classlib: fix String.replace, add optimizations for some corner cases

Fix #932

classlib/src/main/java/org/teavm/classlib/java/lang/TString.java

@@ -501,22 +501,37 @@ public class TString extends TObject implements TSerializable, TComparable<TStri
     }
 
     public String replace(TCharSequence target, TCharSequence replacement) {
-        var sb = new StringBuilder();
+        if (target == replacement) {
-        int sz = length() - target.length();
+            return (String) (Object) this;
-        int i = 0;
+        } else if (target.isEmpty()) {
-        outer:
+            var sb = new StringBuilder();
-        for (; i <= sz; ++i) {
+            for (var i = 0; i < length(); ++i) {
-            for (int j = 0; j < target.length(); ++j) {
+                sb.append(replacement);
-                if (charAt(i + j) != target.charAt(j)) {
+                sb.append(charAt(i));
-                    sb.append(charAt(i));
-                    continue outer;
-                }
             }
             sb.append(replacement);
-            i += target.length() - 1;
+            return sb.toString();
+        } else if (target.length() == 1 && replacement.length() == 1) {
+            return (String) (Object) replace(target.charAt(0), replacement.charAt(0));
+        } else {
+            var sb = new StringBuilder();
+            int sz = length() - target.length();
+            int i = 0;
+            outer:
+            for (; i <= sz; ++i) {
+                for (int j = 0; j < target.length(); ++j) {
+                    if (charAt(i + j) != target.charAt(j)) {
+                        sb.append(charAt(i));
+                        continue outer;
+                    }
+                }
+                sb.append(replacement);
+                i += target.length() - 1;
+            }
+            sb.append(substring(i));
+
+            return sb.toString();
         }
-        sb.append(substring(i));
-        return sb.toString();
     }
 
     public TString trim() {

tests/src/test/java/org/teavm/classlib/java/lang/StringTest.java

@@ -197,6 +197,9 @@ public class StringTest {
     @Test
     public void sequenceReplaced() {
         assertEquals("ba", "aaa".replace("aa", "b"));
+        assertEquals("xaxaxax", "aaa".replace("", "x"));
+        assertEquals("axc", "abc".replace("b", "x"));
+        assertEquals("abc", "abc".replace("bc", "bc"));
     }
 
     @Test