Introduce the Crtierion enum

src/criterion.rs

@@ -0,0 +1,31 @@
+pub enum Criterion {
+    /// Sorted by increasing number of typos.
+    Typo,
+    /// Sorted by decreasing number of matched query terms.
+    Words,
+    /// Sorted by increasing distance between matched query terms.
+    Proximity,
+    /// Documents with quey words contained in more important
+    /// attributes are considred better.
+    Attribute,
+    /// Documents with query words at the front of an attribute is
+    /// considered better than if it was at the back.
+    WordsPosition,
+    /// Sorted by the similarity of the matched words with the query words.
+    Exactness,
+    /// Sorted by the increasing value of the field specified.
+    CustomAsc(String),
+    /// Sorted by the decreasing value of the field specified.
+    CustomDesc(String),
+}
+
+pub fn default_criteria() -> Vec<Criterion> {
+    vec![
+        Criterion::Typo,
+        Criterion::Words,
+        Criterion::Proximity,
+        Criterion::Attribute,
+        Criterion::WordsPosition,
+        Criterion::Exactness,
+    ]
+}

src/lib.rs

@@ -1,4 +1,5 @@
 mod best_proximity;
+mod criterion;
 mod heed_codec;
 mod iter_shortest_paths;
 mod query_tokens;