Expose wasm engine clone and add prepare API

Agent-Logs-Url: https://github.com/microsoft/regorus/sessions/58419b20-1586-4a23-85fb-5aed5f5d5961

Co-authored-by: anakrish <35780660+anakrish@users.noreply.github.com>

bindings/wasm/README.md

@@ -21,3 +21,6 @@ Run `cargo xtask build-wasm` to invoke wasm-pack with sensible defaults, or `car
 ## Usage
 
 See [test.js](https://github.com/microsoft/regorus/blob/main/bindings/wasm/test.js) for example usage.
+
+For best performance with large policies, call `engine.prepare()` after loading
+policy/data, then use `engine.clone()` to create per-request engines.

bindings/wasm/src/lib.rs

@@ -138,6 +138,14 @@ impl Engine {
         self.engine.set_rego_v0(enable)
     }
 
+    /// Clone this engine.
+    ///
+    /// Useful for creating per-request engines after loading policy/data once.
+    #[wasm_bindgen(js_name = "clone")]
+    pub fn cloneEngine(&self) -> Engine {
+        Clone::clone(self)
+    }
+
     /// Add a policy
     ///
     /// The policy is parsed into AST.
@@ -158,6 +166,14 @@ impl Engine {
         self.engine.add_data(data).map_err(error_to_jsvalue)
     }
 
+    /// Prepare the engine for evaluation.
+    ///
+    /// This initializes internal evaluation structures so a cloned engine can
+    /// evaluate without requiring an initial "dummy" evaluation.
+    pub fn prepare(&mut self) -> Result<(), JsValue> {
+        self.engine.prepare().map_err(error_to_jsvalue)
+    }
+
     /// Get the list of packages defined by loaded policies.
     ///
     /// See https://docs.rs/regorus/latest/regorus/struct.Engine.html#method.get_packages
@@ -487,6 +503,9 @@ mod tests {
         )?;
         assert_eq!(pkg, "data.test");
 
+        // Prepare before first evaluation.
+        engine.prepare()?;
+
         let results = engine.evalQuery("data".to_string())?;
         let r = regorus::Value::from_json_str(&results).map_err(error_to_jsvalue)?;
 

bindings/wasm/test.js

@@ -40,6 +40,13 @@ engine.addDataJson(`
  }
 `);
 
+// Prepare internal evaluation structures once.
+engine.prepare();
+
+// Clone a prepared template engine for reuse.
+var template = engine.clone();
+engine = template.clone();
+
 // Set policy input
 engine.setInputJson(`
  {