[Rd] Bug: time complexity of substring is quadratic as string size and number of substrings increases

[Toby Hocking]

Hi all, (and especially hi to Tomas Kalibera who accepted my patch sent
yesterday)

I believe that I have found another bug, this time in the substring
function. The use case that I am concerned with is when there is a single
(character scalar) text/subject, and many substrings to extract. For example

substring("AAAA", 1:4, 1:4)

or more generally,

N=1000
substring(paste(rep("A", N), collapse=""), 1:N, 1:N)

The problem I observe is that the time complexity is quadratic in N, as
shown on this figure
https://github.com/tdhock/namedCapture-article/blob/master/figure-substring-bug.png
source:
https://github.com/tdhock/namedCapture-article/blob/master/figure-substring-bug.R

I expected the time complexity to be linear in N.

The example above may seem contrived/trivial, but it is indeed relevant to
a number of packages (rex, rematch2, namedCapture) which provide functions
that use gregexpr and then substring to extract the text in the captured
sub-patterns. The figure
https://github.com/tdhock/namedCapture-article/blob/master/figure-trackDb-pkgs.png
shows the issue: these packages have quadratic time complexity, whereas
other packages (and the gregexpr function with perl=TRUE after applying the
patch discussed yesterday) have linear time complexity. I believe the
problem is the substring function. Source for this figure:
https://github.com/tdhock/namedCapture-article/blob/master/figure-trackDb-pkgs.R

I suspect that a fix can be accomplished by optimizing the implementation
of substring, for the special case when the text/subject is a single
element (character scalar). Right now I notice that the substring R code
uses rep_len so that the text/subject which is passed to the C code is a
character vector with the same length as the number of substrings to
extract. Maybe the C code is calling strlen for each of these (identical)
text/subject elements?

Anyway, it would be useful to have some feedback to make sure this is
indeed a bug before I post on bugzilla. (btw thanks Martin for signing me
up for an account)

Toby

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