public static ImageFormat guessFormat()

in src/main/java/org/apache/commons/imaging/Imaging.java [676:775]
93 lines of code
29 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;
            }
            return Stream
                .of(ImageFormats.values())
                .filter(imageFormat -> Stream
                    .of(imageFormat.getExtensions())
                    .anyMatch(extension -> {
                        final String fileName = byteSource.getFileName();
                        if (fileName == null || fileName.trim().isEmpty()) {
                            return false;
                        }
                        final String fileExtension = fileName.substring(fileName.lastIndexOf('.') + 1);
                        return extension != null
                                && !extension.trim().isEmpty()
                                && fileExtension.equalsIgnoreCase(extension);
                    }))
                .findFirst()
                .orElse(ImageFormats.UNKNOWN)
            ;
        }
    }