From 54dbf78412f50610ab91e9fd73a6c727049dc0c3 Mon Sep 17 00:00:00 2001
From: +merlan #flirora <uruwi@protonmail.com>
Date: Sun, 29 Sep 2024 20:44:08 -0400
Subject: [PATCH] Add `TrivialWeight`

---
 CHANGELOG.md                            |   1 +
 Cargo.lock                              |   6 +-
 rustfst/src/semirings/mod.rs            |   2 +
 rustfst/src/semirings/trivial_weight.rs | 105 ++++++++++++++++++++++++
 4 files changed, 111 insertions(+), 3 deletions(-)
 create mode 100644 rustfst/src/semirings/trivial_weight.rs

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 090b8b08e..c50707e3b 100644
--- a/CHANGELOG.md
+++ b/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
 
diff --git a/Cargo.lock b/Cargo.lock
index f55b97ab2..155194e58 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -730,7 +730,7 @@ checksum = "adad44e29e4c806119491a7f06f03de4d1af22c3a680dd47f1e6e179439d1f56"
 
 [[package]]
 name = "rustfst"
-version = "1.1.1"
+version = "1.1.2"
 dependencies = [
  "anyhow",
  "bimap",
@@ -760,7 +760,7 @@ dependencies = [
 
 [[package]]
 name = "rustfst-cli"
-version = "1.1.1"
+version = "1.1.2"
 dependencies = [
  "anyhow",
  "clap",
@@ -774,7 +774,7 @@ dependencies = [
 
 [[package]]
 name = "rustfst-ffi"
-version = "1.1.1"
+version = "1.1.2"
 dependencies = [
  "anyhow",
  "downcast-rs",
diff --git a/rustfst/src/semirings/mod.rs b/rustfst/src/semirings/mod.rs
index dcdf216dd..37bdb60bf 100644
--- a/rustfst/src/semirings/mod.rs
+++ b/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};
diff --git a/rustfst/src/semirings/trivial_weight.rs b/rustfst/src/semirings/trivial_weight.rs
new file mode 100644
index 000000000..c70cef546
--- /dev/null
+++ b/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(())
+    }
+}