Add filter operation (#1588)

* add filter operation

* add tests

* add tests

* revert debug

* simplify filter

* fix tests

* remove logs

pkg/yqlib/doc/operators/filter.md

@@ -0,0 +1,18 @@
+
+## Filter array
+Given a sample.yml file of:
+```yaml
+- 1
+- 2
+- 3
+```
+then
+```bash
+yq 'filter(. < 3)' sample.yml
+```
+will output
+```yaml
+- 1
+- 2
+```
+

pkg/yqlib/lexer_participle.go

@@ -37,6 +37,7 @@ var participleYqRules = []*participleYqRule{
 
 	{"MapValues", `map_?values`, opToken(mapValuesOpType), 0},
 	simpleOp("map", mapOpType),
+	simpleOp("filter", filterOpType),
 	simpleOp("pick", pickOpType),
 
 	{"FlattenWithDepth", `flatten\([0-9]+\)`, flattenWithDepth(), 0},

pkg/yqlib/lib.go

@@ -84,6 +84,7 @@ var expressionOpType = &operationType{Type: "EXP", NumArgs: 0, Precedence: 50, H
 
 var collectOpType = &operationType{Type: "COLLECT", NumArgs: 1, Precedence: 50, Handler: collectOperator}
 var mapOpType = &operationType{Type: "MAP", NumArgs: 1, Precedence: 50, Handler: mapOperator}
+var filterOpType = &operationType{Type: "FILTER", NumArgs: 1, Precedence: 50, Handler: filterOperator}
 var errorOpType = &operationType{Type: "ERROR", NumArgs: 1, Precedence: 50, Handler: errorOperator}
 var pickOpType = &operationType{Type: "PICK", NumArgs: 1, Precedence: 50, Handler: pickOperator}
 var evalOpType = &operationType{Type: "EVAL", NumArgs: 1, Precedence: 50, Handler: evalOperator}

pkg/yqlib/operator_filter.go

@@ -0,0 +1,33 @@
+package yqlib
+
+import (
+	"container/list"
+)
+
+func filterOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	log.Debugf("-- filterOperation")
+	var results = list.New()
+
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+		children := context.SingleChildContext(candidate)
+		splatted, err := splat(children, traversePreferences{})
+		if err != nil {
+			return Context{}, err
+		}
+		filtered, err := selectOperator(d, splatted, expressionNode)
+		if err != nil {
+			return Context{}, err
+		}
+
+		selfExpression := &ExpressionNode{Operation: &Operation{OperationType: selfReferenceOpType}}
+		collected, err := collectTogether(d, filtered, selfExpression)
+		if err != nil {
+			return Context{}, err
+		}
+		collected.Node.Style = unwrapDoc(candidate.Node).Style
+		results.PushBack(collected)
+	}
+	return context.ChildContext(results), nil
+}
+

pkg/yqlib/operator_filter_test.go

@@ -0,0 +1,49 @@
+package yqlib
+
+import (
+	"testing"
+)
+
+var filterOperatorScenarios = []expressionScenario{
+	{
+		description: "Filter array",
+		document:   `[1,2,3]`,
+		expression: `filter(. < 3)`,
+		expected: []string{
+			"D0, P[], (!!seq)::[1, 2]\n",
+		},
+	},
+	{
+		skipDoc: true,
+		document:    `[1,2,3]`,
+		expression:  `filter(. > 1)`,
+		expected: []string{
+			"D0, P[], (!!seq)::[2, 3]\n",
+		},
+	},
+	{
+		skipDoc: true,
+		description: "Filter array to empty",
+		document:    `[1,2,3]`,
+		expression:  `filter(. > 4)`,
+		expected: []string{
+			"D0, P[], (!!seq)::[]\n",
+		},
+	},
+	{
+		skipDoc: true,
+		description: "Filter empty array",
+		document:    `[]`,
+		expression:  `filter(. > 1)`,
+		expected: []string{
+			"D0, P[], (!!seq)::[]\n",
+		},
+	},
+}
+
+func TestFilterOperatorScenarios(t *testing.T) {
+	for _, tt := range filterOperatorScenarios {
+		testScenario(t, &tt)
+	}
+	documentOperatorScenarios(t, "filter", filterOperatorScenarios)
+}