added support empty curly block

Author: raydac

src/main/java/com/igormaznitsa/prologparser/PrologParser.java

@@ -475,15 +475,22 @@ private PrologTerm readBlock(final Koi7CharOpMap endOperators) {
                   }
 
                   if (processReadAtom) {
+                    boolean emptyCurly = false;
                     if (readAtom == null) {
+                      if (onlyCharCode == '{') {
+                        emptyCurly = true;
+                      } else
                       throw new PrologParserException("Illegal start of term",
                           readAtomContainer.getLine(), readAtomContainer.getPos());
                     }
 
-                    readAtom.setLine(readAtomContainer.getLine());
-                    readAtom.setPos(readAtomContainer.getPos());
-
-                    readAtom = new PrologStruct(onlyCharCode == '{' ? Op.VIRTUAL_OPERATOR_CURLY_BLOCK : Op.VIRTUAL_OPERATOR_BLOCK, new PrologTerm[] {readAtom}, readAtomContainer.getLine(), readAtomContainer.getPos());
+                    if (emptyCurly) {
+                      readAtom = new PrologStruct(Op.VIRTUAL_OPERATOR_CURLY_BLOCK, EMPTY_TERM_ARRAY, readAtomContainer.getLine(), readAtomContainer.getPos());
+                    }else {
+                      readAtom.setLine(readAtomContainer.getLine());
+                      readAtom.setPos(readAtomContainer.getPos());
+                      readAtom = new PrologStruct(onlyCharCode == '{' ? Op.VIRTUAL_OPERATOR_CURLY_BLOCK : Op.VIRTUAL_OPERATOR_BLOCK, new PrologTerm[] {readAtom}, readAtomContainer.getLine(), readAtomContainer.getPos());
+                    }
 
                     final TokenizerResult token = this.tokenizer.readNextToken();
 

src/main/java/com/igormaznitsa/prologparser/terms/PrologStruct.java

@@ -187,9 +187,17 @@ public String toString() {
     if (this.functor.getTermType() == TermType.OPERATOR) {
       if (this.isBlock()) {
         if (this.isCurlyBlock()) {
-          builder.append('{').append(this.elements[0].toString()).append('}');
+          if (this.isEmpty()) {
+            builder.append("{}");
+          } else {
+            builder.append('{').append(this.elements[0].toString()).append('}');
+          }
         } else {
-          builder.append('(').append(this.elements[0].toString()).append(')');
+          if (this.isEmpty()) {
+            builder.append("()");
+          } else {
+            builder.append('(').append(this.elements[0].toString()).append(')');
+          }
         }
       } else {
         final Op operatorFunctor = (Op) functor;
@@ -321,6 +329,10 @@ public String toString() {
     return builder.toString();
   }
 
+  public boolean isEmpty() {
+    return this.elements.length == 0;
+  }
+
   @Override
   public Stream<PrologTerm> stream() {
     return StreamSupport.stream(

src/test/java/com/igormaznitsa/prologparser/IntegrationTest.java

@@ -588,6 +588,9 @@ private void assertReadSictusTerms(final int expected, final String resource, fi
   public void testCurlyBracket() {
     assertEquals("{1 , 2 , 3 , 4 , 5}", new GenericPrologParser(new StringReader("{1,2,3,4,5}."), of(FLAG_CURLY_BRACKETS)).next().toString());
     assertEquals("{1 , {2 , {3 , {4} , 5}}}", new GenericPrologParser(new StringReader("{1,{2,{3,{4},5}}}."), of(FLAG_CURLY_BRACKETS)).next().toString());
+    assertEquals("[1, {2 , 3}|X]", new GenericPrologParser(new StringReader("[1,{2,3}|X]."), of(FLAG_CURLY_BRACKETS)).next().toString());
+    assertEquals("{}", new GenericPrologParser(new StringReader("{}."), of(FLAG_CURLY_BRACKETS)).next().toString());
+
     assertThrows(PrologParserException.class, () -> new GenericPrologParser(new StringReader("test{1,2,3,4,5}."), of(FLAG_CURLY_BRACKETS)).next());
   }