readBlock

fn readBlock(input: *Reader, allocating: *Writer.Allocating) !void {
    var packed_size: ?u64 = null;
    var unpacked_size: ?u64 = null;

    const header_size = h: {
        // Read the block header via peeking so that we can hash the whole thing too.
        const first_byte: usize = try input.peekByte();
        if (first_byte == 0) return error.SuccessfulEndOfStream;

        const declared_header_size = first_byte * 4;
        try input.fill(declared_header_size);
        const header_seek_start = input.seek;
        input.toss(1);

        const Flags = packed struct(u8) {
            last_filter_index: u2,
            reserved: u4,
            has_packed_size: bool,
            has_unpacked_size: bool,
        };
        const flags = try input.takeStruct(Flags, .little);

        const filter_count = @as(u3, flags.last_filter_index) + 1;
        if (filter_count > 1) return error.Unsupported;

        if (flags.has_packed_size) packed_size = try input.takeLeb128(u64);
        if (flags.has_unpacked_size) unpacked_size = try input.takeLeb128(u64);

        const FilterId = enum(u64) {
            lzma2 = 0x21,
            _,
        };

        const filter_id: FilterId = @fromBackingInt(@intCast(try input.takeLeb128(u64)));
        if (filter_id != .lzma2) return error.Unsupported;

        const properties_size = try input.takeLeb128(u64);
        if (properties_size != 1) return error.CorruptInput;
        // TODO: use filter properties
        _ = try input.takeByte();

        const actual_header_size = input.seek - header_seek_start;
        if (actual_header_size > declared_header_size) return error.CorruptInput;
        const remaining_bytes = declared_header_size - actual_header_size;
        for (0..remaining_bytes) |_| {
            if (try input.takeByte() != 0) return error.CorruptInput;
        }

        const header_slice = input.buffer[header_seek_start..][0..declared_header_size];
        const computed_checksum = Crc32.hash(header_slice);
        const declared_checksum = try input.takeInt(u32, .little);
        if (computed_checksum != declared_checksum) return error.WrongChecksum;
        break :h declared_header_size;
    };

    // Compressed Data

    var lzma2_decode = try lzma2.Decode.init(allocating.allocator);
    defer lzma2_decode.deinit(allocating.allocator);
    const before_size = allocating.writer.end;
    const packed_bytes_read = try lzma2_decode.decompress(input, allocating);
    const unpacked_bytes = allocating.writer.end - before_size;

    if (packed_size) |s| {
        if (s != packed_bytes_read) return error.CorruptInput;
    }

    if (unpacked_size) |s| {
        if (s != unpacked_bytes) return error.CorruptInput;
    }

    // Block Padding
    const block_counter = header_size + packed_bytes_read;
    const padding = try input.take(@intCast((4 - (block_counter % 4)) % 4));
    for (padding) |byte| {
        if (byte != 0) return error.CorruptInput;
    }
}