Introduce the Initial Criterion

milli/src/search/criteria/initial.rs

@@ -0,0 +1,28 @@
+use roaring::RoaringBitmap;
+
+use crate::search::query_tree::Operation;
+use crate::search::WordDerivationsCache;
+
+use super::{Criterion, CriterionResult};
+
+pub struct Initial {
+    answer: Option<CriterionResult>
+}
+
+impl Initial {
+    pub fn new(query_tree: Option<Operation>, mut candidates: Option<RoaringBitmap>) -> Initial {
+        let answer = CriterionResult {
+            query_tree,
+            candidates: candidates.clone(),
+            bucket_candidates: candidates.take().unwrap_or_default(),
+        };
+        Initial { answer: Some(answer) }
+    }
+}
+
+impl Criterion for Initial {
+    #[logging_timer::time("Initial::{}")]
+    fn next(&mut self, _: &mut WordDerivationsCache) -> anyhow::Result<Option<CriterionResult>> {
+        Ok(self.answer.take())
+    }
+}