Java: Use StringBuildler in decodeURI and guard against H(null)L pattern

java/src/name/fraser/neil/plaintext/diff_match_patch.java

@@ -1507,7 +1507,7 @@ private int digit16(char b) throws IllegalArgumentException {
 
   private String decodeURI(String text) throws IllegalArgumentException {
     int i = 0;
-    StringBuffer decoded = new StringBuffer("");
+    StringBuilder decoded = new StringBuilder(text.length());
 
     while (i < text.length()) {
       if (text.charAt(i) != '%') {
@@ -1576,7 +1576,16 @@ private String decodeURI(String text) throws IllegalArgumentException {
       throw new IllegalArgumentException();
     }
 
-    return decoded.toString();
+    // some objective-c versions of the library produced patches with
+    // (null) in the place where surrogates were split across diff
+    // boundaries. if we leave those in we'll be stuck with a
+    // high-surrogate (null) low-surrogate pattern that will break
+    // deeper in the library or consuming application. we'll "fix"
+    // these by dropping the (null) and re-joining the surrogate halves
+    return decoded.toString().replaceAll(
+      "([\\uD800-\\uDBFF])\\(null\\)([\\uDC00-\\uDFFF])",
+      "$1$2"
+    );
   }
 
   /**

java/tests/name/fraser/neil/plaintext/diff_match_patch_test.java

@@ -460,6 +460,12 @@ public static void testDiffDelta() {
       dmp.diff_toDelta(dmp.diff_fromDelta("\ud83c\udd70", "=1\t-1\t+%ED%B5%B1"))
     );
 
+    assertEquals(
+      "diff_fromDelta: Invalid diff from objective-c with (null) string",
+      diffList(new Diff(INSERT, "\ud83c\udd70")),
+      dmp.diff_fromDelta("", "+%ED%A0%BC%28null%29%ED%B5%B0")
+    );
+
     // Verify pool of unchanged characters.
     diffs = diffList(new Diff(INSERT, "A-Z a-z 0-9 - _ . ! ~ * ' ( ) ; / ? : @ & = + $ , # "));
     String text2 = dmp.diff_text2(diffs);