drain

    fn drain(w: *Writer, data: []const []const u8, splat: usize) Error!usize {
        const a: *Allocating = @fieldParentPtr("writer", w);
        assert(data.len != 0);
        const count = countSplat(data, splat);
        a.ensureUnusedCapacity(count + 1) catch return error.WriteFailed;
        for (data[0 .. data.len - 1]) |bytes| {
            @memcpy(a.writer.buffer[a.writer.end..][0..bytes.len], bytes);
            a.writer.end += bytes.len;
        }
        const pattern = data[data.len - 1];
        switch (pattern.len) {
            0 => {},
            1 => {
                @memset(a.writer.buffer[a.writer.end..][0..splat], pattern[0]);
                a.writer.end += splat;
            },
            else => for (0..splat) |_| {
                @memcpy(a.writer.buffer[a.writer.end..][0..pattern.len], pattern);
                a.writer.end += pattern.len;
            },
        }
        return count;
    }