Re: [PATCH 4/5] xdiff: introduce XDF_IGNORE_CASE

Junio C Hamano [off-list ref] writes:

Teach the hash function and per-line comparison logic to compare lines
while ignoring the differences in case.  It is not an ignore-whitespace
option but still needs to trigger the inexact match logic, and that is
why the previous step introduced XDF_INEXACT_MATCH mask.

Nb. how it compares with ignore case in filesystem paths?
 

Assign the 7th bit for this option, and move the bits to select diff
algorithms out of the way in order to leave room for a few bits to add
more variants of ignore-whitespace, such as --ignore-tab-expansion, if
somebody else is inclined to do so later.

Or do a proper Unicode sorting / collation algorithm, with different
levels 

  (4.3 Form a sort key for each string, UTS #10.):

     Level 1: alphabetic ordering
     Level 2: diacritic ordering
     Level 3: case ordering
     Level 4: tie-breaking (e.g. in the case when variable is 'shifted')

We would still need to teach the front-end to flip this bit, for this
change to be any useful.

Signed-off-by: Junio C Hamano <redacted>
---

+static inline int match_a_byte(char ch1, char ch2, long flags)
+{
+	if (ch1 == ch2)
+		return 1;
+	if (!(flags & XDF_IGNORE_CASE) || ((ch1 | ch2) & 0x80))
+		return 0;
+	if (isupper(ch1))
+		ch1 = tolower(ch1);
+	if (isupper(ch2))
+		ch2 = tolower(ch2);
+	return (ch1 == ch2);
+}

<del>
Wouldn't a better solution be a collate algorithm rather than changing
a sorting function?  Or is it a performance hack on typical body of
text under version control (mainly lowercase)?
</del>

"(libc.info)Collation Fuctions" says:

     The functions `strcoll' and `wcscoll' perform this translation
  implicitly, in order to do one comparison.  By contrast, `strxfrm' and
  `wcsxfrm' perform the mapping explicitly.  If you are making multiple
  comparisons using the same string or set of strings, it is likely to be
  more efficient to use `strxfrm' or `wcsxfrm' to transform all the
  strings just once, and subsequently compare the transformed strings
  with `strcmp' or `wcscmp'.

The function match_a_byte (memcoll?) defined here is similar to strcoll;
do we compare single line with more than one other line?

+static inline unsigned long hash_a_byte(const char ch_, long flags)
+{
+	unsigned long ch = ch_ & 0xFF;
+	if ((flags & XDF_IGNORE_CASE) && !(ch & 0x80) && isupper(ch))
+		ch = tolower(ch);
+	return ch;
+}
+

Hmmm... hash_a_byte (memxfrm?) is similar to strxfrm, so you do use
one or the other...

-- 
Jakub Narebski