1 files changed, 149 insertions, 0 deletions
diff --git a/alligator_tvg/src/header.rs b/alligator_tvg/src/header.rs
new file mode 100644
index 0000000..b3be494
--- /dev/null
+++ b/alligator_tvg/src/header.rs
@@ -0,0 +1,149 @@
+use std::io::{self, Read};
+
+use byteorder::{LittleEndian, ReadBytesExt};
+use num_enum::TryFromPrimitive;
+use raise::yeet;
+
+use crate::colors::ColorEncoding;
+use crate::read_varuint;
+use crate::TvgError;
+
+const MAGIC: [u8; 2] = [0x72, 0x56];
+pub const SUPPORTED_VERSION: u8 = 1;
+
+/// The coordinate range defines how many bits a Unit value uses.
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, Hash, TryFromPrimitive)]
+#[repr(u8)]
+pub enum CoordinateRange {
+	/// Each [`Unit`] takes up 16 bits.
+	#[default]
+	Default = 0,
+	/// Each [`Unit`] takes up 8 bits.
+	Reduced = 1,
+	/// Each [`Unit`] takes up 32 bits.
+	Enhanced = 2,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, TryFromPrimitive)]
+#[repr(u8)]
+pub enum Scale {
+	_1 = 0,
+	_2 = 1,
+	_4 = 2,
+	_8 = 3,
+	_16 = 4,
+	_32 = 5,
+	_64 = 6,
+	_128 = 7,
+	_256 = 8,
+	_512 = 9,
+	_1024 = 10,
+	_2048 = 11,
+	_4096 = 12,
+	_8192 = 13,
+	_16384 = 14,
+	_32768 = 15,
+}
+
+/// Each TVG file starts with a header defining some global values for the file
+/// like scale and image size. This is a representation of the header, but not
+/// necessarily an exact representation of the bits of a TVG header.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct TvgHeader {
+	/// Must always be [0x72, 0x56]
+	magic: [u8; 2],
+	/// Must be 1. For future versions, this field might decide how the rest of
+	/// the format looks like.
+	version: u8,
+	/// Defines the number of fraction bits in a Unit value.
+	scale: Scale,
+	/// Defines the type of color information that is used in the
+	/// [`ColorTable`].
+	color_encoding: ColorEncoding,
+	/// Defines the number of total bits in a Unit value and thus the overall
+	/// precision of the file.
+	coordinate_range: CoordinateRange,
+	/// Encodes the maximum width of the output file in *display units*.
+	///
+	/// A value of 0 indicates that the image has the maximum possible width.
+	width: u32,
+	/// Encodes the maximum height of the output file in *display units*.
+	///
+	/// A value of 0 indicates that the image has the maximum possible height.
+	height: u32,
+	/// The number of colors in the color table.
+	color_count: u32,
+}
+
+impl TvgHeader {
+	pub fn read<R: Read>(reader: &mut R) -> Result<Self, TvgError> {
+		// magic number is used as a first line defense against invalid data
+		let magic = [reader.read_u8()?, reader.read_u8()?];
+		if magic != MAGIC {
+			yeet!(TvgError::InvalidFile);
+		}
+
+		// the version of tvg being used
+		let version = reader.read_u8()?;
+		if version != SUPPORTED_VERSION {
+			yeet!(TvgError::UnsupportedVersion(version))
+		}
+
+		// scale, color_encoding, and coordinate_range are stored in one byte
+		let byte = reader.read_u8()?;
+		let scale = Scale::try_from_primitive(byte & 0b0000_1111).expect("invalid scale");
+		let color_encoding = ColorEncoding::try_from_primitive((byte & 0b0011_0000) >> 4)
+			.expect("invalid color encoding");
+		let coordinate_range = CoordinateRange::try_from_primitive((byte & 0b1100_0000) >> 6)
+			.expect("invalid coordinate range");
+
+		// width and height depend on the coordinate range
+		let width = read_unsigned_unit(coordinate_range, reader)?;
+		let height = read_unsigned_unit(coordinate_range, reader)?;
+
+		let color_count = read_varuint(reader)?;
+
+		Ok(Self {
+			magic,
+			version,
+			scale,
+			color_encoding,
+			coordinate_range,
+			width,
+			height,
+			color_count,
+		})
+	}
+
+	/// Defines the number of total bits in a Unit value and thus the overall
+	/// precision of the file.
+	pub fn coordinate_range(&self) -> CoordinateRange {
+		self.coordinate_range
+	}
+
+	/// Defines the type of color information that is used in the [`ColorTable`].
+	pub fn color_encoding(&self) -> ColorEncoding {
+		self.color_encoding
+	}
+
+	/// Defines the number of fraction bits in a Unit value.
+	pub fn scale(&self) -> Scale {
+		self.scale
+	}
+
+	/// The number of colors in the [`ColorTable`].
+	pub fn color_count(&self) -> u32 {
+		self.color_count
+	}
+}
+
+fn read_unsigned_unit<R: Read>(
+	coordinate_range: CoordinateRange,
+	bytes: &mut R,
+) -> io::Result<u32> {
+	Ok(match coordinate_range {
+		CoordinateRange::Reduced => bytes.read_u8()? as u32,
+		CoordinateRange::Default => bytes.read_u16::<LittleEndian>()? as u32,
+		CoordinateRange::Enhanced => bytes.read_u32::<LittleEndian>()?,
+	})
+}