Introduce the Typo criterion

milli/src/search/criteria/mod.rs

@@ -0,0 +1,34 @@
+use crate::Index;
+
+use roaring::RoaringBitmap;
+
+use super::query_tree::Operation;
+
+pub mod typo;
+
+pub trait Criterion {
+    fn next(&mut self) -> anyhow::Result<Option<(Option<Operation>, RoaringBitmap)>>;
+}
+
+/// Either a set of candidates that defines the candidates
+/// that are allowed to be returned,
+/// or the candidates that must never be returned.
+enum Candidates {
+    Allowed(RoaringBitmap),
+    Forbidden(RoaringBitmap)
+}
+
+impl Candidates {
+    fn into_inner(self) -> RoaringBitmap {
+        match self {
+            Self::Allowed(inner) => inner,
+            Self::Forbidden(inner) => inner,
+        }
+    }
+}
+
+impl Default for Candidates {
+    fn default() -> Self {
+        Self::Forbidden(RoaringBitmap::new())
+    }
+}