feat: Introduce the Criteria struct

src/rank/criterion/sum_of_words_position.rs

@@ -0,0 +1,21 @@
+use std::cmp::Ordering;
+use group_by::GroupBy;
+use crate::Match;
+use crate::rank::{match_query_index, Document};
+
+#[inline]
+fn sum_matches_attribute_index(matches: &[Match]) -> u32 {
+    // note that GroupBy will never return an empty group
+    // so we can do this assumption safely
+    GroupBy::new(matches, match_query_index).map(|group| unsafe {
+        group.get_unchecked(0).attribute_index
+    }).sum()
+}
+
+#[inline]
+pub fn sum_of_words_position(lhs: &Document, rhs: &Document) -> Ordering {
+    let lhs = sum_matches_attribute_index(&lhs.matches);
+    let rhs = sum_matches_attribute_index(&rhs.matches);
+
+    lhs.cmp(&rhs)
+}