ARROW-10195: [C++] Add string struct extract kernel using re2

[maartenbreddels]

The second commit adds re2 to the linked libraries. @xhochy how should this be done, should I open a separate issue for this?

[github-actions]

https://issues.apache.org/jira/browse/ARROW-10195

[maartenbreddels]

should re2 be available on all platform? appveyor for instance gives: 're2/re2.h': No such file or directory

[xhochy]

@maartenbreddels Can you rebase this? That would be helpful for me setting up re2 in the toolchain.

[maartenbreddels]

I cleaned it up a bit, but it might fail running the test, hope this is enough for you for now.

[maartenbreddels]

There are two issues with this PR:

empty slices

In #8728 https://github.com/apache/arrow/pull/8728/files#diff-7771ecc138c4ecc2dc8498affe04f5b7f182c4b77b18a512c3dd07a82d45aa3dR116 was added, which adds an empty slice to the test, which breaks my test (which is good). However, my kernel will not be called, meaning i cannot set the output's type (which depends on the input regex). Shall I open a JIRA for that?

The error is:

# Array types differed: struct<letter: string, digit: string> vs struct<>
Null counts differ. Expected -1 but was 0
Expected:
  -- is_valid: all not null
  -- child 0 type: string
    []
  -- child 1 type: string
    []
Actual:
  -- is_valid: all not null

(showing the struct mismatch)

ChunkedArray fails

Commenting out the empty slice test, the next failure is with

arrow/cpp/src/arrow/compute/kernels/test_util.cc

Line 133 in db20c7a

AssertDatumsEqual(std::make_shared<ChunkedArray>(expected_chunks), out);

Failed
Got: 
  [
    -- is_valid: all not null
    -- child 0 type: string
      [
        "a"
      ]
    -- child 1 type: string
      [
        "1"
      ],
    -- is_valid:
          [
        true,
        false,
        false
      ]
    -- child 0 type: string
      [
        "b",
        "",
        ""
      ]
    -- child 1 type: string
      [
        "2",
        "",
        ""
      ]
  ]
Expected: 
  [
    -- is_valid: all not null
    -- child 0 type: string
      [
        "a"
      ]
    -- child 1 type: string
      [
        "1"
      ],
    -- is_valid:
          [
        true,
        false,
        false
      ]
    -- child 0 type: string
      [
        "b",
        "",
        ""
      ]
    -- child 1 type: string
      [
        "2",
        "",
        ""
      ]
  ]

I fail to see a difference in this output (maybe a null value in a child?), and this is difficult to debug in gdb. Maybe an opportunity to improve the printing of differences (since I fail to see any).

[jorisvandenbossche]

@maartenbreddels what's the status of this PR? Ready for review, or are there still open issues? (you listed some above at #8459 (comment))

[nealrichardson]

This needs rebase now that #8468 has merged.

[maartenbreddels]

Rebased, but there are two issues left:

• null values comparison fail, no idea what the issue is there, quite difficult for me to debug
• the compute kernel does not get called on empty input, so i skip that test for now. But I think kernels should have the option to opt-in at least, so we can return an empty struct (all its arrays of length 0), but with field names.

[pitrou]

@maartenbreddels Will you need help here?

[maartenbreddels]

Yes, that would be appriciated, the null values I find difficult to debug, and the second issue I'm not sure that is good solution.

[pitrou]

Ok, I'm looking at this.

[pitrou]

Naming nit @jorisvandenbossche @jonkeane @ianmcook .
Do you think "extract_regex" is the right term for this function? Something else?

[pitrou]

@maartenbreddels For the record:

null values comparison fail, no idea what the issue is there, quite difficult for me to debug

I am not sure what was causing this, but my small rewrite of the kernel fixed it.

I think kernels should have the option to opt-in at least, so we can return an empty struct (all its arrays of length 0), but with field names.

This was solved by defining a Resolver function so that the FunctionOptions instance can participate in output type resolution (instead of specifying an empty struct type when declaring the kernel).

[jorisvandenbossche]

Do you think "extract_regex" is the right term for this function? Something else?

In pandas, this is called "extract" (https://pandas.pydata.org/docs/reference/api/pandas.Series.str.extract.html), so from that point of view this is a good name.
In re, I think one would use re.search, but that's basically both used for "match" as for "extract" purposes, so I think "extract" is a better name for the kernel in this PR.

The _regex might be a bit superfluous, as there is no variant without regex (but on the other hand it explicitly signals it is using regular expressions).

[jorisvandenbossche]

One thing I noted while quickly trying it out:

In [6]: pc.extract_regex(pa.array(['a1', 'b2', 'c3']), regex=r'(?P<letter>[ab])?(?P<digit>\d)')
Out[6]: 
<pyarrow.lib.StructArray object at 0x7f8c1bdf9760>
-- is_valid: all not null
-- child 0 type: string
  [
    "a",
    "b",
    ""
  ]
-- child 1 type: string
  [
    "1",
    "2",
    "3"
  ]

When having an optional group like the above, I would maybe expect a null instead of an empty string in the first child?

[pitrou]

When having an optional group like the above, I would maybe expect a null instead of an empty string in the first child?

Agreed, but unfortunately re2 doesn't give us that information...

[jorisvandenbossche]

Naming nit: I would maybe call this "pattern" instead of "regex"

[pitrou]

+1, will merge

[maartenbreddels]

Thanks for finishing this Antoine!

This was solved by defining a Resolver function so that the FunctionOptions instance can participate in output type resolution (instead of specifying an empty struct type when declaring the kernel).

Good to know, thanks.

[pitrou]

@maartenbreddels You're welcome :-)