[feature] Add support for Count Expression

Author: gabriele-tomassetti

exist-core/src/main/antlr/org/exist/xquery/parser/XQuery.g

@@ -802,14 +802,21 @@ initialClause throws XPathException
 
 intermediateClause throws XPathException
 :
-    ( initialClause | whereClause | groupByClause | orderByClause )
+    ( initialClause | whereClause | groupByClause | orderByClause | countClause )
     ;
 
 whereClause throws XPathException
 :
 	"where"^ exprSingle
 	;
 
+countClause throws XPathException
+{ String varName; }
+:
+	"count"^ DOLLAR! varName=varName!
+	{ #countClause = #(#countClause, #[VARIABLE_BINDING, varName]); }
+	;
+
 forClause throws XPathException
 :
 	"for"^ inVarBinding ( COMMA! inVarBinding )*
@@ -2223,6 +2230,8 @@ reservedKeywords returns [String name]
 	|
 	"array" { name = "array"; }
 	|
+	"count" { name = "count"; }
+	|
 	"copy-namespaces" { name = "copy-namespaces"; }
 	|
 	"empty-sequence" { name = "empty-sequence"; }

exist-core/src/main/antlr/org/exist/xquery/parser/XQueryTree.g

@@ -1752,9 +1752,22 @@ throws PermissionDeniedException, EXistException, XPathException
                     clauses.add(clause);
                 }
             )
-        )+
-        step=expr [(PathExpr) action]
-        {
+            |
+            #(
+            	co:"count"
+            	countVarName:VARIABLE_BINDING
+                {
+                    ForLetClause clause = new ForLetClause();
+                    clause.ast = co;
+                    clause.varName = countVarName.getText();
+                    clause.type = FLWORClause.ClauseType.COUNT;
+                    clause.inputSequence = null;
+                    clauses.add(clause);
+                }
+            )
+		)+
+		step=expr [(PathExpr) action]
+		{
             for (int i= clauses.size() - 1; i >= 0; i--) {
                 ForLetClause clause= (ForLetClause) clauses.get(i);
                 FLWORClause expr;

exist-core/src/main/java/org/exist/xquery/FLWORClause.java

@@ -31,7 +31,7 @@
 public interface FLWORClause extends Expression {
 
     enum ClauseType {
-        FOR, LET, GROUPBY, ORDERBY, WHERE, SOME, EVERY
+        FOR, LET, GROUPBY, ORDERBY, WHERE, SOME, EVERY, COUNT
     }
 
     /**