serde-patch 0.2.3

use serde::{Serialize, de::DeserializeOwned};
use serde_json::{Map, Value};

/// Applies a JSON Merge Patch (RFC 7396).
///
/// Consumes the current value and returns the updated value.
///
/// # Example
///
/// ```
/// #[derive(serde::Serialize, serde::Deserialize, PartialEq, Debug)]
/// struct User { id: u32, name: String }
///
/// let current = User { id: 1, name: "old".to_string() };
/// let patch = r#"{ "name": "new" }"#;
///
/// let updated = serde_patch::apply(current, patch).unwrap();
/// assert_eq!(updated.name, "new");
/// assert_eq!(updated.id, 1);
/// ```
pub fn apply<T>(current: T, patch_json: &str) -> Result<T, serde_json::Error>
where
    T: Serialize + DeserializeOwned,
{
    let mut current_val = serde_json::to_value(current)?;

    let patch_val: Value = serde_json::from_str(patch_json)?;

    merge_patch(&mut current_val, &patch_val);

    serde_json::from_value(current_val)
}

/// Recursively merges a patch into a target JSON value (internal).
fn merge_patch(target: &mut Value, patch: &Value) {
    if let Value::Object(patch_map) = patch {
        if !target.is_object() {
            *target = Value::Object(Map::new());
        }

        let target_map = target.as_object_mut().unwrap();

        for (key, patch_value) in patch_map {
            if patch_value.is_null() {
                target_map.remove(key);
            } else {
                let target_entry = target_map.entry(key.clone()).or_insert(Value::Null);
                merge_patch(target_entry, patch_value);
            }
        }
    } else {
        *target = patch.clone();
    }
}