before a major refactor

I decide to adopt a new approach of recording and updating item
derivation forests.  Since this affects a lot of things, I decide to
commit before the refactor, so that I can create a branch for that
refactor.

1 files changed, 43 insertions, 0 deletions

diff --git a/semiring/src/counting.rs b/semiring/src/counting.rs
new file mode 100644
index 0000000..9095888
--- /dev/null
+++ b/semiring/src/counting.rs
@@ -0,0 +1,43 @@
+//! This module implements the "counting semiring", which counts the
+//! number of ways a sentence can be derived by a grammar.
+
+use super::*;
+
+/// The counting semiring is a thin wrapper around the natural numbers
+/// with the usual addition and multiplication.
+#[derive(Copy, Clone, Debug, Ord, PartialOrd, Hash, Eq, PartialEq)]
+pub struct Count(usize);
+
+impl Count {
+    /// Wrap the count in the wrapper.
+    pub fn new(count: usize) -> Self {
+        Self(count)
+    }
+
+    /// Extract the count out.
+    pub fn value(&self) -> usize {
+        self.0
+    }
+}
+
+impl Semiring for Count {
+    fn zero() -> Self {
+        Self::new(0)
+    }
+
+    fn one() -> Self {
+        Self::new(1)
+    }
+
+    fn add(&mut self, other: &Self) {
+        self.0 = self.value() + other.value();
+    }
+
+    fn mul(&mut self, other: &Self) {
+        self.0 = self.value() * other.value();
+    }
+
+    fn valuation(_pos: usize, _grammar: &Grammar) -> Self {
+        Self::new(1)
+    }
+}