JDK-8302323 Add repeat methods to StringBuilder/StringBuffer

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

@@ -1821,4 +1821,115 @@ private final void appendChars(CharSequence s, int off, int end) {
         }
         count += end - off;
     }
+
+    /**
+     * Appends {@code count} copies of the character {@code c} to this sequence.
+     * <p>
+     * The length of this sequence increases by {@code count}.
+     *
+     * @param c      character to append
+     * @param count  number of times to copy
+     *
+     * @return  a reference to this object.
+     *
+     * @since 21
+     * @throws IllegalArgumentException  if {@code count} is less than zero
+     * @throws StringIndexOutOfBoundsException  if the result overflows the buffer
+     */
+    public AbstractStringBuilder repeat(char c, int count) {
+        if (count < 0) {
+            throw new IllegalArgumentException("count is less than zero: " + count);
+        }
+        if (count == 0) {
+            return this;
+        }
+        ensureCapacityInternal(this.count + count);
+        if (isLatin1() && StringLatin1.canEncode(c)) {
+            int index = this.count;
+            while (count-- != 0) {
+                value[index++] = (byte) c;
+            }
+            this.count = index;
+        } else {
+            if (isLatin1()) {
+                inflate();
+            }
+            int index = this.count;
+            while (count-- != 0) {
+                StringUTF16.putCharSB(value, index++, c);
+            }
+            this.count = index;
+        }
+        return this;
+    }
+
+    private AbstractStringBuilder repeatNull(int count) {
+        if (count < 0) {
+            throw new IllegalArgumentException("count is less than zero: " + count);
+        } else if (count == 0) {
+            return this;
+        }
+        int offset = this.count;
+        appendNull();
+        int length = this.count - offset;
+        int valueLength = length << coder;
+        if ((Integer.MAX_VALUE - offset) / count < valueLength) {
+            throw new OutOfMemoryError("Required length exceeds implementation limit");
+        }
+        int limit = count * length;
+        ensureCapacityInternal(offset + limit);
+        String.repeatCopyRest(value, offset << coder, limit << coder, length << coder);
+        this.count = offset + limit;
+        return this;
+    }
+
+    /**
+     * Appends {@code count} copies of the specified {@code CharSequence} {@code cs}
+     * to this sequence.
+     * <p>
+     * The length of this sequence increases by {@code count} times the
+     * {@code CharSequence} length.
+     *
+     * @param cs     a {@code CharSequence}
+     * @param count  number of times to copy
+     *
+     * @return  a reference to this object.
+     *
+     * @since 21
+     * @throws IllegalArgumentException  if {@code count} is less than zero
+     * @throws StringIndexOutOfBoundsException  if the result overflows the buffer
+     */
+    public AbstractStringBuilder repeat(CharSequence cs, int count) {
+        if (cs == null) {
+            return repeatNull(count);
+        } else if (count < 0) {
+            throw new IllegalArgumentException("count is less than zero: " + count);
+        } else if (count == 0) {
+            return this;
+        } else if (count == 1) {
+            return append(cs);
+        }
+
+        int length = cs.length();
+        if (length == 1) {
+            return repeat(cs.charAt(0), count);
+        }
+        int valueLength = length << UTF16;
+        if ((Integer.MAX_VALUE - this.count) / count < valueLength) {
+            throw new OutOfMemoryError("Required length exceeds implementation limit");
+        }
+        int limit = count * length;
+        int offset = this.count;
+        ensureCapacityInternal(offset + limit);
+        if (cs instanceof String str) {
+            putStringAt(offset, str);
+        } else  if (cs instanceof AbstractStringBuilder asb) {
+            append(asb);
+        } else {
+            appendChars(cs, 0, length);
+        }
+        String.repeatCopyRest(value, offset << coder, limit << coder, length << coder);
+        this.count = offset + limit;
+        return this;
+    }
 }

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

@@ -4453,12 +4453,34 @@ public String repeat(int count) {
         final int limit = len * count;
         final byte[] multiple = new byte[limit];
         System.arraycopy(value, 0, multiple, 0, len);
-        int copied = len;
+        repeatCopyRest(multiple, 0, limit, len);
+        return new String(multiple, coder);
+    }
+
+    /**
+     * Used to perform copying after the initial insertion. Copying is optimized
+     * by using power of two duplication. First pass duplicates original copy,
+     * second pass then duplicates the original and the copy yielding four copies,
+     * third pass duplicates four copies yielding eight copies, and so on.
+     * Finally, the remainder is filled in with prior copies.
+     *
+     * @implNote The technique used here is significantly faster than hand-rolled
+     * loops or special casing small numbers due to the intensive optimization
+     * done by intrinsic {@code System.arraycopy}.
+     *
+     * @param buffer    destination buffer
+     * @param offset    offset in the destination buffer
+     * @param limit     total replicated including what is already in the buffer
+     * @param copied    number of bytes that have already in the buffer
+     */
+    static void repeatCopyRest(byte[] buffer, int offset, int limit, int copied) {
+        // Initial copy is in the buffer.
         for (; copied < limit - copied; copied <<= 1) {
-            System.arraycopy(multiple, 0, multiple, copied, copied);
+            // Power of two duplicate.
+            System.arraycopy(buffer, offset, buffer, offset + copied, copied);
         }
-        System.arraycopy(multiple, 0, multiple, copied, limit - copied);
-        return new String(multiple, coder);
+        // Duplicate remainder.
+        System.arraycopy(buffer, offset, buffer, offset + copied, limit - copied);
     }
 
     ////////////////////////////////////////////////////////////////

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

@@ -708,6 +708,28 @@ public synchronized StringBuffer reverse() {
         return this;
     }
 
+    /**
+     * @since 21
+     * @throws IllegalArgumentException {@inheritDoc}
+     * @throws StringIndexOutOfBoundsException {@inheritDoc}
+     */
+    @Override
+    public synchronized StringBuffer repeat(char c, int count) {
+        super.repeat(c, count);
+        return this;
+    }
+
+    /**
+     * @since 21
+     * @throws IllegalArgumentException {@inheritDoc}
+     * @throws StringIndexOutOfBoundsException {@inheritDoc}
+     */
+    @Override
+    public synchronized StringBuffer repeat(CharSequence cs, int count) {
+        super.repeat(cs, count);
+        return this;
+    }
+
     @Override
     @IntrinsicCandidate
     public synchronized String toString() {

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

@@ -446,6 +446,28 @@ public StringBuilder reverse() {
         return this;
     }
 
+    /**
+     * @since 21
+     * @throws IllegalArgumentException {@inheritDoc}
+     * @throws StringIndexOutOfBoundsException {@inheritDoc}
+     */
+    @Override
+    public StringBuilder repeat(char c, int count) {
+        super.repeat(c, count);
+        return this;
+    }
+
+    /**
+     * @since 21
+     * @throws IllegalArgumentException {@inheritDoc}
+     * @throws StringIndexOutOfBoundsException {@inheritDoc}
+     */
+    @Override
+    public StringBuilder repeat(CharSequence cs, int count) {
+        super.repeat(cs, count);
+        return this;
+    }
+
     @Override
     @IntrinsicCandidate
     public String toString() {

test/jdk/java/lang/StringBuilder/Repeat.java

@@ -0,0 +1,159 @@
+/*
+ * Copyright (c) 2023, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import org.testng.annotations.Test;
+
+import static org.testng.Assert.assertEquals;
+
+import java.util.Arrays;
+
+/**
+ * @test
+ * @bug 8302323
+ * @summary Test StringBuilder.repeat sanity tests
+ * @run testng/othervm -XX:-CompactStrings Repeat
+ * @run testng/othervm -XX:+CompactStrings Repeat
+ */
+@Test
+public class Repeat {
+    private static class MyChars implements CharSequence {
+        private static final char[] DATA = new char[] { 'a', 'b', 'c' };
+
+        @Override
+        public int length() {
+            return DATA.length;
+        }
+
+        @Override
+        public char charAt(int index) {
+            return DATA[index];
+        }
+
+        @Override
+        public CharSequence subSequence(int start, int end) {
+            return new String(Arrays.copyOfRange(DATA, start, end));
+        }
+    }
+
+    private static final MyChars MYCHARS = new MyChars();
+
+    public void sanity() {
+        StringBuilder sb = new StringBuilder();
+        // prime the StringBuilder
+        sb.append("repeat");
+
+        // single character Latin1
+        sb.repeat('1', 0);
+        sb.repeat('2', 1);
+        sb.repeat('3', 5);
+
+        // single string Latin1 (optimized)
+        sb.repeat("1", 0);
+        sb.repeat("2", 1);
+        sb.repeat("3", 5);
+
+        // multi string Latin1
+        sb.repeat("-1", 0);
+        sb.repeat("-2", 1);
+        sb.repeat("-3", 5);
+
+        // single character UTF16
+        sb.repeat('\u2460', 0);
+        sb.repeat('\u2461', 1);
+        sb.repeat('\u2462', 5);
+
+        // single string UTF16 (optimized)
+        sb.repeat("\u2460", 0);
+        sb.repeat("\u2461", 1);
+        sb.repeat("\u2462", 5);
+
+        // multi string UTF16
+
+        sb.repeat("-\u2460", 0);
+        sb.repeat("-\u2461", 1);
+        sb.repeat("-\u2462", 5);
+
+        // CharSequence
+        sb.repeat(MYCHARS, 3);
+
+        // null
+        sb.repeat((String)null, 0);
+        sb.repeat((String)null, 1);
+        sb.repeat((String)null, 5);
+        sb.repeat((CharSequence)null, 0);
+        sb.repeat((CharSequence)null, 1);
+        sb.repeat((CharSequence)null, 5);
+
+
+        String expected = "repeat233333233333-2-3-3-3-3-3\u2461\u2462\u2462\u2462\u2462\u2462\u2461\u2462\u2462\u2462\u2462\u2462-\u2461-\u2462-\u2462-\u2462-\u2462-\u2462abcabcabc" +
+                          "nullnullnullnullnullnullnullnullnullnullnullnull";
+        assertEquals(expected, sb.toString());
+    }
+
+    public void exceptions() {
+        StringBuilder sb = new StringBuilder();
+
+        try {
+            sb.repeat(' ', Integer.MAX_VALUE);
+            throw new RuntimeException("No OutOfMemoryError thrown");
+        } catch (OutOfMemoryError | StringIndexOutOfBoundsException ex) {
+            // Okay
+        }
+
+        try {
+            sb.repeat("    ", Integer.MAX_VALUE);
+            throw new RuntimeException("No OutOfMemoryError thrown");
+        } catch (OutOfMemoryError | StringIndexOutOfBoundsException ex) {
+            // Okay
+        }
+
+        try {
+            sb.repeat(MYCHARS, Integer.MAX_VALUE);
+            throw new RuntimeException("No OutOfMemoryError thrown");
+        } catch (OutOfMemoryError | StringIndexOutOfBoundsException ex) {
+            // Okay
+        }
+
+        try {
+            sb.repeat(' ', -1);
+            throw new RuntimeException("No IllegalArgumentException thrown");
+        } catch (IllegalArgumentException | StringIndexOutOfBoundsException ex) {
+            // Okay
+        }
+
+        try {
+            sb.repeat("abc", -1);
+            throw new RuntimeException("No IllegalArgumentException thrown");
+        } catch (IllegalArgumentException | StringIndexOutOfBoundsException ex) {
+            // Okay
+        }
+
+        try {
+            sb.repeat(MYCHARS, -1);
+            throw new RuntimeException("No IllegalArgumentException thrown");
+        } catch (IllegalArgumentException | StringIndexOutOfBoundsException ex) {
+            // Okay
+        }
+
+    }
+}