Re: [PR] [FLINK-40350][table-runtime] LPAD/RPAD split supplementary-plane characters into unpaired surrogates [flink]

["snuyanzin (via GitHub)" <gi...@apache.org>]

snuyanzin commented on code in PR #29004:
URL: https://github.com/apache/flink/pull/29004#discussion_r3921449114


##########
flink-table/flink-table-runtime/src/main/java/org/apache/flink/table/runtime/functions/SqlFunctionUtils.java:
##########
@@ -345,29 +338,61 @@ public static String rpad(String base, int len, String pad) {
             return "";
         }
 
-        char[] data = new char[len];
-        char[] baseChars = base.toCharArray();
-        char[] padChars = pad.toCharArray();
+        final int baseEnd = endOfCodePoints(base, len);
+        final int padCount = len - base.codePointCount(0, baseEnd);
+        if (padCount == 0) {
+            // The base already holds len code points, so it only needs truncating.
+            return base.substring(0, baseEnd);
+        }
+
+        final int padChars = padLength(pad, padCount);
+        final char[] data = new char[baseEnd + padChars];
+
+        base.getChars(0, baseEnd, data, 0);
+        writePad(data, baseEnd, pad, padChars);
 
-        int pos = 0;
+        return new String(data);
+    }
 
-        // copy the base
-        while (pos < base.length() && pos < len) {
-            data[pos] = baseChars[pos];
-            pos += 1;
+    /** Index just past the first count code points of str, or its end if str holds fewer. */
+    private static int endOfCodePoints(String str, int count) {
+        final int length = str.length();
+        int index = 0;
+        for (int i = 0; i < count && index < length; i++) {
+            index += Character.charCount(str.codePointAt(index));
         }
+        return index;
+    }
 
-        // copy the padding
-        while (pos < len) {
-            int i = 0;
-            while (i < pad.length() && i < len - pos) {
-                data[pos + i] = padChars[i];
-                i += 1;
-            }
-            pos += pad.length();
+    /** Number of chars taken by count code points of pad repeated cyclically. */
+    private static int padLength(String pad, int count) {
+        final int padLen = pad.length();
+        final int cycle = pad.codePointCount(0, padLen);
+        return (count / cycle) * padLen + pad.offsetByCodePoints(0, count % cycle);
+    }
+
+    /** Writes chars characters into data at pos, repeating pad cyclically. */
+    private static void writePad(char[] data, int pos, String pad, int chars) {
+        if (chars == 0) {
+            return;
         }
 
-        return new String(data);
+        final int padLen = pad.length();
+        if (padLen == 1) {
+            // A single char fills directly, which is faster than the doubling copy below.

Review Comment:
   from this comment it is not clear why do we need doubling copy
   need a separate line clarifying this (before the corresponding code)



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