Adding first operator

pkg/yqlib/operator_first.go

@@ -0,0 +1,51 @@
+package yqlib
+
+import "container/list"
+
+func firstOperator(d *dataTreeNavigator, context Context, expressionNode *ExpressionNode) (Context, error) {
+	results := list.New()
+
+	for el := context.MatchingNodes.Front(); el != nil; el = el.Next() {
+		candidate := el.Value.(*CandidateNode)
+
+		// If no RHS expression is provided, simply return the first entry in candidate.Content
+		if expressionNode == nil || expressionNode.RHS == nil {
+			if len(candidate.Content) > 0 {
+				results.PushBack(candidate.Content[0])
+			}
+			continue
+		}
+
+		splatted, err := splat(context.SingleChildContext(candidate), traversePreferences{})
+		if err != nil {
+			return Context{}, err
+		}
+
+		for splatEl := splatted.MatchingNodes.Front(); splatEl != nil; splatEl = splatEl.Next() {
+			splatCandidate := splatEl.Value.(*CandidateNode)
+			// Create a new context for this splatted candidate
+			splatContext := context.SingleChildContext(splatCandidate)
+			// Evaluate the RHS expression against this splatted candidate
+			rhs, err := d.GetMatchingNodes(splatContext, expressionNode.RHS)
+			if err != nil {
+				return Context{}, err
+			}
+
+			includeResult := false
+
+			for resultEl := rhs.MatchingNodes.Front(); resultEl != nil; resultEl = resultEl.Next() {
+				result := resultEl.Value.(*CandidateNode)
+				includeResult = isTruthyNode(result)
+				if includeResult {
+					break
+				}
+			}
+			if includeResult {
+				results.PushBack(splatCandidate)
+				break
+			}
+		}
+
+	}
+	return context.ChildContext(results), nil
+}