2 files changed, 75 insertions, 3 deletions

diff --git a/pkg/query/parser.go b/pkg/query/parser.go
index 5de803c..d7f4fdd 100644
--- a/pkg/query/parser.go
+++ b/pkg/query/parser.go
@@ -53,8 +53,11 @@ type Statement struct {
 	Operator opType
 	Value    Valuer
 }
+
+type Statements []Statement
+
 type Clause struct {
-	Statements []Statement
+	Statements Statements
 	Clauses    []*Clause
 	Operator   clauseOperator
 }
@@ -101,7 +104,7 @@ func (v StringValue) Compare(other Valuer) int {
 }
 
 type DatetimeValue struct {
-	d time.Time
+	D time.Time
 }
 
 func (v DatetimeValue) Type() valuerType {
@@ -114,7 +117,7 @@ func (v DatetimeValue) Compare(other Valuer) int {
 		return 0
 	}
 
-	return v.d.Compare(o.d)
+	return v.D.Compare(o.D)
 }
 
 var _ Valuer = StringValue{}
@@ -230,6 +233,16 @@ func (root *Clause) Flatten() {
 		stack = stack[:top]
 
 		hasMerged := false
+
+		// merge if only child clause
+		if len(node.Statements) == 0 && len(node.Clauses) == 1 {
+			child := node.Clauses[0]
+
+			node.Operator = child.Operator
+			node.Statements = child.Statements
+			node.Clauses = child.Clauses
+		}
+
 		// cannot be "modernized", node.Clauses is modified in loop
 		for i := 0; i < len(node.Clauses); i++ {
 			child := node.Clauses[i]
@@ -293,6 +306,15 @@ func (root Clause) Depth() int {
 	return maxHeight
 }
 
+// Order of clause tree
+func (root Clause) Order() int {
+	count := 0
+	for range root.DFS() {
+		count++
+	}
+	return count
+}
+
 func (root *Clause) DFS() iter.Seq[*Clause] {
 	return func(yield func(*Clause) bool) {
 		stack := make([]*Clause, 0, len(root.Clauses))
diff --git a/pkg/query/parser_test.go b/pkg/query/parser_test.go
index 7ee4987..c1f25d4 100644
--- a/pkg/query/parser_test.go
+++ b/pkg/query/parser_test.go
@@ -19,6 +19,26 @@ func TestClause_Flatten(t *testing.T) {
 			query.Clause{},
 		},
 		{
+			"empty with child",
+			&query.Clause{
+				Operator: query.COP_OR,
+				Clauses: []*query.Clause{
+					{
+						Operator: query.COP_AND,
+						Statements: []query.Statement{
+							{Category: query.CAT_AUTHOR, Operator: query.OP_AP, Value: query.StringValue{"jp"}},
+						},
+					},
+				},
+			},
+			query.Clause{
+				Operator: query.COP_AND,
+				Statements: []query.Statement{
+					{Category: query.CAT_AUTHOR, Operator: query.OP_AP, Value: query.StringValue{"jp"}},
+				},
+			},
+		},
+		{
 			"already flat",
 			&query.Clause{
 				Operator: query.COP_AND,
@@ -170,3 +190,33 @@ func TestClause_Flatten(t *testing.T) {
 		})
 	}
 }
+
+func TestParse(t *testing.T) {
+	tests := []struct {
+		name string // description of this test case
+		// Named input parameters for target function.
+		tokens  []query.Token
+		want    *query.Clause
+		wantErr bool
+	}{
+		// TODO: Add test cases.
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, gotErr := query.Parse(tt.tokens)
+			if gotErr != nil {
+				if !tt.wantErr {
+					t.Errorf("Parse() failed: %v", gotErr)
+				}
+				return
+			}
+			if tt.wantErr {
+				t.Fatal("Parse() succeeded unexpectedly")
+			}
+			// TODO: update the condition below to compare got with tt.want.
+			if true {
+				t.Errorf("Parse() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}