Introduce DecimalDigits.appendPair for efficient two-digit formatting and refactor DateTimeHelper

src/java.base/share/classes/java/lang/AbstractStringBuilder.java

@@ -906,6 +906,31 @@ public AbstractStringBuilder append(char c) {
         return this;
     }
 
+    /**
+     * Appends the two-digit string representation of the {@code int}
+     * argument to this sequence.
+     * <p>
+     * The integer {@code i} is formatted as two decimal digits.
+     * If the value is between 0 and 9, it is formatted with a leading zero
+     * (e.g., 5 becomes "05"). If the value is outside the range 0-99,
+     * the behavior is unspecified and may result in unexpected output.
+     *
+     * @param   i   an {@code int} (should be between 0 and 99 inclusive).
+     * @throws  IndexOutOfBoundsException if {@code i} is outside the range 0-99.
+     */
+    void appendPair(int i) {
+        byte coder = this.coder;
+        int count = this.count;
+        byte[] value = ensureCapacitySameCoder(this.value, coder, count + 2);
+        if (isLatin1(coder)) {
+            DecimalDigits.uncheckedPutPairLatin1(value, count, i);
+        } else {
+            DecimalDigits.uncheckedPutPairUTF16(value, count, i);
+        }
+        this.value = value;
+        this.count = count + 2;
+    }
+
     /**
      * Appends the string representation of the {@code int}
      * argument to this sequence.

src/java.base/share/classes/java/lang/System.java

@@ -2149,6 +2149,10 @@ public byte[] getBytesUTF8NoRepl(String s) {
                 return String.getBytesUTF8NoRepl(s);
             }
 
+            public void appendPair(StringBuilder buf, int v) {
+                buf.appendPair(v);
+            }
+
             public void inflateBytesToChars(byte[] src, int srcOff, char[] dst, int dstOff, int len) {
                 StringLatin1.inflate(src, srcOff, dst, dstOff, len);
             }

src/java.base/share/classes/jdk/internal/access/JavaLangAccess.java

@@ -386,6 +386,22 @@ public interface JavaLangAccess {
      */
     void uncheckedPutCharUTF16(byte[] bytes, int index, int ch);
 
+    /**
+     * Appends the two-digit string representation of the {@code int}
+     * argument to the given {@code StringBuilder}.
+     * <p>
+     * This method is intended for internal use by {@code DecimalDigits.appendPair}.
+     * The integer {@code v} is formatted as two decimal digits.
+     * If the value is between 0 and 9, it is formatted with a leading zero
+     * (e.g., 5 becomes "05"). If the value is outside the range 0-99,
+     * the behavior is unspecified.
+     *
+     * @param buf the {@code StringBuilder} to append to.
+     * @param v the {@code int} value (should be between 0 and 99 inclusive).
+     * @see jdk.internal.util.DecimalDigits#appendPair(StringBuilder, int)
+     */
+    void appendPair(StringBuilder buf, int v);
+
     /**
      * Encode the given string into a sequence of bytes using utf8.
      *

src/java.base/share/classes/jdk/internal/util/DateTimeHelper.java

@@ -49,39 +49,40 @@ public static void formatTo(StringBuilder buf, LocalDateTime dateTime) {
      * Requires extra capacity of 10 to avoid StringBuilder reallocation.
      */
     public static void formatTo(StringBuilder buf, LocalDate date) {
-        int year  = date.getYear(),
-            month = date.getMonthValue(),
-            day   = date.getDayOfMonth();
-        int absYear = Math.abs(year);
+        int year    = date.getYear(),
+            absYear = Math.abs(year);
         if (absYear < 1000) {
             if (year < 0) {
                 buf.append('-');
             }
-            buf.repeat('0', absYear < 10 ? 3 : absYear < 100 ? 2 : 1);
-            buf.append(absYear);
+            int y01 = absYear / 100;
+            DecimalDigits.appendPair(buf, y01);
+            DecimalDigits.appendPair(buf, absYear - y01 * 100);
         } else {
             if (year > 9999) {
                 buf.append('+');
             }
             buf.append(year);
         }
-        buf.append(month < 10 ? "-0" : "-").append(month)
-           .append(day < 10 ? "-0" : "-").append(day);
+        buf.append('-');
+        DecimalDigits.appendPair(buf, date.getMonthValue());
+        buf.append('-');
+        DecimalDigits.appendPair(buf, date.getDayOfMonth());
     }
 
     /**
      * Prints the toString result to the given buf, avoiding extra string allocations.
      * Requires extra capacity of 18 to avoid StringBuilder reallocation.
      */
     public static void formatTo(StringBuilder buf, LocalTime time) {
-        int hour   = time.getHour(),
-            minute = time.getMinute(),
-            second = time.getSecond(),
+        DecimalDigits.appendPair(buf, time.getHour());
+        buf.append(':');
+        DecimalDigits.appendPair(buf, time.getMinute());
+        int second = time.getSecond(),
             nano   = time.getNano();
-        buf.append(hour < 10 ? "0" : "").append(hour)
-           .append(minute < 10 ? ":0" : ":").append(minute);
         if ((second | nano) > 0) {
-            buf.append(second < 10 ? ":0" : ":").append(second);
+            buf.append(':');
+            DecimalDigits.appendPair(buf, second);
             if (nano > 0) {
                 buf.append('.');
                 int zeros = 9 - DecimalDigits.stringSize(nano);

src/java.base/share/classes/jdk/internal/util/DecimalDigits.java

@@ -25,6 +25,7 @@
 
 package jdk.internal.util;
 
+import jdk.internal.access.SharedSecrets;
 import jdk.internal.misc.Unsafe;
 import jdk.internal.vm.annotation.Stable;
 
@@ -443,4 +444,22 @@ private static void uncheckedPutCharUTF16(byte[] buf, int charPos, int c) {
         assert charPos >= 0 && charPos < (buf.length >> 1);
         UNSAFE.putCharUnaligned(buf, ARRAY_BYTE_BASE_OFFSET + ((long) charPos << 1), (char) c);
     }
+
+    /**
+     * Appends the two-digit string representation of the {@code int}
+     * argument to the given {@code StringBuilder}.
+     * <p>
+     * The integer {@code v} is formatted as two decimal digits.
+     * If the value is between 0 and 9, it is formatted with a leading zero
+     * (e.g., 5 becomes "05"). If the value is outside the range 0-99,
+     * the behavior is unspecified.
+     *
+     * @param buf the {@code StringBuilder} to append to.
+     * @param v the {@code int} value (should be between 0 and 99 inclusive).
+     * @see jdk.internal.access.JavaLangAccess#appendPair(StringBuilder, int)
+     */
+    public static void appendPair(StringBuilder buf, int v) {
+        SharedSecrets.getJavaLangAccess()
+                     .appendPair(buf, v);
+    }
 }