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    use std::io;

    /// The kind of encoding used to store sample values
    #[derive(Clone, Copy, PartialEq, Eq, Debug)]
    pub enum SampleEncoding {
        /// Samples are unsigned binary integers in big endian
        Binary,

        /// Samples are encoded as decimal ascii strings separated by whitespace
        Ascii,
    }

    /// Denotes the category of the magic number
    #[derive(Clone, Copy, PartialEq, Eq, Debug)]
    pub enum PNMSubtype {
        /// Magic numbers P1 and P4
        Bitmap(SampleEncoding),

        /// Magic numbers P2 and P5
        Graymap(SampleEncoding),

        /// Magic numbers P3 and P6
        Pixmap(SampleEncoding),

        /// Magic number P7
        ArbitraryMap,
    }

    /// Stores the complete header data of a file.
    ///
    /// Internally, provides mechanisms for lossless reencoding. After reading a file with the decoder
    /// it is possible to recover the header and construct an encoder. Using the encoder on the just
    /// loaded image should result in a byte copy of the original file (for single image pnms without
    /// additional trailing data).
    pub struct PNMHeader {
        pub(crate) decoded: HeaderRecord,
        pub(crate) encoded: Option<Vec<u8>>,
    }

    pub(crate) enum HeaderRecord {
        Bitmap(BitmapHeader),
        Graymap(GraymapHeader),
        Pixmap(PixmapHeader),
        Arbitrary(ArbitraryHeader),
    }

    /// Header produced by a `pbm` file ("Portable Bit Map")
    #[derive(Clone, Copy, Debug)]
    pub struct BitmapHeader {
        /// Binary or Ascii encoded file
        pub encoding: SampleEncoding,

        /// Height of the image file
        pub height: u32,

        /// Width of the image file
        pub width: u32,
    }

    /// Header produced by a `pgm` file ("Portable Gray Map")
    #[derive(Clone, Copy, Debug)]
    pub struct GraymapHeader {
        /// Binary or Ascii encoded file
        pub encoding: SampleEncoding,

        /// Height of the image file
        pub height: u32,

        /// Width of the image file
        pub width: u32,

        /// Maximum sample value within the image
        pub maxwhite: u32,
    }

    /// Header produced by a `ppm` file ("Portable Pixel Map")
    #[derive(Clone, Copy, Debug)]
    pub struct PixmapHeader {
        /// Binary or Ascii encoded file
        pub encoding: SampleEncoding,

        /// Height of the image file
        pub height: u32,

        /// Width of the image file
        pub width: u32,

        /// Maximum sample value within the image
        pub maxval: u32,
    }

    /// Header produced by a `pam` file ("Portable Arbitrary Map")
    #[derive(Clone, Debug)]
    pub struct ArbitraryHeader {
        /// Height of the image file
        pub height: u32,

        /// Width of the image file
        pub width: u32,

        /// Number of color channels
        pub depth: u32,

        /// Maximum sample value within the image
        pub maxval: u32,

        /// Color interpretation of image pixels
        pub tupltype: Option<ArbitraryTuplType>,
    }

    /// Standardized tuple type specifiers in the header of a `pam`.
    #[derive(Clone, Debug)]
    pub enum ArbitraryTuplType {
        /// Pixels are either black (0) or white (1)
        BlackAndWhite,

        /// Pixels are either black (0) or white (1) and a second alpha channel
        BlackAndWhiteAlpha,

        /// Pixels represent the amount of white
        Grayscale,

        /// Grayscale with an additional alpha channel
        GrayscaleAlpha,

        /// Three channels: Red, Green, Blue
        RGB,

        /// Four channels: Red, Green, Blue, Alpha
        RGBAlpha,

        /// An image format which is not standardized
        Custom(String),
    }

    impl PNMSubtype {
        /// Get the two magic constant bytes corresponding to this format subtype.
        pub fn magic_constant(self) -> &'static [u8; 2] {
            match self {
                PNMSubtype::Bitmap(SampleEncoding::Ascii) => b"P1",
                PNMSubtype::Graymap(SampleEncoding::Ascii) => b"P2",
                PNMSubtype::Pixmap(SampleEncoding::Ascii) => b"P3",
                PNMSubtype::Bitmap(SampleEncoding::Binary) => b"P4",
                PNMSubtype::Graymap(SampleEncoding::Binary) => b"P5",
                PNMSubtype::Pixmap(SampleEncoding::Binary) => b"P6",
                PNMSubtype::ArbitraryMap => b"P7",
            }
        }

        /// Whether samples are stored as binary or as decimal ascii
        pub fn sample_encoding(self) -> SampleEncoding {
            match self {
                PNMSubtype::ArbitraryMap => SampleEncoding::Binary,
                PNMSubtype::Bitmap(enc) => enc,
                PNMSubtype::Graymap(enc) => enc,
                PNMSubtype::Pixmap(enc) => enc,
            }
        }
    }

    impl PNMHeader {
        /// Retrieve the format subtype from which the header was created.
        pub fn subtype(&self) -> PNMSubtype {
            match self.decoded {
                HeaderRecord::Bitmap(BitmapHeader { encoding, .. }) => PNMSubtype::Bitmap(encoding),
                HeaderRecord::Graymap(GraymapHeader { encoding, .. }) => PNMSubtype::Graymap(encoding),
                HeaderRecord::Pixmap(PixmapHeader { encoding, .. }) => PNMSubtype::Pixmap(encoding),
                HeaderRecord::Arbitrary(ArbitraryHeader { .. }) => PNMSubtype::ArbitraryMap,
            }
        }

        /// The width of the image this header is for.
        pub fn width(&self) -> u32 {
            match self.decoded {
                HeaderRecord::Bitmap(BitmapHeader { width, .. }) => width,
                HeaderRecord::Graymap(GraymapHeader { width, .. }) => width,
                HeaderRecord::Pixmap(PixmapHeader { width, .. }) => width,
                HeaderRecord::Arbitrary(ArbitraryHeader { width, .. }) => width,
            }
        }

        /// The height of the image this header is for.
        pub fn height(&self) -> u32 {
            match self.decoded {
                HeaderRecord::Bitmap(BitmapHeader { height, .. }) => height,
                HeaderRecord::Graymap(GraymapHeader { height, .. }) => height,
                HeaderRecord::Pixmap(PixmapHeader { height, .. }) => height,
                HeaderRecord::Arbitrary(ArbitraryHeader { height, .. }) => height,
            }
        }

        /// The biggest value a sample can have. In other words, the colour resolution.
        pub fn maximal_sample(&self) -> u32 {
            match self.decoded {
                HeaderRecord::Bitmap(BitmapHeader { .. }) => 1,
                HeaderRecord::Graymap(GraymapHeader { maxwhite, .. }) => maxwhite,
                HeaderRecord::Pixmap(PixmapHeader { maxval, .. }) => maxval,
                HeaderRecord::Arbitrary(ArbitraryHeader { maxval, .. }) => maxval,
            }
        }

        /// Retrieve the underlying bitmap header if any
        pub fn as_bitmap(&self) -> Option<&BitmapHeader> {
            match self.decoded {
                HeaderRecord::Bitmap(ref bitmap) => Some(bitmap),
                _ => None,
            }
        }

        /// Retrieve the underlying graymap header if any
        pub fn as_graymap(&self) -> Option<&GraymapHeader> {
            match self.decoded {
                HeaderRecord::Graymap(ref graymap) => Some(graymap),
                _ => None,
            }
        }

        /// Retrieve the underlying pixmap header if any
        pub fn as_pixmap(&self) -> Option<&PixmapHeader> {
            match self.decoded {
                HeaderRecord::Pixmap(ref pixmap) => Some(pixmap),
                _ => None,
            }
        }

        /// Retrieve the underlying arbitrary header if any
        pub fn as_arbitrary(&self) -> Option<&ArbitraryHeader> {
            match self.decoded {
                HeaderRecord::Arbitrary(ref arbitrary) => Some(arbitrary),
                _ => None,
            }
        }

        /// Write the header back into a binary stream
        pub fn write(&self, writer: &mut io::Write) -> io::Result<()> {
            writer.write_all(self.subtype().magic_constant())?;
            match *self {
                PNMHeader {
                    encoded: Some(ref content),
                    ..
                } => writer.write_all(content),
                PNMHeader {
                    decoded:
                        HeaderRecord::Bitmap(BitmapHeader {
                            encoding: _encoding,
                            width,
                            height,
                        }),
                    ..
                } => writeln!(writer, "\n{} {}", width, height),
                PNMHeader {
                    decoded:
                        HeaderRecord::Graymap(GraymapHeader {
                            encoding: _encoding,
                            width,
                            height,
                            maxwhite,
                        }),
                    ..
                } => writeln!(writer, "\n{} {} {}", width, height, maxwhite),
                PNMHeader {
                    decoded:
                        HeaderRecord::Pixmap(PixmapHeader {
                            encoding: _encoding,
                            width,
                            height,
                            maxval,
                        }),
                    ..
                } => writeln!(writer, "\n{} {} {}", width, height, maxval),
                PNMHeader {
                    decoded:
                        HeaderRecord::Arbitrary(ArbitraryHeader {
                            width,
                            height,
                            depth,
                            maxval,
                            ref tupltype,
                        }),
                    ..
                } => {
                    #[allow(unused_assignments)]
                    // Declared here so its lifetime exceeds the matching. This is a trivial
                    // constructor, no allocation takes place and in the custom case we must allocate
                    // regardless due to borrow. Still, the warnings checker does pick this up :/
                    // Could use std::borrow::Cow instead but that really doesn't achieve anything but
                    // increasing type complexity.
                    let mut custom_fallback = String::new();

                    let tupltype = match *tupltype {
                        None => "",
                        Some(ArbitraryTuplType::BlackAndWhite) => "TUPLTYPE BLACKANDWHITE\n",
                        Some(ArbitraryTuplType::BlackAndWhiteAlpha) => "TUPLTYPE BLACKANDWHITE_ALPHA\n",
                        Some(ArbitraryTuplType::Grayscale) => "TUPLTYPE GRAYSCALE\n",
                        Some(ArbitraryTuplType::GrayscaleAlpha) => "TUPLTYPE GRAYSCALE_ALPHA\n",
                        Some(ArbitraryTuplType::RGB) => "TUPLTYPE RGB\n",
                        Some(ArbitraryTuplType::RGBAlpha) => "TUPLTYPE RGB_ALPHA\n",
                        Some(ArbitraryTuplType::Custom(ref custom)) => {
                            custom_fallback = format!("TUPLTYPE {}", custom);
                            &custom_fallback
                        }
                    };

                    writeln!(
                        writer,
                        "\nWIDTH {}\nHEIGHT {}\nDEPTH {}\nMAXVAL {}\n{}ENDHDR",
                        width, height, depth, maxval, tupltype
                    )
                }
            }
        }
    }

    impl From<BitmapHeader> for PNMHeader {
        fn from(header: BitmapHeader) -> Self {
            PNMHeader {
                decoded: HeaderRecord::Bitmap(header),
                encoded: None,
            }
        }
    }

    impl From<GraymapHeader> for PNMHeader {
        fn from(header: GraymapHeader) -> Self {
            PNMHeader {
                decoded: HeaderRecord::Graymap(header),
                encoded: None,
            }
        }
    }

    impl From<PixmapHeader> for PNMHeader {
        fn from(header: PixmapHeader) -> Self {
            PNMHeader {
                decoded: HeaderRecord::Pixmap(header),
                encoded: None,
            }
        }
    }

    impl From<ArbitraryHeader> for PNMHeader {
        fn from(header: ArbitraryHeader) -> Self {
            PNMHeader {
                decoded: HeaderRecord::Arbitrary(header),
                encoded: None,
            }
        }
    }

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