Introduce the FieldsIdsMap type

src/fields_ids_map.rs

@@ -0,0 +1,108 @@
+use std::collections::{HashMap, BTreeMap};
+use serde::{Serialize, Deserialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct FieldsIdsMap {
+    names_ids: HashMap<String, u8>,
+    ids_names: BTreeMap<u8, String>,
+    next_id: Option<u8>,
+}
+
+impl FieldsIdsMap {
+    pub fn new() -> FieldsIdsMap {
+        FieldsIdsMap {
+            names_ids: HashMap::new(),
+            ids_names: BTreeMap::new(),
+            next_id: Some(0),
+        }
+    }
+
+    /// Returns the number of fields ids in the map.
+    pub fn len(&self) -> usize {
+        self.names_ids.len()
+    }
+
+    /// Returns `true` if the map is empty.
+    pub fn is_empty(&self) -> bool {
+        self.names_ids.is_empty()
+    }
+
+    /// Returns the field id related to a field name, it will create a new field id if the
+    /// name is not already known. Returns `None` if the maximum field id as been reached.
+    pub fn insert(&mut self, name: &str) -> Option<u8> {
+        match self.names_ids.get(name) {
+            Some(id) => Some(*id),
+            None => {
+                let id = self.next_id?;
+                self.next_id = id.checked_add(1);
+                self.names_ids.insert(name.to_owned(), id);
+                self.ids_names.insert(id, name.to_owned());
+                Some(id)
+            }
+        }
+    }
+
+    /// Get the id of a field based on its name.
+    pub fn id(&self, name: &str) -> Option<u8> {
+        self.names_ids.get(name).copied()
+    }
+
+    /// Get the name of a field based on its id.
+    pub fn name(&self, id: u8) -> Option<&str> {
+        self.ids_names.get(&id).map(String::as_str)
+    }
+
+    /// Remove a field name and id based on its name.
+    pub fn remove(&mut self, name: &str) -> Option<u8> {
+        match self.names_ids.remove(name) {
+            Some(id) => self.ids_names.remove_entry(&id).map(|(id, _)| id),
+            None => None,
+        }
+    }
+
+    /// Iterate over the ids and names in the ids order.
+    pub fn iter(&self) -> impl Iterator<Item=(u8, &str)> {
+        self.ids_names.iter().map(|(id, name)| (*id, name.as_str()))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fields_ids_map() {
+        let mut map = FieldsIdsMap::new();
+
+        assert_eq!(map.insert("id"), Some(0));
+        assert_eq!(map.insert("title"), Some(1));
+        assert_eq!(map.insert("description"), Some(2));
+        assert_eq!(map.insert("id"), Some(0));
+        assert_eq!(map.insert("title"), Some(1));
+        assert_eq!(map.insert("description"), Some(2));
+
+        assert_eq!(map.id("id"), Some(0));
+        assert_eq!(map.id("title"), Some(1));
+        assert_eq!(map.id("description"), Some(2));
+        assert_eq!(map.id("date"), None);
+
+        assert_eq!(map.len(), 3);
+
+        assert_eq!(map.name(0), Some("id"));
+        assert_eq!(map.name(1), Some("title"));
+        assert_eq!(map.name(2), Some("description"));
+        assert_eq!(map.name(4), None);
+
+        assert_eq!(map.remove("title"), Some(1));
+
+        assert_eq!(map.id("title"), None);
+        assert_eq!(map.insert("title"), Some(3));
+        assert_eq!(map.len(), 3);
+
+        let mut iter = map.iter();
+        assert_eq!(iter.next(), Some((0, "id")));
+        assert_eq!(iter.next(), Some((2, "description")));
+        assert_eq!(iter.next(), Some((3, "title")));
+        assert_eq!(iter.next(), None);
+    }
+}

src/lib.rs

@@ -1,4 +1,5 @@
 mod criterion;
+mod fields_ids_map;
 mod index;
 mod indexing;
 mod mdfs;