feat(cognitarium): define query engine structure

contracts/okp4-cognitarium/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod contract;
 mod error;
 pub mod msg;
+mod querier;
 mod rdf;
 pub mod state;
 mod storer;

contracts/okp4-cognitarium/src/querier/engine.rs

@@ -0,0 +1,32 @@
+use crate::querier::plan::{PatternValue, QueryNode, QueryPlan};
+use crate::state::{triples, Object, Predicate, Subject, Triple};
+use cosmwasm_std::{Order, StdError, StdResult, Storage};
+use cw_storage_plus::IndexList;
+use std::collections::VecDeque;
+use std::iter;
+
+pub struct QueryEngine<'a> {
+    storage: &'a dyn Storage,
+}
+
+impl<'a> QueryEngine<'a> {
+    pub fn new(storage: &'a dyn Storage) -> Self {
+        Self { storage }
+    }
+
+    pub fn eval_plan(&self, plan: QueryPlan) -> ResolvedVariablesIterator {
+        Box::new(iter::empty())
+    }
+}
+
+type ResolvedVariablesIterator = Box<dyn Iterator<Item = StdResult<ResolvedVariables>>>;
+
+pub enum ResolvedVariable {
+    Subject(Subject),
+    Predicate(Predicate),
+    Object(Object),
+}
+
+pub struct ResolvedVariables {
+    pub variables: Vec<Option<ResolvedVariable>>,
+}

contracts/okp4-cognitarium/src/querier/mod.rs

@@ -0,0 +1,3 @@
+mod engine;
+mod plan;
+mod plan_builder;