Things to do for GNU grep

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===============
Short term work
===============

See where we are with UTF-8 performance.

Merge Debian patches that seem relevant.

Go through patches in Savannah.

Fix --directories=read.

Write better Texinfo documentation for grep.  The manual page would be a
good place to start, but Info documents are also supposed to contain a
tutorial and examples.

Some tests in tests/spencer2.tests should have failed!  Need to filter out
some bugs in dfa.[ch]/regex.[ch].

Multithreading?

GNU grep originally did 32-bit arithmetic.  Although it has moved to
64-bit on 64-bit platforms by using types like ptrdiff_t and size_t,
this conversion has not been entirely systematic and should be checked.

Lazy dynamic linking of the PCRE library.

Check FreeBSD’s integration of zgrep (-Z) and bzgrep (-J) in one
binary.  Is there a possibility of doing even better by automatically
checking the magic of binary files ourselves (0x1F 0x8B for gzip, 0x1F
0x9D for compress, and 0x42 0x5A 0x68 for bzip2)?  Once what to do with
the PCRE library is decided, do the same for libz and libbz2.


===================
Matching algorithms
===================

Take a look at these and consider opportunities for merging or cloning:

   -- http://osrd.org/projects/grep/global-regular-expression-print-tools-grep-variants
   -- ja-grep’s mlb2 patch (Japanese grep)
      <http://distcache.freebsd.org/ports-distfiles/grep-2.4.2-mlb2.patch.gz>
   -- lgrep (from lv, a Powerful Multilingual File Viewer / Grep)
      <http://www.mt.cs.keio.ac.jp/person/narita/lv/>;
   -- cgrep (Context grep) <https://awgn.github.io/cgrep/>
      seems like nice work;
   -- sgrep (Struct grep) <https://www.cs.helsinki.fi/u/jjaakkol/sgrep.html>;
   -- agrep (Approximate grep) <https://www.tgries.de/agrep/>,
      from glimpse;
   -- nr-grep (Nondeterministic reverse grep)
      <https://www.dcc.uchile.cl/~gnavarro/software/>;
   -- ggrep (Grouse grep) <http://www.grouse.com.au/ggrep/>;
   -- freegrep <https://github.com/howardjp/freegrep>;

Check some new algorithms for matching.  See, for example, Faro &
Lecroq (cited in kwset.c).

Fix the DFA matcher to never use exponential space.  (Fortunately, these
cases are rare.)


============================
Standards: POSIX and Unicode
============================

For POSIX compliance issues, see POSIX 1003.1.

Current support for the POSIX [= =] and [. .] constructs is limited to
platforms whose regular expression matchers are sufficiently
compatible with the GNU C library so that the --without-included-regex
option of ‘configure’ is in effect.  Extend this support to non-glibc
platforms, where --with-included-regex is in effect, by modifying the
included version of the regex code to defer to the native version when
handling [= =] and [. .].

For Unicode, interesting things to check include the Unicode Standard
<https://www.unicode.org/standard/standard.html> and the Unicode Technical
Standard #18 (<https://www.unicode.org/reports/tr18/> “Unicode Regular
Expressions”).  Talk to Bruno Haible who’s maintaining GNU libunistring.
See also Unicode Standard Annex #15 (<https://www.unicode.org/reports/tr15/>
“Unicode Normalization Forms”), already implemented by GNU libunistring.

In particular, --ignore-case needs to be evaluated against the standards.
We may want to deviate from POSIX if Unicode provides better or clearer
semantics.

POSIX and --ignore-case
-----------------------

For this issue, interesting things to check in POSIX include the
Open Group Base Specifications, Chapter “Regular Expressions”, in
particular Section “Regular Expression General Requirements” and its
paragraph about caseless matching (this may not have been fully
thought through and that this text may be self-contradicting
[specifically: “of either data or patterns” versus all the rest]).
See:

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_02

In particular, consider the following with POSIX’s approach to case
folding in mind.  Assume a non-Turkic locale with a character
repertoire reduced to the following various forms of “LATIN LETTER I”:

  0049;LATIN CAPITAL LETTER I;Lu;0;L;;;;;N;;;;0069;
  0069;LATIN SMALL LETTER I;Ll;0;L;;;;;N;;;0049;;0049
  0130;LATIN CAPITAL LETTER I WITH DOT ABOVE;Lu;0;L;0049 0307;;;;N;\
    LATIN CAPITAL LETTER I DOT;;;0069;
  0131;LATIN SMALL LETTER DOTLESS I;Ll;0;L;;;;;N;;;0049;;0049

UTF-8 octet lengths differ between U+0049 (0x49) and U+0069 (0x69)
versus U+0130 (0xC4 0xB0) and U+0131 (0xC4 0xB1).  This implies that
whole UTF-8 strings cannot be case-converted in place, using the same
memory buffer, and that the needed octet-size of the new buffer cannot
merely be guessed (although there’s a simple upper bound of five times
the size of the input, as the longest UTF-8 encoding of any character
is five bytes).

We have

  lc(I) = i, uc(I) = I
  lc(i) = i, uc(i) = I
  lc(İ) = i, uc(İ) = İ
  lc(ı) = ı, uc(ı) = I

where lc() and uc() denote lower-case and upper-case conversions.

There are several candidate --ignore-case logics.  Using the

  if (lc(input_wchar) == lc(pattern_wchar))

logic leads to the following matches:

    \in  I  i  İ  ı
  pat\   ----------
   I  |  Y  Y  Y  n
   i  |  Y  Y  Y  n
   İ  |  Y  Y  Y  n
   ı  |  n  n  n  Y

There is a lack of symmetry between CAPITAL and SMALL LETTERs with
this.  Using the

  if (uc(input_wchar) == uc(pattern_wchar))

logic (which is what GNU grep currently does although this is not
documented or guaranteed in the future), leads to the following
matches:

    \in  I  i  İ  ı
  pat\   ----------
   I  |  Y  Y  n  Y
   i  |  Y  Y  n  Y
   İ  |  n  n  Y  n
   ı  |  Y  Y  n  Y

There is a lack of symmetry between CAPITAL and SMALL LETTERs with
this.

Using the

  if (lc(input_wchar) == lc(pattern_wchar)
      || uc(input_wchar) == uc(pattern_wchar))

logic leads to the following matches:

    \in  I  i  İ  ı
  pat\   ----------
   I  |  Y  Y  Y  Y
   i  |  Y  Y  Y  Y
   İ  |  Y  Y  Y  n
   ı  |  Y  Y  n  Y

There is some elegance and symmetry with this.  But there are
potentially two conversions to be made per input character.  If the
pattern is pre-converted, two copies of it need to be kept and used in
a mutually coherent fashion.

Using the

  if (input_wchar  == pattern_wchar
      || lc(input_wchar) == pattern_wchar
      || uc(input_wchar) == pattern_wchar)

logic (a plausible interpretation of POSIX) leads to the following
matches:

    \in  I  i  İ  ı
  pat\   ----------
   I  |  Y  Y  n  Y
   i  |  Y  Y  Y  n
   İ  |  n  n  Y  n
   ı  |  n  n  n  Y

There is a different CAPITAL/SMALL symmetry with this.  But there’s
also a loss of pattern/input symmetry that’s unique to it.  Also there
are potentially two conversions to be made per input character.

Using the

  if (lc(uc(input_wchar)) == lc(uc(pattern_wchar)))

logic leads to the following matches:

    \in  I  i  İ  ı
  pat\   ----------
   I  |  Y  Y  Y  Y
   i  |  Y  Y  Y  Y
   İ  |  Y  Y  Y  Y
   ı  |  Y  Y  Y  Y

This shows total symmetry and transitivity (at least in this example
analysis).  There are two conversions to be made per input character,
but support could be added for having a single straight mapping
performing a composition of the two conversions.

Any optimization in the implementation of each logic must not change
its basic semantic.


Unicode and --ignore-case
-------------------------

For this issue, interesting things to check in Unicode include:

  - The Unicode Standard, Chapter 3
    (<https://www.unicode.org/versions/Unicode9.0.0/ch03.pdf>
    “Conformance”), Section 3.13 (“Default Case Algorithms”) and the
    toCasefold() case conversion operation.

  - The Unicode Standard, Chapter 4
    (<https://www.unicode.org/versions/Unicode9.0.0/ch04.pdf>
    “Character Properties”), Section 4.2 (“Case”) and
    the <https://www.unicode.org/Public/UNIDATA/SpecialCasing.txt>
    SpecialCasing.txt and
    <https://www.unicode.org/Public/UNIDATA/CaseFolding.txt>
    CaseFolding.txt files.

  - The Unicode Standard, Chapter 5
    (<https://www.unicode.org/versions/Unicode9.0.0/ch05.pdf>
    “Implementation Guidelines”), Section 5.18 (“Case Mappings”),
    Subsection “Caseless Matching”.

  - The Unicode case charts <https://www.unicode.org/charts/case/>.

Unicode uses the

  if (toCasefold(input_wchar_string) == toCasefold(pattern_wchar_string))

logic for caseless matching.  Consider the “LATIN LETTER I” example
mentioned above.  In a non-Turkic locale, simple case folding yields

  toCasefold_simple(U+0049) = U+0069
  toCasefold_simple(U+0069) = U+0069
  toCasefold_simple(U+0130) = U+0130
  toCasefold_simple(U+0131) = U+0131

which leads to the following matches:

    \in  I  i  İ  ı
  pat\   ----------
   I  |  Y  Y  n  n
   i  |  Y  Y  n  n
   İ  |  n  n  Y  n
   ı  |  n  n  n  Y

This is different from anything so far!

In a non-Turkic locale, full case folding yields

  toCasefold_full(U+0049) = U+0069
  toCasefold_full(U+0069) = U+0069
  toCasefold_full(U+0130) = <U+0069, U+0307>
  toCasefold_full(U+0131) = U+0131

with

  0307;COMBINING DOT ABOVE;Mn;230;NSM;;;;;N;NON-SPACING DOT ABOVE;;;;

which leads to the following matches:

    \in  I  i  İ  ı
  pat\   ----------
   I  |  Y  Y  *  n
   i  |  Y  Y  *  n
   İ  |  n  n  Y  n
   ı  |  n  n  n  Y

This is just sad!

Having toCasefold(U+0131), simple or full, map to itself instead of
U+0069 is in contradiction with the rules of Section 5.18 of the
Unicode Standard since toUpperCase(U+0131) is U+0049.  Same thing for
toCasefold_simple(U+0130) since toLowerCase(U+0131) is U+0069.  The
justification for the weird toCasefold_full(U+0130) mapping is
unknown; it doesn’t even make sense to add a dot (U+0307) to a letter
that already has one (U+0069).  It would have been so simple to put
them all in the same equivalence class!

Otherwise, also consider the following problem with Unicode’s approach
on case folding in mind.  Assume that we want to perform

  echo 'AßBC' | grep -i 'Sb'

which corresponds to

  input:    U+0041 U+00DF U+0042 U+0043 U+000A
  pattern:  U+0053 U+0062

Following CaseFolding.txt, applying the toCasefold() transformation to
these yields

  input:    U+0061 U+0073 U+0073 U+0062 U+0063 U+000A
  pattern:                U+0073 U+0062

so, according to this approach, the input should match the pattern.
As long as the original input line is to be reported to the user as a
whole, there is no problem (from the user’s point-of-view;
implementation is complicated by this).

However, consider both these GNU extensions:

  echo 'AßBC' | grep -i --only-matching 'Sb'
  echo 'AßBC' | grep -i --color=always  'Sb'

What is to be reported in these cases, since the match begins in the
*middle* of the original input character ‘ß’?

Unicode’s toCasefold() cannot be implemented in terms of POSIX’s
towctrans() since that can only return a single wint_t value per input
wint_t value.
