi) || (len(z.curr) < a1) {
 		return errInvalidOffset
 	}
 	dst := z.curr[z.wi:a1]
 	penColor := z.penColor()
 	for i := range dst {
 		dst[i] = penColor
 	}
 	z.wi = a1
 	z.penColorIsWhite = !z.penColorIsWhite
 	return nil
 }
 
 func readerModeExt(z *reader, arg int) error {
 	return errUnsupportedMode
 }
 
 // DecodeIntoGray decodes the CCITT-formatted data in r into dst.
 //
 // It returns an error if dst's width and height don't match the implied width
 // and height of CCITT-formatted data.
 func DecodeIntoGray(dst *image.Gray, r io.Reader, order Order, sf SubFormat, opts *Options) error {
 	bounds := dst.Bounds()
 	if (bounds.Dx() < 0) || (bounds.Dy() < 0) {
 		return errInvalidBounds
 	}
 	if bounds.Dx() > maxWidth {
 		return errUnsupportedWidth
 	}
 
 	z := reader{
 		br:        bitReader{r: r, order: order},
 		subFormat: sf,
 		align:     (opts != nil) && opts.Align,
 		invert:    (opts != nil) && opts.Invert,
 		width:     bounds.Dx(),
 	}
 	if err := z.startDecode(); err != nil {
 		return err
 	}
 
 	width := bounds.Dx()
 	for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
 		p := (y - bounds.Min.Y) * dst.Stride
 		z.curr = dst.Pix[p : p+width]
 		if err := z.decodeRow(y+1 == bounds.Max.Y); err != nil {
 			return err
 		}
 		z.curr, z.prev = nil, z.curr
 	}
 
 	if err := z.finishDecode(false); err != nil {
 		return err
 	}
 
 	if z.invert {
 		for y := bounds.Min.Y; y < bounds.Max.Y; y++ {
 			p := (y - bounds.Min.Y) * dst.Stride
 			invertBytes(dst.Pix[p : p+width])
 		}
 	}
 
 	return nil
 }
 
 // NewReader returns an io.Reader that decodes the CCITT-formatted data in r.
 // The resultant byte stream is one bit per pixel (MSB first), with 1 meaning
 // white and 0 meaning black. Each row in the result is byte-aligned.
 //
 // A negative height, such as passing AutoDetectHeight, means that the image
 // height is not known in advance. A negative width is invalid.
 func NewReader(r io.Reader, order Order, sf SubFormat, width int, height int, opts *Options) io.Reader {
 	readErr := error(nil)
 	if width < 0 {
 		readErr = errInvalidBounds
 	} else if width > maxWidth {
 		readErr = errUnsupportedWidth
 	}
 
 	return &reader{
 		br:            bitReader{r: r, order: order},
 		subFormat:     sf,
 		align:         (opts != nil) && opts.Align,
 		invert:        (opts != nil) && opts.Invert,
 		width:         width,
 		rowsRemaining: height,
 		readErr:       readErr,
 	}
 }