feat: Introduce the SortBy help structure

This structure help ranking documents using stored attributes.

src/rank/criterion/document_id.rs

@@ -0,0 +1,19 @@
+use std::cmp::Ordering;
+use std::ops::Deref;
+
+use rocksdb::DB;
+
+use crate::rank::criterion::Criterion;
+use crate::database::DatabaseView;
+use crate::rank::Document;
+
+#[derive(Debug, Clone, Copy)]
+pub struct DocumentId;
+
+impl<D> Criterion<D> for DocumentId
+where D: Deref<Target=DB>
+{
+    fn evaluate(&self, lhs: &Document, rhs: &Document, _: &DatabaseView<D>) -> Ordering {
+        lhs.id.cmp(&rhs.id)
+    }
+}

src/rank/criterion/mod.rs

@@ -7,7 +7,9 @@ mod exact;
 
 use std::cmp::Ordering;
 use std::ops::Deref;
+use std::marker;
 
+use serde::de::DeserializeOwned;
 use rocksdb::DB;
 
 use crate::database::DatabaseView;
@@ -69,6 +71,73 @@ where D: Deref<Target=DB>
     }
 }
 
+/// An helper struct that permit to sort documents by
+/// some of their stored attributes.
+///
+/// # Note
+///
+/// If a document cannot be deserialized it will be considered [`None`][].
+///
+/// Deserialized documents are compared like `Some(doc0).cmp(&Some(doc1))`,
+/// so you must check the [`Ord`] of `Option` implementation.
+///
+/// [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
+/// [`Ord`]: https://doc.rust-lang.org/std/option/enum.Option.html#impl-Ord
+///
+/// # Example
+///
+/// ```
+/// use serde_derive::Deserialize;
+/// use meilidb::rank::criterion::*;
+///
+/// #[derive(Deserialize, PartialOrd, Ord, PartialEq, Eq)]
+/// struct TimeOnly {
+///     time: String,
+/// }
+///
+/// let builder = CriteriaBuilder::with_capacity(7)
+///        .add(SumOfTypos)
+///        .add(NumberOfWords)
+///        .add(WordsProximity)
+///        .add(SumOfWordsAttribute)
+///        .add(SumOfWordsPosition)
+///        .add(Exact)
+///        .add(SortBy::<TimeOnly>::new())
+///        .add(DocumentId);
+///
+/// let criterion = builder.build();
+///
+/// ```
+#[derive(Default)]
+pub struct SortBy<T> {
+    _phantom: marker::PhantomData<T>,
+}
+
+impl<T> SortBy<T> {
+    pub fn new() -> Self {
+        SortBy { _phantom: marker::PhantomData }
+    }
+}
+
+impl<T, D> Criterion<D> for SortBy<T>
+where D: Deref<Target=DB>,
+      T: DeserializeOwned + Ord,
+{
+    fn evaluate(&self, lhs: &Document, rhs: &Document, view: &DatabaseView<D>) -> Ordering {
+        let lhs = match view.retrieve_document::<T>(lhs.id) {
+            Ok(doc) => Some(doc),
+            Err(e) => { eprintln!("{}", e); None },
+        };
+
+        let rhs = match view.retrieve_document::<T>(rhs.id) {
+            Ok(doc) => Some(doc),
+            Err(e) => { eprintln!("{}", e); None },
+        };
+
+        lhs.cmp(&rhs)
+    }
+}
+
 pub struct CriteriaBuilder<D>
 where D: Deref<Target=DB>
 {

src/rank/criterion/sort_by.rs

@@ -0,0 +1,77 @@
+use std::cmp::Ordering;
+use std::ops::Deref;
+use std::marker;
+
+use rocksdb::DB;
+use serde::de::DeserializeOwned;
+
+use crate::rank::criterion::Criterion;
+use crate::database::DatabaseView;
+use crate::rank::Document;
+
+/// An helper struct that permit to sort documents by
+/// some of their stored attributes.
+///
+/// # Note
+///
+/// If a document cannot be deserialized it will be considered [`None`][].
+///
+/// Deserialized documents are compared like `Some(doc0).cmp(&Some(doc1))`,
+/// so you must check the [`Ord`] of `Option` implementation.
+///
+/// [`None`]: https://doc.rust-lang.org/std/option/enum.Option.html#variant.None
+/// [`Ord`]: https://doc.rust-lang.org/std/option/enum.Option.html#impl-Ord
+///
+/// # Example
+///
+/// ```no-test
+/// use serde_derive::Deserialize;
+/// use meilidb::rank::criterion::*;
+///
+/// #[derive(Deserialize, PartialOrd, Ord, PartialEq, Eq)]
+/// struct TimeOnly {
+///     time: String,
+/// }
+///
+/// let builder = CriteriaBuilder::with_capacity(8)
+///        .add(SumOfTypos)
+///        .add(NumberOfWords)
+///        .add(WordsProximity)
+///        .add(SumOfWordsAttribute)
+///        .add(SumOfWordsPosition)
+///        .add(Exact)
+///        .add(SortBy::<TimeOnly>::new())
+///        .add(DocumentId);
+///
+/// let criterion = builder.build();
+///
+/// ```
+#[derive(Default)]
+pub struct SortBy<T> {
+    _phantom: marker::PhantomData<T>,
+}
+
+impl<T> SortBy<T> {
+    pub fn new() -> Self {
+        SortBy { _phantom: marker::PhantomData }
+    }
+}
+
+impl<T, D> Criterion<D> for SortBy<T>
+where D: Deref<Target=DB>,
+      T: DeserializeOwned + Ord,
+{
+    fn evaluate(&self, lhs: &Document, rhs: &Document, view: &DatabaseView<D>) -> Ordering {
+        let lhs = match view.retrieve_document::<T>(lhs.id) {
+            Ok(doc) => Some(doc),
+            Err(e) => { eprintln!("{}", e); None },
+        };
+
+        let rhs = match view.retrieve_document::<T>(rhs.id) {
+            Ok(doc) => Some(doc),
+            Err(e) => { eprintln!("{}", e); None },
+        };
+
+        lhs.cmp(&rhs)
+    }
+}