support subtraction

src/ops.rs

@@ -18,6 +18,26 @@ impl std::ops::Add for F8 {
 	}
 }
 
+impl std::ops::Sub for F8 {
+	type Output = Self;
+
+	fn sub(self, rhs: Self) -> Self::Output {
+		let (m1, e1) = self.split_unbias();
+		let (m2, e2) = rhs.split_unbias();
+		if e1 < e2 {
+			return Self(0);
+		}
+		if rhs.0 == 0 {
+			return self;
+		}
+		let e_diff = e1 - e2;
+		let Some(m) = m1.checked_sub(m2 >> e_diff) else {
+			return Self(0);
+		};
+		Self::merge_unbias(m, e1)
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::*;
@@ -32,4 +52,15 @@ mod tests {
 		assert_eq!(F8::from(0b0011) + F8::from(0b0111), F8::from(0b1010));
 		assert_eq!(F8::from(0b1100) + F8::from(0b0001), F8::from(0b1101));
 	}
+
+	#[test]
+	fn test_sub() {
+		assert_eq!(F8::from(0b0011) - F8::from(0b0000), F8::from(0b0011));
+		assert_eq!(F8::from(0b0000) - F8::from(0b0111), F8::from(0));
+		assert_eq!(F8::from(0b0011) - F8::from(0b0101), F8::from(0));
+		assert_eq!(F8::from(0b0101) - F8::from(0b0011), F8::from(0b0010));
+		assert_eq!(F8::from(0b0110) - F8::from(0b0011), F8::from(0b0011));
+		assert_eq!(F8::from(0b0111) - F8::from(0b0011), F8::from(0b0100));
+		assert_eq!(F8::from(0b1100) - F8::from(0b0001), F8::from(0b1011));
+	}
 }