Function decodeZstandardHeader [src]

Decode the header of a Zstandard frame. Errors returned: error.ReservedBitSet if any of the reserved bits of the header are set error.EndOfStream if source does not contain a complete header

Prototype

 pub fn decodeZstandardHeader( source: anytype, ) (@TypeOf(source).Error || error{ EndOfStream, ReservedBitSet })!ZstandardHeader

Source

  pub fn decodeZstandardHeader(
    source: anytype,
) (@TypeOf(source).Error || error{ EndOfStream, ReservedBitSet })!ZstandardHeader {
    const descriptor = @as(ZstandardHeader.Descriptor, @bitCast(try source.readByte()));

    if (descriptor.reserved) return error.ReservedBitSet;

    var window_descriptor: ?u8 = null;
    if (!descriptor.single_segment_flag) {
        window_descriptor = try source.readByte();
    }

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

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

    const header = ZstandardHeader{
        .descriptor = descriptor,
        .window_descriptor = window_descriptor,
        .dictionary_id = dictionary_id,
        .content_size = content_size,
    };
    return header;
}  