[TUHS] Command line options and complexity

Dan Cross crossd at gmail.com
Wed Mar 11 05:38:50 AEST 2020

On Tue, Mar 10, 2020 at 2:43 PM Doug McIlroy <doug at cs.dartmouth.edu> wrote:

> > This begs questions of stability
> Astute question. I had that in my original draft, but eliminited
> it for what I thought was clarity. Anyway, depending on implementation
> of sort, you may need sort -s. Of course it doesn't matter which copy
> among several equal lines uniq produces, nor does it matter in sort
> when there are no comparison options--they're all the same.

Thanks. That's interesting.

Did `sort -s` come later? The idea that you preferred clarity over
stability for `sort -u` would indicate so, otherwise one might imagine that
`-u` would just imply `-s` and that would be that.

> I don't know enough about the
> > internals of sed to know even what algorithm it uses
> > (... a disk-based merge sort?)
> sed is not a sorting program--basically it copies input to
> output, making line-by-line editing changes. That's the
> way I meant to use it in sed s/nonkeys//|sort -keys|uniq.
> (I have added options to sort, hopefully for clarity).
> The argument to sed here means substitute the empty
> string for the nonkey fields (specified by a regular expression).

`sed` in my email was a typo, as you speculated below.

Interestingly, this `sed` construction prior to `sort` loses information,
which perhaps doesn't matter in any given specific case, but is
insufficient in general, which I gathered to be the entire reason you
implemented `sort -u`.

If "sed" was a typo for "sort",

It was.

all versions of sort that
> I know of use an internal sorting algorithm for big chunks
> of the file, then combines the chunks by merge. But internal
> sorting varies all over the map--variations on quicksort,
> radix sort, merge sort, ...

It's the details of the internal sorts that are most interesting in some
sense, as the merges are probably fairly straight forward but the internal
sorts will affect stability and have other interesting characteristics.

As an aside, one must imagine that, in this day and age, a "big chunk" is
probably big enough to hold the vast majority of files entirely in RAM, and
only exceptionally large files actually require merging multiple blocks.

        - Dan C.
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