1 files changed, 60 insertions, 0 deletions
diff --git a/src/util/param_cache.rs b/src/util/param_cache.rs
new file mode 100644
index 0000000..6faced9
--- /dev/null
+++ b/src/util/param_cache.rs
@@ -0,0 +1,60 @@
+use super::SmallCString;
+use std::cell::RefCell;
+use std::collections::BTreeMap;
+
+/// Maps parameter names to parameter indices.
+#[derive(Default, Clone, Debug)]
+// BTreeMap seems to do better here unless we want to pull in a custom hash
+// function.
+pub(crate) struct ParamIndexCache(RefCell<BTreeMap<SmallCString, usize>>);
+
+impl ParamIndexCache {
+    pub fn get_or_insert_with<F>(&self, s: &str, func: F) -> Option<usize>
+    where
+        F: FnOnce(&std::ffi::CStr) -> Option<usize>,
+    {
+        let mut cache = self.0.borrow_mut();
+        // Avoid entry API, needs allocation to test membership.
+        if let Some(v) = cache.get(s) {
+            return Some(*v);
+        }
+        // If there's an internal nul in the name it couldn't have been a
+        // parameter, so early return here is ok.
+        let name = SmallCString::new(s).ok()?;
+        let val = func(&name)?;
+        cache.insert(name, val);
+        Some(val)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    #[test]
+    fn test_cache() {
+        let p = ParamIndexCache::default();
+        let v = p.get_or_insert_with("foo", |cstr| {
+            assert_eq!(cstr.to_str().unwrap(), "foo");
+            Some(3)
+        });
+        assert_eq!(v, Some(3));
+        let v = p.get_or_insert_with("foo", |_| {
+            panic!("shouldn't be called this time");
+        });
+        assert_eq!(v, Some(3));
+        let v = p.get_or_insert_with("gar\0bage", |_| {
+            panic!("shouldn't be called here either");
+        });
+        assert_eq!(v, None);
+        let v = p.get_or_insert_with("bar", |cstr| {
+            assert_eq!(cstr.to_str().unwrap(), "bar");
+            None
+        });
+        assert_eq!(v, None);
+        let v = p.get_or_insert_with("bar", |cstr| {
+            assert_eq!(cstr.to_str().unwrap(), "bar");
+            Some(30)
+        });
+        assert_eq!(v, Some(30));
+    }
+}