Simplify Unicode surrogate pair conversion for generation

src/main/java/com/fasterxml/jackson/core/base/GeneratorBase.java

@@ -6,6 +6,7 @@
 
 import com.fasterxml.jackson.core.*;
 import com.fasterxml.jackson.core.io.IOContext;
+import com.fasterxml.jackson.core.io.UTF8Writer;
 import com.fasterxml.jackson.core.json.DupDetector;
 import com.fasterxml.jackson.core.json.JsonWriteContext;
 import com.fasterxml.jackson.core.json.PackageVersion;
@@ -533,8 +534,7 @@ protected final int _decodeSurrogate(int surr1, int surr2) throws IOException
 "Incomplete surrogate pair: first char 0x%04X, second 0x%04X", surr1, surr2);
             _reportError(msg);
         }
-        int c = 0x10000 + ((surr1 - SURR1_FIRST) << 10) + (surr2 - SURR2_FIRST);
-        return c;
+        return (surr1 << 10) + surr2 + UTF8Writer.SURROGATE_BASE;
     }
 
     /*

src/main/java/com/fasterxml/jackson/core/io/JsonStringEncoder.java

@@ -659,7 +659,7 @@ private static int _convert(int p1, int p2) {
         if (p2 < SURR2_FIRST || p2 > SURR2_LAST) {
             throw new IllegalArgumentException("Broken surrogate pair: first char 0x"+Integer.toHexString(p1)+", second 0x"+Integer.toHexString(p2)+"; illegal combination");
         }
-        return 0x10000 + ((p1 - SURR1_FIRST) << 10) + (p2 - SURR2_FIRST);
+        return (p1 << 10) + p2 + UTF8Writer.SURROGATE_BASE;
     }
 
     private static void _illegal(int c) {

src/main/java/com/fasterxml/jackson/core/io/UTF8Writer.java

@@ -9,6 +9,8 @@ public final class UTF8Writer extends Writer
     final static int SURR2_FIRST = 0xDC00;
     final static int SURR2_LAST = 0xDFFF;
 
+    public static final int SURROGATE_BASE = 0x10000 - UTF8Writer.SURR2_FIRST - (UTF8Writer.SURR1_FIRST << 10);
+
     final private IOContext _context;
 
     private OutputStream _out;
@@ -369,7 +371,7 @@ protected int convertSurrogate(int secondPart)
         if (secondPart < SURR2_FIRST || secondPart > SURR2_LAST) {
             throw new IOException("Broken surrogate pair: first char 0x"+Integer.toHexString(firstPart)+", second 0x"+Integer.toHexString(secondPart)+"; illegal combination");
         }
-        return 0x10000 + ((firstPart - SURR1_FIRST) << 10) + (secondPart - SURR2_FIRST);
+        return (firstPart << 10) + secondPart + UTF8Writer.SURROGATE_BASE;
     }
 
     protected static void illegalSurrogate(int code) throws IOException {

src/test/java/com/fasterxml/jackson/core/io/UTF8WriterTest.java

@@ -3,6 +3,7 @@
 import java.io.*;
 
 import org.junit.Assert;
+import org.junit.Ignore;
 
 public class UTF8WriterTest
     extends com.fasterxml.jackson.core.BaseTest
@@ -136,6 +137,17 @@ public void testSurrogatesFail() throws Exception
         }
     }
 
+    @Ignore
+    public void testSurrogateConversion() {
+        for (int first = UTF8Writer.SURR1_FIRST; first <= UTF8Writer.SURR1_LAST; first++) {
+            for (int second = UTF8Writer.SURR2_FIRST; second <= UTF8Writer.SURR2_LAST; second++) {
+                int expected = 0x10000 + ((first - UTF8Writer.SURR1_FIRST) << 10) + (second - UTF8Writer.SURR2_FIRST);
+                int actual = (first << 10) + second + UTF8Writer.SURROGATE_BASE;
+                assertEquals(Integer.toHexString(first) + " " + Integer.toHexString(second), expected, actual);
+            }
+        }
+    }
+
     private IOContext _ioContext() {
         return testIOContext();
     }