func Decode()

in tiff/reader.go [559:704]
127 lines of code
36 McCabe index (conditional complexity)

func Decode(r io.Reader) (img image.Image, err error) {
	d, err := newDecoder(r)
	if err != nil {
		return
	}

	blockPadding := false
	blockWidth := d.config.Width
	blockHeight := d.config.Height
	blocksAcross := 1
	blocksDown := 1

	if d.config.Width == 0 {
		blocksAcross = 0
	}
	if d.config.Height == 0 {
		blocksDown = 0
	}

	var blockOffsets, blockCounts []uint

	if int(d.firstVal(tTileWidth)) != 0 {
		blockPadding = true

		blockWidth = int(d.firstVal(tTileWidth))
		blockHeight = int(d.firstVal(tTileLength))

		if blockWidth != 0 {
			blocksAcross = (d.config.Width + blockWidth - 1) / blockWidth
		}
		if blockHeight != 0 {
			blocksDown = (d.config.Height + blockHeight - 1) / blockHeight
		}

		blockCounts = d.features[tTileByteCounts]
		blockOffsets = d.features[tTileOffsets]

	} else {
		if int(d.firstVal(tRowsPerStrip)) != 0 {
			blockHeight = int(d.firstVal(tRowsPerStrip))
		}

		if blockHeight != 0 {
			blocksDown = (d.config.Height + blockHeight - 1) / blockHeight
		}

		blockOffsets = d.features[tStripOffsets]
		blockCounts = d.features[tStripByteCounts]
	}

	// Check if we have the right number of strips/tiles, offsets and counts.
	if n := blocksAcross * blocksDown; len(blockOffsets) < n || len(blockCounts) < n {
		return nil, FormatError("inconsistent header")
	}

	imgRect := image.Rect(0, 0, d.config.Width, d.config.Height)
	switch d.mode {
	case mGray, mGrayInvert:
		if d.bpp == 16 {
			img = image.NewGray16(imgRect)
		} else {
			img = image.NewGray(imgRect)
		}
	case mPaletted:
		img = image.NewPaletted(imgRect, d.palette)
	case mNRGBA:
		if d.bpp == 16 {
			img = image.NewNRGBA64(imgRect)
		} else {
			img = image.NewNRGBA(imgRect)
		}
	case mRGB, mRGBA:
		if d.bpp == 16 {
			img = image.NewRGBA64(imgRect)
		} else {
			img = image.NewRGBA(imgRect)
		}
	}

	for i := 0; i < blocksAcross; i++ {
		blkW := blockWidth
		if !blockPadding && i == blocksAcross-1 && d.config.Width%blockWidth != 0 {
			blkW = d.config.Width % blockWidth
		}
		for j := 0; j < blocksDown; j++ {
			blkH := blockHeight
			if !blockPadding && j == blocksDown-1 && d.config.Height%blockHeight != 0 {
				blkH = d.config.Height % blockHeight
			}
			offset := int64(blockOffsets[j*blocksAcross+i])
			n := int64(blockCounts[j*blocksAcross+i])
			switch d.firstVal(tCompression) {

			// According to the spec, Compression does not have a default value,
			// but some tools interpret a missing Compression value as none so we do
			// the same.
			case cNone, 0:
				if b, ok := d.r.(*buffer); ok {
					d.buf, err = b.Slice(int(offset), int(n))
				} else {
					d.buf = make([]byte, n)
					_, err = d.r.ReadAt(d.buf, offset)
				}
			case cG3:
				inv := d.firstVal(tPhotometricInterpretation) == pWhiteIsZero
				order := ccittFillOrder(d.firstVal(tFillOrder))
				r := ccitt.NewReader(io.NewSectionReader(d.r, offset, n), order, ccitt.Group3, blkW, blkH, &ccitt.Options{Invert: inv, Align: false})
				d.buf, err = ioutil.ReadAll(r)
			case cG4:
				inv := d.firstVal(tPhotometricInterpretation) == pWhiteIsZero
				order := ccittFillOrder(d.firstVal(tFillOrder))
				r := ccitt.NewReader(io.NewSectionReader(d.r, offset, n), order, ccitt.Group4, blkW, blkH, &ccitt.Options{Invert: inv, Align: false})
				d.buf, err = ioutil.ReadAll(r)
			case cLZW:
				r := lzw.NewReader(io.NewSectionReader(d.r, offset, n), lzw.MSB, 8)
				d.buf, err = ioutil.ReadAll(r)
				r.Close()
			case cDeflate, cDeflateOld:
				var r io.ReadCloser
				r, err = zlib.NewReader(io.NewSectionReader(d.r, offset, n))
				if err != nil {
					return nil, err
				}
				d.buf, err = ioutil.ReadAll(r)
				r.Close()
			case cPackBits:
				d.buf, err = unpackBits(io.NewSectionReader(d.r, offset, n))
			default:
				err = UnsupportedError(fmt.Sprintf("compression value %d", d.firstVal(tCompression)))
			}
			if err != nil {
				return nil, err
			}

			xmin := i * blockWidth
			ymin := j * blockHeight
			xmax := xmin + blkW
			ymax := ymin + blkH
			err = d.decode(img, xmin, ymin, xmax, ymax)
			if err != nil {
				return nil, err
			}
		}
	}
	return
}