vim-patch:9.1.0645: regex: wrong match when searching multi-byte char case-insensitive

Problem:  regex: wrong match when searching multi-byte char
          case-insensitive (diffsetter)
Solution: Apply proper case-folding for characters and search-string

This patch does the following 4 things:

1) When the regexp engine compares two utf-8 codepoints case
   insensitive it may match an adjacent character, because it assumes
   it can step over as many bytes as the pattern contains.

   This however is not necessarily true because of case-folding, a
   multi-byte UTF-8 character can be considered equal to some
   single-byte value.

   Let's consider the pattern 'ſ' and the string 's'. When comparing and
   ignoring case, the single character 's' matches, and since it matches
   Vim will try to step over the match (by the amount of bytes of the
   pattern), assuming that since it matches, the length of both strings is
   the same.

   However in that case, it should only step over the single byte value
   's' by 1 byte and try to start matching after it again. So for the
   backtracking engine we need to ensure:
   * we try to match the correct length for the pattern and the text
   * in case of a match, we step over it correctly

   There is one tricky thing for the backtracing engine. We also need to
   calculate correctly the number of bytes to compare the 2 different
   utf-8 strings s1 and s2. So we will count the number of characters in
   s1 that the byte len specified. Then we count the number of bytes to
   step over the same number of characters in string s2 and then we can
   correctly compare the 2 utf-8 strings.

2) A similar thing can happen for the NFA engine, when skipping to the
   next character to test for a match. We are skipping over the regstart
   pointer, however we do not consider the case that because of
   case-folding we may need to adjust the number of bytes to skip over.
   So this needs to be adjusted in find_match_text() as well.

3) A related issue turned out, when prog->match_text is actually empty.
   In that case we should try to find the next match and skip this
   condition.

4) When comparing characters using collections, we must also apply case
   folding to each character in the collection and not just to the
   current character from the search string.  This doesn't apply to the
   NFA engine, because internally it converts collections to branches
   [abc] -> a\|b\|c

fixes: vim/vim#14294
closes: vim/vim#14756

https://github.com/vim/vim/commit/22e8e12d9f5034e1984db0c567b281fda4de8dd7

N/A patches:
vim-patch:9.0.1771: regex: combining chars in collections not handled
vim-patch:9.0.1777: patch 9.0.1771 causes problems

Co-authored-by: Christian Brabandt <cb@256bit.org>

2 files changed, 59 insertions, 31 deletions

diff --git a/src/nvim/mbyte.c b/src/nvim/mbyte.c
index a544113a7b..c6cefb8a91 100644
--- a/src/nvim/mbyte.c
+++ b/src/nvim/mbyte.c
@@ -1384,7 +1384,7 @@ bool mb_isalpha(int a)
   return mb_islower(a) || mb_isupper(a);
 }
 
-static int utf_strnicmp(const char *s1, const char *s2, size_t n1, size_t n2)
+int utf_strnicmp(const char *s1, const char *s2, size_t n1, size_t n2)
 {
   int c1, c2;
   char buffer[6];
diff --git a/src/nvim/regexp.c b/src/nvim/regexp.c
index 31384e8124..3f34ca7e0e 100644
--- a/src/nvim/regexp.c
+++ b/src/nvim/regexp.c
@@ -1732,7 +1732,9 @@ static void mb_decompose(int c, int *c1, int *c2, int *c3)
 
 /// Compare two strings, ignore case if rex.reg_ic set.
 /// Return 0 if strings match, non-zero otherwise.
-/// Correct the length "*n" when composing characters are ignored.
+/// Correct the length "*n" when composing characters are ignored
+/// or when both utf codepoints are considered equal because of
+/// case-folding but have different length (e.g. 's' and 'ſ')
 static int cstrncmp(char *s1, char *s2, int *n)
 {
   int result;
@@ -1740,8 +1742,27 @@ static int cstrncmp(char *s1, char *s2, int *n)
   if (!rex.reg_ic) {
     result = strncmp(s1, s2, (size_t)(*n));
   } else {
-    assert(*n >= 0);
-    result = mb_strnicmp(s1, s2, (size_t)(*n));
+    char *p = s1;
+    size_t n2 = 0;
+    int n1 = *n;
+    // count the number of characters for byte-length of s1
+    while (n1 > 0 && *p != NUL) {
+      n1 -= utfc_ptr2len(s1);
+      MB_PTR_ADV(p);
+      n2++;
+    }
+    // count the number of bytes to advance the same number of chars for s2
+    p = s2;
+    while (n2-- > 0 && *p != NUL) {
+      MB_PTR_ADV(p);
+    }
+
+    n2 = (size_t)(p - s2);
+
+    result = utf_strnicmp(s1, s2, (size_t)(*n), n2);
+    if (result == 0 && (int)n2 < *n) {
+      *n = (int)n2;
+    }
   }
 
   // if it failed and it's utf8 and we want to combineignore:
@@ -1799,31 +1820,34 @@ static inline char *cstrchr(const char *const s, const int c)
     return vim_strchr(s, c);
   }
 
-  // Use folded case for UTF-8, slow! For ASCII use libc strpbrk which is
-  // expected to be highly optimized.
+  int cc, lc;
   if (c > 0x80) {
-    const int folded_c = utf_fold(c);
-    for (const char *p = s; *p != NUL; p += utfc_ptr2len(p)) {
-      const int uc = utf_ptr2char(p);
-      // Do not match an illegal byte.  E.g. 0xff matches 0xc3 0xbf, not 0xff.
-      if ((uc < 0x80 || uc != (uint8_t)(*p)) && utf_fold(uc) == folded_c) {
-        return (char *)p;
-      }
-    }
-    return NULL;
-  }
-
-  int cc;
-  if (ASCII_ISUPPER(c)) {
+    cc = utf_fold(c);
+    lc = cc;
+  } else if (ASCII_ISUPPER(c)) {
     cc = TOLOWER_ASC(c);
+    lc = cc;
   } else if (ASCII_ISLOWER(c)) {
     cc = TOUPPER_ASC(c);
+    lc = c;
   } else {
     return vim_strchr(s, c);
   }
 
-  char tofind[] = { (char)c, (char)cc, NUL };
-  return strpbrk(s, tofind);
+  for (const char *p = s; *p != NUL; p += utfc_ptr2len(p)) {
+    const int uc = utf_ptr2char(p);
+    if (c > 0x80 || uc > 0x80) {
+      // Do not match an illegal byte.  E.g. 0xff matches 0xc3 0xbf, not 0xff.
+      // Compare with lower case of the character.
+      if ((uc < 0x80 || uc != (uint8_t)(*p)) && utf_fold(uc) == lc) {
+        return (char *)p;
+      }
+    } else if ((uint8_t)(*p) == c || (uint8_t)(*p) == cc) {
+      return (char *)p;
+    }
+  }
+
+  return NULL;
 }
 
 ////////////////////////////////////////////////////////////////
@@ -6619,11 +6643,9 @@ static bool regmatch(uint8_t *scan, const proftime_T *tm, int *timed_out)
               }
             }
           } else {
-            for (i = 0; i < len; i++) {
-              if (opnd[i] != rex.input[i]) {
-                status = RA_NOMATCH;
-                break;
-              }
+            if (cstrncmp((char *)opnd, (char *)rex.input, &len) != 0) {
+              status = RA_NOMATCH;
+              break;
             }
           }
           rex.input += len;
@@ -13984,19 +14006,25 @@ static int skip_to_start(int c, colnr_T *colp)
 static int find_match_text(colnr_T *startcol, int regstart, uint8_t *match_text)
 {
   colnr_T col = *startcol;
-  const int regstart_len = utf_ptr2len((char *)rex.line + col);
+  const int regstart_len = utf_char2len(regstart);
 
   while (true) {
     bool match = true;
     uint8_t *s1 = match_text;
-    uint8_t *s2 = rex.line + col + regstart_len;  // skip regstart
+    // skip regstart
+    int regstart_len2 = regstart_len;
+    if (regstart_len2 > 1 && utf_ptr2len((char *)rex.line + col) != regstart_len2) {
+      // because of case-folding of the previously matched text, we may need
+      // to skip fewer bytes than utf_char2len(regstart)
+      regstart_len2 = utf_char2len(utf_fold(regstart));
+    }
+    uint8_t *s2 = rex.line + col + regstart_len2;
     while (*s1) {
       int c1_len = utf_ptr2len((char *)s1);
       int c1 = utf_ptr2char((char *)s1);
       int c2_len = utf_ptr2len((char *)s2);
       int c2 = utf_ptr2char((char *)s2);
-      if ((c1 != c2 && (!rex.reg_ic || utf_fold(c1) != utf_fold(c2)))
-          || c1_len != c2_len) {
+      if (c1 != c2 && (!rex.reg_ic || utf_fold(c1) != utf_fold(c2))) {
         match = false;
         break;
       }
@@ -15664,7 +15692,7 @@ static int nfa_regexec_both(uint8_t *line, colnr_T startcol, proftime_T *tm, int
 
     // If match_text is set it contains the full text that must match.
     // Nothing else to try. Doesn't handle combining chars well.
-    if (prog->match_text != NULL && !rex.reg_icombine) {
+    if (prog->match_text != NULL && *prog->match_text != NUL && !rex.reg_icombine) {
       retval = find_match_text(&col, prog->regstart, prog->match_text);
       if (REG_MULTI) {
         rex.reg_mmatch->rmm_matchcol = col;