As Bogdan writes, the problem is repeatedly calling `string-append`.
Instead one can make a list of all the small strings and then use
`string-join` at the end.
#lang racket
(require racket/flonum)
(define (xy->string x y)
(string-append
(~r x #:precision 1) ","
(~r y #:precision 1)))
(define (xy-vectors->strings x-vec y-vec)
(for/list ((x (in-flvector x-vec))
(y (in-flvector y-vec)))
(xy->string x y)))
(define (xy-vectors->string x-vec y-vec)
(string-join (xy-vectors->strings x-vec y-vec) " "))
Den søn. 27. jun. 2021 kl. 15.26 skrev Alessandro Motta <amott...@gmail.com
>:
> Hi racket-users!
>
> I've recently become interested in Lisp/Scheme and have started to hack
> in Racket. The excellent documentation, the fast integrated search, and
> DrRacket have made that a real pleasure.
>
> Thank you for that!
>
> I've been working on a tool to convert notes from the reMarkable 2
> tablet to SVG files. At the core is the conversion of (x, y) coordinate
> pairs from two `flvector`s to a string of the form "x1,y1 x2,y2 x3,y3".
>
> ```
> (define (xy->string x y)
> (string-append
> (~r x #:precision 1) ","
> (~r y #:precision 1)))
>
> (define (xy-vectors->string x-vec y-vec)
> (for/fold ((coordinates "")
> (separator "")
> #:result coordinates)
> ((x (in-flvector x-vec))
> (y (in-flvector y-vec)))
> (values (string-append
> coordinates
> separator
> (xy->string x y))
> " ")))
> ```
>
> This is currently the bottleneck for large conversion jobs.
>
> Profiling these functions with `profile-flame-graph` resulted in
>
> https://gist.githubusercontent.com/amotta/cfe4b19e24455af219521c9e94455c67/raw/dbbc87bd2f6dd4e27c33831749baa90fffdaed55/flvector-to-coordinates-string-flamegraph.svg
>
> The full profiling script is available at
> https://gist.github.com/amotta/e76197082bb1bf63538ede01872917f3
>
> Roughly 90% of time is spent in `contract/private/arrow-val-first.rkt`.
> Based on my very limited understanding of Racket, it seems that ~38% of
> time is spent handling keyword arguments (presumably `#:precision 1`?).
> The `catnp` function (the conversion from flonum to string, I think)
> takes up only ~11% of time.
>
> Is this interpretation of the flame graph correct? If so, are there any
> obvious blunders on my part? Any ideas for how to speed up this code?
>
>
> Best wishes,
> Alessandro
>
> --
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>
--
--
Jens Axel Søgaard
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