apacheGH-2218: Implement LATERAL as per SEP-0006/SEP-0007

jena-arq/src/main/java/org/apache/jena/sparql/core/Substitute.java

@@ -20,16 +20,19 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Set;
 
 import org.apache.jena.graph.Node;
 import org.apache.jena.graph.NodeFactory;
 import org.apache.jena.graph.Triple;
 import org.apache.jena.sparql.algebra.Op;
+import org.apache.jena.sparql.algebra.Transform;
 import org.apache.jena.sparql.algebra.TransformCopy;
 import org.apache.jena.sparql.algebra.Transformer;
 import org.apache.jena.sparql.algebra.op.*;
 import org.apache.jena.sparql.engine.binding.Binding;
 import org.apache.jena.sparql.engine.binding.BindingFactory;
+import org.apache.jena.sparql.engine.iterator.QueryIterLateral;
 import org.apache.jena.sparql.expr.Expr;
 import org.apache.jena.sparql.expr.ExprList;
 import org.apache.jena.sparql.pfunction.PropFuncArg;
@@ -40,12 +43,42 @@
  * Substitution in SPARQL algebra.
  * <p>
  * See also {@link QueryTransformOps} and {@link UpdateTransformOps} which operate on SPARQL syntax.
+ * <p>
+ * {@link #inject} provides the substitution, while leaving a variable present, used by LATERAL.
  */
 public class Substitute {
+    /**
+     * Inject takes an {@link Op} to transform using a {Binding binding}. The
+     * transformation assumes the Ope structure is legal for the operation. The
+     * transformation is to wrap each place a variable is used (BGP, GRAPH, Path and
+     * some equivalent operations) with a {@code BIND} to restrict the vartibale to a specific value
+     * while still retaining the variable (e.g for FILETERs).
+     * <p>
+     * <pre>
+     *   (bgp
+     *     (?s :p 123)
+     *     (?s :q ?a)
+     *   )
+     * </pre>
+     * with binding {@code ?s = :x } becomes
+     * <pre>
+     * (assign (?s :x)
+     *    (bgp
+     *      (:x :p 123)
+     *      (:x :q ?a)
+     *    ))
+     * </pre>
+     */
+    public static Op inject(Op opInput, Binding binding) {
+        Set<Var> injectVars = binding.varsMentioned();
+        Transform transform = new QueryIterLateral.TransformInject(injectVars, binding::get);
+        Op opOutput = Transformer.transform(transform, opInput);
+        return opOutput;
+    }
+
     public static Op substitute(Op op, Binding binding) {
         // Want to avoid cost if the binding is empty
         // but the empty test is not zero-cost on non-empty things.
-
         if ( isNotNeeded(binding) )
             return op;
         return Transformer.transform(new OpSubstituteWorker(binding), op);

jena-arq/src/main/java/org/apache/jena/sparql/core/VarExprList.java

@@ -35,10 +35,10 @@ public class VarExprList {
     private List<Var> vars;
     private LinkedHashMap<Var, Expr> exprs;   // Preserve order.
 
-    public VarExprList(List<Var> vars) {
-        this.vars = new ArrayList<>(vars);
-        this.exprs = new LinkedHashMap<>();
-    }
+//    public VarExprList(List<Var> vars) {
+//        this.vars = new ArrayList<>(vars);
+//        this.exprs = new LinkedHashMap<>();
+//    }
 
     public VarExprList(VarExprList other) {
         this.vars = new ArrayList<>(other.vars);

jena-arq/src/main/java/org/apache/jena/sparql/engine/binding/Binding.java

@@ -19,6 +19,7 @@
 package org.apache.jena.sparql.engine.binding;
 
 import java.util.Iterator;
+import java.util.Set;
 import java.util.function.BiConsumer;
 
 import org.apache.jena.graph.Node;
@@ -56,6 +57,9 @@ public static BindingBuilder builder(Binding parent) {
     /** Iterate over all variables of this binding. */
     public Iterator<Var> vars();
 
+    /** Iterate over all variables of this binding. */
+    public Set<Var> varsMentioned();
+
     /** Operate on each entry. */
     public void forEach(BiConsumer<Var, Node> action);
 

jena-arq/src/main/java/org/apache/jena/sparql/engine/binding/BindingBase.java

@@ -19,6 +19,8 @@
 package org.apache.jena.sparql.engine.binding;
 
 import java.util.Iterator;
+import java.util.LinkedHashSet;
+import java.util.Set;
 import java.util.function.BiConsumer;
 
 import org.apache.jena.atlas.iterator.IteratorConcat;
@@ -45,14 +47,20 @@ protected BindingBase(Binding _parent) {
 //    public Binding getParent() { return parent; }
 
     @Override
-    final public Iterator<Var> vars()
-    {
+    final public Iterator<Var> vars() {
         Iterator<Var> iter = vars1();
         if ( parent != null )
             iter = IteratorConcat.concat(parent.vars(), iter);
         return iter;
     }
 
+    @Override
+    final public Set<Var> varsMentioned() {
+        Set<Var> result = new LinkedHashSet<>();
+        vars().forEachRemaining(result::add);
+        return result;
+    }
+
     /** Operate on each entry. */
     @Override
     public void forEach(BiConsumer<Var, Node> action) {