Make the FieldsIdsMap serialization more stable by using a BTreeMap

src/fields_ids_map.rs

@@ -1,9 +1,9 @@
-use std::collections::{HashMap, BTreeMap};
+use std::collections::BTreeMap;
 use serde::{Serialize, Deserialize};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct FieldsIdsMap {
-    names_ids: HashMap<String, u8>,
+    names_ids: BTreeMap<String, u8>,
     ids_names: BTreeMap<u8, String>,
     next_id: Option<u8>,
 }
@@ -11,7 +11,7 @@ pub struct FieldsIdsMap {
 impl FieldsIdsMap {
     pub fn new() -> FieldsIdsMap {
         FieldsIdsMap {
-            names_ids: HashMap::new(),
+            names_ids: BTreeMap::new(),
             ids_names: BTreeMap::new(),
             next_id: Some(0),
         }
@@ -66,6 +66,12 @@ impl FieldsIdsMap {
     }
 }
 
+impl Default for FieldsIdsMap {
+    fn default() -> FieldsIdsMap {
+        FieldsIdsMap::new()
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;