arc feature to enable using Engine and other data structures from multiple threads (#142)

* `arc` feature to make engine usable from multiple threads.

`arc` is turned on by default. When enabled, std::sync::Arc
will be used instead of std::rc::Rc. The former makes regorus
types like Engine, Value, ast nodes etc Send, allowing for
usability from multiple threads.
Arc would add a performance overhead though since the reference
counting will now become atomic.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

* Make engine and related types Debug

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

* Input, Data as json. Evaluate bool queries.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

---------

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/engine.rs

@@ -17,7 +17,7 @@ use anyhow::{bail, Result};
 
 /// The Rego evaluation engine.
 ///
-#[derive(Clone)]
+#[derive(Debug, Clone)]
 pub struct Engine {
     modules: Vec<Ref<Module>>,
     interpreter: Interpreter,
@@ -120,6 +120,11 @@ impl Engine {
         self.interpreter.set_input(input);
     }
 
+    pub fn set_input_json(&mut self, input_json: &str) -> Result<()> {
+        self.set_input(Value::from_json_str(input_json)?);
+        Ok(())
+    }
+
     /// Clear the data document.
     ///
     /// The data document will be reset to an empty object.
@@ -182,6 +187,10 @@ impl Engine {
         self.interpreter.get_data_mut().merge(data)
     }
 
+    pub fn add_data_json(&mut self, data_json: &str) -> Result<()> {
+        self.add_data(Value::from_json_str(data_json)?)
+    }
+
     /// Set whether builtins should raise errors strictly or not.
     ///
     /// Regorus differs from OPA in that by default builtins will
@@ -256,6 +265,14 @@ impl Engine {
         )
     }
 
+    pub fn eval_bool_query(&mut self, query: String, enable_tracing: bool) -> Result<bool> {
+        let results = self.eval_query(query, enable_tracing)?;
+        if results.result.len() != 1 || results.result[0].expressions.len() != 1 {
+            bail!("query did not produce exactly one value");
+        }
+        results.result[0].expressions[0].value.as_bool().copied()
+    }
+
     #[doc(hidden)]
     fn prepare_for_eval(&mut self, enable_tracing: bool) -> Result<()> {
         self.interpreter.set_traces(enable_tracing);