Don't try to seek when the input is a pipe

[rennex]

The code that estimates n seeks back to the beginning of the input file. This prevents supplying the input from a pipe (e.g. zcat). I suggest adding an option to specify n myself, so the input can come from a pipe. Then supporting reading from stdin would be a natural addition too.

[Freaky]

I don't really need n up-front, it's mainly to avoid the input vec having to resize itself too much. It also serves as an early warning if n*p won't fit in 64-bits. I could just skip it if it's unavailable, and indeed make an optional argument to fill in if the user wants to provide it.

Should probably also support streaming the output, since I have no need to seek in that.

Just need to ponder the interface. Should a create without filenames process stdin to stdout, should I require at least a - for that, or should I provide more explicit flags.