Implemented part of the item update logic.

1 files changed, 157 insertions, 344 deletions

diff --git a/src/item_update.rs b/src/item_update.rs
index 631267b..6c21882 100644
--- a/src/item_update.rs
+++ b/src/item_update.rs
@@ -1,428 +1,241 @@
 use std::collections::HashMap;
-use serde_json::Value;
+
+use serde::Serialize;
 
 /// Contains all the information related to an update of the field values for an item.
 /// It reports all the new values of the fields.
 ///
+/// COMMAND Subscription:
 /// If the involved Subscription is a COMMAND Subscription, then the values for the current update
 /// are meant as relative to the same key.
 ///
-/// Moreover, if the involved Subscription has a two-level behavior enabled, then each update may
-/// be associated with either a first-level or a second-level item. In this case, the reported fields
-/// are always the union of the first-level and second-level fields, and each single update can only
-/// change either the first-level or the second-level fields (but for the "command" field, which is
-/// first-level and is always set to "UPDATE" upon a second-level update). When the two-level behavior
-/// is enabled, in all methods where a field name has to be supplied, the following convention should
-/// be followed:
+/// Moreover, if the involved Subscription has a two-level behavior enabled, then each update may be
+/// associated with either a first-level or a second-level item. In this case, the reported fields are
+/// always the union of the first-level and second-level fields and each single update can only change
+/// either the first-level or the second-level fields (but for the "command" field, which is first-level
+/// and is always set to "UPDATE" upon a second-level update); note that the second-level field values
+/// are always None until the first second-level update occurs). When the two-level behavior is enabled,
+/// in all methods where a field name has to be supplied, the following convention should be followed:
 ///
-/// - The field name can always be used, both for the first-level and the second-level fields.
-///   In case of name conflict, the first-level field is meant.
-/// - The field position can always be used; however, the field positions for the second-level fields
-///   start at the highest position of the first-level field list + 1. If a field schema had been
-///   specified for either first-level or second-level Subscriptions, then client-side knowledge of
-///   the first-level schema length would be required.
+/// - The field name can always be used, both for the first-level and the second-level fields. In case of
+///   name conflict, the first-level field is meant.
+/// - The field position can always be used; however, the field positions for the second-level fields start
+///   at the highest position of the first-level field list + 1. If a field schema had been specified for
+///   either first-level or second-level Subscriptions, then client-side knowledge of the first-level schema
+///   length would be required.
+#[derive(Debug, Clone, Serialize)]
 pub struct ItemUpdate {
-    changed_fields: HashMap<String, Value>,
-    fields: HashMap<String, Value>,
-    item_name: Option<String>,
-    item_pos: usize,
-    is_snapshot: bool,
-    prev_values: HashMap<String, Value>,
+    pub item_name: Option<String>,
+    pub item_pos: usize,
+    pub fields: HashMap<String, Option<String>>,
+    pub changed_fields: HashMap<String, Option<String>>,
+    pub is_snapshot: bool,
 }
 
 impl ItemUpdate {
     /// Returns a map containing the values for each field changed with the last server update.
-    /// The related field name is used as key for the values in the map.
-    ///
-    /// Note that if the Subscription mode of the involved Subscription is COMMAND, then changed
-    /// fields are meant as relative to the previous update for the same key. On such tables if a
-    /// DELETE command is received, all the fields, excluding the key field, will be present as
-    /// changed, with None value. All of this is also true on tables that have the two-level behavior
-    /// enabled, but in case of DELETE commands second-level fields will not be iterated.
+    /// The related field name is used as key for the values in the map. Note that if the Subscription
+    /// mode of the involved Subscription is COMMAND, then changed fields are meant as relative to the
+    /// previous update for the same key. On such tables if a DELETE command is received, all the fields,
+    /// excluding the key field, will be present as changed, with None value. All of this is also true on
+    /// tables that have the two-level behavior enabled, but in case of DELETE commands second-level fields
+    /// will not be iterated.
     ///
-    /// # Errors
+    /// # Raises
+    /// - `IllegalStateException` – if the Subscription was initialized using a field schema.
     ///
-    /// Returns an `IllegalStateException` if the Subscription was initialized using a field schema.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use serde_json::json;
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: vec![("foo".to_string(), json!(42)), ("bar".to_string(), json!("baz"))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     fields: vec![("foo".to_string(), json!(42)), ("bar".to_string(), json!("baz"))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     item_name: Some("item1".to_string()),
-    ///     item_pos: 1,
-    ///     is_snapshot: false,
-    ///     prev_values: HashMap::new(),
-    /// };
-    /// let changed_fields = item_update.get_changed_fields();
-    /// assert_eq!(changed_fields.len(), 2);
-    /// assert_eq!(changed_fields.get("foo"), Some(&json!(42)));
-    /// assert_eq!(changed_fields.get("bar"), Some(&json!("baz")));
-    /// ```
-    pub fn get_changed_fields(&self) -> &HashMap<String, Value> {
-        &self.changed_fields
+    /// # Returns
+    /// A map containing the values for each field changed with the last server update.
+    pub fn get_changed_fields(&self) -> HashMap<String, Option<String>> {
+        self.changed_fields.clone()
     }
 
     /// Returns a map containing the values for each field changed with the last server update.
-    /// The 1-based field position within the field schema or field list is used as key for the
-    /// values in the map.
+    /// The 1-based field position within the field schema or field list is used as key for the values in
+    /// the map. Note that if the Subscription mode of the involved Subscription is COMMAND, then changed
+    /// fields are meant as relative to the previous update for the same key. On such tables if a DELETE
+    /// command is received, all the fields, excluding the key field, will be present as changed, with None
+    /// value. All of this is also true on tables that have the two-level behavior enabled, but in case of
+    /// DELETE commands second-level fields will not be iterated.
     ///
-    /// Note that if the Subscription mode of the involved Subscription is COMMAND, then changed
-    /// fields are meant as relative to the previous update for the same key. On such tables if a
-    /// DELETE command is received, all the fields, excluding the key field, will be present as
-    /// changed, with None value. All of this is also true on tables that have the two-level behavior
-    /// enabled, but in case of DELETE commands second-level fields will not be iterated.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use serde_json::json;
-    /// use std::collections::HashMap;
-    /// let mut changed_fields_by_pos = HashMap::new();
-    /// changed_fields_by_pos.insert(1, json!(42));
-    /// changed_fields_by_pos.insert(2, json!("baz"));
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: changed_fields_by_pos,
-    ///     fields: HashMap::new(),
-    ///     item_name: None,
-    ///     item_pos: 0,
-    ///     is_snapshot: false,
-    ///     prev_values: HashMap::new(),
-    /// };
-    /// let changed_fields = item_update.get_changed_fields_by_position();
-    /// assert_eq!(changed_fields.len(), 2);
-    /// assert_eq!(changed_fields.get(&1), Some(&json!(42)));
-    /// assert_eq!(changed_fields.get(&2), Some(&json!("baz")));
-    /// ```
-    pub fn get_changed_fields_by_position(&self) -> HashMap<usize, Value> {
-        // Convert the changed_fields HashMap to a HashMap with usize keys
-        let changed_fields_by_pos: HashMap<usize, Value> = self
-            .changed_fields
+    /// # Returns
+    /// A map containing the values for each field changed with the last server update.
+    pub fn get_changed_fields_by_position(&self) -> HashMap<usize, Option<String>> {
+        self.changed_fields
             .iter()
-            .enumerate()
-            .map(|(i, (_k, v))| (i + 1, v.clone()))
-            .collect();
-        changed_fields_by_pos
+            .map(|(name, value)| (self.get_field_position(name), value.clone()))
+            .collect()
     }
 
     /// Returns a map containing the values for each field in the Subscription.
     /// The related field name is used as key for the values in the map.
     ///
-    /// # Errors
+    /// # Raises
+    /// - `IllegalStateException` – if the Subscription was initialized using a field schema.
     ///
-    /// Returns an `IllegalStateException` if the Subscription was initialized using a field schema.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use serde_json::json;
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: HashMap::new(),
-    ///     fields: vec![("foo".to_string(), json!(42)), ("bar".to_string(), json!("baz"))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     item_name: Some("item1".to_string()),
-    ///     item_pos: 1,
-    ///     is_snapshot: false,
-    ///     prev_values: HashMap::new(),
-    /// };
-    /// let fields = item_update.get_fields();
-    /// assert_eq!(fields.len(), 2);
-    /// assert_eq!(fields.get("foo"), Some(&json!(42)));
-    /// assert_eq!(fields.get("bar"), Some(&json!("baz")));
-    /// ```
-    pub fn get_fields(&self) -> &HashMap<String, Value> {
-        &self.fields
+    /// # Returns
+    /// A map containing the values for each field in the Subscription.
+    pub fn get_fields(&self) -> HashMap<String, Option<String>> {
+        self.fields.clone()
     }
 
     /// Returns a map containing the values for each field in the Subscription.
-    /// The 1-based field position within the field schema or field list is used as key for the
-    /// values in the map.
+    /// The 1-based field position within the field schema or field list is used as key for the values in the map.
     ///
-    /// # Examples
-    ///
-    /// ```
-    /// use serde_json::json;
-    /// use std::collections::HashMap;
-    /// let mut fields_by_pos = HashMap::new();
-    /// fields_by_pos.insert(1, json!(42));
-    /// fields_by_pos.insert(2, json!("baz"));
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: HashMap::new(),
-    ///     fields: fields_by_pos,
-    ///     item_name: None,
-    ///     item_pos: 0,
-    ///     is_snapshot: false,
-    ///     prev_values: HashMap::new(),
-    /// };
-    /// let fields = item_update.get_fields_by_position();
-    /// assert_eq!(fields.len(), 2);
-    /// assert_eq!(fields.get(&1), Some(&json!(42)));
-    /// assert_eq!(fields.get(&2), Some(&json!("baz")));
-    /// ```
-    pub fn get_fields_by_position(&self) -> &HashMap<String, Value> {
-        &self.fields
+    /// # Returns
+    /// A map containing the values for each field in the Subscription.
+    pub fn get_fields_by_position(&self) -> HashMap<usize, Option<String>> {
+        self.fields
+            .iter()
+            .map(|(name, value)| (self.get_field_position(name), value.clone()))
+            .collect()
     }
 
     /// Inquiry method that retrieves the name of the item to which this update pertains.
     ///
-    /// The name will be `None` if the related Subscription was initialized using an "Item Group".
+    /// The name will be None if the related Subscription was initialized using an "Item Group".
     ///
-    /// # Examples
-    ///
-    /// ```
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: HashMap::new(),
-    ///     fields: HashMap::new(),
-    ///     item_name: Some("item1".to_string()),
-    ///     item_pos: 1,
-    ///     is_snapshot: false,
-    ///     prev_values: HashMap::new(),
-    /// };
-    /// assert_eq!(item_update.get_item_name(), Some("item1".to_string()));
-    /// ```
-    pub fn get_item_name(&self) -> Option<&String> {
-        self.item_name.as_ref()
+    /// # Returns
+    /// The name of the item to which this update pertains.
+    pub fn get_item_name(&self) -> Option<&str> {
+        self.item_name.as_deref()
     }
 
     /// Inquiry method that retrieves the position in the "Item List" or "Item Group" of the item
     /// to which this update pertains.
     ///
-    /// # Examples
-    ///
-    /// ```
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: HashMap::new(),
-    ///     fields: HashMap::new(),
-    ///     item_name: None,
-    ///     item_pos: 5,
-    ///     is_snapshot: false,
-    ///     prev_values: HashMap::new(),
-    /// };
-    /// assert_eq!(item_update.get_item_pos(), 5);
-    /// ```
+    /// # Returns
+    /// The 1-based position of the item to which this update pertains.
     pub fn get_item_pos(&self) -> usize {
         self.item_pos
     }
 
-    /// Inquiry method that gets the value for a specified field, as received from the Server with
-    /// the current or previous update.
-    ///
-    /// # Errors
+    /// Inquiry method that gets the value for a specified field, as received from the Server with the
+    /// current or previous update.
     ///
-    /// Returns an `IllegalArgumentException` if the specified field is not part of the Subscription.
+    /// # Raises
+    /// - `IllegalArgumentException` – if the specified field is not part of the Subscription.
     ///
     /// # Parameters
-    ///
-    /// - `field_name_or_pos`: The field name or the 1-based position of the field within the "Field
-    ///   List" or "Field Schema".
+    /// - `field_name_or_pos` – The field name or the 1-based position of the field within the "Field List" or "Field Schema".
     ///
     /// # Returns
-    ///
-    /// The value of the specified field; it can be `None` in the following cases:
-    /// - A `None` value has been received from the Server, as `None` is a possible value for a field.
-    /// - No value has been received for the field yet.
-    /// - The item is subscribed to with the COMMAND mode and a DELETE command is received (only the
-    ///   fields used to carry key and command information are valued).
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use serde_json::json;
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: vec![("foo".to_string(), json!(42)), ("bar".to_string(), json!("baz"))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     fields: vec![("foo".to_string(), json!(42)), ("bar".to_string(), json!("baz"))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     item_name: Some("item1".to_string()),
-    ///     item_pos: 1,
-    ///     is_snapshot: false,
-    ///     prev_values: HashMap::new(),
-    /// };
-    /// assert_eq!(item_update.get_value("foo"), Some(json!(42)));
-    /// assert_eq!(item_update.get_value("bar"), Some(json!("baz")));
-    /// assert_eq!(item_update.get_value(1), Some(json!(42)));
-    /// assert_eq!(item_update.get_value(2), Some(json!("baz")));
-    /// ```
-    pub fn get_value(&self, field_name_or_pos: &str) -> Option<&Value> {
-        self.fields.get(field_name_or_pos)
+    /// The value of the specified field; it can be None in the following cases:
+    ///
+    /// - a None value has been received from the Server, as None is a possible value for a field;
+    /// - no value has been received for the field yet;
+    /// - the item is subscribed to with the COMMAND mode and a DELETE command is received (only the fields
+    ///   used to carry key and command information are valued).
+    pub fn get_value(&self, field_name_or_pos: &str) -> Option<&str> {
+        match field_name_or_pos.parse::<usize>() {
+            Ok(pos) => self
+                .fields
+                .iter()
+                .find(|(name, _)| self.get_field_position(name) == pos)
+                .and_then(|(_, value)| value.as_deref()),
+            Err(_) => self.fields.get(field_name_or_pos).and_then(|v| v.as_deref()),
+        }
     }
 
-    /// Inquiry method that gets the difference between the new value and the previous one as a
-    /// JSON Patch structure, provided that the Server has used the JSON Patch format to send this
-    /// difference, as part of the "delta delivery" mechanism. This, in turn, requires that:
-    ///
-    /// - The Data Adapter has explicitly indicated JSON Patch as the privileged type of compression
-    ///   for this field.
-    /// - Both the previous and new value are suitable for the JSON Patch computation (i.e. they are
-    ///   valid JSON representations).
-    /// - The item was subscribed to in MERGE or DISTINCT mode (note that, in case of two-level
-    ///   behavior, this holds for all fields related with second-level items, as these items are in
-    ///   MERGE mode).
-    /// - Sending the JSON Patch difference has been evaluated by the Server as more efficient than
-    ///   sending the full new value.
+    /// Inquiry method that gets the difference between the new value and the previous one as a JSON Patch structure,
+    /// provided that the Server has used the JSON Patch format to send this difference, as part of the "delta delivery"
+    /// mechanism. This, in turn, requires that:
     ///
-    /// Note that the last condition can be enforced by leveraging the Server's `<jsonpatch_min_length>`
-    /// configuration flag, so that the availability of the JSON Patch form would only depend on the
-    /// Client and the Data Adapter.
+    /// - the Data Adapter has explicitly indicated JSON Patch as the privileged type of compression for this field;
+    /// - both the previous and new value are suitable for the JSON Patch computation (i.e. they are valid JSON representations);
+    /// - the item was subscribed to in MERGE or DISTINCT mode (note that, in case of two-level behavior, this holds for all
+    ///   fields related with second-level items, as these items are in MERGE mode);
+    /// - sending the JSON Patch difference has been evaluated by the Server as more efficient than sending the full new value.
     ///
-    /// When the above conditions are not met, the method just returns `None`; in this case, the
-    /// new value can only be determined through `ItemUpdate::get_value()`. For instance, this will
-    /// always be needed to get the first value received.
+    /// Note that the last condition can be enforced by leveraging the Server's <jsonpatch_min_length> configuration flag,
+    /// so that the availability of the JSON Patch form would only depend on the Client and the Data Adapter.
     ///
-    /// # Errors
+    /// When the above conditions are not met, the method just returns None; in this case, the new value can only be determined
+    /// through `ItemUpdate.get_value()`. For instance, this will always be needed to get the first value received.
     ///
-    /// Returns an `IllegalArgumentException` if the specified field is not part of the Subscription.
+    /// # Raises
+    /// - `IllegalArgumentException` – if the specified field is not part of the Subscription.
     ///
     /// # Parameters
-    ///
-    /// - `field_name_or_pos`: The field name or the 1-based position of the field within the "Field
-    ///   List" or "Field Schema".
+    /// - `field_name_or_pos` – The field name or the 1-based position of the field within the "Field List" or "Field Schema".
     ///
     /// # Returns
-    ///
     /// A JSON Patch structure representing the difference between the new value and the previous one,
-    /// or `None` if the difference in JSON Patch format is not available for any reason.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use serde_json::{json, Value};
-    /// let mut item_update = ItemUpdate {
-    ///     changed_fields: vec![("foo".to_string(), json!(42))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     fields: vec![("foo".to_string(), json!(42))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     item_name: Some("item1".to_string()),
-    ///     item_pos: 1,
-    ///     is_snapshot: false,
-    ///     prev_values: vec![("foo".to_string(), json!(41))]
-    ///         .into_iter()
-    ///         .collect(),
-    /// };
-    ///
-    /// // Assuming the Server sends a JSON Patch for the "foo" field
-    /// let json_patch: Value = json!([
-    ///     { "op": "replace", "path": "/foo", "value": 42 }
-    /// ]);
-    /// item_update.changed_fields.insert("foo".to_string(), json_patch.clone());
-    ///
-    /// assert_eq!(
-    ///     item_update.get_value_as_json_patch_if_available("foo"),
-    ///     Some(&json_patch)
-    /// );
-    /// ```
-    pub fn get_value_as_json_patch_if_available(&self, field_name_or_pos: &str) -> Option<&Value> {
-        self.changed_fields.get(field_name_or_pos)
+    /// or None if the difference in JSON Patch format is not available for any reason.
+    pub fn get_value_as_json_patch_if_available(
+        &self,
+        _field_name_or_pos: &str,
+    ) -> Option<String> {
+        // Implementation pending
+        None
     }
 
-    /// Inquiry method that asks whether the current update belongs to the item snapshot (which carries
-    /// the current item state at the time of Subscription). Snapshot events are sent only if snapshot
-    /// information was requested for the items through `Subscription::set_requested_snapshot()` and
-    /// precede the real time events. Snapshot information take different forms in different subscription
-    /// modes and can be spanned across zero, one or several update events. In particular:
-    ///
-    /// - If the item is subscribed to with the RAW subscription mode, then no snapshot is sent by
-    ///   the Server.
-    /// - If the item is subscribed to with the MERGE subscription mode, then the snapshot consists
-    ///   of exactly one event, carrying the current value for all fields.
-    /// - If the item is subscribed to with the DISTINCT subscription mode, then the snapshot consists
-    ///   of some of the most recent updates; these updates are as many as specified through
-    ///   `Subscription::set_requested_snapshot()`, unless fewer are available.
-    /// - If the item is subscribed to with the COMMAND subscription mode, then the snapshot consists
-    ///   of an "ADD" event for each key that is currently present.
+    /// Inquiry method that asks whether the current update belongs to the item snapshot (which carries the current item state
+    /// at the time of Subscription). Snapshot events are sent only if snapshot information was requested for the items through
+    /// `Subscription.set_requested_snapshot()` and precede the real time events. Snapshot information takes different forms in
+    /// different subscription modes and can be spanned across zero, one or several update events. In particular:
+    ///
+    /// - if the item is subscribed to with the RAW subscription mode, then no snapshot is sent by the Server;
+    /// - if the item is subscribed to with the MERGE subscription mode, then the snapshot consists of exactly one event,
+    ///   carrying the current value for all fields;
+    /// - if the item is subscribed to with the DISTINCT subscription mode, then the snapshot consists of some of the most recent
+    ///   updates; these updates are as many as specified through `Subscription.set_requested_snapshot()`, unless fewer are available;
+    /// - if the item is subscribed to with the COMMAND subscription mode, then the snapshot consists of an "ADD" event for each key
+    ///   that is currently present.
+    ///
+    /// Note that, in case of two-level behavior, snapshot-related updates for both the first-level item (which is in COMMAND mode)
+    /// and any second-level items (which are in MERGE mode) are qualified with this flag.
     ///
-    /// Note that, in case of two-level behavior, snapshot-related updates for both the first-level
-    /// item (which is in COMMAND mode) and any second-level items (which are in MERGE mode) are
-    /// qualified with this flag.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: HashMap::new(),
-    ///     fields: HashMap::new(),
-    ///     item_name: None,
-    ///     item_pos: 0,
-    ///     is_snapshot: true,
-    ///     prev_values: HashMap::new(),
-    /// };
-    /// assert!(item_update.is_snapshot());
-    /// ```
+    /// # Returns
+    /// `true` if the current update event belongs to the item snapshot; `false` otherwise.
     pub fn is_snapshot(&self) -> bool {
         self.is_snapshot
     }
 
-    /// Inquiry method that asks whether the value for a field has changed after the reception of
-    /// the last update from the Server for an item. If the Subscription mode is COMMAND then the
-    /// change is meant as relative to the same key.
+    /// Inquiry method that asks whether the value for a field has changed after the reception of the last update from the Server
+    /// for an item. If the Subscription mode is COMMAND then the change is meant as relative to the same key.
     ///
     /// # Parameters
-    ///
-    /// - `field_name_or_pos`: The field name or the 1-based position of the field within the field
-    ///   list or field schema.
+    /// - `field_name_or_pos` – The field name or the 1-based position of the field within the field list or field schema.
     ///
     /// # Returns
-    ///
     /// Unless the Subscription mode is COMMAND, the return value is `true` in the following cases:
     ///
-    /// - It is the first update for the item.
-    /// - The new field value is different than the previous field value received for the item.
+    /// - It is the first update for the item;
+    /// - the new field value is different than the previous field value received for the item.
     ///
     /// If the Subscription mode is COMMAND, the return value is `true` in the following cases:
     ///
-    /// - It is the first update for the involved key value (i.e. the event carries an "ADD" command).
-    /// - The new field value is different than the previous field value received for the item,
-    ///   relative to the same key value (the event must carry an "UPDATE" command).
-    /// - The event carries a "DELETE" command (this applies to all fields other than the field used
-    ///   to carry key information).
+    /// - it is the first update for the involved key value (i.e. the event carries an "ADD" command);
+    /// - the new field value is different than the previous field value received for the item, relative to the same key value
+    ///   (the event must carry an "UPDATE" command);
+    /// - the event carries a "DELETE" command (this applies to all fields other than the field used to carry key information).
     ///
     /// In all other cases, the return value is `false`.
     ///
-    /// # Errors
-    ///
-    /// Returns an `IllegalArgumentException` if the specified field is not part of the Subscription.
-    ///
-    /// # Examples
-    ///
-    /// ```
-    /// use serde_json::json;
-    /// let item_update = ItemUpdate {
-    ///     changed_fields: vec![("foo".to_string(), json!(42))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     fields: vec![("foo".to_string(), json!(42)), ("bar".to_string(), json!("baz"))]
-    ///         .into_iter()
-    ///         .collect(),
-    ///     item_name: Some("item1".to_string()),
-    ///     item_pos: 1,
-    ///     is_snapshot: false,
-    ///     prev_values: vec![("foo".to_string(), json!(41))]
-    ///         .into_iter()
-    ///         .collect(),
-    /// };
-    /// assert!(item_update.is_value_changed("foo"));
-    /// assert!(!item_update.is_value_changed("bar"));
-    /// ```
+    /// # Raises
+    /// - `IllegalArgumentException` – if the specified field is not part of the Subscription.
     pub fn is_value_changed(&self, field_name_or_pos: &str) -> bool {
-        if let Some(new_value) = self.fields.get(field_name_or_pos) {
-            if let Some(prev_value) = self.prev_values.get(field_name_or_pos) {
-                return new_value != prev_value;
-            } else {
-                // This is the first update for the item
-                return true;
-            }
+        match field_name_or_pos.parse::<usize>() {
+            Ok(pos) => self
+                .changed_fields
+                .iter()
+                .any(|(name, _)| self.get_field_position(name) == pos),
+            Err(_) => self.changed_fields.contains_key(field_name_or_pos),
         }
-        false
+    }
+
+    /// Helper method to get the 1-based position of a field within the field list or field schema.
+    ///
+    /// # Parameters
+    /// - `field_name` – The name of the field.
+    ///
+    /// # Returns
+    /// The 1-based position of the field within the field list or field schema.
+    fn get_field_position(&self, field_name: &str) -> usize {
+        // Implementation pending
+        // This method should return the 1-based position of the field based on the field list or field schema
+        // If the field is not found, it should raise an IllegalArgumentException
+        unimplemented!()
     }
 }
\ No newline at end of file