gif.fathom

//! # Graphics Interchange Format
//!
//! ## References
//!
//! - [GIF89a Specification](https://www.w3.org/Graphics/GIF/spec-gif89a.txt)
//! - [Wikipedia](https://en.wikipedia.org/wiki/GIF)
//! - [Kaitai Format](https://formats.kaitai.io/gif/)

/// # Logical Screen Descriptor
///
/// ## References
///
/// - [GIF89a Specification: Section 18](https://www.w3.org/Graphics/GIF/spec-gif89a.txt)
def logical_screen_descriptor = {
    image_width <- u16le,
    image_height <- u16le,
    flags <- u8,
    bg_color_index <- u8,
    pixel_aspect_ratio <- u8,

    // TODO: interpret flags
    // has_color_table : Bool = (flags & 0b10000000) != 0,
    // color_table_size : U16 = 2 << (flags & 7),
};

/// # Header
///
/// ## References
///
/// - [GIF89a Specification: Section 17](https://www.w3.org/Graphics/GIF/spec-gif89a.txt)
def header = {
    magic <- repeat_len8 3 u8,   // TODO: where magic == ascii "GIF"`,
    version <- repeat_len8 3 u8,
};

/// # Global Color Table Entry
///
/// ## References
///
/// - [GIF89a Specification: Section 19](https://www.w3.org/Graphics/GIF/spec-gif89a.txt)
def color_table_entry = {
    red <- u8,
    green <- u8,
    blue <- u8,
};

/// # Global Color Table
///
/// ## References
///
/// - [GIF89a Specification: Section 19](https://www.w3.org/Graphics/GIF/spec-gif89a.txt)
def global_color_table (len : U16) = {
    entries <- repeat_len16 len color_table_entry,
};

def main = {
    header <- header,
    screen <- logical_screen_descriptor,
    // global_color_table <- global_color_table screen.color_table_size,    // TODO: if `screen.has_color_table,`
    // blocks <- array 0 block,                                             // TODO: repeat while not EOF or BlockTerminator
};