collations: fix sorting in UCA900 collations (#12555) (#12562)

* collations: fix sorting in UCA900 collations

When using the fast iterator to _compare_ two strings with an UCA
collation, we need to keep in mind that the weights in the collation are
in BIG ENDIAN (this is the output format for the weight strings, so we
store the weights this way), so comparing them directly will not result
in the proper collation order. They need to be byte-swapped before they
can be compared with an arithmetic operation!



* collations: comment



---------

Signed-off-by: Vicent Marti <vmg@strn.cat>

go/mysql/collations/internal/uca/iter_fast_900.go

@@ -66,7 +66,7 @@ func (it *FastIterator900) FastForward32(it2 *FastIterator900) int {
 
 	p1 := it.input
 	p2 := it2.input
-	var w1, w2 uint32
+	var w1, w2 uint16
 
 	for len(p1) >= 4 && len(p2) >= 4 {
 		dword1 := *(*uint32)(unsafe.Pointer(&p1[0]))
@@ -75,17 +75,20 @@ func (it *FastIterator900) FastForward32(it2 *FastIterator900) int {
 
 		if nonascii == 0 {
 			if dword1 != dword2 {
+				// Use the weight string fast tables for quick weight comparisons;
+				// see (*FastIterator900).NextWeightBlock64 for a description of
+				// the table format
 				table := it.fastTable
-				if w1, w2 = table[p1[0]], table[p2[0]]; w1 != w2 {
+				if w1, w2 = uint16(table[p1[0]]), uint16(table[p2[0]]); w1 != w2 {
 					goto mismatch
 				}
-				if w1, w2 = table[p1[1]], table[p2[1]]; w1 != w2 {
+				if w1, w2 = uint16(table[p1[1]]), uint16(table[p2[1]]); w1 != w2 {
 					goto mismatch
 				}
-				if w1, w2 = table[p1[2]], table[p2[2]]; w1 != w2 {
+				if w1, w2 = uint16(table[p1[2]]), uint16(table[p2[2]]); w1 != w2 {
 					goto mismatch
 				}
-				if w1, w2 = table[p1[3]], table[p2[3]]; w1 != w2 {
+				if w1, w2 = uint16(table[p1[3]]), uint16(table[p2[3]]); w1 != w2 {
 					goto mismatch
 				}
 			}
@@ -114,7 +117,8 @@ mismatch:
 		it.unicode++
 		return 0
 	}
-	return int(w1) - int(w2)
+	// The weights must be byte-swapped before comparison because they're stored in big endian
+	return int(bits.ReverseBytes16(w1)) - int(bits.ReverseBytes16(w2))
 }
 
 // NextWeightBlock64 takes a byte slice of 16 bytes and fills it with the next