public static ImageFormat guessFormat()

in src/main/java/org/apache/commons/imaging/Imaging.java [703:813]
102 lines of code
33 McCabe index (conditional complexity)

    public static ImageFormat guessFormat(final ByteSource byteSource) throws IOException {
        if (byteSource == null) {
            return ImageFormats.UNKNOWN;
        }

        try (InputStream is = byteSource.getInputStream()) {
            final int i1 = is.read();
            final int i2 = is.read();
            if (i1 < 0 || i2 < 0) {
                throw new IllegalArgumentException("Couldn't read magic numbers to guess format.");
            }

            final int b1 = i1 & 0xff;
            final int b2 = i2 & 0xff;
            final int[] bytePair = { b1, b2, };

            if (compareBytePair(MAGIC_NUMBERS_GIF, bytePair)) {
                return ImageFormats.GIF;
                // } else if (b1 == 0x00 && b2 == 0x00) // too similar to TGA
                // {
                // return ImageFormat.IMAGE_FORMAT_ICO;
            }
            if (compareBytePair(MAGIC_NUMBERS_PNG, bytePair)) {
                return ImageFormats.PNG;
            }
            if (compareBytePair(MAGIC_NUMBERS_JPEG, bytePair)) {
                return ImageFormats.JPEG;
            }
            if (compareBytePair(MAGIC_NUMBERS_BMP, bytePair)) {
                return ImageFormats.BMP;
            }
            if (compareBytePair(MAGIC_NUMBERS_TIFF_MOTOROLA, bytePair)) {
                return ImageFormats.TIFF;
            }
            if (compareBytePair(MAGIC_NUMBERS_TIFF_INTEL, bytePair)) {
                return ImageFormats.TIFF;
            }
            if (compareBytePair(MAGIC_NUMBERS_PSD, bytePair)) {
                return ImageFormats.PSD;
            }
            if (compareBytePair(MAGIC_NUMBERS_PAM, bytePair)) {
                return ImageFormats.PAM;
            }
            if (compareBytePair(MAGIC_NUMBERS_PBM_A, bytePair)) {
                return ImageFormats.PBM;
            }
            if (compareBytePair(MAGIC_NUMBERS_PBM_B, bytePair)) {
                return ImageFormats.PBM;
            }
            if (compareBytePair(MAGIC_NUMBERS_PGM_A, bytePair)) {
                return ImageFormats.PGM;
            }
            if (compareBytePair(MAGIC_NUMBERS_PGM_B, bytePair)) {
                return ImageFormats.PGM;
            }
            if (compareBytePair(MAGIC_NUMBERS_PPM_A, bytePair)) {
                return ImageFormats.PPM;
            }
            if (compareBytePair(MAGIC_NUMBERS_PPM_B, bytePair)) {
                return ImageFormats.PPM;
            }
            if (compareBytePair(MAGIC_NUMBERS_JBIG2_1, bytePair)) {
                final int i3 = is.read();
                final int i4 = is.read();
                if (i3 < 0 || i4 < 0) {
                    throw new IllegalArgumentException("Couldn't read magic numbers to guess format.");
                }

                final int b3 = i3 & 0xff;
                final int b4 = i4 & 0xff;
                final int[] bytePair2 = { b3, b4, };
                if (compareBytePair(MAGIC_NUMBERS_JBIG2_2, bytePair2)) {
                    return ImageFormats.JBIG2;
                }
            } else if (compareBytePair(MAGIC_NUMBERS_ICNS, bytePair)) {
                return ImageFormats.ICNS;
            } else if (compareBytePair(MAGIC_NUMBERS_DCX, bytePair)) {
                return ImageFormats.DCX;
            } else if (compareBytePair(MAGIC_NUMBERS_RGBE, bytePair)) {
                return ImageFormats.RGBE;
            } else if (compareBytePair(MAGIC_NUMBERS_RIFF_1, bytePair)) {
                final int i3 = is.read();
                final int i4 = is.read();
                if (i3 < 0 || i4 < 0) {
                    throw new IllegalArgumentException("Couldn't read magic numbers to guess format.");
                }

                final int b3 = i3 & 0xff;
                final int b4 = i4 & 0xff;
                final int[] bytePair2 = { b3, b4, };
                if (compareBytePair(MAGIC_NUMBERS_RIFF_2, bytePair2)) {
                    final byte[] bytes = new byte[4];
                    if (is.read(bytes) < 4) { // Skip file size
                        throw new IllegalArgumentException("Couldn't read magic numbers to guess format.");
                    }

                    if (is.read(bytes) == 4 && Arrays.equals(MAGIC_NUMBERS_WEBP, bytes)) {
                        return ImageFormats.WEBP;
                    }
                }
            }
            return Stream.of(ImageFormats.values()).filter(imageFormat -> Stream.of(imageFormat.getExtensions()).anyMatch(extension -> {
                final String fileName = byteSource.getFileName();
                if (StringUtils.isEmpty(fileName)) {
                    return false;
                }
                final String fileExtension = fileName.substring(fileName.lastIndexOf('.') + 1);
                return fileExtension.equalsIgnoreCase(extension);
            })).findFirst().orElse(ImageFormats.UNKNOWN);
        }
    }