func newDecoder()

in tiff/reader.go [399:538]
120 lines of code
31 McCabe index (conditional complexity)

func newDecoder(r io.Reader) (*decoder, error) {
	d := &decoder{
		r:        newReaderAt(r),
		features: make(map[int][]uint),
	}

	p := make([]byte, 8)
	if _, err := d.r.ReadAt(p, 0); err != nil {
		if err == io.EOF {
			err = io.ErrUnexpectedEOF
		}
		return nil, err
	}
	switch string(p[0:4]) {
	case leHeader:
		d.byteOrder = binary.LittleEndian
	case beHeader:
		d.byteOrder = binary.BigEndian
	default:
		return nil, FormatError("malformed header")
	}

	ifdOffset := int64(d.byteOrder.Uint32(p[4:8]))

	// The first two bytes contain the number of entries (12 bytes each).
	if _, err := d.r.ReadAt(p[0:2], ifdOffset); err != nil {
		return nil, err
	}
	numItems := int(d.byteOrder.Uint16(p[0:2]))

	// All IFD entries are read in one chunk.
	p = make([]byte, ifdLen*numItems)
	if _, err := d.r.ReadAt(p, ifdOffset+2); err != nil {
		return nil, err
	}

	prevTag := -1
	for i := 0; i < len(p); i += ifdLen {
		tag, err := d.parseIFD(p[i : i+ifdLen])
		if err != nil {
			return nil, err
		}
		if tag <= prevTag {
			return nil, FormatError("tags are not sorted in ascending order")
		}
		prevTag = tag
	}

	d.config.Width = int(d.firstVal(tImageWidth))
	d.config.Height = int(d.firstVal(tImageLength))

	if _, ok := d.features[tBitsPerSample]; !ok {
		// Default is 1 per specification.
		d.features[tBitsPerSample] = []uint{1}
	}
	d.bpp = d.firstVal(tBitsPerSample)
	switch d.bpp {
	case 0:
		return nil, FormatError("BitsPerSample must not be 0")
	case 1, 8, 16:
		// Nothing to do, these are accepted by this implementation.
	default:
		return nil, UnsupportedError(fmt.Sprintf("BitsPerSample of %v", d.bpp))
	}

	// Determine the image mode.
	switch d.firstVal(tPhotometricInterpretation) {
	case pRGB:
		if d.bpp == 16 {
			for _, b := range d.features[tBitsPerSample] {
				if b != 16 {
					return nil, FormatError("wrong number of samples for 16bit RGB")
				}
			}
		} else {
			for _, b := range d.features[tBitsPerSample] {
				if b != 8 {
					return nil, FormatError("wrong number of samples for 8bit RGB")
				}
			}
		}
		// RGB images normally have 3 samples per pixel.
		// If there are more, ExtraSamples (p. 31-32 of the spec)
		// gives their meaning (usually an alpha channel).
		//
		// This implementation does not support extra samples
		// of an unspecified type.
		switch len(d.features[tBitsPerSample]) {
		case 3:
			d.mode = mRGB
			if d.bpp == 16 {
				d.config.ColorModel = color.RGBA64Model
			} else {
				d.config.ColorModel = color.RGBAModel
			}
		case 4:
			switch d.firstVal(tExtraSamples) {
			case 1:
				d.mode = mRGBA
				if d.bpp == 16 {
					d.config.ColorModel = color.RGBA64Model
				} else {
					d.config.ColorModel = color.RGBAModel
				}
			case 2:
				d.mode = mNRGBA
				if d.bpp == 16 {
					d.config.ColorModel = color.NRGBA64Model
				} else {
					d.config.ColorModel = color.NRGBAModel
				}
			default:
				return nil, FormatError("wrong number of samples for RGB")
			}
		default:
			return nil, FormatError("wrong number of samples for RGB")
		}
	case pPaletted:
		d.mode = mPaletted
		d.config.ColorModel = color.Palette(d.palette)
	case pWhiteIsZero:
		d.mode = mGrayInvert
		if d.bpp == 16 {
			d.config.ColorModel = color.Gray16Model
		} else {
			d.config.ColorModel = color.GrayModel
		}
	case pBlackIsZero:
		d.mode = mGray
		if d.bpp == 16 {
			d.config.ColorModel = color.Gray16Model
		} else {
			d.config.ColorModel = color.GrayModel
		}
	default:
		return nil, UnsupportedError("color model")
	}

	return d, nil
}