6840246: Lightweight implementation of String.split for simple use case

Summary: Added a fastpath for simple use case
Reviewed-by: alanb, martin

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

@@ -2301,6 +2301,54 @@ public final class String
      * @spec JSR-51
      */
     public String[] split(String regex, int limit) {
+        /* fastpath if the regex is a
+           (1)one-char String and this character is not one of the
+              RegEx's meta characters ".$|()[{^?*+\\", or
+           (2)two-char String and the first char is the backslash and
+              the second is not the ascii digit or ascii letter.
+        */
+        char ch = 0;
+        if (((regex.count == 1 &&
+             ".$|()[{^?*+\\".indexOf(ch = regex.charAt(0)) == -1) ||
+             (regex.length() == 2 &&
+              regex.charAt(0) == '\\' &&
+              (((ch = regex.charAt(1))-'0')|('9'-ch)) < 0 &&
+              ((ch-'a')|('z'-ch)) < 0 &&
+              ((ch-'A')|('Z'-ch)) < 0)) &&
+            (ch < Character.MIN_HIGH_SURROGATE ||
+             ch > Character.MAX_LOW_SURROGATE))
+        {
+            int off = 0;
+            int next = 0;
+            boolean limited = limit > 0;
+            ArrayList<String> list = new ArrayList<String>();
+            while ((next = indexOf(ch, off)) != -1) {
+                if (!limited || list.size() < limit - 1) {
+                    list.add(substring(off, next));
+                    off = next + 1;
+                } else {    // last one
+                    //assert (list.size() == limit - 1);
+                    list.add(substring(off, count));
+                    off = count;
+                    break;
+                }
+            }
+            // If no match was found, return this
+            if (off == 0)
+                return new String[] { this };
+
+            // Add remaining segment
+            if (!limited || list.size() < limit)
+                list.add(substring(off, count));
+
+            // Construct result
+            int resultSize = list.size();
+            if (limit == 0)
+                while (resultSize > 0 && list.get(resultSize-1).length() == 0)
+                    resultSize--;
+            String[] result = new String[resultSize];
+            return list.subList(0, resultSize).toArray(result);
+        }
         return Pattern.compile(regex).split(this, limit);
     }
 

test/java/lang/String/Split.java

@@ -23,14 +23,18 @@
 
 /**
  * @test
+ * @bug 6840246
  * @summary test String.split()
  */
+import java.util.Arrays;
+import java.util.Random;
 import java.util.regex.*;
 
 public class Split {
 
     public static void main(String[] args) throws Exception {
         String source = "0123456789";
+
         for (int limit=-2; limit<3; limit++) {
             for (int x=0; x<10; x++) {
                 String[] result = source.split(Integer.toString(x), limit);
@@ -80,5 +84,48 @@ public class Split {
             throw new RuntimeException("String.split failure 8");
         if (!result[0].equals(source))
             throw new RuntimeException("String.split failure 9");
+
+        // check fastpath of String.split()
+        source = "0123456789abcdefgABCDEFG";
+        Random r = new Random();
+
+        for (boolean doEscape: new boolean[] {false, true}) {
+            for (int cp = 0; cp < 0x11000; cp++) {
+                Pattern p = null;
+                String regex = new String(Character.toChars(cp));
+                if (doEscape)
+                    regex = "\\" + regex;
+                try {
+                    p = Pattern.compile(regex);
+                } catch (PatternSyntaxException pse) {
+                    // illegal syntax
+                    try {
+                        "abc".split(regex);
+                    } catch (PatternSyntaxException pse0) {
+                        continue;
+                    }
+                    throw new RuntimeException("String.split failure 11");
+                }
+                int off = r.nextInt(source.length());
+                String[] srcStrs = new String[] {
+                    "",
+                    source,
+                    regex + source,
+                    source + regex,
+                    source.substring(0, 3)
+                        + regex + source.substring(3, 9)
+                        + regex + source.substring(9, 15)
+                        + regex + source.substring(15),
+                    source.substring(0, off) + regex + source.substring(off)
+                };
+                for (String src: srcStrs) {
+                    for (int limit=-2; limit<3; limit++) {
+                        if (!Arrays.equals(src.split(regex, limit),
+                                           p.split(src, limit)))
+                            throw new RuntimeException("String.split failure 12");
+                    }
+                }
+            }
+        }
     }
 }