Add fold filter optimizations

Author: antonmedv

bench_test.go

@@ -66,21 +66,36 @@ func Benchmark_expr_reuseVm(b *testing.B) {
 }
 
 func Benchmark_filter(b *testing.B) {
-	params := make(map[string]interface{})
-	params["max"] = 50
+	type Item struct {
+		Value int
+	}
+	type Env struct {
+		Items []Item
+	}
 
-	program, err := expr.Compile(`filter(1..100, {# > max})`, expr.Env(params))
+	program, err := expr.Compile(`filter(filter(Items, {.Value % 2 == 0}), {.Value % 3 == 0})`, expr.Env(Env{}))
 	if err != nil {
 		b.Fatal(err)
 	}
 
+	env := Env{
+		Items: make([]Item, 100),
+	}
+	for i := 1; i <= 100; i++ {
+		env.Items[i-1] = Item{Value: i}
+	}
+
+	var out interface{}
 	for n := 0; n < b.N; n++ {
-		_, err = vm.Run(program, params)
+		out, err = vm.Run(program, env)
 	}
 
 	if err != nil {
 		b.Fatal(err)
 	}
+	if len(out.([]interface{})) != 16 {
+		b.Fail()
+	}
 }
 
 func Benchmark_access(b *testing.B) {

optimizer/fold.go

@@ -129,5 +129,26 @@ func (fold *fold) Exit(node *Node) {
 			}
 
 		}
+
+	case *BuiltinNode:
+		switch n.Name {
+		case "filter":
+			if len(n.Arguments) != 2 {
+				return
+			}
+			if base, ok := n.Arguments[0].(*BuiltinNode); ok && base.Name == "filter" {
+				patch(&BuiltinNode{
+					Name: "filter",
+					Arguments: []Node{
+						base.Arguments[0],
+						&BinaryNode{
+							Operator: "&&",
+							Left:     base.Arguments[1],
+							Right:    n.Arguments[1],
+						},
+					},
+				})
+			}
+		}
 	}
 }