rebaseInner

    fn rebaseInner(w: *Writer, preserve: usize, capacity: usize, eos: bool) Writer.Error!void {
        const r: *Raw = @fieldParentPtr("writer", w);
        assert(preserve + capacity <= w.buffer.len);
        if (eos) assert(capacity == w.buffer.len);

        var partial_header: [5]u8 = undefined;
        var footer_buf: [8]u8 = undefined;
        const preserved = @min(w.end, preserve);
        var remaining = w.buffer[0 .. w.end - preserved];

        var vecs: [16][]const u8 = undefined;
        var vecs_n: usize = 0;
        while (remaining.len > max_block_size) { // not >= so there is always a block down below
            if (vecs_n == vecs.len) {
                try r.output.writeVecAll(&vecs);
                vecs_n = 0;
            }
            vecs[vecs_n + 0] = &full_header;
            vecs[vecs_n + 1] = remaining[0..max_block_size];
            r.hasher.update(vecs[vecs_n + 1]);
            vecs_n += 2;
            remaining = remaining[max_block_size..];
        }

        // eos check required for empty block
        if (w.buffer.len - (remaining.len + preserved) < capacity or eos) {
            // A partial write is necessary to reclaim enough buffer space
            const block_size: u16 = @intCast(remaining.len);
            partial_header[0] = BlockHeader.int(.{ .final = eos, .kind = .stored });
            mem.writeInt(u16, partial_header[1..3], block_size, .little);
            mem.writeInt(u16, partial_header[3..5], ~block_size, .little);

            if (vecs_n == vecs.len) {
                try r.output.writeVecAll(&vecs);
                vecs_n = 0;
            }
            vecs[vecs_n + 0] = &partial_header;
            vecs[vecs_n + 1] = remaining[0..block_size];
            r.hasher.update(vecs[vecs_n + 1]);
            vecs_n += 2;
            remaining = remaining[block_size..];
            assert(remaining.len == 0);

            if (eos and r.hasher != .raw) {
                // the footer is done here instead of `flush` so it can be included in the vector
                var footer_w: Writer = .fixed(&footer_buf);
                r.hasher.writeFooter(&footer_w) catch unreachable;
                assert(footer_w.end != 0);

                if (vecs_n == vecs.len) {
                    try r.output.writeVecAll(&vecs);
                    return r.output.writeAll(footer_w.buffered());
                } else {
                    vecs[vecs_n] = footer_w.buffered();
                    vecs_n += 1;
                }
            }
        }

        try r.output.writeVecAll(vecs[0..vecs_n]);
        _ = w.consume(w.end - preserved - remaining.len);
    }