bug#37393: 26.2.90; [PATCH] Speed up 'csv-align-fields'

[Stefan Monnier]

> The attached patch attempts to speed up the 'csv-align-fields' command
> by avoiding expensive calls to 'current-column', instead reusing field
> widths already computed by 'csv--column-widths'.

Sounds good.  I rarely use large CSV files, but I know the operation is slow.

I'm OK with the patch, tho please see my comment below.

> I felt an urge to speed up the command a bit while working with large
> (100 000+ lines) CSV files. Below are benchmarks produced by running
>
>   (benchmark 3 '(csv-align-fields nil (point-min) (point-max)))
>
> in three CSV files from the real world of various sizes. In these cases
> the speedup seems to be around 1.5x—2x.
>
> ~400 line file:
>   Before: Elapsed time: 0.175867s
>   After:  Elapsed time: 0.086809s
>
> ~50 000 line file:
>   Before: Elapsed time: 34.665853s (7.480686s in 35 GCs)
>   After:  Elapsed time: 24.349081s (7.154716s in 27 GCs)
>
> ~110 000 line file:
>   Before: Elapsed time: 82.444038s (19.799686s in 51 GCs)
>   After:  Elapsed time: 40.184331s (9.037813s in 25 GCs)

40s is still slow, but a factor of 2 is good, thanks.

If you're interested in this line, I think there are two avenues to
improve the behavior further:
- align lazily via jit-lock (this way the time is determined by the
  amount of text displayed rather than the total file size).
- make align-fields' into a mode, where fields are kept aligned even while
  the buffer is modified.

>  (defun csv--column-widths ()
> -  (let ((widths '()))
> +  (let ((column-widths '())
> +        (field-widths '()))

I think the return value is now sufficiently complex that the function
deserves a docstring describing it.


        Stefan