Add TrivialWeight

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## Added
 - Binary serialization & deserialization support for FST caches.
 - Binary serialization & deserialization support for Compose FST op state table.
+- Add `TrivialWeight`
 
 ## [0.8.0] - 2020-16-10
 

rustfst/src/semirings/mod.rs

@@ -13,6 +13,7 @@ mod probability_weight;
 mod product_weight;
 mod string_variant;
 mod string_weight;
+mod trivial_weight;
 mod tropical_weight;
 mod union_weight;
 pub(crate) mod utils_float;
@@ -33,5 +34,6 @@ pub(crate) use self::string_variant::StringWeightVariant;
 pub use self::string_weight::{
     StringType, StringWeightLeft, StringWeightRestrict, StringWeightRight,
 };
+pub use self::trivial_weight::TrivialWeight;
 pub use self::tropical_weight::TropicalWeight;
 pub use self::union_weight::{UnionWeight, UnionWeightOption};

rustfst/src/semirings/trivial_weight.rs

@@ -0,0 +1,105 @@
+use anyhow::Result;
+
+use crate::semirings::{CompleteSemiring, ReverseBack, Semiring, SemiringProperties, StarSemiring};
+use std::borrow::Borrow;
+
+use super::WeaklyDivisibleSemiring;
+
+/// Trivial semiring: (..., ..., (), ()).
+///
+/// This is useful for defining unweighted transducers.
+#[derive(Clone, Debug, PartialEq, PartialOrd, Default, Eq, Copy, Hash)]
+pub struct TrivialWeight;
+
+impl Semiring for TrivialWeight {
+    type Type = ();
+
+    type ReverseWeight = TrivialWeight;
+
+    fn zero() -> Self {
+        Self
+    }
+
+    fn one() -> Self {
+        Self
+    }
+
+    fn new(_value: Self::Type) -> Self {
+        Self
+    }
+
+    fn plus_assign<P: Borrow<Self>>(&mut self, _rhs: P) -> anyhow::Result<()> {
+        Ok(())
+    }
+
+    fn times_assign<P: Borrow<Self>>(&mut self, _rhs: P) -> anyhow::Result<()> {
+        Ok(())
+    }
+
+    fn approx_equal<P: Borrow<Self>>(&self, _rhs: P, _delta: f32) -> bool {
+        true
+    }
+
+    fn value(&self) -> &Self::Type {
+        &()
+    }
+
+    fn take_value(self) -> Self::Type {}
+
+    fn set_value(&mut self, _value: Self::Type) {}
+
+    fn reverse(&self) -> anyhow::Result<Self::ReverseWeight> {
+        Ok(*self)
+    }
+
+    fn properties() -> SemiringProperties {
+        SemiringProperties::LEFT_SEMIRING
+            | SemiringProperties::RIGHT_SEMIRING
+            | SemiringProperties::COMMUTATIVE
+            | SemiringProperties::IDEMPOTENT
+            | SemiringProperties::PATH
+    }
+}
+
+impl ReverseBack<TrivialWeight> for TrivialWeight {
+    fn reverse_back(&self) -> Result<TrivialWeight> {
+        Ok(*self)
+    }
+}
+
+impl CompleteSemiring for TrivialWeight {}
+
+impl StarSemiring for TrivialWeight {
+    fn closure(&self) -> Self {
+        Self
+    }
+}
+
+impl WeaklyDivisibleSemiring for TrivialWeight {
+    fn divide_assign(&mut self, _rhs: &Self, _divide_type: super::DivideType) -> Result<()> {
+        bail!("Division by 0")
+    }
+}
+
+impl From<()> for TrivialWeight {
+    fn from(_value: ()) -> Self {
+        Self
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_trivial_weight() -> Result<()> {
+        let trivial = TrivialWeight;
+
+        // Test plus
+        assert_eq!(trivial.plus(trivial)?, trivial);
+
+        // Test times
+        assert_eq!(trivial.times(trivial)?, trivial);
+        Ok(())
+    }
+}