lift clauses in LogicalAst

(a OR b) OR (c OR d) can be simplified to (a OR b OR c OR d)
(a AND b) AND (c AND d) can be simplified to (a AND b AND c AND d)

This directly affects how queries are executed

src/query/query_parser/logical_ast.rs

@@ -39,6 +39,33 @@ impl LogicalAst {
             LogicalAst::Boost(Box::new(self), boost)
         }
     }
+
+    pub fn simplify(self) -> LogicalAst {
+        match self {
+            LogicalAst::Clause(clauses) => {
+                let mut new_clauses: Vec<(Occur, LogicalAst)> = Vec::new();
+
+                for (occur, sub_ast) in clauses {
+                    let simplified_sub_ast = sub_ast.simplify();
+
+                    // If clauses below have the same `Occur`, we can pull them up
+                    match simplified_sub_ast {
+                        LogicalAst::Clause(sub_clauses)
+                            if sub_clauses.iter().all(|(o, _)| *o == occur) =>
+                        {
+                            for sub_clause in sub_clauses {
+                                new_clauses.push(sub_clause);
+                            }
+                        }
+                        _ => new_clauses.push((occur, simplified_sub_ast)),
+                    }
+                }
+
+                LogicalAst::Clause(new_clauses)
+            }
+            LogicalAst::Leaf(_) | LogicalAst::Boost(_, _) => self,
+        }
+    }
 }
 
 fn occur_letter(occur: Occur) -> &'static str {

src/query/query_parser/query_parser.rs

@@ -379,7 +379,7 @@ impl QueryParser {
         if !err.is_empty() {
             return Err(err.swap_remove(0));
         }
-        Ok(ast)
+        Ok(ast.simplify())
     }
 
     /// Parse the user query into an AST.
@@ -1434,7 +1434,7 @@ mod test {
         );
         test_parse_query_to_logical_ast_helper(
             "(+title:a +title:b) title:c",
-            r#"(+(+Term(field=0, type=Str, "a") +Term(field=0, type=Str, "b")) +Term(field=0, type=Str, "c"))"#,
+            r#"(+Term(field=0, type=Str, "a") +Term(field=0, type=Str, "b") +Term(field=0, type=Str, "c"))"#,
             true,
         );
     }
@@ -1470,7 +1470,7 @@ mod test {
     pub fn test_parse_query_to_ast_two_terms() {
         test_parse_query_to_logical_ast_helper(
             "title:a b",
-            r#"(Term(field=0, type=Str, "a") (Term(field=0, type=Str, "b") Term(field=1, type=Str, "b")))"#,
+            r#"(Term(field=0, type=Str, "a") Term(field=0, type=Str, "b") Term(field=1, type=Str, "b"))"#,
             false,
         );
         test_parse_query_to_logical_ast_helper(