struct Header [src]

Fields

descriptor: Descriptor
window_descriptor: ?u8
dictionary_id: ?u32
content_size: ?u64
Members

decode (Function)
DecodeError (Error Set)
Descriptor (struct)
windowSize (Function)
Source

  pub const Header = struct {
    descriptor: Descriptor,
    window_descriptor: ?u8,
    dictionary_id: ?u32,
    content_size: ?u64,

    pub const Descriptor = packed struct {
        dictionary_id_flag: u2,
        content_checksum_flag: bool,
        reserved: bool,
        unused: bool,
        single_segment_flag: bool,
        content_size_flag: u2,
    };

    pub const DecodeError = Reader.Error || error{ReservedBitSet};

    pub fn decode(in: *Reader) DecodeError!Header {
        const descriptor: Descriptor = @bitCast(try in.takeByte());

        if (descriptor.reserved) return error.ReservedBitSet;

        const window_descriptor: ?u8 = if (descriptor.single_segment_flag) null else try in.takeByte();

        const dictionary_id: ?u32 = if (descriptor.dictionary_id_flag > 0) d: {
            // if flag is 3 then field_size = 4, else field_size = flag
            const field_size = (@as(u4, 1) << descriptor.dictionary_id_flag) >> 1;
            break :d try in.takeVarInt(u32, .little, field_size);
        } else null;

        const content_size: ?u64 = if (descriptor.single_segment_flag or descriptor.content_size_flag > 0) c: {
            const field_size = @as(u4, 1) << descriptor.content_size_flag;
            const content_size = try in.takeVarInt(u64, .little, field_size);
            break :c if (field_size == 2) content_size + 256 else content_size;
        } else null;

        return .{
            .descriptor = descriptor,
            .window_descriptor = window_descriptor,
            .dictionary_id = dictionary_id,
            .content_size = content_size,
        };
    }

    /// Returns the window size required to decompress a frame, or `null` if it
    /// cannot be determined (which indicates a malformed frame header).
    pub fn windowSize(header: Header) ?u64 {
        if (header.window_descriptor) |descriptor| {
            const exponent = (descriptor & 0b11111000) >> 3;
            const mantissa = descriptor & 0b00000111;
            const window_log = 10 + exponent;
            const window_base = @as(u64, 1) << @as(u6, @intCast(window_log));
            const window_add = (window_base / 8) * mantissa;
            return window_base + window_add;
        } else return header.content_size;
    }
}  