Refactoring for MDEV-27042 and MDEV-27009

This patch prepares the code for upcoming changes:

MDEV-27009 Add UCA-14.0.0 collations
MDEV-27042 UCA: Resetting contractions to ignorable does not work well

1. Adding "const" qualifiers to return type and parameters in functions:
- my_uca_contraction2_weight()
- my_wmemcmp()
- my_uca_contraction_weight()
- my_uca_scanner_contraction_find()
- my_uca_previous_context_find()
- my_uca_context_weight_find()

2. Adding a helper function my_uca_true_contraction_eq()

3. Changing the way how scanner->wbeg is set during context weight handling.
   It was previously set inside functions:
   - my_uca_scanner_contraction_find()
   - my_uca_previous_context_find()
   Now it's set inside scanner_next(), which makes the code more symmetric
   for context-free and context-dependent sequences.
   This makes then upcoming fix for MDEV-27042 simpler.

include/m_ctype.h

@@ -135,8 +135,8 @@ typedef struct my_contraction_list_t
 
 my_bool my_uca_can_be_contraction_head(const MY_CONTRACTIONS *c, my_wc_t wc);
 my_bool my_uca_can_be_contraction_tail(const MY_CONTRACTIONS *c, my_wc_t wc);
-uint16 *my_uca_contraction2_weight(const MY_CONTRACTIONS *c,
-                                   my_wc_t wc1, my_wc_t wc2);
+const uint16 *my_uca_contraction2_weight(const MY_CONTRACTIONS *c,
+                                         my_wc_t wc1, my_wc_t wc2);
 
 
 /* Collation weights on a single level (e.g. primary, secondary, tertiarty) */

strings/ctype-uca.c

@@ -31358,7 +31358,7 @@ my_uca_can_be_contraction_part(const MY_CONTRACTIONS *c, my_wc_t wc, int flag)
   @retval   ptr  - contraction weight array
 */
 
-uint16 *
+const uint16 *
 my_uca_contraction2_weight(const MY_CONTRACTIONS *list, my_wc_t wc1, my_wc_t wc2)
 {
   MY_CONTRACTION *c, *last;
@@ -31443,13 +31443,29 @@ my_uca_needs_context_handling(const MY_UCA_WEIGHT_LEVEL *level, my_wc_t wc)
   @retval       non-zero - strings are different
 */
 
-static int
-my_wmemcmp(my_wc_t *a, my_wc_t *b, size_t len)
+static inline int
+my_wmemcmp(const my_wc_t *a, const my_wc_t *b, size_t len)
 {
   return memcmp(a, b, len * sizeof(my_wc_t));
 }
 
 
+/*
+  Test if the MY_CONTRACTION instance is equal to the wide
+  string with the given length.
+  Note, only true contractions are checked,
+  while previous context pairs always return FALSE.
+*/
+static inline my_bool
+my_uca_true_contraction_eq(const MY_CONTRACTION *c,
+                           const my_wc_t *wc, size_t len)
+{
+  return (len >= MY_UCA_MAX_CONTRACTION || c->ch[len] == 0) &&
+         !c->with_context &&
+         !my_wmemcmp(c->ch, wc, len);
+}
+
+
 /**
   Check if a string is a contraction,
   and return its weight array on success.
@@ -31463,7 +31479,7 @@ my_wmemcmp(my_wc_t *a, my_wc_t *b, size_t len)
   @retval       ptr  - contraction weight array
 */
 
-static inline uint16 *
+static inline const uint16 *
 my_uca_contraction_weight(const MY_CONTRACTIONS *list, my_wc_t *wc, size_t len)
 {
   MY_CONTRACTION *c, *last;
@@ -31471,9 +31487,7 @@ my_uca_contraction_weight(const MY_CONTRACTIONS *list, my_wc_t *wc, size_t len)
 
   for (c= list->item, last= c + list->nitems; c < last; c++)
   {
-    if ((len >= MY_UCA_MAX_CONTRACTION || c->ch[len] == 0) &&
-        !c->with_context &&
-        !my_wmemcmp(c->ch, wc, len))
+    if (my_uca_true_contraction_eq(c, wc, len))
       return c->weight;
   }
   return NULL;
@@ -31495,12 +31509,15 @@ my_uca_contraction_weight(const MY_CONTRACTIONS *list, my_wc_t *wc, size_t len)
   @retval         ptr  - contraction weight array
 */
 
-static uint16 *
-my_uca_scanner_contraction_find(my_uca_scanner *scanner, my_wc_t *wc)
+static const uint16 *
+my_uca_scanner_contraction_find(my_uca_scanner *scanner, my_wc_t currwc)
 {
   size_t clen= 1;
   int flag;
   const uchar *s, *beg[MY_UCA_MAX_CONTRACTION];
+  my_wc_t wc[MY_UCA_MAX_CONTRACTION];
+  wc[0]= currwc;
+
   memset((void*) beg, 0, sizeof(beg));
 
   /* Scan all contraction candidates */
@@ -31520,13 +31537,12 @@ my_uca_scanner_contraction_find(my_uca_scanner *scanner, my_wc_t *wc)
   /* Find among candidates the longest real contraction */
   for ( ; clen > 1; clen--)
   {
-    uint16 *cweight;
+    const uint16 *cweight;
     if (my_uca_can_be_contraction_tail(&scanner->level->contractions,
                                        wc[clen - 1]) &&
         (cweight= my_uca_contraction_weight(&scanner->level->contractions,
                                             wc, clen)))
     {
-      scanner->wbeg= cweight + 1;
       scanner->sbeg= beg[clen - 1];
       return cweight;
     }
@@ -31549,19 +31565,15 @@ my_uca_scanner_contraction_find(my_uca_scanner *scanner, my_wc_t *wc)
   @retval   ptr  - contraction weight array
 */
 
-static uint16 *
-my_uca_previous_context_find(my_uca_scanner *scanner,
+static const uint16 *
+my_uca_previous_context_find(const MY_CONTRACTIONS *list,
                              my_wc_t wc0, my_wc_t wc1)
 {
-  const MY_CONTRACTIONS *list= &scanner->level->contractions;
   MY_CONTRACTION *c, *last;
   for (c= list->item, last= c + list->nitems; c < last; c++)
   {
     if (c->with_context && wc0 == c->ch[0] && wc1 == c->ch[1])
-    {
-      scanner->wbeg= c->weight + 1;
       return c->weight;
-    }
   }
   return NULL;
 }
@@ -31584,10 +31596,11 @@ my_uca_previous_context_find(my_uca_scanner *scanner,
   @retval          NULL if could not find any contextual weights for wc[0]
   @retval          non null pointer to a zero-terminated weight string otherwise
 */
-static inline uint16 *
-my_uca_context_weight_find(my_uca_scanner *scanner, my_wc_t *wc)
+static inline const uint16 *
+my_uca_context_weight_find(my_uca_scanner *scanner, my_wc_t currwc)
 {
-  uint16 *cweight;
+  const uint16 *cweight;
+  my_wc_t prevwc;
   DBUG_ASSERT(scanner->level->contractions.nitems);
   /*
     If we have scanned a character which can have previous context,
@@ -31599,21 +31612,22 @@ my_uca_context_weight_find(my_uca_scanner *scanner, my_wc_t *wc)
     context at the moment. CLDR does not have longer sequences.
   */
   if (my_uca_can_be_previous_context_tail(&scanner->level->contractions,
-                                          wc[0]) &&
+                                          currwc) &&
       scanner->wbeg != nochar &&     /* if not the very first character */
       my_uca_can_be_previous_context_head(&scanner->level->contractions,
-                                          (wc[1]= ((scanner->page << 8) +
+                                          (prevwc= ((scanner->page << 8) +
                                                     scanner->code))) &&
-      (cweight= my_uca_previous_context_find(scanner, wc[1], wc[0])))
+      (cweight= my_uca_previous_context_find(&scanner->level->contractions,
+                                             prevwc, currwc)))
   {
     scanner->page= scanner->code= 0; /* Clear for the next character */
     return cweight;
   }
   else if (my_uca_can_be_contraction_head(&scanner->level->contractions,
-                                          wc[0]))
+                                          currwc))
   {
     /* Check if w[0] starts a contraction */
-    if ((cweight= my_uca_scanner_contraction_find(scanner, wc)))
+    if ((cweight= my_uca_scanner_contraction_find(scanner, currwc)))
       return cweight;
   }
   return NULL;

strings/ctype-uca.ic

@@ -52,28 +52,31 @@ MY_FUNCTION_NAME(scanner_next)(my_uca_scanner *scanner)
   do
   {
     const uint16 *wpage;
-    my_wc_t wc[MY_UCA_MAX_CONTRACTION];
     int mblen;
+    my_wc_t currwc;
 
     /* Get next character */
 #if MY_UCA_ASCII_OPTIMIZE
     /* Get next ASCII character */
     if (scanner->sbeg < scanner->send && scanner->sbeg[0] < 0x80)
     {
-      wc[0]= scanner->sbeg[0];
+      currwc= scanner->sbeg[0];
       scanner->sbeg+= 1;
 
 #if MY_UCA_COMPILE_CONTRACTIONS
-      if (my_uca_needs_context_handling(scanner->level, wc[0]))
+      if (my_uca_needs_context_handling(scanner->level, currwc))
       {
-        uint16 *cweight= my_uca_context_weight_find(scanner, wc);
+        const uint16 *cweight= my_uca_context_weight_find(scanner, currwc);
         if (cweight)
+        {
+          scanner->wbeg= cweight + 1;
           return *cweight;
+        }
       }
 #endif
 
       scanner->page= 0;
-      scanner->code= (int) wc[0];
+      scanner->code= (int) currwc;
       scanner->wbeg= scanner->level->weights[0] + scanner->code * scanner->level->lengths[0];
       if (scanner->wbeg[0])
         return *scanner->wbeg++;
@@ -82,8 +85,8 @@ MY_FUNCTION_NAME(scanner_next)(my_uca_scanner *scanner)
     else
 #endif
     /* Get next MB character */
-    if (((mblen= MY_MB_WC(scanner, wc, scanner->sbeg,
-                                       scanner->send)) <= 0))
+    if (((mblen= MY_MB_WC(scanner, &currwc, scanner->sbeg,
+                                            scanner->send)) <= 0))
     {
       if (scanner->sbeg >= scanner->send)
         return -1; /* No more bytes, end of line reached */
@@ -105,7 +108,7 @@ MY_FUNCTION_NAME(scanner_next)(my_uca_scanner *scanner)
     }
 
     scanner->sbeg+= mblen;
-    if (wc[0] > scanner->level->maxchar)
+    if (currwc > scanner->level->maxchar)
     {
       /* Return 0xFFFD as weight for all characters outside BMP */
       scanner->wbeg= nochar;
@@ -113,17 +116,20 @@ MY_FUNCTION_NAME(scanner_next)(my_uca_scanner *scanner)
     }
 
 #if MY_UCA_COMPILE_CONTRACTIONS
-    if (my_uca_needs_context_handling(scanner->level, wc[0]))
+    if (my_uca_needs_context_handling(scanner->level, currwc))
     {
-      uint16 *cweight= my_uca_context_weight_find(scanner, wc);
+      const uint16 *cweight= my_uca_context_weight_find(scanner, currwc);
       if (cweight)
+      {
+        scanner->wbeg= cweight + 1;
         return *cweight;
+      }
     }
 #endif
 
     /* Process single character */
-    scanner->page= wc[0] >> 8;
-    scanner->code= wc[0] & 0xFF;
+    scanner->page= currwc >> 8;
+    scanner->code= currwc & 0xFF;
 
     /* If weight page for w[0] does not exist, then calculate algoritmically */
     if (!(wpage= scanner->level->weights[scanner->page]))